VTechWorks Repository :: Browsing by Author "Ramseyer, Craig A."

Browsing by Author "Ramseyer, Craig A."

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Advances in tropical climatology – a review
Moraes, Flávia D. S.; Ramseyer, Craig A.; Miller, Paul W.; Trepanier, Jill C. (Informa, 2024-02-12)
Understanding tropical climatology is essential to comprehending the atmospheric connections between the tropics and extratropical latitudes weather and climate events. In this review paper, we emphasize the advances in key areas of tropical climatology knowledge since the end of the 20th century and offer a summary, assessment, and discussion of previously published literature. Among the key areas analyzed here, we explore the advances in tropical oceanic and atmospheric variability, such as El Niño – Southern Oscillation and the Madden-Julian Oscillation, and how those teleconnection events have helped us to better understand variabilities in tropical monsoons, tropical cyclones, and drought events. We also discuss new concepts incorporated into the study of tropical cyclones, such as rapid intensification, and how those studies are evolving and helping scientists to better prepare and predict hurricanes. Regarding tropical aerosols, we discuss how satellite-based dust detection has improved the comprehension of Saharan dust as a driver of drought in locations far from the dust source region while simultaneously altering tropical cyclone variability. Finally, our review shows that there have been significant advances in tropical hydroclimatic studies in order to better investigate monsoons, flooding, and drought, helping scholars of tropical climatology to better understand its extreme events.
Analysis of Urban Heat Island Intensity Through Air Mass Persistence
Van Tol, Zachary Charles (Virginia Tech, 2021-05-24)
The bulk of synoptic weather type research related to urban climate focuses on human health impacts; however, recent studies have begun to quantify urban heat island (UHI) magnitudes by weather type, or air mass classification. This study presents an analysis of UHI intensity through synoptic-scale air mass persistence during the spring season for four UHI-prone United States cities. Historical daily weather types for Birmingham, Alabama; Charlotte, North Carolina; Louisville, Kentucky; and St. Louis, Missouri were extracted from the Spatial Synoptic Classification database for 40 years from 1980 through 2019. Daily minimum surface air temperature data were downloaded from the Global Historical Climate Network to compute UHI. The historical daily weather type data were converted into a record of persistence, or the length of consecutive days that a synoptic weather type was in place at each location. A descriptive climatology of SSC weather types and UHI at each location was constructed before UHI magnitudes were segregated by day of persistence and examined for differences in intensity. Climatologically, the four urban areas experienced an increase in warm weather types at the expense of cool weather types throughout the study period. Specifically, the persistence of moist tropical weather types increased at a statistically significant rate at Birmingham, Charlotte, and Louisville, consistent with the theorized northward migration of the mid-latitude jet stream. Also evident is a statistically significant increase in UHI frequency and intensity at Birmingham, Charlotte, and Louisville during the study period. Results show that the moisture character of a weather type is an important differentiating factor in UHI intensification, as the mean UHI was found to increase with the persistence of dry weather types and decrease with the persistence of moist weather types, presumably reflecting differences in radiational heating and cooling with atmospheric moisture content. The most intense UHIs and the largest UHI magnitude increase by day of persistence are associated with dry weather types, which have become more frequent since 1980. The findings suggest that larger magnitude UHIs may become more common in the future should dry weather type persistence continue to increase. Higher urban temperatures put human health at risk due to a well-linked relationship between heat and mortality and morbidity rates. The effects of heat are cumulative; the more common persistent, oppressive days become, the larger the impact.
Atmospheric Flash Drought in the Caribbean
Ramseyer, Craig A.; Miller, Paul W. (American Meteorological Society, 2023-09-13)
Despite the intensifying interest in flash drought both within the U.S. and globally, moist tropical landscapes have largely escaped the attention of the flash drought community. Because these ecozones are acclimatized to receiving regular, near-daily precipitation, they are especially vulnerable to rapid-drying events. This is particularly true within the Caribbean basin where numerous small islands lack the surface and groundwater resources to cope with swiftly developing drought conditions. This study fills the tropical flash drought gap by examining the pervasiveness of flash drought across the pan-Caribbean region using a recently proposed criterion based on the Evaporative Demand Drought Index (EDDI). The EDDI identifies 46 instances of widespread flash drought “outbreaks” in which significant fractions of the pan-Caribbean encounter rapid drying over 15 days and then maintain this condition for another 15 days. Moreover, a self-organizing maps (SOM) classification reveals a tendency for flash drought to assume recurring typologies concentrated in either the Central American, South American, or Greater Antilles coastlines, though a simultaneous, Caribbean-wide drought is never observed within the 40-year (1981-2020) period examined. Further, three of the six flash drought typologies identified by the SOM initiate most often during Phase 2 of the Madden-Julian Oscillation. Collectively, these findings motivate the need to more critically examine the transferability of flash drought definitions into the global tropics, particularly for small water-vulnerable islands where even island-wide flash droughts may only occupy a few pixels in most reanalysis datasets.
