Browsing by Author "van den Heever, Susan, committee member"
Now showing 1 - 11 of 11
Results Per Page
Sort Options
Item Open Access A tropical radiation and cloud system feedback modulated by sea surface temperature(Colorado State University. Libraries, 2011) Igel, Matthew R., author; Stephens, Graeme, advisor; van den Heever, Susan, committee member; Eykholt, Richard, committee memberA large domain, high resolution cloud system resolving model set up in the tropics over fixed sea surface temperatures (SST) of 298 K and 302 K and run to radiative convective equilibrium has been analyzed with the focus on well equilibrated, domain mean results. The Regional Atmospheric Modeling System (RAMS) is used. The modeled convection organizes into disturbed, convective and undisturbed, subsidence regions. The mean profiles of state variables such as temperature, relative humidity (RH), and convective mass flux are analyzed and found to depend on SST in both predictable and unpredictable ways. The characteristics of rain depend on SST such that higher surface temperatures produce greater variability in intensity and lesser frequency. Next, the large-scale mean state is used to understand the convective system-scale setup. A focus is on the controls in the undisturbed regions of the disturbed region, deep convective anvil detrainment. Upper tropospheric radiation, through diabatic convergence, is used as a paradigm to understand the height at which detrainment occurs. The dependence of upper tropospheric radiation on RH is derived explicitly for the first time. From this new equation, temperature and RH are found to control anvil detrainment. The addition of RH as an anvil detrainment control explains why the model leads to an understanding of cooler anvils with higher SST - a positive climate feedback on the system. Other anvil feedbacks exhibited by the model are similar to those proposed in the Iris and Thermostat hypotheses. The convective system components are shown to enhance one another such that the overall system dependence on SST is nonlinear. To understand the circulation system, a heat engine analogue is made that shows the warmer state is able to more efficiently circulate or move heat. Finally, observational evidence from Cloudsat and CALIPSO shows that some of the modeled results are also apparent in nature.Item Open Access CSU-MLP GEFS day-1 "first-guess" excessive rainfall forecasts: aggregate evaluation and synoptic regimes of best- and worst-performing forecasts(Colorado State University. Libraries, 2022) Escobedo, Jacob A., author; Schumacher, Russ, advisor; van den Heever, Susan, committee member; Cooley, Daniel, committee memberForecasting excessive rainfall, particularly flash flood-producing rainfall, is an important problem that remains difficult due to the small spatial scales and varying temporal scales at which they occur. One important operational product that highlights areas for potential excessive rainfall and flash flood occurrences is the Excessive Rainfall Outlook (ERO) issued by the NOAA Weather Prediction Center (WPC), which provides outlooks for lead times of 1-3 days. To address the need for additional tools for WPC forecasters while forming a given ERO, the Colorado State University Machine Learning Probabilities (CSU-MLP) system, a probabilistic forecast system for excessive rainfall (and other convective hazards), was developed to produce forecasts to be used as a "first-guess" ERO. CSU-MLP employs the use of a random forest (RF) algorithm trained using NOAA's Second-Generation Global Ensemble Forecast System Reforecast (GEFS/R) and precipitation observations, while using the operational GEFS with the trained model to produce real-time forecasts. Initially developed as a medium range guidance (2-3 day lead time), CSU-MLP has produced day-1 forecasts that have been evaluated favorably during the 4-week Flash Flood and Intense Rainfall Experiment (FFaIR) in the summer of 2020. However, CSU-MLP day-1 forecasts have been observed to have daily forecast skill that can vary widely between days. This work will include an aggregate evaluation of CSU-MLP day-1 forecasts over a longer period of study (3 March 2019 – 15 October 2020) than that of FFaIR, and an identification of synoptic regimes for which these forecasts tend to perform at their best and worst. Results show that CSU-MLP day-1 forecasts are reliable, provide adequate discrimination of excessive rainfall events (AuROC =0.819), and have comparable performance, evaluated by use of the Brier skill score (BSS), to that of the ERO (CSU-MLP BSS = 0.081; ERO BSS = 0.085). However, CSU-MLP forecasts have a higher frequency of categorical probabilities (≥ 0.05) which results in larger variations of daily BSS. Synoptic regimes of best-performing daily forecasts reveal