Ph.D. - Atmospheric Sciences

Permanent URI for this collectionhttps://hdl.handle.net/10125/50831

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Quantification of the presence and production of giant sea salt aerosol in the global marine atmosphere
(University of Hawai'i at Manoa, 2025) Ackerman, Katherine Louise; Nugent, Alison D.; Atmospheric Sciences
Giant sea salt aerosol (GSSA, dry radius > 1 μm) plays a crucial role in atmospheric processes, yet significant uncertainties remain regarding its production, vertical transport, and global distribution. These large particles influence global radiative balance and cloud interactions, but observational limitations have hindered accurate quantification of their production and consequently global presence in the marine atmosphere. This study integrates in-situ measurements and models to build upon our understanding of GSSA production, presence, and dynamics that control their existence in the atmosphere. Recognizing the aerosol concentrations are built up over time, HYSPLIT back-trajectories and ERA5 reanalysis data are used to augment information from instantaneous observations, and ultimately account for the cumulative effects of historical environmental conditions. I assessed the influence of past wind speeds and ocean conditions on GSSA concentrations and determined that traditional correlations with instantaneous environmental variables may overestimate the significance of local conditions, underscoring the need to consider air mass history when interpreting GSSA measurements. Expanding on this idea of utilizing historical information to inform observations, I developed a new GSSA source function based on over 700 size distributions collected across four field campaigns from regions around the world. GSSA particle sizes are often neglected in traditional sea salt aerosol source functions, or their size ranges have gone untested. This new function, derived using a novel, 1-D quasi-concentration build-up model, incorporates information from historical environmental influences to better capture GSSA production. Comparison of this new parameterization alongside existing source functions from earth system models demonstrates a reduction in root mean squared error, ultimately enhancing our ability to estimate SSA contributions and their atmospheric impacts more accurately. Lastly, this research investigates how local effects, like coastal wave breaking, may alter GSSA concentrations in places like Hawai‘i. Historical studies determined that coastal processes and orographic effects can enhance atmospheric concentrations of GSSA particles by orders of magnitude, but the extent of these contributions remains poorly constrained. Field observations from the Hawaiian Island of O‘ahu confirm that coastal wave action and orographic uplift significantly increase GSSA concentrations and facilitate vertical mixing. Measurements collected reveal that coastal concentrations of GSSA particles are 2.7–5.4 times greater than open-ocean levels. Furthermore, significant wave height exhibits the strongest correlation with SSA variability, emphasizing the role of coastal processes in modifying aerosol distributions. This research highlights the importance of accounting for air mass history in aerosol observations, the need for refined source functions to improve global GSSA modeling, and the critical role of coastal environments in GSSA production, ultimately providing important context to highly nuanced observations that have been historically understudied.
THE OROTE POINT THUNDERSTORM: LEESIDE CONVECTION OVER GUAM’S NEARSHORE WATERS DURING THE SEASONAL TRANSITION
(University of Hawai'i at Manoa, 2024) Hitzl, David Eugene; Chen, Yi-Leng; Atmospheric Sciences
The Orote Point Thunderstorm (OPT) is a recurring, mesoscale convective system which forms over Guam’s leeside waters during the monsoon seasonal transition and poses a hazard to local aviation. Historically underpredicted, OPT forecasting can be improved by greater understanding of its mechanisms through climatological analysis and the application of high-resolution mesoscale models. The OPT shares characteristics with small heat island convection and tropical, quasi-stationary, recurring thunderstorms found globally. Like these, it is supported by a seasonal transition pattern, including light to moderate trade winds (<= 7 m s-1), moderate to high CAPE (>1000 J kg-1), and high precipitable water (>40 mm) values. Uniquely, the OPT is triggered by island induced leeside convergence, which is optimized by easterly trades coupled with yearly maximum surface temperatures preceding and following the rainy season. A radar based OPT catalogue indicates greatest OPT frequency during the dry to wet season transition. A 2015-2020 ERA5 OPT climatology identifies the coinciding synoptic features as: ENE to ESE airflow, a weak upper-level trough, moderate to high CAPE and PWAT values (> 1000 J kg-1 and 40 mm, respectively) and steep mid-tropospheric e lapse rates. WRF 36-h daily, high-resolution (500-m) model runs with the Unified Noah Land Surface Model, Coastal Change Analysis Program, (C-CAP)) ground cover, and updated soil moisture are used to construct a 5-year mesoscale, seasonal transition (JJA), climatology. An OPT case study within the climatology demonstrates WRF’s skill in simulating the timing, location, and evolution of an OPT event as verified by multi-spectral satellite imagery. The mesoscale climatology reveals the interdependence of local pressure gradients, differential heating, mesoscale airflows, sensible and latent heat fluxes, and mesoscale vertical motions as they contribute to the development and maintenance of offshore convective cells. A model sensitivity test in which the terrain is replaced with a uniform surface at sea level reveals that the tip jet mechanism leading to local convergence and convection is primarily the result of afternoon land surface heating rather than local topography.
A TALE OF TWO FORCES. How Thermodynamic and Dynamic Forces Contributed to the Record-Breaking Rainfall in the 2018 Kauai Rainfall Event.
(University of Hawai'i at Manoa, 2024) Corrigan, Terrence John; Businger, Steven; Atmospheric Sciences
The interactions of deep moist convection, particularly supercell storms, in the vicinity of substantial terrain, is a relatively under-investigated research topic, especially for sub-tropical coastal locations. While terrain-influenced supercell storms are rare in the subtropics, when they do occur, they often result in prolific rainfall and other weather-related hazards. The islands of Hawaii and their complex terrain features have each hosted many terrain-influenced storms and subsequently produced weather-related hazards ranging from bow echoes, hail, flash-flooding, and tornadoes. Perhaps the most notable case, the 14-15th of April 2018, Kauai rainfall event consisted of a series of three distinct rainfall periods, each supporting radar-inferred evidence of meso-cyclonic rotation, and together breaking the U.S. 24-h rainfall record, with 1262 mm (just under 50 inches) of rainfall reported at Waip ̄a Garden. To this end, Chapter 1 introduces heavy rainfall events, their properties, and their occurrences in Hawai‘i. Chapter 2 thoroughly investigates the observational details of the 2018 Kaua‘i storm event to uncover how a storm of this magnitude evolved, persisted, and produced historic rainfall totals. To substantiate our observational analysis, Chapter 3 investigates this event further with a high-resolution WRF simulation and focuses on the heaviest rainfall period, occurring overnight between 0700-1300 UTC, 15 April 2018. Analyses will focus on properties and features within the upwind storm environment and connect the evolution of convection, vorticity, and cold pools with the trends in rainfall magnitude and location. Lastly, Chapter 4 uses results from Chapter 3 to decompose the total pressure field into its constituent parts related to dynamic and thermodynamic processes using discrete Fourier transformations. With this data, a component analysis of the vertical momentum equation is completed to identify and quantify the mechanisms most responsible for the highly localized, persistent, and historic rainfall during the 2018 Kaua‘i event. Lastly, a summary of the conclusions and discussions is presented in Chapter 5.
