Ph.D. - Atmospheric Sciences

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https://hdl.handle.net/10125/50831

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    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.
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    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.
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    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.
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    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.
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    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.
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    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.