Eugene W. McCaul

Buoyancy and shear characteristics of hurricane-tornado environments

Abstract Detailed composite profiles of temperature, moisture, and wind are constructed for tornado environments in tropical cyclones that affected the United States during the period 1948–86. Winds are composited in components radial and tangential to the tropical cyclone center at observation time. Guided by observed patterns of tornado occurrence, composites are constructed for a variety of different stratifications of the data, including proximity to tornadoes, position relative to the cyclone center, time of day, time after cyclone landfall, cyclone translation speed, and location of landfall. The composites are also compared to composite soundings from Great Plains tornado environments. A variety of sounding parameters am examined to see which are most closely related to the patterns of tornado distribution. Lower-tropospheric vertical shears are generally stronger in the tropical cyclone tornado environments than on the Great Plains. Vertical shear and helicity parameters, along with 700-hPa wind s...

The 1997–98 El Nino event and related wintertime lightning variations in the southeastern United States

The El Nino Southern Oscillation (ENSO) is a climate anomaly responsible for worldwide weather impacts ranging from droughts to floods. In the United States, warm episode years are known to produce above normal rainfall along the Southeast U.S. Gulf Coast and into the Gulf of Mexico, with the greatest response observed in the October–March period of the warm episode year. The 1997–98 warm episode is notable for being the strongest event since 1982–83. With the recent launch of a lightning sensor on NASAs Tropical Rainfall Measuring Mission (TRMM) in November 1997 and the detailed coverage of the U.S. National Lightning Detection Network (NLDN), such interannual changes in lightning activity can be examined with far greater detail than ever before. For the 1997–98 ENSO event the most significant year-to-year changes in lightning frequency worldwide occurred along the Gulf Coast and within the Gulf of Mexico basin during the Northern Hemisphere winter. Within a broad swath across the northern Gulf of Mexico basin there is a 100–150% increase in lightning days year-to-year (a peak of 33 days in the winter of 1997–98 vs. only 15 days or fewer in both the 1996–97 and 1998–99 winter). In addition, there is a nearly 200% increase in lightning hours (a peak of 138 hours in 1996–97 vs. 50 hours in both 1996–97 and 1998–99). The increase in lightning activity during ENSO occurs in association with a 100% increase in the number of synoptic scale cyclones that developed within or moved through the Gulf basin. The primary variables controlling these enhancements in thunderstorm activity are the position and strength of the jet stream.

Wavelength dependence of backscatter by use of aerosol microphysics and lidar data sets: application to 2.1-µm wavelength for space-based and airborne lidars

An aerosol microphysics dataset was used to model backscatter in the 0.35-11-mum wavelength range, with the results validated by comparison with measured cw and pulsed lidar backscatter obtained during two NASA-sponsored airborne field experiments. Different atmospheric features were encountered, with aerosol backscatter ranging over 4 orders of magnitude. Modeled conversion functions were used to convert existing lidar backscatter datasets to 2.1 mum. Resulting statistical distribution shows the midtropospheric aerosol backscatter background mode of beta(2.1) to be between ~3.0 x 10(-10) and ~1.3 x 10(-9) m(-1) sr(-1), ~10-20 times higher than that for beta(9.1); and a beta(2.1) boundary layer mode of ~1.0 x 10(-7) to ~1.3 x 10(-6) m(-1) sr(-1), ~3-5 times higher than beta(9.1).

Regional Comparison of GOES Cloud-Top Properties and Radar Characteristics in Advance of First-Flash Lightning Initiation

Lightning initiation (LI) events over Florida and Oklahoma are examined and statistically compared to understand the behavior of observed radar and infrared satellite interest fields (IFs) in the 75-min time frame surrounding LI. Lightning initiation is defined as the time of the first lightning, of any kind, generated in a cumulonimbus cloud. Geostationary Operational Environmental Satellite (GOES) infrared IFs, contoured frequency by altitude diagrams (CFADs) of radar reflectivity, and model sounding data, analyzed in concert, show the mean characteristics over time for 36 and 23 LI events over Florida and Oklahoma, respectively. CFADs indicate that radar echoes formed 60 min before Florida LI, yet Oklahoma storms exhibited a ;30-min delayed development. Large ice volumes in Florida developed from the freezing of loftedliquidhydrometeorsformedbylong-lived(;45 min) warm rain processes, whichare mostlyabsentin Oklahoma. However, ice volumes developed abruptly in Oklahoma storms despite missing a significant warm rain component. GOES fields were significantly different before 30 min prior to LI between the two locations. Compared to Florida storms, lower precipitable water (PW), higher convective available potential energy, and higher 3.9-mm reflectance in Oklahoma, suggest stronger and drier updrafts producing a greater abundance of small ice particles. Somewhat larger 15-min 10.7-mm cooling rates in Oklahoma confirm stronger updrafts, while clouds in the 60‐30-min pre-LI period show more IF variability (e.g., in the 6.5‐10.7-mm difference). Florida storms (high PW, slower growth) offer more lead time for LI predictability, compared to Oklahoma storms (low PW, explosive growth), with defined anvils being obvious at the time of LI.