a tendency for these regimes to be characterized by moderate to strong large-scale forcing and relatively high low-level and column moisture. This would include warm-season regimes with moderate amplitude upper-level troughs, tropical cyclones, cut-off lows, and cool-season regimes where strong forcing is co-located near an abundant moisture source. Forecasts tend to perform worst when there is strong large-scale forcing and low-level and column moisture is relatively low, such as cool-season regimes with large amplitude troughs and surface cyclones but higher levels of atmospheric moisture are not present nor as widespread. This work has implications for WPC forecasters as they use the "first-guess" forecasts while developing the ERO for a given day, as well as implications for future CSU-MLP system model iterations and/or designs.Item Open Access Development of a polarimetric radar based hydrometeor classification algorithm for winter precipitation(Colorado State University. Libraries, 2012) Thompson, Elizabeth Jennifer, author; Rutledge, Steven A., advisor; Dolan, Brenda, committee member; Chandrasekar, V., committee member; van den Heever, Susan, committee memberThe nation-wide WSR-88D radar network is currently being upgraded for dual-polarized technology. While many convective, warm-season fuzzy-logic hydrometeor classification algorithms based on this new suite of radar variables and temperature have been refined, less progress has been made thus far in developing hydrometeor classification algorithms for winter precipitation. Unlike previous studies, the focus of this work is to exploit the discriminatory power of polarimetric variables to distinguish the most common precipitation types found in winter storms without the use of temperature as an additional variable. For the first time, detailed electromagnetic scattering of plates, dendrites, dry aggregated snowflakes, rain, freezing rain, and sleet are conducted at X-, C-, and S-band wavelengths. These physics-based results are used to determine the characteristic radar variable ranges associated with each precipitation type. A variable weighting system was also implemented in the algorithm's decision process to capitalize on the strengths of specific dual-polarimetric variables to discriminate between certain classes of hydrometeors, such as wet snow to indicate the melting layer. This algorithm was tested on observations during three different winter storms in Colorado and Oklahoma with the dual-wavelength X- and S-band CSU-CHILL, C-band OU-PRIME, and X-band CASA IP1 polarimetric radars. The algorithm showed success at all three frequencies, but was slightly more reliable at X-band because of the algorithm's strong dependence on specific differential phase. While plates were rarely distinguished from dendrites, the latter were satisfactorily differentiated from dry aggregated snowflakes and wet snow. Sleet and freezing rain could not be distinguished from rain or light rain based on polarimetric variables alone. However, high-resolution radar observations illustrated the refreezing process of raindrops into ice pellets, which has been documented before but not yet explained. Persistent, robust patterns of decreased correlation coefficient, enhanced differential reflectivity, and an inflection point around enhanced reflectivity occurred over the exact depth of the surface cold layer indicated by atmospheric soundings during times when sleet was reported at the surface. It is hypothesized that this refreezing signature is produced by a modulation of the drop size distribution such that smaller drops preferentially freeze into ice pellets first. The melting layer detection algorithm and fall speed spectra from vertically pointing radar also captured meaningful trends in the melting layer depth, height, and mean correlation coefficient during this transition from freezing rain to sleet at the surface. These findings demonstrate that this new radar-based winter hydrometeor classification algorithm is applicable for both research and operational sectors.Item Open Access Factors affecting lightning behavior in various regions of the United States(Colorado State University. Libraries, 2014) Fuchs, Brody Robert, author; Rutledge, Steven, advisor; Lang, Timothy, committee member; van den Heever, Susan, committee member; Eykholt, Richard, committee memberLightning activity varies greatly on a global scale. Global maps of total flash density show a strong tendency for lightning to favor continental areas over the open ocean, even in regions with similar instability. Previous studies have attributed the difference to thermodynamic and aerosol differences over continental regions, but the exact cause is still elusive. While this is not a global study, we attempt to characterize lightning