Ph.D. - Meteorology

Permanent URI for this collectionhttps://hdl.handle.net/10125/2100

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Large-scale disturbances in a conditionally unstable tropical troposphere
(University of Hawaii at Manoa, 1972) Hoyer, Ruben Roy.; Meteorology
A study of Rossby-type disturbances in a conditionally unstable tropical atmosphere is made using an energy consistent balance-system which includes planetary boundary layer friction. The effect of condensation heating is parameterized by means of the CIS
Orographic Precipitation Over the Island of Oahu
(University of Hawaii at Manoa, 2018-08) Robinson, Thomas E.; Meteorology
To demonstrate the relationship between orographic vertical motion and precipitation, a linear diagnostic upslope model has been developed to provide a high-resolution picture of terrain induced vertical motion and precipitation over the Island of Oahu. The model is called the Hawaiian Orographic Precipitation (HOP) model. The HOP model demonstrates that vertical motions often exist near the surface that are on the order of 10 m s-1, agreeing with direct observations along the Ko‘olau Mountain ridge. The HOP model vertical motion and Orographic Rainfall Index (ORI) are correlated with precipitation during the month of February 2015, but the microphysics of the HOP model is insufficient to produce a meaningful diagnosis of the actual precipitation during that time period. The ORI and orographic vertical motion are closely related indicating that the vertical motion is more important than the moisture in determining where precipitation occurs in the area of the mountains. Three case studies demonstrate that the linear model performs well when the Froude Number is above 1, and is at its best when the precipitation is light and falls out over the mountain. The HOP model does not perform well in light wind and synoptically forced weather conditions. The high-resolution terrain gradient from the HOP model is smoothed and applied to the Weather Research and Forecasting (WRF) model in a new method for calculating the lowest level vertical motion. During the month of January 2016, the WRF model produces less precipitation than is observed at rain gauges around the Island of Oahu, but the new method produces more precipitation than the original. During a stormy period on 09 March 2012, the new method produces a different vertical motion pattern that affects the predicted weather over the island. The skill of the modified WRF model is improved during the 09 March 2012 storm by using the new method.
Bacterial Coral Pathogens of the Genus Vibrio
(University of Hawaii at Manoa, 2016-08) Ushijima, Blake
Coral reefs play an important role in numerous marine ecosystems, however, their survival is threatened by outbreaks of disease. On their own, reefs have the ability to regenerate after destructive events like natural disasters; however recent threats have pushed coral reefs past the point of recovery and many reefs are now under threat of disappearing forever. Outbreaks of diseases specific to corals have already decimated the reefs of the Florida Keys and the Caribbean. In addition, the baseline levels of disease are increasing, as well as the expansion of disease outbreaks into new regions and the broadening of coral species that are affected. Unfortunately, not all of the characterized diseases have a pathogen positively identified and many of the mechanisms of disease for the known pathogens have yet to be determined. This piece of work describes the isolation, identification, and characterization of three virulent Vibrio strains that infect and cause tissue lysis in Hawaiian corals and species at Palmyra Atoll. First, Vibrio oswensii strain OCN002 causes chronic Montipora white syndrome (cMWS) among the Hawaiian Rice coral (Montipora capitata), a major reef building species, in Kāne‘ohe Bay, Hawai‘i. Second, Vibrio coralliilyticus strain OCN008 causes a comparably faster-spreading disease called acute Montipra white syndrome (aMWS). Third, V. coralliilyticus strain OCN014 is a cause of Acropora white syndrome among the table coral (Acopora cytherea) at Palmyra Atoll. In addition to characterizing infection, common virulence factors between the two V. coralliilyticus were investigated and a direct link between rising global sea surface temperatures and increased virulence of a coral pathogen was established. This work also describes the identification of a novel virulence mechanism utilized by strain OCN008, which may represent the evolution of this pathogenic species in response to the protective properties conferred to coral by the microorganisms normally associated with it.