Hawaiian wet-season regional climate variability associated with the El Niño and Pacific Meridional Mode
(University of Hawai'i at Manoa, 2024) LU, BO-YI; Chu, Pao-Shin; Atmospheric Sciences
This dissertation aims to delve into the intricate interactions between the El Niño-Southern Oscillation (ENSO) and the Pacific Meridional Mode (PMM), examining their impact on the wet-season regional climate in Hawaii.While previous studies have offered a comprehensive understanding of the influence of canonical El Niño events on Hawaii's climate, the first part of this research focuses on the nuanced interplay within the large-scale atmospheric circulation of the North Pacific during two specific types of El Niño events – eastern Pacific (EP) and central Pacific (CP). The analysis reveals distinctive patterns associated with EP and CP El Niño winters, influencing Hawaii's rainfall and temperature variability. EP El Niño events, characterized by active convective heating in the equatorial EP, create unfavorable conditions for winter rainfall in Hawaii. This is manifested through the enhanced Hadley circulation, anomalous sinking motion, and the eastward shift of the subtropical jet stream in the North Pacific, resulting in drier climates. Conversely, CP El Niño events, featuring weaker and westward-shift equatorial ocean warming and variable sinking motion near Hawaii, lead to a larger variation in rainfall patterns. Additionally, the second part of this dissertation focuses on the PMM’s atmospheric features and their impact on Hawaiian rainfall variability in the preceding winter and concurrent spring. Positive PMM in winter is linked to anomalous surface westerly winds, inducing an east-west rainfall dipole pattern in the Hawaiian Islands. In spring, (+) PMM enhances moisture and rising motion over the tropical North EP, leading to a widespread rainfall in the state. Our assessment shows that moderate daily rainfall intensity over the windward sides and extreme rainfall events over lee sides are elevated in winter in (+)PMM. In contrast, springtime windward sides see a decrease in light daily rainfall frequency and an increase in moderate daily rainfall frequency. For the leeward side in spring, a substantial higher occurrence of heavy/extreme rainfall events is found in (+) PMM. Through the multiple linear regression, ENSO dominates the Hawaiian rainfall pattern in winter, whereas PMM plays a more critical role in spring. The final section of this dissertation delves into the oceanic dynamical processes underlying the evolution of PMM-related equatorial warming from 1950 to 2020. Emphasis is placed on identifying the dominant factors contributing to PMM-related equatorial warming in both the prior and posterior winter, as these factors play a crucial role in modulating ENSO diversity and intensity. The preexisting PMM-related equatorial warming in the prior winter is linked to anomalous positive vertical temperature gradients generated by convergence and descending motions via Western North Pacific Oscillation-related anomalous westerlies and Ekman transports. Moreover, the extensive PMM-related El Niño-like equatorial warming in the following winter is associated with meridional temperature advection, thermocline feedback, and Bjerknes feedback. The study also identifies interdecadal changes in PMM-related equatorial warming and examines potential influences of the Interdecadal Pacific Oscillation (IPO) and global warming effects on this interdecadal variation.
Influence of El Niño Southern Oscillation on Tropical Eastern Pacific Mean State and Annual Cycle
(University of Hawai'i at Manoa, 2023) Xue, Manrui; Li, Tim TL; Atmospheric Sciences
This dissertation explores the role of El Niño Southern Oscillation (ENSO) in modulating the mean state and annual cycle in the tropical eastern Pacific. In the first part, characteristics of the tropical climate mean state, annual cycle, and ENSO along with their associated essential mechanisms are introduced. On the one hand, the tropical Pacific mean state is critical to determine the preferred location of maximum sea surface temperature (SST) variability associated with ENSO. The eastern equatorial Pacific (EEP) permits an effective air-sea coupling on the interannual timescale, leading to ENSO growth. On the other hand, the impact of the mean state on ENSO can also be inferred from its phase locking to the annual cycle. To understand the capability of ENSO in modulating the climate state over the tropical eastern Pacific, the second part with observational analysis reveals that the linear ODH associated with ENSO has a considerable amplitude. However, its impact on the long-term SST is small due to changing signs between El Niño and La Nina phases. The nonlinear oceanic dynamic heating (ODH) exhibits a consistent sign during warm and cold episodes, but its weak amplitude limits its cumulative effect on the mean SST. As for thermodynamic processes, surface heat flux shows weak effects, with downward longwave radiative heating offsetting the net shortwave radiative heating. In the third part, the OGCM numerical experiments confirm the observational findings, indicating a modest rectification effect of ENSO on the mean state. In the fourth part, the modulation effect of ENSO on the annual cycle is investigated in the equatorial eastern Pacific by applying the analytical heat budget model. The analysis reveals that ENSO primarily modulates the annual cycle through ODH processes, including anomalous temperature advection by the mean vertical and meridional currents, anomalous zonal advection associated with the geostrophic current, and anomalous upwelling induced vertical advection. Thermodynamic processes induced by ENSO also exert modulation effects, with downward longwave radiative flux reinforcing the ocean dynamic impact and surface latent heat flux offsetting it. These ENSO impacts on the EEP annual cycle persist from fall to winter and weaken during spring. Overall, the rectification effect of ENSO on the mean SST in the tropical eastern Pacific is found to be moderate. However, ENSO exhibits a significant impact on the annual cycle, primarily through ODH processes. The relationships between ENSO, mean state, and annual cycle are analyzed using a quantitative diagnostic methodology.