activity in 4 different regions of the United States with high resolution Lightning Mapping Array (LMA) networks over one warm season. The regions of study are Washington, D.C. (DC), northern Alabama, central Oklahoma and northeast Colorado. A wide spectrum of environmental characteristics is afforded by these regions. Lightning characteristics include storm total flash rates, positive cloud-to-ground (+CG) strikes and intra-cloud (IC) to CG ratio (IC:CG). This is accomplished by using the CSU Lightning, Environmental, Aerosol and Radar (CLEAR) framework, first developed by Lang and Rutledge (2011), to objectively analyze large amounts of storm data. Lightning activity is provided by a new flash clustering algorithm, which produces total flash rates and IC flash rates when combined with NLDN CG data. The results have shown that lightning behavior has high variability throughout the regions of study. Median total storm flash rates range from approximately 1 flash/min in Alabama and DC to near 8 flashes/min in Colorado. Positive CG flash fractions exhibit a similar relationship with 10% of all CG flashes being positive polarity in Alabama and DC up to 40% in Colorado. The anomalous nature of the Colorado region is evident in all lightning metrics. Colorado is also characterized by an anomalous environment with high cloud base storms and coincident shallow warm cloud depths. Examination of all storms simultaneously has shown that relationships exist between total flash rate and environmental parameters. The similarity of these results to other studies on global scales is striking and provides evidence for the robustness of these relationships. Examination of relationships between radar and lightning intensity metrics are also performed. Similar behaviors between these intensity metrics are observed in all regions.Item Open Access Gravity wave and microphysical effects on bow echo development(Colorado State University. Libraries, 2012) Selin, Rebecca Denise Adams, author; Johnson, Richard, advisor; van den Heever, Susan, committee member; Bienkiewicz, Bogusz, committee member; Schumacher, Russ, committee memberNumerical simulations of the 13 March 2003 bow echo over Oklahoma are used to evaluate bow echo development and its relationship with gravity wave generation and microphysical heating profile variations. The first part of the research is directed at an explanation of recent observations of surface pressure surges ahead of convective lines prior to the bowing process. Multiple fast-moving n = 1 gravity waves are generated in association with fluctuations in the first vertical mode of heating in the convective line. The surface impacts of four such waves are observed in Oklahoma mesonet data during this case. A slower gravity wave is also produced in the simulation, which is responsible for the pre-bowing pressure surge in the model. This gravity wave is generated by an increase in low-level microphysical cooling associated with an increase in rear-to-front flow and low-level downdrafts shortly before bowing. The wave moves ahead of the convective line and is manifested at the surface by a positive pressure surge ahead of the convective line. The low-level upward vertical motion associated with this wave, in conjunction with higher-frequency gravity waves generated by the multicellularity of the convective line, increases the immediate pre-system CAPE by approximately 250 J kg-1. Two-dimensional heating profiles from this idealized, full-physics bow echo simulation are placed as a constant heat source in another simulation without moisture, to evaluate what type of gravity waves are produced by a heating profile from a given instance in time. A one-dimensional vertical mean heating profile is calculated from each two-dimensional profile, and a statistical method is used to evaluate the significance of each vertical mode. A number of gravity waves are produced in the dry simulation despite their vertical mode lacking statistical significance in the one-dimensional profile, suggesting that horizontal variations in the heating profile are important to consider. Microphysical sensitivity tests further elucidate the importance of the horizontal distribution of the microphysical heating profile. The tests used variations in the graupel parameter to evaluate its effect on bowing development and related forecasting parameters. Idealized and case study simulations showed that simulations using a larger, heavier, more "hail-like" graupel parameter with faster fallspeeds have decreased evaporation and melting rates concentrated closer behind the convective line, compared to simulations with a smaller, slower-falling, more "graupel-like" graupel parameter. This resulted in increased precipitation efficiency but a smaller stratiform region, weaker cold