Cloud and rainfall distributions in urban spaces have implications for planning, hydrological response, reservoir management, renewable energy generation, transportation, and agricultural productivity. Studies have confirmed that large urban areas can initiate or modify precipitation, but there are still questions about the role of city size and atmospheric interactions. The majority of case study approaches have focused on large cities or urban clusters and have largely ignored small to moderate sized cities. Herein, an analysis of the Augusta, Georgia metropolitan statistical area is conducted. Using a gridded, daily multi-sensor precipitation dataset and satellite-based cloud cover climatology, the warm seasons (June, July, and August) covering the period from 2002 to 2019 were analyzed using spatial comparisons within an upwind-downwind framework and z-score statistics. Such methodologies have been published for larger urban areas. We confirmed that a moderate-sized city like Augusta, Georgia and neighboring Aiken, South Carolina is associated with spatial patterns consistent with the “urban rainfall effect” (URE) and possibly an “urban cloud effect” (UCE). Contextual analysis of other local mesoscale signatures related to nearby water bodies are also provided as a sanity check on process identification.

Monitoring and quantifying high-latitude atmospheric circulation variability and trends are important towards understanding the anatomy of extreme events and constraining their probabilities under continued Arctic change. The greater Alaska region stands out as one region of enhanced warming and environmental changes over the Arctic amplification era, which has shown proclivity to extreme events and speaks to the need to consistently monitor overlying atmospheric variability. In this paper, we describe the creation and analysis of an updated, daily mid-tropospheric Alaska Blocking Index (ABI) time series from 1948 to 2020. Sensitivity testing and index modifications from the initial dataset are discussed, and the new ABI is evaluated over the full period and subperiods. Additionally, we assess the role of the ABI as a regional climate indicator by analysing its variability against surface air temperature and precipitation from 13 climate divisions across Alaska as well as broader-scale hemispheric temperatures. Months of the highest ABI means mainly transpired in the most recent climatological period, 1991-2020, and most notably during the spring and autumn transition seasons, reflecting the nonstationary nature of the jet stream magnitude through time atop Alaska. ABI trends are positive across all months, seasons, and annually only in the 1991-2020 period. Contemporaneous ABI values are strongly and positively correlated with air temperature across all Alaskan climate divisions during the warm season, but negatively correlated with winter temperatures over southeastern Alaska. Meanwhile, the ABI is significantly anticorrelated with summer precipitation over Interior and southern Alaska, but only over southeastern Alaska in other seasons. The ABI is statistically differentiated from the primary modes of atmospheric variability as shown by its generally weak correlations with the Arctic Dipole and the Arctic Oscillation (r <= 0.40).

Observational and numerical modeling studies continue to affirm the existence of the “urban rainfall effect” (URE), or a discernible anomaly in warm season precipitation due to urbanization. However, the literature has been lacking a progression towards the predictability of the URE. Atlanta, Georgia has consistently appeared in the literature because of its well-studied urban rainfall anomalies. Using the Multi-sensor Precipitation Estimates (MPE) dataset and the ERA-Interim reanalysis dataset, an 18-year period (2002–2019) is examined. Three similar but distinct methods are used to define urban rainfall days (URDs), or periods when the precipitation in the urbanized areas of Atlanta are greater than the surrounding rural areas. A combination of compositing, wind rose, and k-means cluster analyses are employed to extract the synoptic framework supportive of the URE in Atlanta, Georgia. The synoptic-scale compositing analysis reveals that there are a consistent set of meteorological ingredients that are needed to produce an URD, including weaker-than-average southwesterly-to-northwesterly flow at 700 hPa, copious amounts of moisture throughout the tropospheric column, and a background low-level convergent flow. Composite atmospheric soundings reveal that there is enhanced moisture throughout the tropospheric column on URDs, leading to marginal instability that favors localized convection across the Atlanta metropolitan area. The study also provides clarity on how often the URE is present (roughly 8% of the time) during warm season days across the Atlanta metropolitan area. Taken together, this synoptic framework will aid in the forecasting of the URE in Atlanta and can be easily applied to other cities.

Research on the impact of heat on pregnant women has focused largely on outcomes following extreme temperature events, such as particular heat waves or spells of very cold weather on pregnant women. Consistently, the literature has shown a statistically significant relationship between heat with shortened gestational age with studies concentrated largely in the western states of the USA or other nations. The association between heat and shortened gestational age has not been examined in the Southeastern US where maternal outcomes are some of the most challenging in the nation. Unlike previous studies that focus on the impacts of a single heat wave event, this study seeks to understand the impact of high heat over a 5-year period during the annual warm season (May–September). To achieve this goal, a case-crossover study design is employed to understand the impact of heat on preterm labor across regions in North Carolina (NC). Temperature thresholds for impact and the underlying relationships between preterm labor and heat are investigated using generalized additive models (GAM). Gridded temperature data (PRISM) is used to establish exposure classifications. The results reveal significant impacts to pregnant women exposed to heat with regional variations. The exposure variable with the most stable and significant result was minimum temperature, indicating high overnight temperatures have the most impact on preterm birth. The magnitude of this impact varies across regions from a 1% increase in risk to 6% increase in risk per two-degree increment above established minimum temperature thresholds.