Subtropical Monsoon Moist Dynamics And Its Future Change
(University of Hawaii at Manoa, 2022) Yang, Guang; Li, Tim; Atmospheric Sciences
Different from classic mid-latitude dry baroclinic instability theory, atmospheric motions over the subtropical Meiyu front in boreal summer are dominated by synoptic-scale disturbances coupled with precipitation and moisture under a weaker background vertical shear. This moisture-precipitation-circulation interactive feature, along with a preferred zonal wavelength of about 3400 km and eastward phase propagation is explained by a moist baroclinic instability theoretical framework. The new framework is an extension of a traditional 2-level baroclinic model by considering a prognostic moisture equation, the moisture-precipitation-circulation feedback and an interactive planetary boundary layer. An eigenvalue analysis of the model shows that the most unstable mode has a preferred zonal wavelength of 3400 km, a westward tilted vertical structure, and a phase leading of maximum moisture and precipitation anomalies relative to a lower-tropospheric trough, all of which are in good agreement with observations. Both anomalous horizontal and vertical advection processes contribute to the moisture increase. Further sensitivity tests show that the instability and the zonal scale selection primarily arise from the moisture-convection-circulation feedback, while the vertical shear provides an additional energy source for the perturbation growth. This moist baroclinic instability theory explains well the observed characteristics of the development of synoptic-scale disturbances along the Meiyu front. Dominant summertime disturbances along the subtropical Meiyu front are eastward-propagating synoptic-scale waves coupled with precipitation and moisture under a moderate background vertical shear. To what extent the intensity and structure of such synoptic disturbances would change under global warming is investigated by diagnosing 18 models from Phase 6 of the Coupled Model Intercomparison Project (CMIP6). The model diagnosis reveals that there is a robust increase in the intensity of synoptic-scale motions along the Meiyu front, while the wavelength and phase speed remain unchanged. The cause of such changes of the synoptic-scale variability in the future warmer climate is investigated through the analysis of a moist baroclinic instability model. It is found that the growth rate of the most unstable mode strengthens in the future warmer climate, while the zonal wavenumber and phase speed of the most unstable mode remain unchanged, which is consistent with the CMIP6 projections. The enhanced synoptic-scale variability is primarily attributed to the increase of background meridional and vertical moisture gradients under a warmer climate through strengthened positive moisture-convection-circulation feedback, while the changes of background vertical shear and convective adjustment times are insignificant. The relative roles of the land and ocean surface warming in causing strengthened East Asian summer monsoon (EASM) circulation but weakened South Asian summer monsoon (SASM) circulation under global warming are investigated with an intermediate atmospheric model. By extending a 2.5-layer intermediate atmospheric model to cover both tropical ocean and land regions, we examine how the EASM and SASM circulations respond to idealized land and ocean surface warming patterns. The model is able to reproduce the gross features of the summer monsoon circulation in the present-day climate state, with pronounced southerlies over the EASM region and westerlies over the SASM region. Forced by the land and ocean surface warming patterns derived from Phase 5 of the Coupled Model Intercomparison Project (CMIP5) and CMIP6 model ensembles, the intermediate model simulates successfully a strengthened EASM circulation and a weakened SASM circulation, consistent with CMIP5 and CMIP6 projections. Idealized model experiments are further designed to reveal the relative roles of a greater land than ocean warming and an El Niño-like warming pattern in causing the distinctive circulation changes in South and East Asia. It is found that the strengthened EASM circulation is mainly attributed to the land-ocean warming contrast, whereas the weakened SASM circulation is primarily caused by an El Niño-like warming pattern in the tropical Pacific and an Indian Ocean Dipole-like warming pattern in the Indian Ocean.
The Rapid Change Of Arctic Sea Ice Over The Past Three Decades
(University of Hawaii at Manoa, 2022) Zhou, Xiao; Wang, Bin; Atmospheric Sciences
The rapid change of Arctic sea ice since the end of the 20th century has drawn much attention as an indicator of local and global climate. General global warming and amplified Arctic temperature rise have made the sea ice more vulnerable to natural fluctuation in the atmospheric and oceanic forcing. The area extent of the ice cover has been decreasing at an accelerating rate from 1998 to 2007, exhibiting more than seven times faster than from 1988 to 1997. However, the relative contribution of external forcing to the long-term decline of sea ice is far from clear. The reason for accelerated sea ice loss still remains unresolved. In addition, coupled global climate models are the primary objective tools to investigate the underlying mechanisms of sea ice response and provide future projections based on physical laws. However, substantial uncertainty persists in models’ simulated sea ice loss rate. Understanding the uncertainty source and selecting realistic models to make a reliable projection with reduced inter-model spread remains a frontline challenge.It is well recognized that the rapid change of sea ice has both external drivers and internal variabilities, but their relative importance is poorly known. A set of numerical experiments are conducted to detect the impact of external forcing on the sea ice. The numerical results reveal that the Arctic near-surface air temperature and its related longwave radiative and turbulent sensible heat play a principal role in melting Arctic sea ice and capture about 90% long-term decline trend. The sea ice thickness (SIT) also regulates the seasonal sea ice evolution in response to the Arctic warming. The observation result proves that the April SIT determines the sea ice area reduction rate in the melting season and the ice-albedo feedback further amplifies the sea ice retreat. The abrupt warming in the freezing season (DJFM) since 1998 significantly reduced the sea ice growth and further drove a rapid thinning of sea ice in April. The thinned sea ice favors a faster ice area reduction and accelerates the sea ice retreat in the melting season. Therefore, the September sea ice area (SIA) has dramatically declined since 1998. Extensive attention has been given to the performance of the sea ice model. Forty updated CMIP6 sea ice simulations are examined in this study. As with CMIP3 and CMIP5, a wide inter-model spread persists in the simulated September sea ice area. This huge uncertainty makes the all-model ensemble mean less meaningful. Therefore, selecting realistic models to create a reliable projection with reduced uncertainty is imperative. The existing SIA-based evaluation metrics are deficient due to observational uncertainty, prominent internal variability, and indirect Arctic response to global forcing. The result reveals that the model uncertainty may derive from the underestimation of near-surface air temperature and the inadequate intensity of ice-thickness adjustment in the melting season. Considering the dominant role of sea ice thickness in determining Arctic ice variation throughout the seasonal cycle, two SIT-based metrics are proposed, the April mean SIT and summer SIA response to April SIT, to assess climate models’ capability to reproduce the historical change of the Arctic sea ice area. The selected 11 good models reduce the uncertainty in the projected first ice-free Arctic by 70% relative to the 11 poor models. The chosen models’ ensemble mean projects the first ice-free year in 2049 (2043) under the SSP2-4.5 (SSP5-8.5) scenario with one standard deviation of the inter-model spread of 12.0 (8.9) years. Understanding the dominant role of sea ice thickness (SIT) could be an essential step in figuring out the sea ice seasonal evolution and the long-term decline in response to Arctic warming. SIT is determined by the near-surface air temperature in the winter, and it affects the sea ice melting rate in the following summer. The April SIT acts as a bridge, linking the winter freezing and summer melting processes together. Furthermore, the uncertainty of the sea ice model may also derive from the incorrect simulation of sea ice thickness. Given the notable spread in the CMIP6 models’ simulation, modelers should pay specific attention to improving sea ice thickness simulation, including the winter growth and summer thinning processes. Although some progress has been made, some critical issues still remain unresolved. The summer sea ice melting significantly slowed down after 2008 and showed a shallow mean state with strong interannual oscillation until the present. The reason is still unclear. In addition, the abrupt Arctic warming in the freezing season since 1998 remains a problem that currently receives extensive attention in the Arctic community. Both of these obstacles need to be addressed. A deep understanding of the sea ice behavior in response to Arctic warming could, to some degree, prepare us for a potentially ice-free Arctic in the future.