pool, weaker downdrafts and surface wind gusts, rear-to-front flow that remained elevated until close behind the convective line, and delayed bowing development in the "hail-like" simulations. Output from the case study sensitivity tests were compared to data from the Oklahoma Mesonet, which showed "hail-like" microphysical variations can cause significant variations in simulated forecasting parameters, including a 90 minute delay in onset of bowing, 150% weaker surface wind gusts, and a 600% increase in storm-total precipitation. Results from this work emphasize the importance of microphysical heating and cooling profiles in development of bow echoes, be it through the generation of multiple gravity waves and their feedback to the convection, or through direct modification of convective features such as the rear-inflow circulation and the cold pool strength. The pressure surge gravity wave generated by low-level cooling prior to bowing, and associated destabilization of the environment immediately in advance of the system, improves understanding of the cause of convective intensification as the system bows. However, the strong connection shown between bow echo development and microphysical processes, and the highly diverse nature of microphysical parameterizations, presents a challenge to the prediction of these severe weather phenomena.Item Open Access Ice nucleating particles in the Arctic: measurement and source tracking(Colorado State University. Libraries, 2024) Barry, Kevin Robert, author; Kreidenweis, Sonia, advisor; DeMott, Paul, advisor; van den Heever, Susan, committee member; Fischer, Emily, committee member; Trivedi, Pankaj, committee memberThe Arctic landscape is rapidly changing in a warming climate, with sea ice melting and permafrost thawing. Its near-surface air temperature is warming 3.8 times faster than other regions around the world. This rapid warming is known as Arctic amplification. Clouds contribute to this amplification, with their presence and phase is important for determining the surface energy budget. Arctic mixed-phase clouds can last for several days but are not represented well in climate models. Special aerosols, called ice nucleating particles (INPs) trigger ice formation in the atmosphere at temperatures warmer than -38 °C, and thus are important for determining the initiation, lifetime, and radiative properties of these clouds. Observations of INPs, especially over the central Arctic, are limited, and many sources are unknown. This dissertation has the overarching goal of increasing understanding of Arctic INPs. This is achieved through first presenting a full year of INP measurements in the central Arctic, as well as a full year of their composition, using coincident sampling of bacteria and fungi to gain insight into airmass origin. Next, some of the potentially most active Arctic INP sources are explored. Permafrost, which was known previously to contain high levels of INPs, was tested for its activity and persistence in water, and ability to be aerosolized through bubble bursting over several weeks. Then, sources of INPs were surveyed in a region that is controlled by permafrost (a thermokarst landscape). This included field measurements of permafrost, vegetation, sediment, active layer soil, water, and aerosol samples. A high temperature heat test was developed as a diagnostic tool to differentiate sources. Coincidentally, clean working methods to measure INPs were optimized, as efforts to reduce contamination are needed to accurately sample in this region. The main findings from this work suggest a regionally relatively homogenous population of Arctic INPs at most times of year, which is encouraging for efforts to represent them in numerical models across scales and understand their changes in the future. Permafrost-sourced INPs showed high activity and were enhanced near the coast. Unexpectedly, other components of the thermokarst landscape were found to be rich, organic INP reservoirs, emphasizing that the Arctic tundra is a diverse collection of potential contributors to the aerosol.Item Open Access Land surface sensitivity of mesoscale convective systems(Colorado State University. Libraries, 2016) Tournay, Robert C., author; Schumacher, Russ, advisor; Vonder Haar, Thomas, advisor; van den Heever, Susan, committee member; Nelson, Peter, committee memberMesoscale convective systems (MCSs) are important contributors to the hydrologic cycle in many regions of the world as well as major sources of severe weather. MCSs continue to challenge forecasters and researchers alike, arising from difficulties in understanding system initiation, propagation, and demise. One distinct type of MCS is that formed from individual convective cells initiated primarily by daytime heating over high terrain. This