Walter A. Petersen

Regional Variability in Tropical Convection: Observations from TRMM

Abstract Observation of the vertical profile of precipitation over the global Tropics is a key objective of the Tropical Rainfall Measuring Mission (TRMM) because this information is central to obtaining vertical profiles of latent heating. This study combines both TRMM precipitation radar (PR) and Lightning Imaging Sensor (LIS) data to examine “wet-season” vertical structures of tropical precipitation across a broad spectrum of locations in the global Tropics. TRMM-PR reflectivity data (2A25 algorithm) were utilized to produce seasonal mean three-dimensional relative frequency histograms and precipitation ice water contents over grid boxes of approximately 5°–10° in latitude and longitude. The reflectivity histograms and ice water contents were then combined with LIS lightning flash densities and 2A25 mean rainfall rates to examine regional relationships between precipitation vertical structure, precipitation processes, and lightning production. Analysis of the reflectivity vertical structure histograms ...

On the relationship between cloud‐to‐ground lightning and convective rainfall

Ratios of area mean rainfall and cloud-to-ground lightning flash count (termed rain yields) were computed for several different locations around the globe, over temporal and spatial scales of 1 month and 10 4 -10 5 km 2 , respectively. Values of the rain yield clustered near 10 8 kg/fl for a large portion of the midcontinental United States. Rain yields were slightly lower over the arid southwestern United States, averaging ∼6 × 10 7 kg/fl. In tropical locations the rain yields increased systematically from a tropical continental value of 4 × 10 8 kg/fl to a value of 10 10 kg/fl for the tropical western Pacific Ocean. The observed stability of the rain yield, coupled with demonstrated positive correlations between cloud-to-ground flash density and rainfall amount, suggests that cloud-to-ground lightning data may be useful for inferring monthly convective rainfall statistics in certain rainfall regimes.

Cloud-to-Ground Lightning Observations from TOGA COARE: Selected Results and Lightning Location Algorithms

Abstract Recently, observations of electrified oceanic convection and associated cloud-to-ground (CG) lightning were obtained over the tropical western Pacific Ocean during TOGA COARE (Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment). During COARE, observations of convection were made using a variety of instrument platforms including ship and airborne Doppler radars, an advanced lightning direction finder (ALDF) network, and a shipborne inverted electric field mill. This study focuses on data collected by the COARE ALDF network, fusion of those data with observations, and the methods used to calculate accurate CG return stroke locations. Analysis of CG lightning data and Doppler radar data indicates that lightning-producing oceanic convection is characterized by deep, vertically developed convective cells with radar reflectivities exceeding 30-dBZ above the height of the −10°C level. In several cases a peak in CG frequency occurred coincident with the descent of precipitation...

Radar observations of the kinematic, microphysical, and precipitation characteristics of two MCSs in TRMM LBA

[1] Dual-Doppler and polarimetric radar observations are used to analyze two mesoscale convective systems (MCSs) that occurred during the Tropical Rainfall Measuring Mission Large-Scale Biosphere-Atmosphere field campaign. The MCSs formed in different meteorological regimes, based on profiles of atmospheric wind and thermodynamic data. The first MCS event (26 January 1999) was a squall line that formed in low-level easterly flow and had an intense leading line of convection. In contrast, the 25 February 1999 MCS formed in low-level westerly flow and was best characterized by stratiform precipitation with embedded convective elements. The radar analyses suggest that the MCSs were distinct in terms of overall vertical structure characteristics. In particular, polarimetric radar cross sections indicated the presence of an active mixed phase zone in the easterly MCS that was largely absent in the westerly case. The easterly MCS had considerably more precipitation ice in the middle to upper troposphere compared to the westerly MCS. Composite analyses showed that the easterly MCS had higher peak reflectivities and a smaller reflectivity gradient above the 08C level in convective regions of the storm compared to the westerly MCS event. Moreover, mean profiles of both vertical air motion and vertical mass transport in the convective portion of the easterly MCS were larger (over a factor of 2 at some heights below the 08 C level) than those in the westerly event. These observations suggest that the easterly and westerly wind regimes in the southwest Amazon region produce convection with different vertical structure characteristics, similar to regimes elsewhere in the global tropics (e.g., maritime continent). INDEX TERMS: 3314 Meteorology and Atmospheric Dynamics: Convective processes; 3329 Meteorology and Atmospheric Dynamics: Mesoscale meteorology; 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854); 3374 Meteorology and Atmospheric Dynamics: Tropical meteorology KEYWORDS: Microphysics, tropical, precipitation, polarimetric radar, mesoscale convective system