A Climatological Analysis of Upper-Tropospheric Velocity Potential Fields using Global Weather Reanalysis, 1958-2020
Stanfield, Tyler Jarrett (Virginia Tech, 2022-05-26)
Upper-tropospheric (200 hPa) velocity potential is useful in identifying areas of rising or sinking atmospheric motions on varying temporal scales (e.g., weekly, seasonal, interannual) especially in the global tropics. These areas are associated with enhancement (rising motion) or suppression (sinking motion) of tropical convection and subsequent weather phenomena dependent on these processes (e.g., tropical cyclones). This study employed three commonly used global weather reanalysis datasets (NCEP/NCAR Reanalysis 1, JMA JRA-55, ECMWF ERA5) to calculate and compare upper-tropospheric velocity potential fields on varying temporal scales and quantify any differences that existed between them from 1958 to 2020 over four key regions of variability (Equatorial Africa, Amazon Basin, Equatorial Central Pacific, and Equatorial Indonesia). To supplement this analysis, the highly correlated variables to velocity potential of outgoing longwave radiation (OLR) and daily precipitation rate were used and directly compared with independent OLR and precipitation datasets to determine the reanalysis' level of agreement with the independent data. The ECMWF ERA5 held the highest agreement to these data over all regions examined and was reasoned to have the highest confidence in capturing the variability of upper-tropospheric velocity potential fields for the study period. Confidence was decreased in the usefulness of the NCEP/NCAR Reanalysis 1 as it consistently performed poorly over much of the study domain. The results of this study also emphasized the usefulness in ensemble-based approaches to assessing climate variability and understanding potential biases and uncertainties that are inherent in the data sources.
Detection of Tornado Damage via Convolutional Neural Networks and Unmanned Aerial System Photogrammetry
Carani, Samuel James (Virginia Tech, 2021-10-21)
Disaster damage assessments are a critical component to response and recovery operations. In recent years, the field of remote sensing has seen innovations in automated damage assessments and UAS collection capabilities. However, little work has been done to explore the intersection of automated methods and UAS photogrammetry to detect tornado damage. UAS imagery, combined with Structure from Motion (SfM) output, can directly be used to train models to detect tornado damage. In this research, we develop a CNN that can classify tornado damage in forests using SfM-derived orthophotos and digital surface models. The findings indicate that a CNN approach provides a higher accuracy than random forest classification, and that DSM-based derivatives add predictive value over the use of the orthophoto mosaic alone. This method has the potential to fill a gap in tornado damage assessment, as tornadoes that occur in wooded areas are typically difficult to survey on the ground and in the field; an improved record of tornado damage in these areas will improve our understanding of tornado climatology.
The effects of projected climate change on crop water availability in the US Caribbean
Moraes, Flavia D. S.; Ramseyer, Craig A.; Gamble, Douglas (IWA Publishing, 2023-04)
Anthropogenic climate change affects small islands, and farming systems in the Caribbean are vulnerable to climate change due to their high dependence on rainfall. Therefore, this work evaluated how temperature and precipitation projections affect water crop needs in Puerto Rico and St. Croix. We used Daymet data to create a baseline climatology (1981-2010) and the Coupled Model Intercomparison Project Phase 6 (CMIP6) to create future climatologies (2041-2070 and 2071-2100). A water budget model estimated the water deficit, and the crop risk (CROPRISK) model determined crop suitability for sweet pepper, banana, and plantain. Results indicated an increase in water stress after 2041 for most of the region from June to August, except for western Puerto Rico, where it will occur from January to March. For sweet pepper, banana, and plantain, the most water-stressed season is projected to be January-July. November will be the only month during which all crops are projected to be highly suitable through the end of the 21st century. These findings suggested that Puerto Rico and St. Croix crop water stress may be more sensitive to changes in temperature than precipitation.