work is aimed at improving our understanding of the land surface sensitivity of this class of MCS in the contiguous United States. First, a climatology of mesoscale convective systems originating in the Rocky Mountains and adjacent high plains from Wyoming southward to New Mexico is developed through a combination of objective and subjective methods. This class of MCS is most important, in terms of total warm season precipitation, in the 500 to 1300m elevations of the Great Plains (GP) to the east in eastern Colorado to central Nebraska and northwest Kansas. Examining MCSs by longevity, short lasting MCSs (<12 hrs), medium (12-15 hrs) and long lasting MCSs (>15 hrs) reveals that longer lasting systems tend to form further south and have a longer track with a more southerly track. The environment into which the MCS is moving showed differences across commonly used variables in convection forecasting, with some variables showing more favorable conditions throughout (convective inhibition, 0-6 km shear and 250 hPa wind speed) ahead of longer lasting MCSs. Other variables, such as convective available potential energy, showed improving conditions through time for longer lasting MCSs. Some variables showed no difference across longevity of MCS (precipitable water and large-scale vertical motion). From subsets of this MCS climatology, three regions of origin were chosen based on the presence of ridgelines extending eastward from the Rocky Mountains known to be foci for convection initiation and subsequent MCS formation: Southern Wyoming (Cheyenne Ridge), Colorado (Palmer divide) and northern New Mexico (Raton Mesa). Composite initial and boundary conditions were developed from reanalysis data, from which control runs of regional MCSs were made as well a series of idealized experiments with imposed large scale soil moisture (SM) anomalies to study to impact to each regional MCS on SM variations in initiation region as well down stream in the GP. Another idealized experiment was made to study the impact of varying the planetary boundary layer (PBL) parameterization in the context of the idealized SM variations. While the distribution of SM has a major impact on CAPE and the location and magnitude of CI, also important is the differences in shear driven by the differences in large scale SM, playing a major, and varying depending on where the regional MCSs interact with the shear anomalies. Utilizing a different PBL parameterization impacts the timing and amount of initial CI, impacting the total precipitation produced by the MCSs, but not nearly the magnitude of alteration to the MCS as varying the SM distribution. A climatology of CI in the Rocky Mountains and adjacent high plains is made using a high resolution observational dataset. From this climatology, the sensitivity of CI to land surface variables, including SM and vegetation is studied. It was found that the timing of CI had a stronger relationship with SM, with earlier CI over wetter than average soils, with the greatest difference in May in the north of the domain, nearly all statistical significant values across regions from north to south in June and July with little difference in August in the northern regions. Outside of May, which showed a strong relationship of earlier CI over less vegetated regions, the relationship was similar, but weaker than, that between SM and CI timing. Examining the CAPE, CIN and PW at CI and null points reveal that the values are generally more conducive to CI over wet soils and anomalously vegetated areas at both CI and null points, with stronger difference in the high plains in the east of regions. Examining the covariance of SM and vegetation at CI points revealed that July and August showed expected covariance relationships with concurrently measured convective variables (i.e., high SM/vegetation associated with high CAPE and vice versa for low SM/vegetation) while May and June higher CAPE and CIN over low vegetation anomalies. A climatology of elevated mixed layers in the central GP was conducted, revealing that the greatest number of EMLS occurred in the northern GP. Back trajectories (BT) were conducted from the radiosonde point of detection for 18 and 36 hours, revealing that the BT point mean for days with severe weather were further west and south from the origin point. The SM and vegetation was sampled at the BT point, revealing a negative, significant correlation with EML depth when pooling the northern stations in 18-hr BTs, and a significant, negative correlation with EVI when pooling the southern sites. A modeling case study was conducted in which an idealized SM anomaly was imposed over the EML origin region. Experiments were also conducted to test the sensitivity of ML formation and EML transport using different PBL parameterizations. While