Supercell-type thunderstorms, which embody a complex arrangement of long-lasting rotating updrafts and intense downdrafts, are known to be responsible for generating most intense tornadoes. However, considerable uncertainty exists regarding mechanisms culminating in the actual focus and downward extension of stormscale rotation in the form of a damaging tornadic vortex. Some working hypotheses link this process to the strength, thermodynamic stability (temperature) and spatial configuration of downdrafts in the lowest reaches of these storms, which are in-turn dependent on rates of evaporative cooling influenced by the type and size distributions of falling hydrometeors (chiefly rain and hail.) Rigorous attempts have not yet been made to investigate the range of these microphysical precipitation characteristics beneath supercell thunderstorms. The overarching goal of this project is to i) deploy multiple mobile disdrometers (instruments that measure the characteristic sizes and fallspeeds of precipitation particles) beneath supercell thunderstorms during the second Verification of the Origins of Rotation in Tornadoes EXperiment (VORTEX2; a field program conducted over the central United States known as "tornado alley") in Spring 2010, ii) conduct a comprehensive analysis of both existing and newly-obtained disdrometer observations to determine microphysical characteristics in tornadic compared to nontornadic supercell thunderstorms, and iii) relate these results to contemporaneous high-resolution polarimetric Doppler radar observations. Both disdrometers and polarimetric Doppler radar can detect (or in the case of radar, infer) the size distributions of falling hydrometeors. While preliminary analyses have provided some insights about hydrometeor distributions and their impacts on evolution of supercell storm features, a comprehensive analysis of a number of cases using well-placed high-resolution measurements has yet to be conducted and will be achieved in the course of this work.
Intellectual Merit: This project will augment our knowledge about microphysical processes within supercell thunderstorms and may lead to improved short-term forecasts and warnings of life-threatening severe weather. Since VORTEX2 hosts the largest number of polarimetric Doppler radars to ever monitor the full lifecycle of supercell thunderstorms, this experiment is an ideal laboratory to address these scientific questions. At present, the lack of skill in forecasting and understanding microphysical processes is largely due to both the inadequate representation of microphysical processes and the lack of measurements. This mobile deployment of disdrometers in VORTEX2 will provide by far the most comprehensive dataset of disdrometer and radar observations and analysis in supercell thunderstorms ever collected.
Broader Impact: The improvement of short-term forecasts and warnings of severe weather is strongly linked to the representation and understanding of the microphysical processes, which will be substantially extended by this work. Results will be shared with the modeling community and integrated with other, existing VORTEX2 data sets. Results will also be disseminated through presentations at conferences, seminars, and workshops as well as through publications in relevant professional journals. Additional Broader Impacts will come through direct involvement of graduate students in collection and analysis of field datasets, as well as through enhanced classroom education at both undergraduate and graduate levels.
Rigorous attempts have not yet been made to investigate the range of microphysical characteristics of supercell thunderstorms. The overarching goal of this research was to i) deploy six mobile disdrometers within supercell thunderstorms during the second Verification of the Origins of Rotation in Tornadoes EXperiment (VORTEX2) in 2010, ii) conduct a comprehensive analysis of all disdrometer observations to determine microphysical characteristics in tornadic compared to nontornadic supercell thunderstorms, and iii) relate the results to high-resolution polarimetric Doppler radar observations. While recent analyses of disdrometers and polarimetric radar observations have provided some insights about hydrometeor types within supercell thunderstorms, a comprehensive analysis of a number of cases using high-resolution measurements has yet to be established. The specific objective of the research proposed herein is to analyze particle size distribution, fall velocity, and surface observations in conjunction with dual-polarization Doppler radar measurements during Radar Observations of Tornadoes and Thunderstorm Experiment (ROTATE) 2008 and VORTEX2 to i) characterize PSD and fall velocities in different areas of the storm and at various lifetimes of supercell thunderstorms, ii) create a PSD-parameter data base for understanding microphysical processes within supercell thunderstorms and evaluating storm-scale numerical model outputs, and iii) studying steady-state and intermittent microphysical processes relevant for ice production and determine the effect of the evaporative cooling between tornadic and nontornadic storms by additionally analyzing 3-dimensional dual-polarization and dual-Doppler radar data. Major findings related to the data analysis of the VORTEX2 2009 and 2010 data can be summarized as followed: i) Surface disdrometer observations can be used to validate the performance of a differential phase-based attenuation correction scheme that is applied to data recorded by the National Oceanic and Atmospheric Administration’s (NOAA), X-band, dual-Polarized (NOXP) mobile radar. Disdrometer observations can be used to quantify the attenuation-corrected radar reflectivity, which is particularly useful for X-band radar observations in thunderstorms (Kalina et al. 2014a). ii) Raindrop-size distributions were used to study particle size-sorting and microphysical processes in thunderstorms. The study revealed that the raindrop size distribution changes rapidly in time and space in convective thunderstorms. Graupel, hailstones, and large raindrops were primarily observed close to the updraft region of thunderstorms in the forward- and rear-flank downdrafts and in the reflectivity hook appendage. Close to the updraft, large raindrops were usually accompanied by an increase in small-sized raindrops (Friedrich et al. 2013a). iii) Strong winds affect the quality of optical PARSIVEL disdrometer measurements, which is characterized by a large number concentration of raindrops with large diameters (> 5 mm) and unrealistic fall velocities (< 1 m s-1). It is correlated with high wind speeds, and is consistently observed by stationary disdrometers but is not observed by articulating disdrometers (instruments whose sampling area is rotated into the wind). Most of the time, this effect occurs when wind speed exceeds 20 m s-1, although it was also observed when wind speed was as low as 10 m s-1 (Friedrich et al. 2013b). iv) Idealized high-resolution (1 km) supercell simulations using the Weather Research and Forecasting Model (WRF) were used to analyze the affect of aerosol concentration and drop breakup on cold pool, drop size distribution, collision and coalescence, and the updraft/downdraft in supercell thunderstorms. The results indicate that the maximum perturbation in the microphysical process rates (relative to the cleanest simulation) is achieved by CCN = 3000 cm-3, with further increases in CCN concentration having negligible effect. The response in the cold pool size and mean temperature to CCN concentration is non-monotonic and highly dependent on the environmental conditions. Except in the sounding with dry low-level relative humidity, domain-averaged precipitation is found to increase with CCN concentration before peaking between 500 cm-3 and 5000 cm-3 (depending on the sounding), after which it remains constant or slowly decreases. Spatial plots of domain-averaged precipitation reveal that much of the precipitation enhancement (locally up to 25 mm) occurs near and to the immediate left of the paths of the left- and right-moving updrafts (Kalina et al. 2014b). The improvement of short-term forecasting and warnings of severe weather is strongly linked to the representation and understanding of the microphysical processes. The results of this research is fundamental in providing needed information about the microphysical structure within supercell thunderstorms for storm-scale numerical model validation, radar data assimilation, and radar-based rainfall rate estimation.