Test and Establishment of Using Monocytes and Monocyte-Derived Macrophages as a Novel Therapeutic Delivery System to the Brain
(University of Hawaii at Manoa, 2016-08) Tong, Hsin-I
The Meridional SST Gradient, Low-Level Clouds, and the Latitudinal Location of the ITCZ in the Eastern Pacific: Climatology, Seasonal Cycle, and Interannual Variability
(University of Hawaii at Manoa, 2015-05) Zhou, Li
The meridional shift of the Intertropical Convergence Zone (ITCZ) in the eastern Pacific does not completely follow that of the surface convergence or the local maximum SST, especially in cold season and equatorial cold years when the SST front in the eastern tropical Pacific is strong. This study focuses on the physical processes and mechanisms that predominantly control the meridional shifts of the ITCZ in the eastern Pacific based on high resolution satellite observational data and regional atmospheric model simulations. Observational analyses show that the latitudinal location of the ITCZ in the tropical eastern Pacific is highly correlated with the intensity of the SST gradient (SSTG). The SSTG over the sharp SST front is responsible for the formation of a shallow meridional circulation (SMC) from the ITCZ to the cold tongue. The SMC is significant characterized by a returning flow at around 700 hPa from the ITCZ to the cold tongue, descending over the cold tongue with a maximum at around 0-2oN, and a strong near-surface southerly between the cold tongue and the ITCZ. In the ITCZ, the upward branch of the SMC and the typical Hadley circulation are associated two maximums of vertical velocity at the 850 hPa and 300 hPa. The SMC provides moisture to deep convection and thus largely controls the latitudinal location of the ITCZ and its meridional shift. It is evidenced by the stronger SMC in September, associating surface southerly maxima from the SST front near 3oN to the ITCZ, compared with that in June. Meanwhile, low-level clouds form downstream of the SST front under the returning flow of the SMC and just above the maximum surface northward wind. The low-cloud-top radiative cooling increases the meridional pressure gradient between the SST front and the ITCZ and thus enhances the SMC. The stronger meridional SSTG together with the denser low clouds would lead to stronger and more northward ITCZ when SSTG is stronger. Since the SSTG across the eastern Pacific cold tongue displays a strong seasonal cycle, the control of ITCZ convection by the SMC and the related cloud-radiative forcing also experiences a strong seasonal cycle. The SMC is significant year-round except in boreal spring when SSTG weakens. In boreal spring, the SSTG is relatively weak and provides a weak control on the meridional location of the ITCZ, and low-level and high-level clouds are collocated with surface convergence, indicating the dominant control of surface convergence on the ITCZ convection/precipitation. The overlaying of high-level and low-level clouds persists until June when deep convection in the ITCZ is largely affected by the continental monsoon over the Central and North America. The vertical structure of the ITCZ is stronger coupled in boreal summer. In fall when the SSTG is the strongest, the SMC is also the strongest, associated with more northern location of the ITCZ with a large northward vertical tilting from the surface to the upper troposphere. In winter, the ITCZ moves southward when the gap wind from Central America is stronger. The more northward tilted convergence and vertical cloud structure in the ITCZ may be related the tropical waves. In cold seasons when the SSTG is stronger, the meridional gradient of CRF within the atmosphere is amplified due to the increased low-level clouds downstream of the SST front together with the increased middle/high-level clouds in the ITCZ, positively feedback to the SMC and the ITCZ precipitation. The above relations are also applicable in the interannual timescale with more northern location of the ITCZ in the equatorial cold years and vice versa. The relative location changes of the ITCZ to normal years are asymmetrical between El Nino years and La Nina years with larger amplification in El Nino years. The SMC seems more significant in the La Nina events and it is not significant in the El Nino events when the tropical convections are stronger and the SSTG is weaker. iRAM simulations are used to help to understand the physical processes. The iRAM results are evaluated in climatological mean, seasonal, and interannual time scale. It simulated well in these timescale on multiple variables including the latitudinal location of the ITCZ, clouds, SMC, and the CRF. Comparison in the meridional circulation between the ENSO years and Non-ENSO years apparently show the enhanced SMC in the La Nino events. Sensitivity experiments with SSTA cooling or warming are added over the cold tongue to produce different intensity of the SSTG. The analyses of the heating budget show that the low-level cloud radiative cooling and the vertical mixing due to change of the low-level cloud produce similar magnitude as the diabatic heating in the change of the SMC, which contribute the northward or southward shift of the ITCZ.