the YSU PBL parameterization produced the deeper PBL over anonymously dry soils in the EML origin region, the EML was not transported to the east as it was in those experiments using the MYNN parameterization, impacting the timing and extent of precipitation in the model runs.Item Open Access Microphysical, dynamical, and lightning processes associated with anomalous charge structures in isolated convection(Colorado State University. Libraries, 2017) Fuchs, Brody, author; Rutledge, Steven, advisor; Dolan, Brenda, committee member; van den Heever, Susan, committee member; Pierce, Jeffrey, committee member; Eykholt, Richard, committee memberInternal storm charge structures are linked to storm microphysics and dynamics. This study leverages available radar-based microphysical and dynamical information from recent field campaigns to investigate the processes that influence storm-scale charge structures. Nine normal polarity (mid-level negative charge) cases that occurred in northern Alabama, and six anomalous polarity (mid-level positive charge) cases that occurred in northeastern Colorado are studied in detail. The results suggest the presence of positively charged mid-level graupel in anomalous polarity storms, which is consistent with large amounts of supercooled liquid water (SCLW). Even though the normal polarity storms have more thermodynamic instability, the anomalous polarity storms have broader and stronger updrafts in addition to more robust mixed-phase microphysics. We expect the broader and stronger updrafts in anomalous Colorado storms are more resistant to dilution by entrainment. Using representative updraft speeds and warm cloud depths, the amount of time a parcel spends in the warm phase of a cloud was estimated for each storm observation. This metric is found to be the key discriminator between the two storm populations as the stronger updrafts and shallower warm cloud depths in Colorado lead to much shorter warm cloud residence time in those storms. We hypothesize this parameter strongly influences the amount of SCLW in the mid-levels because it impacts the loss of liquid water in the warm phase of the cloud via autoconversion and coalescence. Using a recently developed automated flash clustering algorithm on multiple years of ground-based lightning mapping array (LMA) data, approximately 63 million lightning flashes were identified and analyzed from Washington DC, northern Alabama, and northeast Colorado. While LMA-based average annual flash density values in Washington DC (~ 20 flashes km-2 yr-1) and Alabama (~ 35 flashes km-2 yr-1) are within 50% of corresponding satellite estimates, LMA-based estimates are approximately a factor of 3 larger (~ 50 flashes km-2 yr-1) than satellite estimates in northeast Colorado. By estimating the initiation and propagation of lightning channels with LMA data, we find that flashes were produced at lower altitudes in Colorado, compared to Alabama or Washington DC. This is a result of the storm charge structures in these regions as normal polarity storms (common in Alabama and Washington DC) produce systematically higher altitude flashes and anomalous storms (common in Colorado) produce systematically lower altitude flashes.Item Open Access Regional aerosol effects on precipitation: an observational study(Colorado State University. Libraries, 2011) Boyd, Kathryn J., author; Kummerow, Christian, advisor; van den Heever, Susan, committee member; Reising, Steven, committee memberThere have been a multitude of studies on the effects increased amounts of aerosols may have on clouds. The connection between increased cloud condensation nuclei (CCN) and cloud microphysics has been established by in situ observations as well as modeling studies. However, the impact on precipitation is less well established. Of the studies that have assessed aerosol effects on precipitation most have been limited to modeling studies or global studies using satellite data. The few observational studies that have examined these relationships have been mainly limited to data from short-lived field campaign, such as oceanic stratocumulus decks or biomass burning areas. This study attempts to examine regional aerosol effects on precipitation in areas not previously examined in field campaigns, using data from two different sites, one from an Atmospheric Radiation Measurement (ARM) Program permanent facility in Oklahoma and the other from a mobile facility located in the Azores. These two sites were chosen in order to illustrate the differences between a marine and a continental location. Meteorological conditions were taken into account in both locations through surface and sounding data and trends in precipitation were found with increasing aerosol concentrations. The marine site witnessed a suppression of