Kinematic and microphysical evolution of the 29 June supercell observed during STEPS

Spring 2003.Also issued as Sarah A. Tessendorf's thesis (M.S.) -- Colorado State University, 2003.Includes bibliographical references.The focus of this thesis is to examine the kinematic and microphysical properties of a severe storm using polarimetric and Doppler radar data. The data were collected during the Severe Thunderstorm Electrification and Precipitation Study (STEPS) that took place between 17 May 2000 and 20 July 2000 in eastern Colorado and western Kansas. One goal of STEPS is to find a relationship between the microphysics and kinematics of severe storms on the High Plains and their unusual positive cloud-to­ ground lightning production. The severe storm observed on 29 June 2000 produced large hail, frequent positive cloud-to-ground lightning, an F1 tornado, and displayed characteristic storm splitting evolution during the sampling period. Unprecedented measurements from three Doppler radars were used to describe the kinematics and rnicrophysics of this storm. Radial components of the wind fields relative to the three Doppler radars were combined to produce the three-dimensional winds in the storm. Bulk precipitation types (e.g., rain, hail) were objectively determined using the multi­ parameter variables available on two of the radars. The Doppler-derived kinematic fields were compared with the microphysical classifications over a nearly three-hour period to examine trends during the lifecycle of the supercell. Results showed that the supercell intensified rapidly while storm splitting occurred. Prior to splitting, there was little cloud-to-ground lightning and little evidence of hail aloft. After storm splitting. hail volume and cloud-to-ground lightning activity quickly intensified. The updraft of this storm pulsated, with maximum speeds to nearly 50 m s·1. The peaks in hail production aloft, largely around -10° C, were well correlated with the updraft fluctuations as well as with peaks in the frequency of positive cloud-to­ ground lightning flashes. These results are consistent with experimental work that shows positive charging in ice-ice collisions around -10° C. The dynamics of the storm-splitting process, in terms of radar-derived updraft and vorticity fields, were shown to be consistent with current conceptual models. The results of this thesis advance our knowledge of supercell evolution and will be used to help determine the electrification mechanisms of severe storms that produce predominantly positive cloud-to-ground lightning.Sponsored by the National Science Foundation under grant ATM-9912051

Radar perspective on the variability of tropical convection characteristics over the southwestern Amazon and eastern Pacific regions

Spring 2003.Includes bibliographical references.The focus of this study is to evaluate the intra-regional and inter-regional variability of tropical convection characteristics in the southwestern Amazon and eastern Pacific warm-pool regions. Convection is examined using radar data collected during two tropical field experiments: TRMM-LBA (Tropical Rainfall Measuring Mission - Large Scale Biosphere-Atmosphere in Amazonia) and EPIC (Eastern Pacific Investigation of Climate Processes in the Coupled Ocean-Atmosphere System). In each of the two tropical regions studied, two distinct wind regimes have been observed to occur. In the southwest Amazon, one wind regime was called easterly regime and the other westerly regime. These changes in wind regime have been previously shown to be associated with the passage of baroclinic waves in the subtropical parts of Brazil and the formation of a synoptic-scale feature known as the South Atlantic Convergence Zone. In the east Pacific, one wind regime was called the northerly regime and the other southerly regime. The changes in wind regime over the east Pacific have been observed to be associated with the passage of easterly waves. The variability of the convective characteristics is evaluated in each region as a function of time of day and wind circulation. Some of the features used to evaluate the characteristics of convection include convective area, convective fraction, reflectivity profiles, rain rates, warm rain statistics and ice fraction. The results presented in this thesis showed that the easterly and northerly wind regimes more frequently featured characteristics of stronger convection: greater rain rates, greater reflectivities and convective fractions, deeper convective cores and smaller fractions of warm-rain-producing areas. The results also showed that the easterly regime was associated with higher ice fractions. The diurnal cycle results indicated that convection initiates in the morning and peaks in the afternoon over the southwest portion of the Amazon, whereas in the east Pacific the convection initiates after sunset and peaks just before sunrise. Variations associated with wind regime were shown to be more intense in the southwest Amazon. It was also found that the east Pacific region presented larger convective areas and convective fractions than the southwest portion of the Amazon. The eastern Pacific region was also associated with larger fractions of warm rain areas, but overall the fraction of rainfall owed to warm processes was very small and approximately the same in both regions.Sponsored by the National Science Foundation ATM-0002256

Microphysical characteristics of ice crystals and snowflakes as revealed by polarimetric radar measurements