Formation of Precipitation-Circulation-Sea Surface Temperature Patterns under Global Warming
(University of Hawaii at Manoa, 2015-05) Zhang, Lei
Most of CMIP5 models projected a weakened Walker Circulation and an El Nino like warming in the tropical Pacific, but what causes such a change is still an open question. We hypothesize that the following three mechanisms are responsible. The first mechanism is a so called “longwave radiative – evaporative damping” mechanism. A simple analytical model was constructed to understand the formation mechanisms of future SST warming pattern under global warming. It is demonstrated that the future SST warming pattern is primarily determined by present-day mean SST and surface latent heat flux (𝑄#$) fields through a longwave radiative – evaporative damping mechanism. Assume a local thermodynamic equilibrium limit without circulation and cloud changes, a uniform GHG forcing would lead to a smaller (larger) warming in the regions where mean SST and 𝑄#$ are large (small). This longwave radiative – evaporative damping mechanism can explain a much greater warming in high latitudes than in tropics, and an El Nino like warming in tropical Pacific. In addition, cloud decreases (increases) in eastern (central) Pacific due to weakened Walker circulation (warmer SST), which also contributes to the El Nino like warming. The second mechanism is “the richest get richer”. In response to a uniform surface warming, the Asia-western North Pacific (WNP) monsoon system is enhanced by competing moisture with other large-scale ascending systems. The strengthened WNP monsoon induces surface westerlies in the western-central equatorial Pacific, weakening the Walker circulation. Idealized AGCM experiments that separate the effect of the mean warming and the relative SST warming pattern clearly demonstrate the effect of the mean warming on change of the equatorial wind. The third mechanism is extra land surface warming, that is, the land obtains a larger warming than the ocean. In particular, a great thermal contrast between American continent and tropical Pacific causes a zonal pressure gradient and westerly anomalies in the eastern equatorial Pacific. This weakens the Walker circulation. A traditional view of weakened Walker circulation is attributed to a slower increase rate of global mean precipitation than moisture. However, by analyzing a uniform warming experiment in an aqua-planet setting, it is demonstrated that the Walker circulation is strengthened, even though global averaged upward motion is weakened. This result suggests that the global moisture budget argument may not be sufficient to explain the change of the Walker circulation in the tropics. By conducting numerical simulations in a realistic land-ocean distribution, we demonstrated that the weakening of the Walker Circulation is attributed to both the monsoon and land forcing effects. The relative SST warming pattern also plays a role, but it is just an amplifier, not a fundamental cause.
El Niño/Annual Cycle Combination Mode Dynamics
(University of Hawaii at Manoa, 2015-05) Stuecker, Malte-Fabian
The intricate interaction between the El Ni~no-Southern Oscillation (ENSO) and the annual cycle has been a long-standing problem in climate science. Here I demonstrate that the nonlinear atmospheric interaction between the Western Pacific warm pool seasonal cycle and ENSO creates a previously overlooked mode of Indo-Pacific climate variability. This combination mode (C-mode) is characterized by near-annual and sub-annual timescales (combination tones) and contributes substantially to the observed low-level atmospheric circulation and precipitation anomalies. The meridionally anti-symmetric warm pool annual cycle is the cause for the characteristic meridionally anti-symmetric C-mode response, while the linear ENSO response is quasi-symmetric with regard to the equator. Several climate phenomena are shown to be manifestations of the C-mode: (i) zonal South Pacific Convergence Zone (SPCZ) events, (ii) the southward shift of anomalous westerlies at the end of the calendar year during El Ni~no events, (iii) the anomalous low-level Northwest Pacific Anticyclone (NWP-AC), and (iv) prolonged low sea-level events in the Southwest Paci c (El Ni~no Taimasa). The NWP-AC bridges the impacts of ENSO to the Asian Monsoon system. Previous research highlighted the importance of thermodynamic air-sea coupling for the genesis and persistence of the NWP-AC. However, I show that the phase transition information for the anomalous Northwest Pacific circulation originates solely from the atmospheric C-mode. Air-sea coupling only contributes amplification and persistence to the C-mode response. The ENSO/annual cycle interaction outlined here can be elegantly viewed as a frequency cascade, which transfers power from the interannual band to higher frequencies (combination tones). This extended ENSO response is not only characterized by different timescales but also by unique circulation and rainfall patterns. Importantly, this high frequency climate variability is purely deterministic in its nature and hence as potentially predictable as ENSO and the seasonal cycle themselves. Finally, the frequency cascade and combination mode frameworks are universal concepts that can be used to understand many nonlinear interactions between different timescale phenomena in the climate system and dynamical systems in general.