An observational and numerical study of a mini-supercell storm

Abstract The structure and evolution of a small, moderately intense supercell storm, observed over northern Alabama on 17 July 1992, is described through a combined observational analysis and numerical modeling study. Wind profiler measurements show that the storm formed within a favorable low-level wind shear regime confined only to northern Alabama over a limited time interval between conventional NWS rawinsonde times. While not severe, this cloud system was long-lived and exhibited characteristics similar to those of classic supercells, including a unicellular echo with multicellular substructures, an extended lifetime, a collar cloud and a flanking line of cumulus. Moreover, this supercell did not produce cloud-to-ground lightning over its lifetime, nor was thunder from intracloud lightning heard over a 20-min period as the storm moved by. Numerical simulations of the cloud system duplicate the general observational features, including horizontal and vertical dimensions, cyclic behavior and convective core and anvil characteristics. Simulated distributions of microphysical fields indicate that a juxtaposition of cloud ice, graupel and supercooled water did not occur in sufficient concentrations to support effective charge separation by the non-inductive charge mechanism, despite updraft magnitudes of 15–20 m s −1 within the −10 to −20°C levels of the cloud.

Why do oceanic negative cloud-to-ground lightning exhibit larger peak current values?

This study examines the temporal (monthly) and spatial climatology (2004–2010) of the first return stroke of the cloud-to-ground (CG) lightning flash peak current (Ip) across various land/water boundaries over the contiguous United States. Four regions are examined: the Gulf of Mexico (region 1), the Florida peninsula (region 2), Lake Michigan (region 3), and part of the U.S. Mid-Atlantic (region 4). The crosss across the coastlines of regions 1, 2, and 4 show a gradual oceanward increase in the mean negative polarity CG peak current values (−Ip). This transition along the respective land/ocean boundaries is not sharp but gradual. In direct contrast with ocean, there is no consistent behavior in −Ip values as we move from land out across the fresh water of Lake Michigan (region 3). Meanwhile, the positive CG flash peak current (+Ip) values do not exhibit a consistent variation across any coastal boundary. For region 1, the −Ip values increase as we move toward the coast (southwards) especially during the wet season (June–October). This finding is in direct contrast with studies that documented winter as the season of maximum −Ip values. The zonal and seasonal variations of −Ip values across region 4 are not quite as pronounced, but the oceanic −Ip values are still larger than over the adjoining landmass. We explore in turn which up to date hypotheses pertinent to the oceanic −Ip enhancement are supported or refuted by our findings. It is concluded that the oceanic −Ip enhancement is not an artifact related to CG detection or Ip retrieval methods, nor is it likely related to the cloud top heights or CG activity. The study cannot refute the role of electrical conductivity and its contribution to CG leader attachment processes. However, given the observed “blurred transition” of the Ip values across the coastlines this paper suggests that likely the main physical mechanism is acting on the thundercloud potential. The recently suggested role of sodium chloride (NaCl) but also the role of ice crystal size (implicated herein), as possible modulators of the thundercloud potential, exhibit distinct pros and cons. Their candidacy is supported by their strong physical links to the electrostatic charging and thundercloud electric potential buildup but also by the exhibited blurred −Ip transition across the coastlines. In contrast, the suggested mechanisms cannot individually explain the observed −Ip enhancement in terms of season, NaCl concentrations, and absence of similar behavior in the respective +Ip values.

Diurnal Characteristics of Lightning Flashes Detected Over the São Paulo Lightning Mapping Array

This study examines diurnal variations of lightning flash characteristics observed by the Lightning Mapping Array in Sao Paulo, Brazil. The diurnal flash counts exhibit the typical afternoon convective maximum. The mean source altitude demonstrates a discrete increase that is temporally coincident with the local sunrise. The mean horizontal and vertical flash extents each attain a maximum (minimum) around local sunrise (afternoon, i.e., 13:00–17:00 local solar time). In addition, joint histograms of flash horizontal and vertical extents show that the majority of the flashes occurring during the afternoon convection are shorter and more comparable in size, and the differences between the horizontal and vertical extents are reduced. Conversely, flashes preceding and following the peak in afternoon convection are less symmetric, with larger horizontal than vertical extents. We discuss whether these observations could be partially explained by the diurnal variations in the convectively induced mixing regimes that control thundercloud charge regions and associated charge separation distances. The documented diurnal flash characteristics closely match recently published findings on the diurnal variation of the peak currents of cloud-to-ground flashes. Possible physical mechanisms for these observations are discussed.