precipitation, consistent with past studies and proposed theories of aerosol effects. This was not true for clouds with liquid water paths exceeding 200g/m2. These clouds appear to contain sufficient amounts of water to overcome the aerosol effect. The continental site, however, experienced an opposite trend, with enhancement of precipitation witnessed in all clouds examined in this study. This is thought to be due to a buffering mechanism in these types of clouds, as introduced by Stevens and Feingold (2009). Results were separated by season and cloud type using the horizontal variability of radar reflectivity at cloud top height. The seasonal results generally either were in line with the year round results or were too noisy to interpret. The results separated by cloud type give a concrete result, illustrating the fact that differing cloud dynamics may lead to opposing trends in precipitation with increasing aerosols. Competing effects of aerosols within clouds appear to dampen any effect on precipitation to the point that it is not detectable from the in-situ observations considered here.Item Open Access Using convection-allowing ensembles to understand the predictability of extreme rainfall(Colorado State University. Libraries, 2016) Nielsen, Erik R., author; Schumacher, Russ, advisor; van den Heever, Susan, committee member; Ramirez, Jorge, committee memberThe meteorological community has well established the usefulness of ensemble-based numerical weather prediction for precipitation guidance, since trusting one possible atmospheric solution can lead to, in some cases, particularly bad forecasts for precipitation guidance, owing to inherent uncertainties in precipitation processes that make deterministic prediction impractical. However, continued predictive challenges associated with intense convective rainfall has led to an increasing need to determine the most effective use of these ensemble systems in high impact, extreme precipitation events. Further, it cannot be assumed that ensembles will evolve similarly in both extreme precipitation and more benign events, due to the importance and error growth associated with convective-scale motions. This error growth associated with the chaotic nature of moist convective dynamics can also serve to limit the predictability of an extreme rainfall event (known as intrinsic predictability), in addition to predictability limits imposed by deficiencies in observing systems and numerical models (known as practical predictability). This research will focus on using a recently developed, operationally based ensemble dataset, specifically the National Oceanic and Atmospheric Administration's (NOAA) Second Generation Global Medium-Range Ensemble Reforecast Dataset (Reforecast-2), to create downscaled ensemble reforecasts of the extreme precipitation events. Some events examined during the course of this research are the inland movement of tropical storm Erin in 2007 and flooding associated with mesoscale convective vortices in Arkansas in 2010 and San Antonio, Texas in 2013. The global reforecasts are used to force an ensemble of convection-allowing WRF-ARW numerical simulations for the purpose of evaluating ensemble-based precipitation forecasts associated with specific extreme rainfall events. Using these ensemble forecasts, we address several questions related to the practical versus the intrinsic predictability of the extreme rainfall events examined. Experiments that vary the magnitude of the perturbations to the initial and lateral boundary conditions (ICs and LBCs) reveal a seemingly proportional scaling of ensemble spread early in the simulations associated with the magnitude of the perturbation, but this scaling is not maintained throughout the simulations. Additionally, a diurnal cycle in ensemble spread growth is observed with large growth associated with afternoon convection, but the growth rate then reduced after convection dissipates the next morning rather than continuing to grow. The specific characteristics of the diurnal cycle, however, vary based upon region and flow regime. Lastly, the ensemble spread was found to be influenced by the size of the IC perturbations out to at least 48 hours. These spread evolution characteristics speak to the viability of running convection-allowing ensembles for prediction on multi-day timescales, since no saturation of the ensemble spread is seen despite extreme precipitation within the modeled time period. In addition to the overall ensemble characteristics, terrain-induced precipitation variability associated with the terrain feature known as the Balcones Escarpment, located in central Texas, is analyzed in multiple instances of heavy rainfall in San Antonio and the surrounding area. Simulations in which the Balcones Escarpment is removed reveal that when