Water Quality Influences on Declining Coral Settlement from the Great Barrier Reef Region: Biofilms as Key Players
(University of Hawaii at Manoa, 2015-05) Prescott, Rebecca
Biofilms (surface attached microbial communities in mucus) are common in coastal marine ecosystems but among the least studied. Their pivotal roles in the resilience of coral reefs stem from their interactions with coral larvae that must select permanent sites for attachment before metamorphosing into coral polyps, a process which can be disrupted by local watershed disturbances. In the laboratory, biofilm communities were exposed to moderately turbid water (50 mg/L sediment) and a doubling of particulate organic matter (POM). Acropora millepora and Pocillopora acuta coral larvae that were provided biofilms from each treatment settled less on those that had been exposed to sediments and organic matter. Pyrosequenced 16S and 18S rRNA gene fragments and scanning electron microscopy showed how community changes in the biofilms in each treatment related to settlement of both coral species’ larvae. We concluded that: 1) particulate carbon levels explained more variation in the community composition of biofilms and in coral settlement response than did turbidity changes; 2) community composition of the microbial eukaryote community shifts from diatom to fungal-dominated; 3) spatial distribution of settlement is driven more by negative cues from biofilms than it is by positive cues; 4) the complex, three dimensional structure of biofilm surfaces diminishes with increased POM and turbidity. Phaeobacter spp. affiliated sequences dominated biofilms before and after treatments. These and other Rhodobacteraceae affiliated sequences were positively associated with A. millepora settlement, but not with that of P. acuta. Phaeobacter spp. occur globally in biofilm communities, produce antibiotics, stimulate algal growth and then produce algaecides to digest algae as it ages, and decrease epibiont settlement on some algae. They may provide key functional roles in coral reef biofilms.
Interactions between Aerosol and Marine Stratocumuli Over the South East Pacific
(University of Hawaii at Manoa, 2015-05) Freitag, Steffen
During the VOCALS Regional Experiment in 2008 fourteen research flights were conducted over the South East Pacific (SEP) to study the interaction between aerosol and cloud covering a wide range of conditions from a more polluted coastal troposphere to the remote marine environment. Size-resolved aerosol physiochemistry was measured above and below stratocumulus (Sc) clouds along with in-cloud observations of cloud droplet size spectra. Because the nature and variability of aerosol effective as cloud condensation nuclei (CCN) impact cloud microphysical properties through cloud droplet number concentration (Nd), size-resolved aerosol measurements were utilized to establish air mass characteristics and their CCN activity over the SEP. The aerosol number size distribution measurements were clustered into groups using a simple k-means technique that recognizes similar pattern over the aerosol size range assessed. This cluster technique yields four distinct aerosol size distributions for the free troposphere (FT). These are attributed to two local coastal pollution sources and long-range transport of aerosol from the South Pacific and Australia based upon back trajectory analysis and investigation of aerosol physiochemistry. Marine boundary layer (MBL) observations reveal six distinct clusters associated with different stages of aging and processing of coastal combustion sources, clean South Pacific and heavy drizzling air masses. All air masses show CCN activity is strongly dependent on aerosol number concentration and size distribution shape, while aerosol hygroscopicity plays a smaller role. This confirms earlier studies although observed MBL hygroscopicity considerably exceeds the previously recommended value of 0.7. Derived MBL CCN also reveal a 1:1 relationship to Nd over the range of air mass characteristics observed once droplet concentrations are corrected for instrumental artifacts that tend to undercount Nd in polluted clouds. This is contrary to some previous estimates of aerosol-cloud interaction and could result in changes in local cloud radiative forcing of -3 to -10 W m−2. In-cloud measurements from VOCALS campaign also show a robust dependency of Nd with the empirical correction factor k∗ utilized in cloud radiative transfer codes in climate models to account for droplet spectral properties. This relationship can be traced to aerosol size distributions below the clouds. Measurements for both clean marine and pollution influenced aerosol populations indicate that as they undergo cloud processing, reducing the number of CCN and Nd, their droplet mean radius (rµ) increases while spectrum width (rσ ) is unaffected. The associated k∗ increases, as it is roughly proportional to rµ / rσ . If this dependency is not accounted for, local forcing could be overestimated by 3 to 6 W m−2 in polluted clouds close to the Chilean coastline. In-cloud measurements also showed that the highest drizzle rates occur in the absence of typical pollution indicators as carbon monoxide and black carbon.
