Dissertation on the relationship between precipitation and lightning as revealed by multiparameter radar observations
Date
1999
Authors
Carey, Lawrence D., author
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Abstract
Part I. Using data from the 11 cm, CSU-CHILL multiparameter radar, the simultaneous evolution of the radar inferred precipitation structure and electrical characteristics of a severe hailstorm is investigated. We compare the sub-storm point discharge current, intracloud (IC) lightning flash rate, cloud-to-ground (CG) lightning flash rate, ground strike location, and flash polarity to the progression of precipitation types and amounts. This study is unique in that it presents multiparameter radar observations of a thunderstorm complex which exhibited an extremely high IC-to-CG ratio (IC/CG, 20 - 70) and predominantly positive CG lightning (over 74%) after it became severe, producing large hail and weak tornadoes. In particular, we investigate the reported relationship between large hail and positive CG lightning. Although a direct correlation is found between a rapid increase in IC/CG, the first positive CG lightning flashes, and the initial production of hail aloft, the temporal and spatial behavior of large hail and positive CG lightning appear to be anti-correlated, as broad peaks in the positive CG flash rate lag relative maxima in the fallout of large hail by up to thirty minutes. The majority of positive ground strikes were adjacent to the main precipitation core in a region of light rain and little or no hail at the surface. Aloft, radar data indicated that ice crystals were aligned vertically in a strong electric field. Corona point observations at the surface indicate that these regions adjacent to the convective core were characterized by net positive charge aloft. Possible mechanisms to explain these observations are discussed. Part II. A propagation correction algorithm utilizing the differential propagation phase ($dp) was developed and tested on C-band polarimetric radar observations of tropical convection obtained during the Maritime Continent Thunderstorm Experiment (MCTEX). An empirical procedure was refined to estimate the mean coefficient of proportionality, a (b), in the linear relationship between $dp and the horizontal (differential) attenuation throughout each radar volume. The empirical estimates of these coefficients were a factor of 1.5 to 2 times larger than predicted by prior scattering simulations. This discrepancy was attributed to the routine presence of large drops ( e.g., Zc1r 3 dB) within the tropical convection observed during MCTEX that were not included in prior theoretical studies. Scattering simulations demonstrated that the coefficients a and b are nearly constant for small-to-moderate drops ( e.g., 0.5 Zc1r 2 dB) but actually increase with the differential reflectivity for drop size distributions characterized by Zc1r > 2 dB. As a result, the presence of large drops 1) bias the mean coefficients upward, and 2) increase the standard error associated with the mean empirical coefficients down range of convective cores which contain large drops. To reduce this error, we proposed and demonstrated a 'large drop correction' which utilizes enhanced coefficients a* and b* in large drop cores. Validation of the propagation correction algorithm was accomplished with cumulative rain gauge data and internal consistency among the polarimetric variables. The bias and standard error of the cumulative radar rainfall estimator R(Zii) [R(~p,~r)] were substantially reduced after the application of the attenuation [differential attenuation] correction procedure utilizing ~dp• Similarly, scatterplots of uncorrected Zh (~r) versus :Kip substantially underestimated theoretical expectations. After application of the propagation correction algorithm, the bias present in observations of both Zh~p) and Zdr(:Ktp) were removed and the standard errors relative to scattering simulation results were significantly reduced. In Part III of this study, the corrected polarimetric data set was then utilized to locate and roughly quantify regions of rain and precipitation sized-ice. This microphysical information provided valuable insight into the couplings between ice phase precipitation and storm electrification/lightning in tropical convection. Part III. One of the primary scientific objectives of the Maritime Continent Thunderstorm Experiment (MCTEX) was to study cloud electrification processes and to investigate the coupling between ice phase precipitation and lightning production in tropical island convection. To accomplish this goal, a C-band polarimetric radar (BMRC C-pol) was deployed in the tropics (11.6° S, 130.8° E) for the first time, accompanied by a suite of lightning measurements. Using observations of the propagation corrected horizontal reflectivity and differential reflectivity, along with specific differential phase, three dimensional rain and ice mass were estimated during the entire life cycle of an electrically active tropical convective complex (known locally as Hector) over the Tiwi Islands on 28 November 1995. Hector's precipitation structure as inferred from these raw and derived radar fields was then compared in time and space to the measured surface electric field, cloud-to-ground and total lightning flash rates, and ground strike locations. During Hector's developing stage, precipitating convective cells along two island sea breezes were dominated by warm rain processes. No significant electric fields or lightning were associated with this stage of Hector, despite substantial rainfall rates. Aided by gust front forcing, the cumulus merger process resulted in larger, taller, and more intense convective complexes which were dominated by mixed phase ice processes. During the mature phase of Hector, lightning and the surface electric field were strongly correlated to the mixed phase ice mass and convective rainfall. Merged convective complexes produced 97% of the rainfall and mixed phase ice mass and 100% of the cloud-to-ground lightning. As Hector dissipated, the stratiform rain and area fractions increased as the cloud-to-ground lightning flash rate decreased significantly. The multicell nature of Hector resulted in the continuous lofting of supercooled drops to the -20° C level in discrete updraft cores during both the early and mature phases. The freezing of these drops provided instantaneous precipitation sized ice particles which may have subsequently rimed and participated in thunderstorm electrification via the noninductive charging mechanism.
Description
Spring 1999.
Rights Access
Subject
Radar meteorology
Thunderstorm electricity
Lightning
Precipitation (Meteorology)