Mace L. Bentley

A Climatology of Derecho-Producing Mesoscale Convective Systems in the Central and Eastern United States, 1986–95. Part I: Temporal and Spatial Distribution

Abstract In 1888, Iowa weather researcher Gustavus Hinrichs gave widespread convectively induced windstorms the name “derecho”. Refinements to this definition have evolved after numerous investigations of these systems; however, to date, a derecho climatology has not been conducted. This investigation examines spatial and temporal aspects of derechos and their associated mesoscale convective systems that occurred from 1986 to 1995. The spatial distribution of derechos revealed four activity corridors during the summer, five during the spring, and two during the cool season. Evidence suggests that the primary warm season derecho corridor is located in the southern Great Plains. During the cool season, derecho activity was found to occur in the southeast states and along the Atlantic seaboard. Temporally, derechos are primarily late evening or overnight events during the warm season and are more evenly distributed throughout the day during the cool season.

Urban-induced thunderstorm modification in the Southeast United States

This study provides the first climatological synthesis of how urbanization augments warm-season convection among a range of cities in the southeastern U.S. By comparing the location of convection in these cities and adjacent control regions via high-resolution, radar reflectivity and lightning data, we illustrate that demographic and land-use changes feed back to local atmospheric processes that promote thunderstorm formation and persistence. Composite radar data for a 10-year, June–August period are stratified according to specific “medium” and “high” reflectivity thresholds. As surrogates for potentially strong (medium reflectivity) and severe (high reflectivity) thunderstorms, these radar climatologies can be used to determine if cities are inducing more intense events. Results demonstrate positive urban amplification of thunderstorm frequency and intensity for major cities. Mid-sized cities investigated had more subtle urban effects, suggesting that the urban influences on thunderstorm development and strength are muted by land cover and climatological controls. By examining cities of various sizes, as well as rural counterparts, the investigation determined that the degree of urban thunderstorm augmentation corresponds to the geometry of the urban footprint. The research provides a methodological template for continued monitoring of anthropogenically forced and/or modified thunderstorms.

A synoptic climatology of derecho producing mesoscale convective systems in the North-Central Plains

Synoptic-scale environments favourable for producing derechos, or widespread convectively induced windstorms, in the North-Central Plains are examined with the goal of providing pattern-recognition/diagnosis techniques. Fifteen derechos were identified across the North-Central Plains region during 1986–1995. The synoptic environment at the initiation, mid-point and decay of each derecho was then evaluated using surface, upper-air and National Center for Atmospheric Research (NCAR)/National Center for Environmental Prediction (NCEP) reanalysis datasets. Results suggest that the synoptic environment is critical in maintaining derecho producing mesoscale convective systems (DMCSs). The synoptic environment in place downstream of the MCS initiation region determines the movement and potential strength of the system. Circulation around surface low pressure increased the instability gradient and maximized leading edge convergence in the initiation region of nearly all events regardless of DMCS location or movement. Other commonalities in the environments of these events include the presence of a weak thermal boundary, high convective instability and a layer of dry low-to-mid-tropospheric air. Of the two corridors sampled, northeastward moving derechos tend to initiate east of synoptic-scale troughs, while southeastward moving derechos form on the northeast periphery of a synoptic-scale ridge. Other differences between these two DMCS events are also discussed. Copyright

Midwestern High Dew Point Events 1960-2000

Daily average dew points (DADPs) computed for 46 Midwestern first-order stations (FOS) were examined from 1960 to 2000 to identify and characterize extreme warm-season high dew point events. To be classified as an extreme event, more than 50% of the FOS had to experience a DADP of 22°C (72°F) or higher for two or more consecutive days within the event. Nine events were found to have occurred during the 41-year period. The length of the events varied from 5 to 13 days, while the number of stations involved in each event ranged from 24 to 40. Two summers, 1995 and 1999, each experienced two events. Event intensity, based on the percentage of all station hours during each event with dew points ≥ 22°C, was greatest in the events that occurred in the 1990s. An examination of the event diurnal cycle identified that: (1) the minimum number of stations experiencing an hourly dew point value ≥ 22°C occurred at 0300 and 0600 local time, while the maximum number of stations meeting this dew point threshold generally occurred at 0900 and 1200; (2) the biggest dew point increases in terms of spatial coverage of values ≥ 22° occurred between 0600 and 0900; and (3) in 10 of 73 (14%) event days dew points remained ≥ 22°C at ≥ 50% of the stations for 24 consecutive hours. Developing a greater understanding of the spatial and temporal evolution of widespread and intense high dew point events should assist those involved in the design and operation of air conditioning systems that rely on evaporative processes to cool air.

A SYNOPTIC CLIMATOLOGY OF COOL-SEASON DERECHO EVENTS

Synoptic-scale environments favorable for producing derechos in the cool season (September through February) are examined with the goal of providing useful techniques for identifying commonalities within derecho activity corridors. Fourteen derechos were identified from two activity corridors located in the southeastern United States and Atlantic seaboard regions between 1986 and 1995. The synoptic environment at the initiation and midpoint of each derecho was then evaluated using surface, upper-air, and the NCAR/NCEP reanalysis data sets. Models are provided in order to illustrate the synoptic-scale environment and to assist meteorologists in recognizing conditions favorable for cool-season derecho formation. Marginal instability and strong synoptic-scale forcing characterized the environments of events in both corridors. The overall synoptic patterns associated with cool-season derecho-producing mesoscale convective systems (DMCSs) resembled environments found with cool-season tornado episodes. Recognition of key elements in this environment could lead to improvements in cool-season severe weather prediction. [Key words: derecho, windstorm, mesoscale convective system (MCS), synoptic climatology, southeast United States.]

Radar identification of urban-enhanced thunderstorm activity for Atlanta, Georgia, USA

The distribution of warm season (June through August) thunderstorm activity surrounding Atlanta, Georgia from 1997-2006 was identified from composite reflectivity data obtained from the network of National Weather Service radars (WSR-88D). Maxima of medium- to high-reflectivity episodes were identified to the north of and within downtown Atlanta and immediately east of the primary urban expansion of the central business district. Additional enhanced, high-reflectivity areas are found south of downtown Atlanta. These regions are also correlated with high-density urban expansion south of the Atlanta CBD.

Lightning at "The Master's": An Evaluation of April Thunderstorms in and Near the Augusta National Golf Club

April lightning production in Richmond County, Georgia, from 1995 to 2003 is considered with regard to the associated risk to golfers and spectators at the Masters Golf Tournament and courses throughout the county. An examination of the April lightning climatology of Georgia, focusing on Richmond County, reveals a minimum in lightning activity during the time of the tournament (the first full week of April). A discriminant analysis of Richmond County lightning production utilizes several meteorological variables in order to discern between three categories of April lightning days: little or no lightning activity, minimal lightning activity, and high lightning activity. The analysis distinguishes relatively well between the categories with only 13.5% of the events misclassified. A composite regional analysis of the three lightning categories illustrates the environmental differences between these events. Although the little or no lightning and high lightning categories are somewhat similar environmentally, the intensity of features separates these events and allows for interpretation. The statistical and synoptic analyses together emphasize the importance of low-level instability and frontal proximity and intensity in enhancing lightning production in thunderstorms during April in and surrounding Richmond County. A multitude of meteorological conditions can lead to thunderstorm development. Localized studies of these environments, especially during times of major outdoor sporting events, are needed to help mitigate the risk associated with lightning.