A comparison of two heavy rainfall events and impacts of data assimilation on a coastal heavy rainfall event during TiMREX (2008)
(University of Hawaii at Manoa, 2013-12) Tu, Chuan-Chi
Two contrasting localized heavy rainfall events during Taiwan's early summer rainy season with the daily rainfall maximum along the windward mountain range and coast were studied and compared using a combination of observations and numerical simulations. Both events occurred under favorable large-scale settings including the existence of a moisture tongue from the tropics. For the 31 May case, heavy rainfall occurred in the afternoon hours over the southwestern windward slopes after a shallow surface front passed central Taiwan. The orographic lifting of the prevailing warm, moist, west-southwesterly flow aloft, combined with a sea breeze-upslope flow at the surface provided the localized lifting needed for the development of heavy precipitation. On 16 June before sunrise, pronounced orographic blocking of the warm, moist, south/southwesterly flow occurred because of the presence of relatively cold air at low levels with an offshore wind component as a result of nocturnal and rain evaporative cooling. This also caused convective systems to intensify as they moved toward the southwestern coast. During the daytime, the cold pool remained over southwestern Taiwan without the development of onshore/upslope flow. Furthermore, with a southsouthwesterly flow aloft parallel to terrain contours, orographic lifting was almost absent, and pre-existing rain cells offshore diminished after they moved inland. Over northern Taiwan on the leeside, a sea-breeze/onshore flow developed in the afternoon hours, resulting in heavy thundershowers. Our results demonstrate the importance of diurnal and local effects on determining the location and timing for the occurrences of localized heavy precipitation during the early summer rainy season over Taiwan. Constellation Observing System for Meteorology, Ionosphere, and Climate global positioning system (GPS) radio-occultation (RO) soundings were used to depict the prefrontal moist tongue for a heavy rainfall case (16 June 2008) during the early summer season over Taiwan. Cycling model runs assimilate Terrain-influenced Monsoon Rainfall Experiment (TiMREX) data, global telecommunications system (GTS) data and GPS RO sounding data in order to improve the initial conditions for the outermost domain as well as all nested domains. This leads to better representations of the prefrontal moist tongue over the open ocean, a weak 500-hPa prefrontal trough, and the modifications of the planetary boundary layer structure by the antecedent rains of 14-15 June. The GPS RO data have positive impacts on subsynoptic features including the upper-level low/trough location and orientation resulting in a better initial condition for sequence forecast for upward motion and rainfall patterns. The initial air temperature in the boundary layer over southwestern Taiwan in the early morning of 16 June is relatively cold with significant orographic blocking as compared with the control run initialized with the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) data. As a result, local circulations over Taiwan as well as rainfall along the southwestern coast and afternoon heavy showers in the wake zone over northern Taiwan are better predicted with cycling runs that start 36 hours before the model forecasts.
Weather and dispersion modeling of the Fukushima Daiichi nuclear power station accident
(University of Hawaii at Manoa, 2013-12) Dunn, Thomas Joel
Contamination due to the surface deposition of radioactive material from the accident at the Fukushima Daiichi nuclear power station was investigated for 11 March to 17 March 2011. A coupled weather and dispersion modeling system was developed and simulations of the accident performed using two independent source terms that differed in emission rate and height and in the total amount of radioactive material released. Results show significant differences in the distribution of cumulative surface deposition of 137Cs due to wet and dry removal processes. Parameterizations for precipitation scavenging by rain, snow, and graupel were implemented to investigate surface wet deposition fields . Results show aerosols from a source term with emission heights at 50, 300, and 1000 meters above ground level (AGL) were scavenged by rain (70%) and graupel (30%) compared with a source term with emission heights at 20 and 120 meters AGL in which material was scavenged preferentially by rain (95%). A sensitivity study was performed that broadened the particle size distribution (PSD) of the source terms during explosive events of the accident. Results for the source term with elevated emission heights show enhanced wet deposition due to scavenging by snow (5%, 35%, 51%) compared with scavenging by rain and graupel as the effluent PSD was increased (0.5, 1.0, and 10m, respectively). In contrast, the source term with relatively lower emission heights remained preferentially scavenged by rain (90%). A second study that investigated the complexity of the cloud microphysics scheme showed that precipitation scavenging of radioactive material was not very sensitive to the choice of single-vs. double-moment cloud microphysics parameterization. A comparison of 137Cs deposition predicted by the model with aircraft observations of surface-deposited gamma radiation showed reasonable agreement in surface contamination patterns during the dry phase of the accident. During the wet phase the pattern is not as well predicted. It is suggested that this discrepancy is because of differences between model predicted and observed precipitation distributions. Dry deposition was the dominant removal process, accounting for the majority of surface contamination (12 orders of magnitude over that due to wet deposition near the source).
The evolution of Hurricane Humberto (2001)
(University of Hawaii at Manoa, 2010-12) Dolling, Klaus P.
The causes of intensity change in tropical cyclones have continued to be a major challenge for the meteorological community. A lack of sampling throughout the life cycle of a tropical cyclone has contributed to our lack of understanding. In September of 2001, NOAA and NASA marshaled their resources and deployed over 200 Global Positioning System dropwindsondes within a 300 km radius of Tropical Cyclone Humberto on 3 consecutive days in the Convection and Moisture Experiment. This dissertation presents the results of an observational study of the aforementioned storm, combining aircraft data with the Global Positioning System dropwindsondes, airborne expendable bathythermographs, lower fuselage, and tail radar. Kinematic and thermodynamic structures are examined in an attempt to better understand intensification processes for a high latitude storm traveling over a variable SST field, in a dry environment, and under increasing vertical wind shear. As Humberto is sampled as a tropical storm, the warm core develops from an area of subsidence on the trailing edge of an anvil. This warming acts to cap the boundary layer and allows for an increase in the energy content under the nascent eye. Throughout the experiment, thermodynamic and kinematic variables exhibit persistent relationships with the vertical wind shear vector. Vertical cross-sections of the warm core show that it has the highest temperature perturbation in the lower troposphere, contrary to past observations and theory. The present study investigates the evolution of an asymmetric system and the causes for the unusual warm core structures.