ABSTRACTOngoing climatic and cryospheric changes observed throughout the greater Alaska region are interconnected and often linked to oceanic and atmospheric patterns and processes that operate on varying spatiotemporal scales. To evaluate the long‐term, mid‐tropospheric circulation field across Alaska, and possible connections to climate and environmental change in the Pacific sector of the Arctic, the Alaska Blocking Index (ABI) is developed over the domain (54°–76°N, 125°–180°W) using daily gridded 500 hPa geopotential height fields derived from the ERA‐40 (1958–1978) and ERA‐Interim (1979–2014) data sets, 1958–2014. Climatological characteristics of the seasonal and annual ABI conditions are evaluated and periods of prevalent blocking conditions are identified and subsequently linked to possible local and large‐scale forcing mechanisms. The ABI has exhibited positive trends during all seasons and annually since 1979. Many of the extreme high ABI values occurred since 2000, including the highest annual values in 2013 and 2014. Anomalous blocking patterns in all seasons are associated with diminished snow depth and sea‐ice cover, positive near‐surface air temperature anomalies, and anticyclonic flows of heat and moisture across the domain. The ABI is also shown to differ from the long‐term variability and atmospheric circulation responses associated with phases of the Pacific–North American pattern and Pacific Decadal Oscillation, revealing some notable spatial and temporal disconnects between the region‐centric, high‐latitude blocking flow and some of the predominant modes of sea surface temperature and middle tropospheric circulation variability in the Northern Hemisphere.

Spatio-temporal patterns in mean and extreme rainfall are examined around the city of Atlanta, Georgia using the Multi-sensor Precipitation Estimates (MPE) and ERA-Interim reanalysis datasets. The analysis spans the period 2002 to 2015 and employs a 9-cell gridded framework centered on downtown Atlanta. Statistically significant anomalies in daily precipitation were found over and downwind (predominately east to northeast) of Atlanta. The pattern of rainfall anomalies is most evident in the early evening hours of the day and is hypothesized to be related to the evolution of the skin or surface urban heat island (UHI), rather than the canopy layer UHI. The study formally proposes the term “flow regime dependent” downwind anomaly regions. Like previous results, the study reveals that downwind anomaly regions can vary as a function of prevailing wind regime. Using a metric called the Wet Millimeter Day (WMD), the study also finds that there is a tendency for extreme rainfall to cluster in the climatological downwind area of Atlanta. The work builds upon previous findings while employing different datasets to provide novel additional contributions related to the temporal evolution of the “urban rainfall effect” and the patterns of extreme rainfall.

A 30 year climatology of North Atlantic cyclones from 1979 to 2008 is examined within the context of extreme Greenland blocking and accelerated surface melting across the Greenland ice sheet (GrIS). A distinct class of North Atlantic cyclones, known as precursor cyclones, was identified as any extratropical cyclones originating to the west of Greenland blocks prior to the peak of blocking intensity. Composite map analysis reveals that precursor cyclones contributed to a significant intensification of extreme Greenland blocking episodes (GBEs) through the process of upper level wave amplification. Across all seasons, most extreme GBEs are associated with multiple precursor cyclones prior to peaking in intensity, and a majority of these cyclones have continental rather than oceanic origins. Over both the western and eastern sectors of Greenland, daily meltwater production simulated by the Modele Atmospherique Regional regional climate model is greater during extreme GBEs accompanied by precursor cyclones compared to extreme GBEs lacking a precursor cyclone. Based on an analysis of air parcel trajectories and North Atlantic SST anomalies, enhanced surface melting during the summer, particularly over southern and western Greenland, is strongly linked to the combination of vigorous adiabatic warming generated by subsiding air within the blocking anticyclones and warm air advection supplied by the precursor cyclones. With the increased frequency of extreme GBEs accompanied by precursor cyclones observed during a strong positive phase of the Atlantic Multidecadal Oscillation, recent long-term increases in GrIS surface melting can be partially attributed to the interaction of these atmospheric and oceanic processes.