An In Situ Analysis Of Marine Stratocumulus Environments And The Associated Boundary Layer, Turbulent, And Preferential Concentration Characteristics
(University of Hawaii at Manoa, 2022) Dodson, Dillon; Small Griswold, Jennifer D.; Atmospheric Sciences
It is well known that fluid turbulence can affect cloud droplet motion, leading to preferential concentration (or droplet clustering), which in turn can impact precipitation formation through influences on collision-coalescence. Previous work suggests that droplet clustering occurs on the order of the Kolmogorov length scale η, with the magnitude of said clustering depending on the Stokes number St. The accuracy of these theories remains largely unquantified for in situ atmospheric clouds however. Along with this, developing a better understanding of processes occurring on subgrid scales is important for better representing stratocumulus (Sc) decks, with most models struggling to resolve boundary layer vertical structure and turbulence, important variables in determining Sc cloud properties. Although turbulence is a critical component to boundary layer and microphysical processes, literature describing cloud-related turbulence based on in situ data is scarce. This work therefore analyzes (1) boundary layer and turbulent characteristics, along with synoptic-scale changes in these properties over time; (2) the in situ characteristics of droplet clustering and the associated effects of drop size d and turbulence. This is done using data collected onboard the CIRPAS Twin Otter Aircraft during the Variability of the American Monsoon Systems (VAMOS) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) through 18 research flights at Point Alpha (20S, 72W) in October and November 2008. This campaign sampled the weakly turbulent Peruvian marine Sc deck. Spatial statistics of cloud droplet arrival times recorded by the phase-Doppler interferometer are analyzed by means of the one-dimensional pair-correlation function η(l). The average boundary layer depth is found to be 1148-m, with 28% of the boundary layer profiles analyzed displaying decoupling. An increase in boundary layer height zi is accompanied by a decrease in turbulence within the boundary layer. As zi increases, cooling near cloud top cannot sustain mixing over the entire depth of the boundary layer, resulting in less turbulence and boundary layer decoupling. As the latent heat flux (LHF) and sensible heat flux (SHF) increase, zi increases, along with the cloud thickness decreasing with increasing LHF. This suggests that an enhanced LHF results in enhanced entrainment, which acts to thin the cloud layer while deepening the boundary layer. A total of 10 of the 18 flights display two peaks in turbulent kinetic energy (TKE) within the cloud layer, one near cloud base and another near cloud top, signifying evaporative and radiational cooling near cloud top and latent heating near cloud base. Decoupled boundary layers tend to have a maximum in turbulence in the sub-cloud layer, with only a single peak in turbulence within the cloud layer. Measures of TKE averaged in-cloud over all 18 flights display a maximum near cloud middle (between normalized in-cloud height Z∗ values of 0.25 and 0.75). Droplet clustering occurs in 95% of cases analyzed, with the magnitude of said clustering becoming significant in the turbulence dissipation range, at a length scale of ∼2η. Analyzing η(l) as a function of Z∗ indicates (i) a maximum in the magnitude of average droplet clustering occurs near Sc middle, at Z∗ = 0.47, (ii) droplet clustering and St are found to be strongly correlated at a statistically significant rate. An enhanced (reduced) aerosol number concentration Na is found to result in enhanced (reduced) clustering at spatial scales be- low 1-mm, but a reduction (enhancement) in the length scale at which clustering becomes significant. The presence of droplet polydispersity results in a saturation value in the preferential concentration being reached, where enhanced (reduced) polydispersity for low (high) Na environments leads to a reduction (enhancement) in clustering below 1-mm.
Summer Atmospheric Heat Sources Over The Tibetan Plateau
(University of Hawaii at Manoa, 2021) Xie, Zhiling; Wang, Bin; Atmospheric Sciences
The spatial-temporal characteristics of the summer atmospheric heat sources over the Tibetan Plateau (TP) are revisited in the first part of this dissertation, applying various bias-corrected datasets, including reanalyses, gauge observations, and satellite products. Verification-based selection and ensemble-mean methods are taken to combine multiple datasets. Compared to previous studies focused on the eastern TP, this study pays special attention to the heat sources over the data-void western plateau. A climatological minimum in the total heat is found in the high-altitude region of the northwestern TP. The TP total heat showed insignificant trends over the eastern and central TP (ETP/CTP) during 1984–2006, whereas exhibited an evident increasing trend over the western TP (WTP). The interannual variation of total heat over the central-eastern TP is dominated by the variation of latent heat from precipitation. However, over the western TP, the variation of the total heat is highly correlated with net radiation and surface sensible heat. The remote forcings and impacts of the interannual variations in summer heat sources over the eastern, central, and western TP are investigated in the second and third parts with observational analyses and numerical model experiments. The summer heat source variability is affected by different remote forcings across the TP from east to west. The ETP precipitation (i.e., latent heating) is likely modulated by North Atlantic Oscillation (NAO) and associated SST anomalies through large-scale wave trains propagating from Western Europe to East Asia. On the other hand, the increased CTP precipitation is primarily driven by a developing La Niña through generating southerly wind anomalies to the south of the CTP, enhancing moisture transport and precipitation over the southern CTP. The increased WTP sensible heating is linked to the tropical western Pacific cooling, central Pacific warming, and North Atlantic cooling. These anomalous SST conditions produce a high-pressure anomaly over the WTP, raising the ground-air temperature difference, thereby enhancing the WTP sensible heat. The results in the third part show that the ETP, CTP, and WTP heat sources have different impacts on regional climate and teleconnection. A warming center in northwestern Asia and a cooling center in western Europe are connected with the ETP heating. The CTP heating is related to northeastern Asian warming and East Asia cooling. The WTP heating is linked to the warming in southeastern China and the polar region of Asia. The linear wave-train responses to the TP heating forcings exhibit notable differences. The ETP heating generates an upper-level wave train propagating eastward to the northwestern Pacific. The wave train excited by the CTP heating propagated far eastward to central North Pacific. The WTP heating produces a wave train that splits into two branches, the northern one propagating northeastward to the Arctic region and the southern one propagating eastward to coastal northwestern Pacific. The fourth part of this dissertation presents 22 CMIP6 models’ performances and future projections for the eastern-TP summer precipitation and sensible heat flux. Nearly all models can well simulate the observed climatological precipitation pattern (1979–2014) but overestimate the mean by 65%. For sensible heat, nearly half of the models can hardly capture the spatial structure. The multimodel ensemble mean of selected high-performance models projects that, under the medium emission scenario (SSP2-4.5), the summer precipitation will likely increase by 2.7% per degree Celsius global warming due to the future enhancement in surface evaporation and vertical moisture transport that are partially offset by weakening ascending motion. The projected sensible heat will likely remain unchanged, associated with the likely unaltered surface wind speed.