Observed and simulated air-sea feedbacks associated with ENSO and monsoon
(University of Hawaii at Manoa, 2011-12) Xiang, Baoqiang
Associated with the double ITCZ (Inter-tropical convergence zone) problem, a dipole SST bias pattern (cold in the equatorial central Pacific and warm in the southeast tropical Pacific) remains a common problem in current coupled models. Based on a newly-developed model, we demonstrated that a serious consequence of this SST bias is to suppress the thermocline feedback in El Niño/Southern Oscillation (ENSO) simulation. Firstly, the excessive cold tongue extension pushes the anomalous convection far westward, diminishing the convection-low level wind feedback and thus the air-sea coupling strength. Secondly, the equatorial surface wind anomaly exhibits weak meridional gradient, leading to a weakened wind-thermocline feedback. Thirdly, the equatorial cold SST bias induces a weakened upper-ocean stratification, yielding the underestimation of the thermocline-subsurface temperature feedback. Finally, the dipole SST bias underestimates the mean upwelling through both dynamic and thermodynamic effects. In recent decades, El Niño events have occurred more frequently over the equatorial central Pacific (CP Warming, CPW). Here, we ascribe this predominance of the CPW to a dramatic decadal change in the Pacific mean state and annual cycle. The mean state change characterized by a decadal La Niña-like pattern tends to anchor convection and surface zonal wind anomalies to the vicinity of the dateline, facilitating surface warming to occur in the CP. The annual cycle change, with the trade winds intensifying during boreal winter and spring, prevents the warming development but helps the warming decay in the EP. More CPW events are expected in the coming decade if the La-Niña-like pattern persists. The western North Pacific (WNP) Subtropical High (SH) has profound impacts on Asian summer monsoon, North Pacific storms. The cause of the interannual variability of WNPSH, however, remains controversial. Here we show that the anomalous WNPSH is primarily determined by a remote cooling/warming in the equatorial central Pacific and a positive thermodynamic feedback between the local circulation and a dipole sea surface temperature in the Indo-Pacific warm pool. We demonstrate that a physical-empirical prediction model built on these physical understandings has comparable performance with those of three state-of-the-art coupled climate models in re-forecast of the strength of the WNPSH.
Dynamics and uncertainties of global warming patterns: sea surface temperature, precipitation, and atmospheric circulation
(University of Hawaii at Manoa, 2012-12) Ma, Jian
Precipitation and atmospheric circulation changes in response to global warming have profound impacts on the environment for life but are highly uncertain. This study investigates fundamental mechanisms controlling these changes and relates them to the effects of sea surface temperature (SST) change, using Coupled Model Intercomparison Project simulations. The SST warming is decomposed into a spatially uniform SST increase (SUSI) and deviations from it. The SST pattern effect is found important in explaining both the multi-model ensemble mean distribution and inter-model variability of rainfall change over tropical oceans. In ensemble mean, the annual rainfall change follows a "warmer-getwetter" pattern, increasing where the SST warming exceeds the tropical mean, and vice versa. Two SST patterns stand out: an equatorial peak that anchors a local precipitation increase, and a meridional dipole mode with increased rainfall and weakened trade winds over the warmer hemisphere. These two modes of inter-model variability in SST account for up to one third of inter-model spread in rainfall projection. Tropospheric warming follows the moist adiabat in the tropics, and static stability increases globally. A diagnostic framework is developed based on a linear baroclinic model (LBM) of the atmosphere. The mean advection of stratification change (MASC) by climatological vertical motion, often neglected in interannual variability, is an important thermodynamic term for global warming. MASC and SST pattern effects are on the same order of magnitude in LBM simulations. Once MASC effect is included, LBM shows skills in reproducing general circulation model (GCM) results by prescribing latent heating diagnosed from the GCMs. Common to all GCMs, MASC causes both the Hadley and Walker circulation to slow down as articulated by previous studies. The weakening of the Walker circulation is robust across models as the SST pattern effect is weak. The Hadley circulation change, by contrast, is significantly affected by SST warming patterns. As a result, near and south of the equator, the Hadley circulation change is weak in the multi-model ensemble mean and subject to large inter-model variability due to the differences in SST warming patterns, explaining up to four fifth of the inter-model variability in changes of the overturning circulation.
