Andrew I. Watson

Diurnal Cloud-to-Ground Lightning Patterns in Arizona during the Southwest Monsoon

Abstract Cloud-to-ground (CG) lightning shows great variability across Arizona from one year to the next as well as from one day to the next. Availability of moisture, location of the subtropical ridge axis, transitory troughs in both the westerlies and easterlies, and low-level moisture surges from the Gulf of California can affect thunderstorm occurrence, which, in turn, will affect lightning production. Diurnal CG lightning patterns in Arizona are also determined by daily heating cycles and topography. Six years of Bureau of Land Management CG flash data are used in this investigation. In Arizona, lightning usually starts first, on a daily basis, in the plateau region and extends in an arc from the White Mountains of eastern Arizona westward across the Mogollon Rim and then northward onto the Kaibab Plateau of northern Arizona. Flash activity moves in a more or less continuous fashion off the plateau, south and westward down the topography gradient, and enters the lower desert by early evening. At the ...

Warm Season Lightning Distributions over the Florida Peninsula as Related to Synoptic Patterns

Abstract Cloud-to-ground lightning data from the National Lightning Detection Network are examined over the Florida peninsula during the warm seasons of 1989 through 1998. The lightning data are stratified according to the location of the subtropical ridge (i.e., north of Florida, south of Florida, and within Florida) as well as other common flow types. Each day is placed into a flow regime based on radiosonde-derived low-level winds at three stations within the study area. Maps of lightning flash density are generated for each flow regime over hourly, daily (24 h), and nocturnal periods. Results for the 24-h period indicate that complexities in the Florida coastline produce four areas of relatively large flash densities: near Tampa, Fort Myers, West Palm Beach, and Cape Canaveral. Nocturnal lightning is found to occur mostly offshore—related to the Gulf Stream, coastline orientations, the prevailing flow, and land breezes. The location of the subtropical ridge with respect to the Florida peninsula is fou...

The Life Cycle of Lightning and Severe Weather in a 3–4 June 1985 PRE-STORM Mesoscale Convective System

Abstract Cloud-to-ground lightning flash characteristics of a series of four mesoscale convective systems (MCS) that occurred in Oklahoma and Kansas on 3–4 June 1985 during the Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central project are described. A total of 23 490 flashes were detected by the network from all four MCSs; 96% of them lowered negative charge to ground. Because the second MCS (MCS II) spent nearly all of its lifetime within the optimal region of coverage of the lightning and radar networks, trends in ground-flash characteristics could be documented throughout the systems life cycle. Lightning trends were analyzed relative to rainfall parameters based on radar network data and were stratified by the flashes’ polarity and locations according to their association with convective and stratiform radar echoes. Most flashes in the second MCS were negative ground strikes within convective radar echoes. In convective regions the flashes were primarily negative; in stratiform region...

Cloud-to-Ground Lightning and Upper-Air Patterns during Bursts and Breaks in the Southwest Monsoon

Abstract Convective bursts and breaks in the southwest U.S. monsoon are investigated in a lightning context because cloud-to-ground (CG) lightning is an excellent indicator of deep convection. Bursts and breaks are identified using six years of Bureau of Land Management CG lightning information. Composited upper-air analyses for 12 bursts and 10 breaks are developed to examine the synoptic-scale differences between these two regimes. Anomaly patterns are investigated, and average burst and break regimes am presented. This investigation shows the importance of moisture, the location of the subtropical ridge axis, and the high-plateau thermal low. For the burst, die ridge axis is displaced northward across Arizona and New Mexico and moisture is usually abundant in the southwestern United States. During the break, the ridge retreats southward into northern Mexico, giving way to dry westerly winds across Arizona. The high-plateau thermal low is firmly in place during July and August, and it pulls low-level mo...

An Eight-Year Lightning Climatology of the Southeast United States Prepared for the 1996 Summer Olympics

Abstract The 1996 Summer Olympics will be held in the Atlanta, Georgia, vicinity and several other sites in the southeast United States between 19 July and 4 August 1996. This period coincides with the peak thunderstorm season, so the threat of lightning casualties cannot be taken lightly since Georgia and surrounding states with Olympic venues rank quite high in the United States in annual lightning casualties and the casualty rate per population. Flash density and thunderstorm day estimates for July and August show that the greatest cloud-to-ground (CG) lightning activity (>5 flashes per square kilometer) occurs near the seacoasts and over the Florida peninsula. Cloud-to-ground flash activity decreases as distances increase from the coastlines, reaching a minimum ( 3 flashes per square kilometer) in and around Atlanta. The frequency of CG lightning reaches a maximum in the late afternoon a...