Dynamics of El Niño-Southern Oscillation Diversity in an Intermediate Coupled Model
(University of Hawaii at Manoa, 2021) Geng, Licheng; Jin, Fei-Fei; Atmospheric Sciences
The El Niño-Southern Oscillation (ENSO) phenomenon features rich sea surface temperature (SST) spatial pattern variations dominated by the central Pacific (CP) and eastern Pacific (EP) patterns at its warm phase. Understanding of such ENSO pattern diversity has been a subject under extensive research activity. The fundamental dynamics for ENSO diversity, however, remains elusive after decades of effort. In this dissertation, an intermediate coupled model based on the Cane-Zebiak-type framework whereas with revised model formulation and well-tuned parameterization schemes, denoted as R-CZ, has been independently built to unveil the dynamics and sensitivity of ENSO diversity. The main findings of this dissertation are as follows. Firstly, through the linear stability analysis, it is demonstrated that there exists a unique ENSO-like leading oscillatory mode within the R-CZ framework. This demonstration precludes the possibility suggested in some earlier studies that the observed CP and EP ENSO may be randomly excited from two coexisting linear ENSO modes under the same climate conditions. The ENSO mode derived in R-CZ exhibits significant sensitivity to feedback processes and mean states. As noted in earlier studies, the ENSO mode is rooted in either the recharge-oscillator (RO) mode or the wave-oscillator (WO) mode. This study further illustrates that the RO and WO modes compete for predominance depending on the relative intensity of the zonal advective feedback and the thermocline feedback. It is the competition between these two generic modes that determine the uniqueness of the ENSO mode. Secondly, a generalized nonlinearity/noise-induced regime transition (NIRT) mechanism is proposed for the pathway from the linear ENSO mode to ENSO diversity. In the subcritical regime where the ENSO mode is stable, ENSO events are excited with external forcing and feature spatial patterns similar to that of the ENSO mode characterized by maximum SSTA over the central-eastern Pacific. In the strongly supercritical regime where the ENSO mode has a large growth rate, the nonlinear growth of the ENSO mode leads to strong ENSO cycles and induces a mean climate drift due to the nonlinear rectification effect. The mean climate drift features a west (cold)-east (warm) dipole of SST and weakens the climatological cold tongue. EP ENSO-like oscillation with a longer period than expected from the ENSO mode is favored under the drifted mean climate. Consequently, EP ENSO manifests as a strong attractor in this supercritical regime. Within the in-between near-critical regime, also named the diversity regime, CP and EP ENSO resembling those observed coexist as nonlinearity allows transition between the two ENSO regimes (i.e., the linear ENSO regime and the nonlinear EP ENSO regime). Such a diversity regime is broadened with stochastic processes. NIRT serves as a unified paradigm for ENSO diversity as it encompasses the most relevant processes suggested in the literature, namely the atmospheric nonlinear convective heating, the oceanic nonlinear dynamical heating, and the stochastic excitation. Thirdly, the sensitivity of ENSO diversity is examined in idealized mean state space. With global warming-like mean state change, the ENSO mode tends to shift towards being more unstable, primarily attributed to the weakened zonal advective feedback and the strengthened Ekman feedback. Correspondingly, EP (CP) ENSO is projected to experience more (less) frequent occurrences. However, the exact impact of global warming on ENSO diversity depends on how close the ENSO mode stability of the climate system is to the criticality and deserves further study.
Inter-basin teleconnection between the tropical Pacific and tropical Atlantic
(University of Hawaii at Manoa, 2021) Jiang, Leishan; Li, Tim; Atmospheric Sciences
In this dissertation, the inter-basin teleconnection between the tropical Pacific and the tropical Atlantic is investigated through observational analyses and numerical simulations. In the first part, the impact of the El Niño-Southern Oscillation (ENSO) on tropical North Atlantic (TNA) sea surface temperature anomaly (SSTA) is discussed. During El Niño decaying spring, the TNA region displays a significant warm SSTA, which is mainly caused by trade wind-induced surface latent heat flux anomaly. ENSO can generate anomalous southwesterlies over the TNA region through an extratropical pathway (via the Pacific - North American pattern) and a tropical pathway (remote Gill response with suppressed tropical Atlantic rainfall). Both a partial regression analysis and numerical simulations indicate that the extratropical (tropical) pathway contributes to approximately 60% (40%) of the observed wind anomaly in TNA.In the second part, the relationship between the ENSO and equatorial Atlantic (EA) variability is explored. While boreal summer EA SSTA has an insignificant correlation with the preceding ENSO, it exists a robust simultaneous negative correlation with the Pacific SSTA (Niño3.4 index). A further analysis shows that both the El Niño and La Niña events in boreal winter precede a warm EA event. The physical cause of the asymmetric ENSO impacts is explored. It is found that El Niño impact is primarily through the preconditioning of the Atlantic SSTA during El Niño developing fall and mature winter, whereas the La Niña impact is mainly via the remote teleconnection pattern during boreal spring and summer after the peak of the La Niña. The season-dependent feature is attributed to distinctive phase evolution characteristics between El Niño and La Niña. In the third part, how the tropical Atlantic SSTA variability affects subsequent ENSO evolution is investigated. It is shown that the TNA (EA) forcing tends to generate a CP-type (EP-type) ENSO event due to the relative longitudinal location of the TNA and EA SSTA forcing. While a basin-wide warming pattern in the Atlantic exerts a robust influence on the Pacific, a meridional dipole pattern cannot. By comparing four different forcing experiments (TNA, EA, basin warming and dipole pattern), we demonstrated the essential role of the Kelvin wave response and the associated atmosphere-ocean interaction over the northern Indian Ocean (NIO) and Maritime Continent (MC) in connecting the Atlantic SSTA forcing and zonal wind anomalies and thus ENSO over the equatorial Pacific. When the Atlantic-induced Kelvin wave process is absent, the zonal wind anomaly generated by the Rossby wave process only remained in the off-equatorial Pacific region, which hardly affects the subsequent ENSO evolution. Further sensitivity experiments show that the TNA/EA forcing is not strong enough to alter the sign of a pre-existing ENSO, but can modulate the amplitude of the ENSO events.
Island Effects on Rainfall for the Hawaiian Islands with Mountaintops below the Trade Wind Inversion
(University of Hawaii at Manoa, 2020) Hsiao, Feng; Chen, Yi-Leng; Atmospheric Sciences
The goal of this research is to investigate the island-scale rainfall, weather, and climate over various local communities in Hawai‘i, especially for islands (O‘ahu and Kaua‘i) with mountain heights below the typical trade wind inversion. High-resolution modeling and satellite observations during July–August 2013 are used to the physical processes responsible for wake circulation and clouds over O‘ahu in the lee-side wake zone under trade wind conditions. In the morning, orographic clouds are more significant on the windward side of the Koʻolau Range when trades are stronger, and are advected downstream by trade winds aloft. During the daytime, warmer air over the island interior is advected off the western leeside coast by downslope winds under strong trades. In contrast, upslope/sea-breeze flow occurs along the leeside coast under weak trades with orographic clouds on the western leeside slopes after sunrise under weak trades. These clouds are advected westward by the combined trade wind and the easterly return flow aloft, resulting in more significant cloudiness in the wake zone with larger horizontal extent in the early afternoon when trades are weaker. Short-lived afternoon heavy rainfall events may form over Central Oʻahu during seasonal transition periods (June and October) under favorable large-scale settings. These include a deep moist layer with relatively high total precipitable water (TPW, > 40 mm), a blocking pattern in mid-latitudes with a northeast–southwest moist tongue from low latitudes ahead of an upper-level trough, the absence of a trade wind inversion, and variable (< 3 m s-1) low-level winds. Our high-resolution (1.5 km) model results show that daytime land surface heating deepens the mixed layer over Central Oʻahu while the top of the mixed layer reaches the lifted condensation level. Meanwhile, the development of onshore/sea-breeze flows, driven by land–sea thermal