A systematic approach to quantifying the noise-ENSO relationship and its effect on extreme El Niño events
(University of Hawaii at Manoa, 2014-08) Levine, Aaron Falk Zacharius
State-dependent noise forcing, where properties such as amplitude are dependent on the background conditions, has been shown as a potential cause for the El Niño--La Niña amplitude asymmetry. Complex characterization of the state dependency of El Niño noise forcing as documented in most existing research has not been readily applicable to the outputs from the current generation of coupled climate models (GCMs). Here a simple method for determining the overall strength of the state-dependence factor of the El Niño noise forcing is proposed and tested. This method is shown to be independent of the data sampling from monthly mean to daily data, which make it readily applicable to monthly climate data archives. Using a reanalysis product and two coupled GCMs, the method is then applied to the equatorial zonal windstress, which is a known noise source for the El Niño-Southern Oscillation (ENSO) phenomenon. The windstress in all three cases is shown to have strong state-dependent noise forcing. The state-dependent component of the noise forcing is further isolated and shown to be enhanced by the Pacific Warm Pool and Westerly Wind Burst interaction. The coupled process acts to increase the low-frequency component of state-dependent windstress noise forcing, a part of noise forcing that is most essential in the excitation and strengthening of ENSO. The methodology is then further applied to the outputs from a suite of Coupled Model Intercomparison Project (CMIP)5 model simulations under various scenarios. It is found that most of the CMIP5 models underestimate the state-dependence factor of the equatorial windstress on sea-surface temperature (SST). There is a consistent relationship with the zonal equatorial SST gradient and the state-dependence factor. However, the models fail to agree on changes of the state-dependence factor due to global warming. Finally, using the conceptual model, the state dependence factor is shown to control the ENSO skewness and is a predictor of the frequency of occurrence of extreme El Niño events. To a large extent, the CMIP5 simulations are in support of these theoretical relationships.
Tropical cyclone energy dispersion in a baroclinic model and its associated cyclogenesis
(University of Hawaii at Manoa, 2008) Ge, Xuyang
A case study on typhoon Prapiroon (2000) is performed to examine the role of TCED in the following cyclogenesis in nature. Sensitivity experiments suggest that the previous TC would modulate the large-scale environmental circulations, thus affect the formation of its sequential storm. A remarkable asymmetry appears in the perturbation growth of the wave train in the presence of vertical wind shears. That is, an easterly (westerly) wind shear confines the wave to the lower (upper) level. It is suggested that the vertical shear may impact the Rossby wave train development through both the barotropic-baroclinic mode coupling and the modulation of the group velocity by the mean flow through a "Doppler shift effect". The destabilization of Rossby wave train by regional easterly vertical shears has important implications.
Tropical-extratropical interaction associated with the Indian summer monsoon
(University of Hawaii at Manoa, 2008) Ding, Qinghua
On the intraseasonal timescale, extreme active and break phases of the ISM often bring about devastating floods and severe droughts. The concurrent buildup of the anomalous high over Central Asia and the arrival of tropical convection over northern India increase the likelihood of occurrence of a heavy rainy period over the NISM region. Two predictors may be used to predict the extreme active/break phases of the northern ISM: normalized 200-hPa geopotential height over Central Asia and outgoing longwave radiation over southern India. Once the mean of the two predictors exceeds a threshold unit 1.0, an extreme phase is anticipated to occur over northern India after six to seven days. This study also reveals a significant, coupled intraseasonal variation between a Rossby wavetrain across the Eurasian continent and the summer monsoon convection in northwestern India and Pakistan (referred to as NISM hereafter). The time-lagged SVD analysis shows that the mid-latitude wavetrain originates from the northeastern Atlantic and traverses Europe to central Asia. The wavetrain enhances the upper-level high pressure and reinforces the convection over the NISM region; meanwhile, it propagates further toward East Asia along the wave guide provided by the westerly jet. After an outbreak of NISM convection, the anomalous central Asian high retreats westward.