Mesoscale and Radar Observations of the Fort Collins Flash Flood of 28 July 1997

Abstract On the evening of 28 July 1997 the city of Fort Collins, Colorado, experienced a devastating flash flood that caused five fatalities and over 200 million dollars in damage. Maximum accumulations of rainfall in the western part of the city exceeded 10 in. in a 6-h period. This study presents a multiscale meteorological overview of the event utilizing a wide variety of instrument platforms and data including rain gauge, CSU—CHILL multiparameter radar, Next Generation Radar, National Lightning Detection Network, surface and Aircraft Communication Addressing and Reporting System observations, satellite observations, and synoptic analyses. Many of the meteorological features associated with the Fort Collins flash flood typify those of similar events in the western United States. Prominent features in the Fort Collins case included the presence of a 500-hPa ridge axis over northeastern Colorado; a weak shortwave trough on the western side of the ridge; postfrontal easterly upslope flow at low levels; w...

TRMM Observations of Intraseasonal Variability in Convective Regimes over the Amazon

This study utilizes the Tropical Rainfall Measuring Mission (TRMM) satellite precipitation radar (PR), lightning imaging sensor (LIS), and passive microwave imager (TMI) data together with ground-based lightning data to investigate the vertical structure, lightning, and rainfall characteristics of Amazonian and subtropical South American convection for three separate wet seasons. These characteristics are partitioned as a function of 850mb zonal wind direction, motivated by observations collected during the 6-week TRMM‐Large-scale Biosphere‐ Atmosphere Experiment in Amazonia (LBA) field campaign. The TRMM‐LBA field campaign observations suggest that systematic variations in Amazonian convective vertical structure, lightning, and rainfall are all linked to bimodal variations in the low-level zonal wind (e.g., easterly and westerly regimes). The more spatially and temporally comprehensive TRMM dataset used in this study extends the TRMM‐LBA observations by examining regime variability in Amazonian and South American convective structure over a continental-scale domain. On a continental scale, patterns of east and west regime 850‐700-mb winds combined with LIS lightning flash densities suggest the presence of synoptic-scale controls [e.g., intrusion of extratropical frontal systems and interaction with the South Atlantic Convergence Zone (SACZ)] on regional-scale variability in convective vertical structure. TRMM PR, TMI, and ground-based lightning data suggest that regional variability in wetseason convective structure is most evident over the southern Amazon, Mato Grosso, Altiplano, southern Brazil, and eastern coastal regions of central and southern South America. Convective vertical structure, convective rainfall rates, and lightning activity are all more pronounced during easterly (westerly) regimes over the southern Amazon and Mato Grosso (Altiplano, and southern Brazil). Importantly, when considered with case study results from TRMM‐LBA, the systematic differences in convective structure that occur as a function of regime suggest that associated regime differences may exist in the vertical distribution of diabatic heating. Hence the discrimination of convective vertical structure ‘‘regimes’’ over parts of the Amazon and vicinity based on a resolved variable such as the 850‐700-mb zonal wind direction, while far from being perfect, may have important applications to the problems of cumulus parameterization, rainfall estimation, and retrievals of latent heating over the Amazon.

Lightning and Severe Weather: A Comparison between Total and Cloud-to-Ground Lightning Trends