contrast, brings in moist maritime air over the island interior. Finally, convergence of the onshore flows over Central Oʻahu provides the localized lifting required for the release of instability. Based on synoptic and observational analyses, nowcasting with a lead time of 2–3 hours ahead of this type of event is possible. In the absence of orographic effects after removing model topography, the destabilizing effects of daytime heating, horizontal advection of moist maritime air inland by the onshore/sea-breeze flows, convergence over Central Oʻahu, and the subsequent development of the heavy showers over land are still simulated. However, when surface fluxes are turned off in the NF run, convective cells are not simulated in the area. These results indicate that daytime heating is crucial for the development of this type of heavy rainfall event under favorable large-scale settings. During typical summer trade wind conditions, orographic precipitation occurs frequently over the mountainous Hawaiian Islands. The mountaintop of Kaua‘i, one of the wettest spots on earth, has the highest rainfall amount during the summer months among the Hawaiian Islands. Based on model sensitivity test, our results show that the orographically induced moisture flux convergence and orographic lifting at the slope surface are the two most dominant terms for the rainfall production. In addition, rainfall production also related to TPW in the environment. Under undisturbed summer trade-wind weather, the moisture flux convergence is related to orographic and local effects, including: flow deceleration on the windward side due to orographic blocking, interactions between the incoming flow and offshore flow due to nocturnal and/or evaporative cooling, and convergence between the incoming winds and opposing sea breezes in the leeside. Furthermore, moisture flux convergence is enhanced by latent heat release. The vertical lifting by winds on the slope surface could be attributed to mechanical lifting and modified by daytime upslope and nighttime downslope winds. For previous theoretical studies on orographic precipitation based on Froude number and orographic rainfall index, the orographically induced moisture convergence and convective feedbacks as well as diurnal variations in land surface heating are largely ignored. The El Niño-Southern Oscillation (ENSO) is a prominent mode of climate variability at the inter-annual time scale. During the negative phase of ENSO (El Niño), Hawai‘i frequently experiences droughts in winter, which continue into the following spring. Less is known about the impact of El Niño on local spatial patterns of rainfall, temperature, moisture and winds. Recent studies have shown that there are two different flavors of El Niño [east Pacific type/Cold Tongue (CT) and central Pacific type/warm pool (WP)]. With large spatial variations of local climate, better understanding of the impact of El Niño favors on different island communities is investigated with regional and island-scale simulations. During El Niño winters, especially under CT events, synoptic subsidence is greater with lower TPW over Hawai‘i than long-term seasonal mean due to a merged inter-tropical convergence zone and South Pacific convergence zone and eastward shift of the Walker Circulation. During El Niño springs, the enhanced tropical convergence weakens with less significant impacts on subsidence over the Hawai‘i than during winters. The results from 6-km simulations during 1979–2017 shows drier conditions during the CT events than during the WP events, and the dry conditions are more significant in winter. During El Niño winters, the Hawaiian region has positive temperature anomalies at the 700 hPa level and are more significant (1.0 K) during the CT winters than during the WP winters (0.5 K). Five CT, six WP, and six neutral events during winter and spring are simulated with an island-scale model over O‘ahu and Hawai‘i. The simulated rainfall patterns are comparable to the Rainfall Atlas of Hawai‘i, however, the high-resolution model overestimates the rainfall amount. During both types of El Niño events, the nighttime cooling accentuated by less atmospheric water vapor affects the simulated surface temperatures over the regions below the trade wind inversion. For island regions above the trade wind inversion, surface warming is accentuated by increased subsidence associated with the Hadley circulation. For Hawai‘i Island, drought conditions are expected during the CT winters over the mountaintops of Mauna Kea and Mauna Loa and in the leeside of the Kohala Mountains due decreased trade wind and low TPW. During the CT springs, persistently weak trade wind speed and low TPW result in maximum rainfall deficiencies over the windward slopes and mountaintops below the trade wind inversion. However, the dry conditions over the windward slopes are less significant during the CT springs than during the CT winters. The only region with greater daily rainfall during the CT springs is off the Kona coast, before sunrise, resulting from the convergence of katabatic flows with the westerly winds offshore. The rainfall distributions during the WP events are quite similar to those during the CT events, however, the dry conditions are less significant due to higher TPW in the environment. For islands with mountaintops below the trade-wind inversion, during the CT winters, increased synoptic subsidence and low TPW are crucial in resulting in drier or even drought conditions over the mountaintops, e.g. the Ko‘olau Mountains on O‘ahu. Daily rainfall over the Ko‘olau Mountains and windward coast are lower during the CT winters than the neutral winters. During the CT springs, the dry conditions are still simulated over mountainous regions. For WP events, mountainous regions with drier than neutral conditions are less significant. During the WP springs, more rainfall is only simulated over the southwestern coasts in the afternoon hours than during the neutral springs due to enhanced thermally driven circulations over the leeside coasts due to warmer surface daytime temperature over land than during the neutral springs.
Interactions between the Madden Julian Oscillation and High-Frequency Waves
(University of Hawaii at Manoa, 2020) Zhu, Yan; Li, Tim; Atmospheric Sciences
The Madden Julian Oscillation (MJO), characterized by the pattern of a large-scale convective envelope, a planetary-scale circulation and a slow eastward phase speed of 5 m/s, is the dominant intraseasonal mode in the tropics. Although numerous theories have been proposed to explain its eastward propagation, the physical mechanisms behind the phenomenon remain open. Many state-of-the-art climate models have difficulty in capturing the realistic MJO eastward propagation, which suggests that some critical process involved is yet unrevealed. The observed multi-scale structure of the MJO convective envelope implies that the interactions between MJO and high-frequency waves (HFW) might play an important role in promoting the eastward propagation. Both the diagnosis of observational and model data and idealized model simulations are utilized to address the MJO-HFW interaction problem. Several major findings are discussed in this paper. (1) Observed HFW activity is enhanced over and to the west of the MJO convective center and weakened to the east of the MJO convective center, due to the impact of MJO-scale vertical zonal wind shear and specific humidity. (2) The weakened HFW activity to the east of the MJO convective center moistens the lower troposphere through reduced dry air mixing from extratropics, which promotes the eastward propagation of MJO convection through the development of the shallow and congestus clouds preceding the MJO convection. (3) The increased (decreased) HFW activity west (east) of the MJO convective center contributes to a positive (negative) eddy zonal momentum flux, which favors the eastward propagation of MJO flows by inducing a positive intraseasonal zonal wind tendency. (4) The aforementioned observed phase relationship between the MJO and HFW is only found in a “good” model group, which is defined as models that capture the observed eastward propagation. (5) Unrealistic distribution of HFW activity in the “poor” model group leads to unrealistic nonlinearly rectified condensational heating and eddy momentum flux divergence distributions, which promote a westward-moving tendency. (6) Whereas mixed Rossby-gravity (MRG) waves are enhanced (weakened) to the west (east) of the MJO convection due to the modulation of MJO-scale vertical wind shear, inertio-gravity waves are strengthened only over the MJO convection region due to their strong dependence on MJO-scale specific humidity. Kelvin waves, on the other hand, are modulated by both the vertical wind shear and specific humidity. It is anticipated that these findings may shed light on improving our current understanding of MJO and the representation of realistic MJO structure and eastward propagation in climate models.