Summer 1998.Also issued as Christine Musick Reese Butler's thesis (M.S.) -- Colorado State University, 1998.Includes bibliographical references.This research encompassed both observational and theoretical aspects of copolar and differential reflectivity in the less explored, yet important, winter season precipitation. The observational portion was conducted with the multiparameter, CSU-CHILL radar and supplemented by observers at the Fort Collins Weather Station on the Campus of Colorado State University (FCL) who recorded microphysical features of the snowfall such as snow type, composition, size, and degree of riming. Additionally, a 2-D video disdrometer, located at FCL, made particle size distribution measurements. In order to compare the appropriate radar data with the ground observations, the approximate trajectory of the snow was computed from the height it was interrogated by the radar to the surface. The trajectory, applied in reverse from FCL, identified the source region of the observed snow in the 0.5° and 1° elevation scans of the radar. The results of the observational analyses suggest that nearly homogeneous populations of aggregates can be distinguished from platelike crystals (i.e. dendrites, stellar crystals, and plates) using a combination of co-polar and differential reflectivity (Z and ZoR) radar observations. Furthermore, it appears possible to discern whether or not the platelike crystals are intensely rimed. Additionally, the results challenge the validity of the common assumption that aggregates always produce a ZoR value of O dB. Scattering model studies based on T-matrix theory and the Mueller matrix method were conducted to demonstrate the consistency of the observed radar variables with theoretical values and to test our speculations on which hydrometeor microphysical characteristics were responsible for the observed variations in those variables. Our modeling results suggest several conclusions. First, the aggregate shapes are more relevant and have more impact on ZoR than generally expected; therefore, the modeling assumption that all aggregates are nearly spherical can produce erroneously low ZoR values. Secondly, the size-dependent density formulas for aggregates which predict that bulk densities decrease with size may not always be applicable. Next, the canting of hydrometeors can overcome the influence of microphysical characteristics on Zoa; thus, in a model, it is important to include canting for turbulent situations. when it likely occurred and to exclude it for calmer situations where it was not likely to have occurred. Lastly, the model results validated the idea that intensely rimed platelike crystals could be distinguished from other platelike crystals and highlighted the problems that can be encountered if modelers depend upon size-dependent axis ratio formulas without considering the effects of riming.Sponsored by the National Science Foundation ATM-9612519

Polarimetric and Multi-Doppler Radar Observations of Sprite-producing Storms

Sprites are caused by luminous electrical breakdown of the upper atmosphere, and frequently occur over large mesoscale precipitation systems. Two spriteproducing storms (on 8 and 25 June) were observed in Colorado during the summer of 2012. Unlike most past studies of sprites, these storms were observed by a polarimetric radar the CSUCHILL facility which provided both PPI and RHI scans of the cases. Also available were multipleDoppler syntheses from CSUCHILL, local NEXRAD radars, and the CSUPawnee radar; as well as data from the Colorado Lightning Mapping Array (COLMA), high speed cameras, and other lightningdetection instrumentation. This unique dataset provided an unprecedented look at the detailed kinematic and microphysical structures of the thunderstorms as they produced sprites, including electrical alignment signatures in the immediate location of the charge layers neutralized by spriteparent positive cloudtoground lightning strokes. One of the spriteproducing cases (25 June) featured an anomalous charge structure and may serve as a model for how sprites can be produced over convection rather than the more typical stratiform regions. Also to be presented will be evidence for advection of charge into a common stratiform precipitation region (on 8 June), which was then tapped by lightning originating from multiple different convective cores to produce sprites. Depending on the outcome of the 2013 convective season, polarimetric data from additional storms that produce sprites and other transient luminous events (TLEs) may be presented

Comparison Of Single- And Dual-polarization-based Rainfall Estimates Using NEXRAD Data For The NASA Iowa Flood Studies Project

This study compares and evaluates single-polarization (SP)- and dual-polarization (DP)-based radar rainfall (RR) estimates using NEXRAD data acquired during Iowa Flood Studies (IFloodS), a NASA GPM ground validation field campaign carried out in May-June 2013. The objective of this study is to understand the potential benefit of the DP quantitative precipitation estimation, which selects different rain-rate estimators according to radar-identified precipitation types, and to evaluate RR estimates generated by the recent research SP and DP algorithms. The Iowa Flood Center SP (IFC-SP) and Colorado State University DP(CSU-DP) products are analyzed and assessed using two high-density, high-quality rain gauge networks as ground reference. The CSU-DP algorithm shows superior performance to the IFC-SP algorithm, especially for heavy convective rains. We verify that dynamic changes in the proportion of heavy rain during the convective period are associated with the improved performance of CSU-DP rainfall estimates. For a lighter rain case, the IFC-SP and CSU-DP products are not significantly different in statistical metrics and visual agreement with the rain gauge data. This is because both algorithms use the identical NEXRAD reflectivity- rain rate (Z-R) relation that might lead to substantial underestimation for the presented case