Many studies over the past several decades have attempted to correlate trends in lightning (e.g., rates, polarity) to severe weather occurrence. These studies mainly used cloud-to-ground (CG) lightning information due to the ease of data availability, high detection efficiency, and broad coverage across the United States, with somewhat inconclusiveresults.Conversely, it has been demonstrated that trendsin total lightning aremorerobustlycorrelatedtosevereweatheroccurrence,withrapidincreasesintotallightningobserved10s of minutes prior to the onset of severe weather. Unfortunately, total lightning observations are not as numerous, or available over the same areal coverage domain, as provided by CG networks. Relatively few studieshaveexamined concurrent trendsin bothtotalandCG lightningwithinthesameseverethunderstorm, or even large sets of thunderstorms using an objective lightning jump algorithm. Multiple studies have shown thatthetotalflashraterapidlyincreasespriortotheonsetofsevereweather.Whatisuntestedwithinthesame framework is the use of CG information to perform the same task. Herein, total and CG lightning trends for 711 thunderstorms occurring in four regions of the country were examined to demonstrate the increased utilitythattotallightningprovides overCGlightning,specifically withintheframeworkofdevelopingauseful lightning-based severe weather warning decision support tool. Results indicate that while both lightning datasets demonstrate the presence of increased lightning activity prior to the onset of severe weather, the use oftotal lightningtrendswasmore effectivethanCG trends[probability ofdetection(POD),79% versus66%; false alarm rate (FAR), 36% versus 53%; critical success index (CSI), 55% versus 38%; Heidke skill score (HSS), 0.71 versus 0.55]. Moreover, 40% of false alarms associated with total lightning, and 16% of false alarms with CG lightning trends, occurred when a lightning jump associated with a severe weather ‘‘warning’’ was already in effect. If these false alarms are removed, the FAR drops from 36% to 22% for total lightning and from 53% to 44% for CG lightning. Importantly, average lead times prior to severe weather occurrence werehigherusingtotallightningascomparedwithCGlightning(20.65versus13.54 min).Theultimategoalof this study was to demonstrate the increased utility of total lightning information that the Geostationary Lightning Mapper (GLM) will provide to operational meteorology in anticipation of severe convective weather on a hemispheric scale once Geostationary Operational Environmental Satellite-R (GOES-R) is deployed in the next decade.

Microphysical observations of tropical clouds

Abstract The results from airborne in situ sampling of convective tropical storms in the Amazon and Kwajalein are presented. Three cases from the Amazon and two from Kwajalein are compared and provide examples of the much larger dataset that was obtained from field campaigns in these two regions during 1999. The strength of the updraft was a major factor in determining the microphysical characteristics of hydrometeors. Weak updrafts exhibited a well-developed warm rain process by the time droplets had reached the freezing level. Stronger updrafts (>5 m s−1) contained smaller droplets or ice particles at cloud midlevels than regions with the weaker updrafts. Significant supercooled liquid water was found only at temperatures warmer than −12°C, although traces of liquid water were observed at temperatures as cold as −18°C. In deep stratiform anvil regions, aggregation was observed to be a major growth mechanism. These clouds did not contain appreciable amounts of supercooled water. Clouds with similar updra...

Preliminary Development and Evaluation of Lightning Jump Algorithms for the Real-Time Detection of Severe Weather

Abstract Previous studies have demonstrated that rapid increases in total lightning activity (intracloud + cloud-to-ground) are often observed tens of minutes in advance of the occurrence of severe weather at the ground. These rapid increases in lightning activity have been termed “lightning jumps.” Herein, the authors document a positive correlation between lightning jumps and the manifestation of severe weather in thunderstorms occurring across the Tennessee Valley and Washington D.C. A total of 107 thunderstorms from the Tennessee Valley; Washington, D.C.; Dallas, Texas; and Houston, Texas, were examined in this study. Of the 107 thunderstorms, 69 thunderstorms fall into the category of nonsevere and 38 into the category of severe. From the dataset of 69 isolated nonsevere thunderstorms, an average, peak, 1-min flash rate of 10 flashes per minute was determined. A variety of severe thunderstorm types were examined for this study, including a mesoscale convective system, mesoscale convective vortex, tor...

Comparison of Raindrop Size Distribution Measurements by Collocated Disdrometers

AbstractAn impact-type Joss–Waldvogel disdrometer (JWD), a two-dimensional video disdrometer (2DVD), and a laser optical OTT Particle Size and Velocity (PARSIVEL) disdrometer (PD) were used to measure the raindrop size distribution (DSD) over a 6-month period in Huntsville, Alabama. Comparisons indicate event rain totals for all three disdrometers that were in reasonable agreement with a reference rain gauge. In a relative sense, hourly composite DSDs revealed that the JWD was more sensitive to small drops (<1 mm), while the PD appeared to severely underestimate small drops less than 0.76 mm in diameter. The JWD and 2DVD measured comparable number concentrations of midsize drops (1–3 mm) and large drops (3–5 mm), while the PD tended to measure relatively higher drop concentrations at sizes larger than 2.44 mm in diameter. This concentration disparity tended to occur when hourly rain rates and drop counts exceeded 2.5 mm h−1 and 400 min−1, respectively. Based on interactions with the PD manufacturer, the p...