Characterization of Non-El Niño Induced Dry Conditions across the U.S. Affiliated Pacific Islands
(University of Hawaii at Manoa, 2017-12) Ludert, Alejandro T.; Atmospheric Sciences
The U.S. A liated Paci c Islands (USAPIs), located in the Western Paci c, have limited water resources making them very susceptible to severe drought conditions. The annual cycle and ENSO response of rainfall di ers between USAPIs north of 7 N and those to the south. Southern stations show a canonical negative correlation between dry season (December to May) rainfall and ENSO. Northern stations, on the other hand, show little correlation with ENSO if the three super El Ni~nos are excluded. Instead, they exhibit two distinct rainfall regimes, the Canonical regime, and a Non-Canonical regime, in which the dry season rainfall is positively correlated with ENSO. Non-canonical years pose an important forecasting challenge. Cool Dry events are of particular interest because they have coincided with several emergency and disaster-level droughts across the Northern USAPIs. Composite analysis of the Canonical and Non-Canonical regimes show stark di erences between dry season atmospheric and SST patterns. Compared to Canonical composites, the Non-Canonical composites show clear and previously undescribed anomaly patterns during the dry season. In Cool Dry events, circulation anomalies over the Western Paci c are anticyclonic, with a band of anomalous dry conditions extending from the central Paci c towards Micronesia that causes unexpected droughts in the Northern USAPIs. Canonical Cool Wet events, on the other hand, show cyclonic West Paci c circulation anomalies and a La Ni~na like horseshoe rainfall pattern over the Paci c Basin. Non-canonical Cool events also show SST anomalies narrowly constrained near the dateline, while Canonical Cool events show their largest anomaly magnitude east of the dateline. Both Non-Canonical and Canonical Cool events show negative rainfall and Western Paci c anticyclonic anomalies before the onset of the Dec-May dry season. In Non-Canonical events, these anomalies persist throughout the dry season, while for Canonical events they shift, rapidly becoming positive rainfall and cyclonic circulation anomalies during the dry season. SST anomalies also evolve di erently, with Non-Canonical Cool events showing anomalies that extend eastward from the central Paci c rather than intensifying in place over the eastern Paci c. The features are similar and opposite for Canonical and Non-CanonicalWarm events. Di erences in the evolution of anomalies suggest that the physical mechanisms governing the Non-Canonical and Canonical ENSO regimes are distinct. These di erences have been leveraged to develop a novel 2-tier forecasting methodology that combines logistic and linear regression to forecast the Dec-May dry season Standardized Precipitation Index in the Northern USAPIs. This 2-tier methodology achieves signi cant improvement in the forecast of Dec-May rainfall anomalies as compared to a benchmark forecast. This type of improved forecasts will help provide local governments and decision makers with guidance for mitigation and relief during Non-Canonical events.
The Role of Boundary Layer Dynamics in Tropical Cyclone Intensification
(University of Hawaii at Manoa, 2018-12) Li, Tsung-Han; Wang, Yuqing; Atmospheric Sciences
This study examines the role of boundary layer dynamics in tropical cyclone (TC) intensification. The hypothesis is that although surface friction has a negative effect on TC intensification by the frictional dissipation (dissipation effect), it contributes positively to TC intensification by modifying the strength and radial location of eyewall updrafts/convection and also enhancing eyewall contraction (indirect effect). To test the hypothesis and isolate the direct dissipation effect and indirect effect, three models with different complexities are used to conduct idealized experiments with a varying surface drag coefficient (CD) and TC vortex structure. Results from a simplified dynamical framework, which includes three layers: a multi-level boundary layer model below, a shallow-water-equation model as the middle layer, and an upper layer, are discussed first. In this framework, the mass sink in the middle layer is parameterized by a mass-flux at the top of the boundary layer to mimic eyewall heating such that the indirect effect of boundary layer dynamics on TC intensification can be evaluated. Namely, the frictional boundary layer in response to gradient wind distribution above the boundary layer controls the strength and radial location of eyewall convection, which in turn contributes to eyewall contraction and intensification of the gradient wind. Results show that through the indirect effect of surface friction, TCs with larger surface drag coefficient, smaller radius of gale wind (RGW), and higher intensity display faster eyewall contraction and more rapid intensification, and that the fastest intensification occurs for TCs with the initial radius of maximum wind (RMW) at around 80 km. Results from full-physics model simulations using the Tropical Cyclone Model version 4 (TCM4) are discussed with the focus on the relative importance and the combined direct dissipation and indirect effect of boundary layer dynamics due to the presence of surface friction on TC intensification. Results show that the intensification rate of a TC during the primary intensification stage is insensitive to CD (including the surface wind-dependent CD), suggesting that the positive contribution due to the indirect effect of surface friction to TC intensification is almost offset by the negative contribution resulting from the direct dissipation effect of surface friction. However, greater surface friction can significantly shorten the initial spin-up stage (the onset of the primary intensification stage) through faster moistening of the inner core of the TC vortex but would lead to a weaker storm in the mature stage. The results thus strongly suggest that the boundary layer dynamics contributes significantly to the onset of the primary intensification while has important but dual opposite effects on the subsequent intensification rate during the primary intensification stage. Results from further sensitivity experiments demonstrate that the TC vortex initially with a smaller RMW or a smaller RGW has a shorter initial spin-up stage and intensifies more rapidly during the primary intensification stage through the indirect effect of the boundary layer dynamics discussed in the simplified framework.
The Asian Summer Precipitation Over The Past Five Centuries
(University of Hawaii at Manoa, 2018-12) Shi, Hui; Wang, Bin; Atmospheric Sciences
Sparse long-term Asian monsoon (AM) records have limited our ability to understand and accurately model low-frequency AM variability. This dissertation presents a gridded 544-year (from AD 1470 to 2013) Reconstructed Asian summer Precipitation (RAP) dataset by the weighted merging of two complementary proxies including 453 tree ring width chronologies and 71 historical documentary records. Verification against observations and evaluation with various proxies (speleothem, ice core and upwelling) supports the RAP as a valuable dataset for study of large-scale low-frequency Asian summer precipitation variability. Four major modes of variability of the Asian summer precipitation are identified with the long record of RAP, including a biennial El Niño-Southern Oscillation (ENSO) mode, a low-frequency ENSO mode, a central Pacific El Niño-like decadal mode, and an interdecadal mode. It is shown that the relationship between the RAP and ENSO is ENSO phase-dependent since 1470. Two major modes of interannual variability are found to be associated with the ENSO developing and decaying phases, respectively. The mechanisms behind the modern monsoon-ENSO relationship can reasonably well explain the past monsoon behavior. In response to a developing ENSO, precipitation anomalies from the Maritime Continent (MC) via India to northern China are in phase, and this “chain reaction” tends to be largely steady since around 1620 AD; during the decaying phase, however, the summer rainfall-ENSO relationship over the Yangtze River Valley-southern East China (YRV-SEC), the MC and central Asia, has gone through large multidecadal to centennial changes over the past five centuries. The Pacific Decadal Oscillation and ENSO intensity are speculated to be associated with these multidecadal to centennial changes of rainfall-ENSO relations. An area-averaged All Asian Rainfall Index (AARI) was constructed with the RAP, and significant low-frequency periodicities of the AARI are found on decadal (8-10 years), quasi-bidecadal (22 years), and semi-centennial or multidecadal (50-54 years), as well as centennial time scales. A remarkable abrupt frequency shift from semi-centennial to decadal is detected around AD 1700 across the entire Asian land area, which nearly concurs with a dramatic upswing of the Indian summer monsoon and AAR. The leading EOF modes on the decadal, multidecadal, and centennial timescales all exhibit a similar spatially uniform structure, suggesting a tendency of in-phase variations among the rainfall over South Asia, East Asia, and MC across the three time scales. The leading mode of the decadal variation of AAR is associated with a mega-El Niño-Southern Oscillation (ENSO) in the Pacific, a cool western Indian Ocean, and a warm North Atlantic and cool tropical South Atlantic. The leading mode of semi-centennial (or multi-decadal) variation exhibits a more spatially uniform pattern and significantly correlated with the reconstructed Atlantic Multidecadal Oscillation (AMO) and the proxy mega-ENSO. Centennial variation of the AAR follows more closely the volcanic forcing variation, while is uncorrelated with solar forcing variability. Both the AARI-mega-ENSO and AARI-AMO relationships are nonstationary and experience significant centennial changes, especially around AD 1700; so is the AARI-volcanic forcing relation.