Evaluating the Role of Atmospheric Stability in Generating Asymmetrical Precipitation During the Landfall of Hurricane Florence (2018)
Morrison, Lindsey Paige (Virginia Tech, 2021-01-11)
Hurricane Florence (2018) was unique due to its slow storm motion during landfall, causing convective rainbands to produce high amounts of precipitation along the coast of North Carolina. This study focuses on the relationship between precipitation asymmetries and atmospheric stability surrounding the tropical cyclone (TC) during the landfall period of a nearly-stationary TC. Previous research with idealized hurricane simulations suggests that atmospheric stability may vary surrounding a TC during landfall, with the atmosphere destabilizing offshore and stabilizing onshore. However, this finding has not been studied using a realistic approach. Due to Hurricane Florence's slow motion, the storm was situated at the land-ocean boundary for multiple days, providing an ideal opportunity to examine the role of atmospheric stability in modifying hurricane precipitation during landfall. This study uses the Advanced Research Weather Research and Forecasting (WRF-ARW) version 3.6.1 to produce high-resolution simulations to examine the variations in precipitation and atmospheric stability surrounding Hurricane Florence. Precipitation accumulation at different temporal scales was used to determine that asymmetries existed during the landfall period. Observed and model-simulated Convective Available Potential Energy (CAPE) were used to measure stability surrounding the TC. Simulated CAPE indicates that there was a significant difference between stability right- and left-of-track. In addition to a control simulation, two experimental simulations were conducted by modifying the land surface to vary the heat and moisture exchange coefficient (HS) and hold the surface roughness (Z0) constant. By isolating the HS to be more moist or dry, the altered low-level moisture was hypothesized to cause the precipitation and convection distributions to become more symmetrical or asymmetrical, respectively. The results from the experimental simulations showed that the altered land surface affects the relative humidity from the surface to 950 mb, which has an immediate impact on stability off-shore left-of-track. Overall, the precipitation and stability asymmetries were not significantly impacted by the altered near-surface moisture, indicating other physical factors contribute to the asymmetries. The results of this study provide insight into the role of atmospheric instability in generating asymmetrical precipitation distributions in landfalling TCs, which may be particularly important in slow-moving TCs like Hurricane Florence.
Evaluating the Skillfulness of the Hurricane Analysis and Forecast System (HAFS) Forecasts for Tropical Cyclone Precipitation using an Object-Based Methodology
Stackhouse, Shakira Deshay (Virginia Tech, 2022-05-24)
Tropical cyclones (TCs) are destructive, natural occurring phenomena that can cause the loss of lives, extensive structural damage, and negative economic impacts. A major hazard associated with these tropical systems is rainfall, which can result in flood conditions, contributing to the death and destruction. The role rainfall plays in the severity of the TC aftermath emphasizes the importance for models to produce reliable precipitation forecasts. Hurricane model precipitation forecasts can be improved through precipitation verification as the model weaknesses are identified. In this study, the Hurricane Analysis and Forecast System (HAFS), an experimental NOAA hurricane model, is evaluated for its skillfulness in forecasting TC precipitation. An object-based verification method is used as it is demonstrated to more accurately represent the model skill compared to traditional point-based verification methods. A 600 km search radius is implemented to capture the TC rainfall and the objects are defined by 2, 5, and 10 mm/hr rain rate thresholds. The 2 mm/hr threshold is chosen to predominantly represent stratiform precipitation, and the 5 and 10 mm/hr thresholds are used as approximate thresholds between stratiform and convective precipitation. Shape metrics such as area, closure, dispersion, and fragmentation, are calculated for the forecast and observed objects and compared using a Mann Whitney U test. The evaluation showed that model precipitation characteristics were consistent with storms that are too intense due to forecast precipitation being too central and enclosed around the TC center at the 2 mm/hr threshold, and too cohesive at the 10 mm/hr threshold. Changes in the model skill with lead time were also investigated. The model spin-up negatively impacted the model skill up to six hours at the 2 mm/hr threshold and up to three hours at the 5 mm/hr threshold, and the skill was not affected by the spin-up at the 10 mm/hr threshold. This indicates that the model took longer to realistically depict stratiform precipitation compared to convective precipitation. The model skill also worsened after 48 hours at the 2 and 10 mm/hr thresholds when the precipitation tended to be too cohesive. Future work will apply the object-based verification method to evaluate the TC precipitation forecasts of the Basin-Scale Hurricane Weather Research and Forecasting (HWRF-B) model.