The Cool-Season Tornadoes of California and Southern Australia

Abstract Examples of cool-season tornadic thunderstorms in California and southern Australia are examined. Almost one-half of the reported Australian tornadoes and the majority of those in California occur in the cool season. It is shown that in both areas the typical synoptic pattern shows an active midlatitude trough just upstream, with a strong jet streak aloft. In both areas the tornadic thunderstorms occur with weak to moderate levels of thermodynamic instability in the lower troposphere but with extremely high values of low-level positive and bulk shear. Statistical tests on null cases (nontornadic thunderstorms) in the Central Valley of California indicate that large values of 0–1-km shear provide a discriminator for more damaging (F1–F3) tornadoes, whereas bulk measures of buoyancy, such as CAPE, do not. Australian case studies and tornado proximity soundings show similar characteristics. A “cool-season tornadic thunderstorm potential” diagnostic for Australian conditions, based on regional NWP an...

Lightning from Two National Detection Networks Related to Vertically Integrated Liquid and Echo-Top Information from WSR-88D Radar

Abstract Two national cloud-to-ground (CG) lightning-detection systems were operating across the contiguous United States during 1993. These two networks are compared to each other and to WSR-88D radar information collected by the Twin Lakes, Oklahoma, radar on 9 June 1993 as a squall line moved through central Oklahoma. Next Generation Weather Radar Information Dissemination Service data are used to investigate the relationship of CG lightning to WSR-88D echo tops and vertically integrated liquid (VIL). Such relationships appear not to have been investigated in such fine time and space resolution to date. When lightning is normalized by the frequency of occurrence of 4 km × 4 km resolution echo-top areas, the greatest percentage of echoes with lightning occurs when echo-top heights exceed 50 000 ft (15.2 km). The percentage of echoes with lightning drops significantly as echo tops decrease. The relationship of VIL with lightning is not as clearly defined. The frequency of echoes with lightning increases ...

Convective Evolution and Merger in the FACE Experimental Area: Mesoscale Convection and Boundary Layer Interactions

Abstract This paper investigates the interactions between the various scales of motion and, specifically, the inter-actions between convection and the surface boundary layer in the development of a mesoscale convective system within the Florida Area Cumulus Experiment (FACE) experimental area. Data used in the analysis are from a surface mesonetwork covering a 1500 km2 area which consisted of wind measuring stations, raingages, hygrothermographs, microbarographs and temperature, humidity and pressure transducers. Surface convergence was shown to exist up to 2 h before the development of precipitation over the convergence area within the mesonetwork. Convergence was not being balanced by divergence within the network, which implies mesoscale and/or synoptic-scale forcing. The subsidence warming and drying in the near environment of the mesoscale convective system appeared to play an important role in its evolution from the mature to the dissipating stage. Soundings taken in the near environment of the conv...

Lightning during Two Central U.S. Winter Precipitation Events

Abstract Network-detected cloud-to-ground lightning coincident with mainly frozen precipitation (freezing rain, sleet, snow) was studied over the central United States during two outbreaks of arctic air in January 1994. During the first event, the ratio of positive to total flashes was 59%, flashes were few and disorganized in area, and no surface observer reported thunder. For the other event the ratio was 52% during the first few hours in subfreezing surface air, then decreased when flashes formed in the nearby region above freezing. Also, flashes in this case were linearly aligned and coincided with conditional symmetric instability; thunder was heard infrequently by surface observers. On radar, reflectivity cores grew from weak to moderate intensity within a few hours of the lightning during both cases. Echo area increased greatly before flashes in one case, while the area increase coincided with flashes in the other. Some base-scan reflectivities were strong in both thunderstorm regions due to the ra...

Surface Wind Convergence as a Short-Term Predictor of Cloud-to-Ground Lightning at Kennedy Space Center

Abstract Cloud-to-ground lightning is a significant forecast problem at the Kennedy Space Center (KSC) in Florida. In this study, cloud-to-ground lightning is related in time and space to surface convergence for 244 days during the convective seasons of 1985 and 1986 over a 790-km2 network at KSC. The method uses surface convergence, particularly the average over the area, to identify the potential for new, local thunderstorm growth, and it can be used to specify the likely time and location of lightning during the life cycle of the convection. A threshold of 75×10−6 s−1 change in divergence is the main criterion used to define a convergence event, and a set of flashes less than 30 min apart defines a lightning event. Time intervals are found from the study to be approximately 1 h from beginning convergence to first flash, and another hour to the end of lightning. The influences of low-level winds and midlevel moisture in determining the location and intensity of convection are discussed. This is the firs...