ABSTRACTIn late October 2012, an extreme area of high pressure centred near Greenland, known as a ‘Greenland block’, forced Hurricane Sandy to turn westward into the Northeast coast of the United States. In light of this unusual and catastrophic event, the Greenland blocking conditions associated with Sandy and previous North Atlantic hurricane tracks are examined from a climatological perspective. Two primary questions raised by Sandy are investigated: (1) How anomalous were the Greenland blocking conditions observed prior to Sandy's landfall? (2) Were North Atlantic hurricane tracks in the historical record affected by Greenland blocking conditions in a manner similar to Sandy?The measure of blocking strength used to answer these questions is the Greenland Blocking Index (GBI), which is calculated as the spatial average of 500 hPa heights over the Greenland region. The GBI prior to Sandy's landfall was found to be more typical of late June conditions, exceeding the 90th percentile of late October climatology during the entire preceding week and peaking at near‐record values (99.8th percentile) on 25 October. Analysis of the GBI in relation to past North Atlantic hurricane tracks shows that above‐normal GBI values were associated with a southward displacement of hurricane tracks and an increased concentration of tracks near the Northeast US coast. Additionally, composites of atmospheric conditions for ‘anomalous’ hurricane tracks (defined as tracks with an initial bearing angle between 90° and 360°) revealed an atypical area of high pressure centred over the Canadian Maritimes during these events, whereas near‐climatological conditions of atmospheric pressure over the North Atlantic prevailed for ‘normal’ hurricane tracks. However, variations in Greenland blocking conditions were not associated with significant changes in the frequency of anomalous North Atlantic hurricane tracks.

ABSTRACTAtmospheric blocking commonly occurs over the high latitudes of the Northern Hemisphere, resulting from the development of persistent areas of high pressure that lead to warmer‐than‐average surface temperatures west of the high centre. While the variability and trends in anticyclonic circulation patterns (including blocking) over Greenland have been previously documented, an analysis of the most extreme blocking events within the observational record is lacking. In this study, a historical climatology of extreme Greenland blocking episodes (GBEs) from 1958 to 2013 is examined within the context of anomalous anticyclonic circulation patterns over the North Atlantic region during recent years. Based on a combination of the ERA‐40 (1958–1978) and ERA‐Interim (1979–2013) reanalysis data sets, the Greenland Blocking Index (GBI) is used to quantify 500 hPa geopotential height anomalies for the identification of extreme GBEs. The annual rate of extreme blocking days has doubled since 1958, reaching an average of approximately 20 days per year by 2013. The frequency and, to some extent, duration of extreme GBEs were unprecedentedly high from 2007 to 2013 compared to the 56‐year period of record, with a majority of the increase occurring during the spring (MAM) and summer (JJA). A multiple linear regression analysis reveals that interannual variability in extreme blocking and the Atlantic Multidecadal Oscillation (AMO) are the two predominant drivers of surface meltwater production across the entire Greenland ice sheet (GrIS), but Arctic sea ice extent and North Atlantic cyclone activity can also influence the extent of summer melting over portions of the GrIS. Thus, in addition to the larger‐scale atmospheric and oceanic variability, smaller‐scale features such as extratropical cyclones can play a significant role in modulating GrIS surface melting each summer.

Although advances have been made regarding the influence of aerosols on precipitation processes, the ability of power plant aerosol emissions to alter cloud microphysics and enhance atmospheric convection is not yet fully understood. By analyzing the relationship between proximity to coal power plants and lightning flash density in Georgia from 1992–2003, we find that lightning strike frequency is not substantially enhanced near power plants in the long-term warm season climatology. If existent, any signal was likely masked by the more dominant mechanisms of the sea breeze circulation, Sandhills-Fall Line convection, and Atlanta urban environment. Despite the lack of a definitive signal in the climatology, several cases of potential lightning amplification were identified for Plant Scherer. Therefore, power plant lightning enhancement may potentially occur in isolated events, but it is difficult to link such events conclusively to power plant aerosol emissions without a more detailed analysis of the causal mechanisms.

The calendar year 2011 was an extraordinary year for tornadoes across the United States, as it marked the second highest annual number of tornadoes since 1950 and was the deadliest tornado year since 1936. Most of the fatalities in 2011 occurred in a series of outbreaks, highlighted by a particularly strong outbreak across the southeastern United States in late April and a series of outbreaks over the Great Plains and Midwest regions in late May, which included a tornado rated as a category 5 event on the enhanced Fujita scale (EF5) that devastated the town of Joplin, Missouri. While most tornado-related fatalities often occur in outbreaks, very few studies have examined the climatological characteristics of outbreaks, particularly those of varying strength. In this study a straightforward metric to assess the strength, or physical magnitude, of tornado outbreaks east of the Rocky Mountains from 1973 to 2010 is developed. This measure of outbreak strength, which integrates the intensity of tornadoes [Fujita (F)/EF-scale rating] over their distance traveled (pathlength), is more highly correlated with injuries and fatalities than other commonly used variables, such as the number of significant tornadoes, and is therefore more reflective of the potential threat of outbreaks to human life. All outbreaks are then ranked according to this metric and their climatological characteristics are examined, with comparisons made to all other tornadoes not associated with outbreaks. The results of the ranking scheme are also compared to those of previous studies, while the strongest outbreaks from 2011 are ranked among other outbreaks in the modern record, including the April 1974 Super Outbreak.