Radar-derived Thermodynamic Structure of a Major Hurricane in Vertical Wind Shear
(University of Hawaii at Manoa, 2016-12) Foerster, Annette
Investigating the physical mechanisms that determine the location and timing of eyewall convection is critical to understand intensity changes in tropical cyclones. Observations within the eyewall are essential to identify these physical mechanisms, but high resolution measurements of both kinematic and thermodynamic quantities in the eyewall are limited. Direct thermodynamic observations in particular are limited spatially to aircraft tracks and dropsonde paths. This study presents a thermodynamic retrieval tailored specifically toward rapidly rotating vortices, which provides an unprecedented view of the three-dimensional temperature and pressure structure of the inner core region of tropical cyclones using air-borne Doppler radar data. The retrieval is applied to observations in Hurricane Rita collected on 23 September 2005 during the RAINEX field campaign. The retrieved pressure and temperature fields along with the wind and precipitation structure of Hurricane Rita emphasize the impact of vertical wind shear on the azimuthal location of convection in the eyewall and show the dynamic and thermodynamic processes that act to balance the vortex tilt. Analysis of the contributions of the retrieved pressure and temperature fields to different azimuthal wavenumbers suggests the interpretation of eyewall convection within a three-level framework of balanced, quasibalanced, and unbalanced motions. The axisymmetric, wavenumber-0 structure is determined by both gradient wind and hydrostatic balance, resulting in a pressure drop and temperature increase toward the center. The wavenumber-1 structure, and perhaps in part the wavenumber-2 structure, is determined by the interaction of the storm with environmental vertical wind shear, resulting in a quasi-balance between shear and shear-induced kinematic and thermodynamic anomalies. The higher-order wavenumbers are connected to unbalanced motions and convective cells within the eyewall.
Estimating Volcanic Sulfur Dioxide to Sulfate Aerosol Conversion Rates in Hawai‘i
(University of Hawaii at Manoa, 2016-05) Pattantyus, Andre
Volcanic smog, known as vog, has been a persistent issue on the island of Hawai‘i since the eruption of Kīlauea Volcano began in 1983. Vog, made up of sulfur dioxide (SO2) and sulfate aerosols (collectively referred to as SO4) poses a significant health risk to communities surrounding the volcano and on the leeward coast of Hawai‘i. The Vog Measurement and Prediction Project (VMAP) was launched by the University of Hawai‘i at Mānoa (UHM) and United States Geologic Survey in 2010 as a feasibility study to evaluate whether vog forecasts are achievable and usable. The UHM Vog Model has been operational since 2010 producing forecasts of SO2 and SO4 for the entire state of Hawai‘i. The UHM Vog Model’s purpose is to warn the public, particularly sensitive groups including the elderly, children, and those with respiratory problems, to avoid high concentrations of vog. More recently, the Hawai‘i Department of Health has expressed interest in using Vog Model forecasts to issue air quality alerts when projected concentrations exceed certain thresholds. An early evaluation of the model performance suggested that forecasts are poor for high-end concentrations of both SO2 and SO4, bringing the reliability of the model into question for issuing warnings. To address this shortcoming, a number of improvements to the model were considered. The most promising improvement was including a more comprehensive sulfur chemistry scheme to represent the conversion of SO2 to SO4. A new sulfur chemistry scheme was constructed for inclusion into the UHM Vog Model. This scheme was based on theory and past research of sulfur chemistry. To provide a baseline for the new scheme a brief field experiment was conducted in July 2015 to estimate the conversion rate of SO2 to SO4. The new sulfur chemistry scheme, along with other available schemes, were evaluated against observations of SO2 and PM2.5 around the island of Hawai‘i during November 2015. The results of this evaluation revealed the new sulfur chemistry scheme improved forecasts for SO2 and SO4. Also, as forecast windows extended from one hour to six hours the probability of detection for SO2 increased from 10-30% to 50-70%.
The Predictability of Anomalous Interannual Boreal Summer Arctic Sea Ice Patterns
(University of Hawaii at Manoa, 2016-05) Grunseich, Gary
There is an abundance of interest in the anomalous year-to-year melting patterns of summer sea ice but ongoing prediction efforts have been a struggle because the factors controlling interannual sea ice variability remain unresolved. Dynamical and statistical modeling techniques incorrectly predict annual minimum sea ice extents and historical simulations of CMIP 5 models fail to represent the magnitude and timing of summer interannual variability in the Arctic. The models underestimate the interannual variability along the Arctic sea ice margins and over predict the magnitude in the dormant inner core. These shortcomings indicate a new approach may be appropriate. While the Arctic Oscillation may be the dominant mode of climate variability shaping sea ice patterns, additional remote forcings have been found. Widespread summer Arctic sea ice anomalies are shaped by wind-forced sea ice transport modulated by unique monsoon-Arctic Rossby wave trains. The anomalous dipole behavior between East Asian and Western North Pacific monsoon rainfall induces a meridional teleconnection, which propagates poleward into the Arctic and influences sea ice patterns with a single barotropic circulation center. Anomalous Indian summer monsoon rainfall excites an eastward propagating circumglobal teleconnection that extends into the North Atlantic before bifurcating into the Arctic. This bifurcation produces a barotropic dipole circulation pattern, which drives distinct sea ice patterns. The combined influences of the two remote Asian monsoon variations throughout the summer induce a sea ice melt pattern in which the North Atlantic-European Arctic contrasts the Siberian-North American Arctic and are comparable in magnitude to the leading interannual mode of sea ice variability that is driven by local forcing (Arctic Oscillation). A new prediction approach utilizing a Physical-Empirical model, which has been previously applied to monsoon rainfall and other climatological phenomena, is applied to anomalous interannual sea ice pattern prediction during the annual minimum extent. The newly discovered monsoon-driven sea ice connections are used to establish predictors of Asian Summer Monsoon rainfall early in the year, in addition to other physically meaningful predictors to independently predict each interannual mode during the end of the Arctic melt period. Using stepwise regressions to develop models relating the predictors to the first four leading sea ice modes, which account for nearly 60% of the variance, exhibits high skill in replicating historical ice concentrations along the Arctic periphery.

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