Long-range detection of sferics over the Pacific Ocean: Thunderstorm characteristics and data assimilation into NWP models
(University of Hawaii at Manoa, 2008) Pessi, Antti
The performance of the PacNet was assessed. Lightning detection efficiency (DE) and location accuracy (LA) model calibration procedures are detailed, and comparisons of model results with lightning observations from the PacNet in correlation with Tropical Rainfall Measuring Mission (TRMM) satellite's Lightning Imaging Sensor (LIS) are presented. The results of the data analysis show a consistent logarithmic increase in convective rainfall rate with increasing lightning rates. Moreover, other storm characteristics, such as radar reflectivity and ice water path, show a similar logarithmic increase. The waveguide between the earth's surface and the ionosphere allows very low frequency emissions generated by lightning, called sferics, to propagate over long distances. The new Pacific Lightning Detection Network (PacNet) utilizes this attribute to monitor lightning activity over the North Pacific Ocean with a network of ground-based lightning detectors. These results were utilized to assimilate lightning data from PacNet into a NWP model. A lightning data assimilation (LDA) system was programmed, which nudged the model's latent heating rates according to rainfall derived from lightning observations.
The influence of Tutt cells on tropical cyclone motion in the northwest Pacific Ocean
(University of Hawaii at Manoa, 2008) Patla, Jason E.
A conceptual model of how a TUTT cell can influence TC track is presented. The model provides quantified, decision-grade operational guidance for TC forecasters using pattern recognition scenarios. Application of the conceptual model at the JTWC is currently underway.Eleven tropical cyclones (TCs) are examined using the latest ECMWF reanalysis (ERA-40) and JTWC best track data to determine how tropical upper tropospheric trough (TUTT) cells influence TC tracks. This type of interaction has led to enormous TC track forecast errors at 72 hour (2000+ km) in the northwest Pacific and are often overlooked and under-forecast frequently due to poor numerical model TUTT cell forecasts. Cases are selected because a TC exhibited a "non-standard" track, a TUTT cell was the sole large-scale transient feature within 2000 km of the TC's center, and the TC intensity was >17 m s -1. Analysis shows that the circulations' separation distance, orientation, intensity, and the depth and breadth of the TUTT cell's closed circulation are critical characteristics in determining the likelihood of a TUTT cell influencing a TC's track. Interactions occur at distances greater than 1700 km, continue for periods ranging from 24 to 48 hours and happen 2-3 times per year in this active TC basin.
A two-dimensional, time dependent numerical model of atmospheric boundary layer flow over in homogeneous terrain
(University of Hawaii at Manoa, 1966) Wagner, Norman Keith
A two-dimensional, time dependent, numerical model of the atmospheric boundary layer is formulated and used to investigate various characteristics of air motion when a change occurs in the roughness of the underlying surface. The model assumes a dry adiabatic atmosphere and adiabatic changes of state, no dynamic pressure effects due to flow accelerations and no vertical displacement of the zero-velocity level at the lower boundary when a discontinuity in surface roughness is encountered. Numerical evaluation of all terms in the resulting equations shows little change in the computed fields of velocity if an incompressible atmosphere is assumed and horizontal mixing is neglected. A computational stability analysis is made which provides the relationship between time-step and grid spacing necessary to insure convergence of the numerical solution. For horizontally homogeneous conditions, comparison of the numerical solutions with various analytical solutions shows that the errors inherent in the finite difference formulation can be reduced to any desirable level by reducing the grid spacing, particularly near the lower boundary. Computed quantities include the two-dimensional distributions of horizontal velocity, vertical velocity and eddy shearing stress. Increasing surface roughness causes a decrease in the horizontal velocity near the lower boundary. This results in upward vertical motion. For a prevailing geostrophic wind speed of 10 m/sec and roughness parameters varying from 1 cm through 10 cm, maximum vertical motion on the order of 5 cm/sec is found over the roughness discontinuity at heights ranging from 50 to 100 meters. Vertical velocities in excess of 0.1 cm/sec are found several meters upstream from the discontinuity and persist for several. hundred meters downstream for the cases considered. The computations also show that a nearly logarithmic wind profile with associated constant shearing stress becomes established throughout a layer which increases in depth with distance downwind from the roughness discontinuity. The ratio of depth to downwind distance of this layer lies between 1/50 and 1/100. Reestablishment of the logarithmic profile does not imply equilibrium flow, however. It is found that a fetch on the order of several kilometers is necessary before the surface eddy stress differs by less than ten percent from the final. equilibrium values. Excellent quantitative' agreement is found with Panofsky and Townsend (1964), who employed data of Kutzbach (1961) to show that their theory agreed with observations. Because of the inability of the numerical model to consider zero-plane displacement and non-neutral atmospheric stability, both of which are probably significant and certainly present in the observations available from the University of Wisconsin, no further quantitative comparisons were made. However, the general features of the computed velocity and stress distributions agree very well with results of Stearns and Lettau (1963), hence there is at least good qualitative agreement with observations. Suggestions are made for generalizing and improving the numerical model.

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