The Evolution and Distribution of Precipitation during Tropical Cyclone Landfalls using the GPM IMERG Product
Sauda, Samrin Sumaiya (Virginia Tech, 2023-06-07)
Landfalling tropical cyclone (TC) induced precipitation poses a great risk to the rising coastal population globally. However, the impacts of tropical cyclone precipitation (TCP) are still difficult to predict due to rapid structural changes during landfall. This study applies a shape metric methodology to quantify the spatiotemporal evolution of TCP in the North Indian (NI), Western Pacific (WP), and North Atlantic (NA) basins. The International Best Track Archive for Climate Stewardship (IBTrACS) data and the Global Precipitation Mission (GPM)'s advanced Integrated Multisatellite Retrievals for GPM (IMERG) dataset is employed to study the 2014-2020 landfalling TCP at three analysis times: pre-landfall, landfall, and post-landfall. We examine three thresholds (2, 5, and 10 mm hr-1) and use six spatial metrics (area, closure, solidity, fragmentation, dispersion, and elongation) to quantify the shape of the precipitation pattern. To identify precipitation changes among the three analysis times and three basins, the Kruskal-Wallis test is applied. The three basins show important differences in size evolution. The greatest structural changes occur during post-landfall when the rainfall extent shrinks. The WP has the largest area of TCP and generates the highest maximum TCP of all basins. NA is the only basin where the precipitation area expands after landfall. NA also has the lowest closure for the three precipitation thresholds. NI precipitation has the lowest dispersion and maximum closure. Shape metrics such as closure and dispersion show a consistent inverse correlation. The maximum precipitation direction within the TCs is also examined in each basin. These results can inform guidelines that contribute to improved TCP forecasting and disaster mitigation strategies for vulnerable coastal populations globally. Future studies can apply shape metrics to the sub-basins in NI and WP to examine regional variability as there has been no such study in these basins. Future work can also investigate if the location of heavy rainfall within the TC structure affects flooding or other water hazards.
Historical trends in the trade wind inversion in the tropical North Atlantic Ocean and Caribbean
Ramseyer, Craig A.; Miller, Paul W. (Wiley, 2021-05-03)
The trade wind inversion (TWI) serves as an important stabilizing mechanism in the tropical North Atlantic (TNA) region, including the Caribbean basin. Previous studies have diagnosed the TWI using in situ observations and radiosondes, typically over tropical islands. However, studies relying on these point measurements are unable to discern the climatology and evolution of the TWI over the rest of the TNA. This study addresses this gap in the literature through the use of high-resolution ERA5 reanalysis model level data. Due to the advances in the ERA line of reanalysis products, ERA5 now provides vertical level resolution as fine as ~4 mb in the lower troposphere, enabling the identification of shallow inversions, such as the TWI, consistently on a climatological time scale in remote regions of the world. While still coarser than observed soundings, this reanalysis-based approach provides a first attempt in understanding TNA TWI variability and its strength and frequency trends from 1979 to 2019. The TWI climatology constructed here finds consilience with previous modelling and observational studies in terms of the spatial variability of the TWI base and strength across this domain. Stronger and more frequent TWIs are noted across the central TNA across all seasons. Results from a Mann–Kendall analysis reveals increasing trends in TWI frequency and strength that vary spatially across the domain based on season. The most widespread and strongest increasing TWI frequency and strength signal is over the central TNA from December to July. Due to the regionalization of trends noted, potential regional forcing mechanisms responsible for these changes are discussed.