the synoptic to mesoscale forcing for heavy rainfall are in place over the Balcones Escarpment, the terrain does not directly affect the occurrence or magnitude of precipitation. It does affect the spatial distribution of the precipitation in a small but consistent way. This shift in precipitation associated with removing the Balcones Escarpment, when compared to a WRF-ARW ensemble for the event, is smaller than shifts associated with typical atmospheric variability. The combined results of these experiments demonstrate that downscaled ensemble NWP systems using readily available global ensemble forecasts can faithfully represent previously unresolved mesoscale features, precipitation totals, and depict ensemble-spread characteristics associated with moist convection.Item Open Access Vertically resolved weak temperature gradient analysis of the Madden-Julian Oscillation(Colorado State University. Libraries, 2017) Wolding, Brandon, author; Maloney, Eric, advisor; Randall, David, committee member; van den Heever, Susan, committee member; Kiladis, George, committee member; Ham, Jay, committee memberInteractions between moisture, convection, and large-scale circulations are thought to play an important role in destabilizing the Madden-Julian Oscillation (MJO). A simplified framework for understanding such interactions is developed, building upon the work of Chikira (2014). Tropical weak temperature gradient (WTG) balance is used to diagnose intraseasonal variations in large-scale vertical velocity from variations in apparent heating, allowing intraseasonal variations in large-scale vertical moisture advection to be decomposed into contributions from various apparent heating processes (e.g. radiative heating, microphysical processes). The WTG diagnosis captures the vertical structure and magnitude of large-scale vertical velocity and vertical moisture advection with exceptional accuracy throughout the free troposphere. Moisture and moisture variance budgets are used to investigate the MJO in ERA-interim (ERAi) reanalysis and the Superparameterized Community Earth System Model (SP-CESM). Moisture budgets indicate that, during the enhanced phase of the MJO, anomalous moistening by large-scale vertical moisture advection exceeds anomalous drying by microphysical processes and sub-grid scale (SGS) eddy fluxes, such that the net effect of these large and opposing processes (hereafter the column process) is to further moisten regions that are anomalously moist. Moisture variance budgets indicate that the column process helps grow moisture variance, acting to destabilize the MJO. Horizontal advective damping of moisture variance, associated with the modulation of higher frequency convective variability on intraseasonal timescales, acts to stabilize the MJO. The vertically resolved WTG balance framework is used to assess the contribution various apparent heating processes make to the column process, and its ability to destabilize the MJO. Intraseasonal variations in longwave radiative heating enhance variations in large-scale vertical moisture advection at low and mid levels, strongly supporting destabilization of the MJO in both ERAi and SP-CESM. The effect of convection alone (i.e. without radiative and surface flux feedbacks) is to weakly grow moisture variance in SP-CESM, and weakly damp moisture variance in ERAi, suggesting that the MJO is unrealistically unstable in the former. Surface flux feedbacks appear to play a more important role in destabilizing the real world MJO. Moisture variance budget analysis of periods of weak, moderate, and strong MJO activity suggests that changes in the vertical structure of apparent heating do not play a dominant role in limiting the amplitude of the MJO in SP-CESM in the current climate. WTG balance provides a useful framework for investigating how the MJO, and its impacts, may change as the climate system warms. Two simulations of SP-CESM, one at pre-industrial levels of CO2 (280 ppm, hereafter PI) and one where CO2 levels have been quadrupled (1120 ppm, hereafter 4xCO2), were analyzed. MJO convective variability increases considerably in the 4xCO2 simulation, a consequence of more favorable mean state moist thermodynamic conditions. A steepened mean state vertical moisture gradient allows MJO convective heating to drive stronger variations in large-scale vertical moisture advection, helping to support enhanced MJO convective variability in the 4xCO2 simulation. The dynamical response to MJO convective heating weakens in the warmer climate, a result of increased tropical static stability. One consequence of this weakened dynamical response is that the MJO's ability to influence the extratopics, which is closely tied to the strength of its associated divergence, is reduced considerably in the 4xCO2 simulation.