Identifying Eastern US Atmospheric River Types and Evaluating Historical Trends
Ramseyer, Craig A.; Stanfield, Tyler J.; Van Tol, Zachary; Gingrich, Tyler; Henry, Parker; Forister, Peter; Lamkin, Bradley; Stackhouse, Shakira; Sauda, Samrin Samaiya (AGU/Wiley, 2022-09-16)
An atmospheric river (AR) is the primary moisture transport forcing in the Western United States, making ARs the predominant producer of extreme precipitation events in this region. A growing body of evidence suggests similar impacts for the Central and Eastern US. This study determines the most prominent types of ARs in the Central and Eastern US study domain through the implementation of a machine learning methodology. Self-organizing maps (SOMs) are leveraged to determine what “flavors” of ARs exist in the study domain. Four atmospheric river detection criteria are utilized to investigate the variability AR types. Mann-Kendall trend analyses on AR strength and size are produced to evaluate changes over the study period. The results confirm extratropical cyclones as the most common driver of ARs, however, limited kinematic forcing can also instigate the development of AR events. Results show coastal cyclones and lee-side cyclones are responsible for producing the strongest ARs. The trend analysis results suggest that ARs associated with Nor'easters and ARs originating in the Gulf of Mexico are exhibiting increasing trends in intensity and/or size. Increasing moisture transport by mature cyclones across the Central and Eastern US have important implications for flooding in highly populated corridors. Areas of concern include the Northeast and Southeast US, while localized enhancement of rainfall is seen along the eastern and southern slopes of the Appalachian Mountains. In addition to the physical findings, this research highlights the importance and sensitivity of statistically significant results to the specific atmospheric river detection criteria that was leveraged.
The Influence of the North Atlantic Subtropical High on Atmospheric Rivers Over the Eastern United States
Finkhauser, Julia Elizabeth Rose (Virginia Tech, 2024-07-22)
This study addresses the susceptibility of atmospheric rivers (ARs) to the behavior of the North Atlantic Subtropical High (NASH). ARs are a major mechanism for meridional moisture transport often connected to heavy precipitation and mid-latitude troughs. The NASH, a semi-permanent anticyclone over the subtropical North Atlantic Ocean, has been shown to be significantly influential on precipitation variability over the southeastern United States. A self-organizing map (SOM) was trained on a 4 x 3 regular grid over 250 iterations using ERA5 derived 6-hourly 850 hPa Geopotential Heights ≥ 1535 gpm from 1979-2020. The 12 resulting "nodes" were analyzed with respect to ARs defined by objects of ERA5 derived integrated water vapor transport (IVT) > 500 m-1 s-1 with lengths > 2000 km. Composites of thresholded 850 hPa heights, AR-concurrent PRISM precipitation, AR spatial frequency distribution maps, and seasonal AR frequency histograms per node illustrate seasonal interactions between the NASH and ARs that demonstrate a tendency of more frequent ARs and higher mean AR-driven precipitation over the Mississippi embayment and Ohio River Valley in the summer months, believed to be representative of extreme moisture transport events, when the NASH exhibits increased intensity, spatial expansion, and southwestward migration. Conversely, AR frequency and AR-concurrent precipitation composites suggest wintertime events are mainly supported by dynamically-driven nor'easter and bomb type cyclones when the NASH is constricted, at higher latitudes, and further east. Findings suggest that extreme summertime water vapor transport events associated with an AR are enhanced by the warm season NASH due to its increased intensity and proximity to the eastern US that acts as a supplementary lifting mechanism amidst low dynamic influence.
The Relationship Between the Saharan Air Layer, Convective Environmental Conditions, and Precipitation in Puerto Rico
Miller, Paul W.; Ramseyer, Craig A. (American Geophysical Union, 2024-01-04)
The Saharan Air Layer (SAL) is a hot, dry, and dust-laden feature that advects large concentrations of dust across the Atlantic annually to destination regions in the Americas and Caribbean. However, recent work has suggested the SAL may be a contributing factor to high-impact drought in the Caribbean basin. While the SAL's characteristic dust loadings have been the focus of much previous research, fewer efforts have holistically engaged the co-evolution of the dust plume, its associated convective environment, and resultant rainfall in Caribbean islands. This study employs a self-organizing map (SOM) classification to identify the common trans-Atlantic dust transport typologies associated with the SAL during June and July 1981–2020. Using the column-integrated dust flux, termed integrated dust transport (IDT), from MERRA-2 reanalysis as a SAL proxy, the SOM resolved two common patterns which resembled trans-Atlantic SAL outbreaks. During these events, the convective environment associated with the SAL, as inferred by the Gálvez-Davison Index, becomes less conducive to precipitation as the SAL migrates further away from the west African coast. Simultaneously, days with IDT patterns grouped to the SAL outbreak typologies demonstrate island-wide negative precipitation anomalies in Puerto Rico. The SOM's most distinctive SAL outbreak pattern has experienced a statistically significant increase during the 40-year study period, becoming roughly 10% more frequent over that time. These results are relevant for both climate scientists and water managers wishing to better anticipate Caribbean droughts on both the long and short terms.
Spatial Patterns and Variations of Tornado Damage as Related to Southeastern Appalachian Forests and Terrain from the Franklin County, Virginia EF-3 Tornado
Forister, Peter Harding (Virginia Tech, 2021-06-24)
Strong tornadoes have impacted the central Appalachian Mountains multiple times in recent years. The topography of this region leads to unique spatial patterns of tornado damage as the tornado vortices pass over ridges in forested areas, and this damage can be detected with vegetation indices derived from remotely sensed imagery. The objectives of this study were to 1) Classify forest damage from the April 19, 2019 EF-3 tornado in Franklin County, VA using remotely-sensed images, 2) Quantify the spatial patterns of forest damage intensity across the path using derived vegetation indices and terrain variables (primarily slope, aspect, elevation, and exposure), and 3) Use regression models to determine if relationships exist among terrain variables along the and forest damage patterns. I generated EVI and NDII vegetation indices from Sentinel-2 imagery and compared the derived damage to the underlying terrain variables. Results revealed that the two vegetation indices were effective for classifying tornado damage, and discrete damage classes aligned well with NWS EF-scale tornado intensity estimations. ANOVA testing suggested that EF-3 equivalent damage was more likely to occur on downslope topography, leeward of the tornado's direction of travel. OLS and geographically weighted regression (GWR) modeling performed poorly, suggesting that an alternative method may be more suitable for modeling, the scale of assessment was inadequate, or that important predictor variables were not captured. Overall, the intensity of the tornado was clearly modified by terrain interactions, and the remote sensing methodology used was effective for reliably identifying and rating damage in forested areas.
Station-based Analysis of Variability and Change in the Nigerian Hydroclimate
Samson, Bright Chukwuca (Virginia Tech, 2024-05-22)
The atmospheric effect of greenhouse gas emissions is posing an increasing threat to the stability of the global climate. Like many developing nations, the western Africa nation of Nigeria faces risks from climate change, with potential effects on the environment upon which Nigerians rely and on broader social constructs, including the national economy. Nigeria's diverse topography, which stretches from dry northern regions of the sub-Sahara to lush southern rainforests along the Gulf of Guinea, accentuates susceptibility to a variety of climate-related hazards, including warming, irregular rainfall patterns, and extreme weather occurrences. Driven by the influence of tropical climates on the global climate system and the importance of climate variability and change specifically within Nigeria, this study of the Nigerian hydroclimate explicitly characterizes historical variability and change through analysis of in-situ daily climate data. Daily maximum and minimum air temperature and total precipitation data from 1982 through 2011 were obtained from the Nigeria Meteorological Service for 20 locations across the country. Given the limited temporal extent of the data, two popular satellite-derived precipitation products were tested for usability as supplements to the in-situ data. Each of the satellite-derived products depicts rainfall with an unrealistically high frequency and with a temporal trend that is opposite reality. Only in-situ data were analyzed further, beginning with a methodology to define the climatological wet and dry seasons across the country. The critical wet season across Nigeria was found to last between 120 days (north) and 200 days (south), beginning April/May and ending September/October, with wetness migrating from nearer the southern coastline northward through the country during the Northern Hemisphere summer, before retreating south again. As with seasonality, the spatial distribution of precipitation amount and frequency relates to distance northward from the southern coast. Wet season precipitation approaches 2500 mm from an average of more than 115 wet days along the coast, to only about 350 mm and 35 days across far northern Nigeria. Conversely, the dry season produces 300 mm from 30 wet days across the south, and only 80 mm from less than 10 days across the north. The wet season in Nigeria accounts for greater than 90% of annual precipitation and number of wet days. Nigeria experienced a warming and wetting of the climate during the 30-year study period, during both the wet and dry seasons. However, a change in the equitable distribution of precipitation across wet days (i.e., daily intensity) is not greatly evident, as it is for many other regions of the world. Thus, the likely benefit of greater precipitation does not appear to be mitigated by the risks associated with an increase in the frequency of high-intensity rainfall events. But tempering the positive precipitation signal is the likely detrimental effect of warming. Inter-annual variability in the wetness of the critical wet season is evident in the synoptic atmospheric expression of the inter-tropical convergence zone/discontinuity, but also in sea surface temperatures within the Gulf of Guinea. Historically, sea surface temperatures are considerably higher during the wettest wet season years compared to the driest years, possibly indicating a short-distance teleconnection that may offer seasonal predictability.
Synoptic-Scale Atmospheric Conditions Associated with Flash Drought Initiation in Puerto Rico and the Caribbean
Gingrich, Tyler Michael (Virginia Tech, 2022-05-26)
While conventional drought has been studied for many years, new research focuses on different aspects and types of drought. Flash Drought is a relatively new area of research in drought literature, dating back to the last ten to twenty years in the United States. Flash drought in the Caribbean has received minimal attention from researchers, but it has been studied in the United States primarily because of the 2012 flash drought event over the Great Plains. This study focuses on flash drought events in Puerto Rico and the Caribbean. Because the rapid onset and intensity of flash drought can potentially cause more devastation without established prediction methods, this research seeks to understand the synoptic scale atmospheric drivers of flash drought events. Recent occurrences of a flash drought event in this region include the 2015 event in Puerto Rico, which resulted in water rationing and shortages for residents of the island (Mote et al., 2017). The primary goal of this study is to understand how flash drought initiates and propagates for Puerto Rico and the Caribbean using two definitions of flash drought. One definition is based on soil moisture deficit, and the second definition is based on the Evaporative Demand Drought Index (EDDI), an experimental drought monitoring tool. Results suggest that an anomalous convection and positive moisture event followed by negative moisture anomalies and persistent subsidence contribute to flash drought event initiation and propagation. Additionally, large scale flash drought events seem to be initiating more frequently, suggesting that the island is becoming more susceptible to the devastations of flash drought.
Using Self-Organizing Maps to Calculate Chilling Hours as an Indicator of Temperature Shifts During Winter in the Southeastern United States
Henry, Parker Wade (Virginia Tech, 2022-05-24)
Several warm winter events have occurred across the Southeast in the past decade, including 2 major events in 2017 and 2018 in Georgia and South Carolina. Plants will begin their spring growth sooner than climatology would suggest and then be damaged by early spring frosts in what is commonly known as a "false spring" event. Some species of plants, like peaches and blueberries, which produce buds early in the season, are just an example of some of the agricultural products more at risk than others. As an important measure of dormancy time in plants, chill hours present a measurement capable of tracking phenological shifts in plants. While a lack of required chill hours can delay spring emergence, intense warm periods can override the chilling hour requirement and induce spring emergence. This project involves training self-organizing maps (SOMs) to identify periods of anomalous winter warming based on a reduced number of chill hours within a 5-day temporal period compared to the period's climatological average. A second SOM is nested in the node that produced the most anomalous events to identify the range of warming that occurs in the most anomalous events, the synoptic setups of these events, and when these occurred. Hourly 2-meter temperature from ERA5 is used to conduct this analysis over a domain centered primarily over South Carolina and Georgia with a temporal period of 1980-2020. Climatological examination of chill hour accumulations in the past 4 decades show an overall decrease in chill hour accumulation across the past decade (2011-2020) Results indicated that periods of higher-than-average temperatures are increasing with time while periods of average or lower than average temperatures are decreasing with time. Both results were statistically significant by Mann-Kendall test. The results of the nested SOMs suggest that an increase in patterns of southerly flow (a common pattern for warmer temperatures) is occurring through time. A third SOM investigating early spring hard freezes was inconclusive but illustrated that some years had more early spring frosts than others independent of how many warmer than average periods occurred in the main winter. The use of SOMs for investigating climatological and synoptic changes in winter and early spring proved successful and effective. Future modifications to these SOMs could be used to identify more trends that exist within these seasons.

Browsing by Author "Ramseyer, Craig A."

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