The Microphysical Structure of Mesoscale Convective Systems

Thesis (Ph.D.)--University of Washington, 2016-05

Mesoscale convective systems (MCSs) are large, long-duration complexes of clouds that are composed of a mixture of convective and stratiform components united by a mesoscale circulation. By developing an innovative spatial compositing technique that combines dual-polarimetric and Doppler radar data obtained during the Dynamics of the Madden-Julian Oscillation/ARM MJO Investigation Experiment (DYNAMO/AMIE), it is shown that hydrometeors are systematically organized around the mesoscale airflow patterns in MCSs in manner that is consistent with their known dynamical structure. Nine different hydrometeor types are identified by applying the National Center for Atmospheric Research (NCAR) particle identification (PID) algorithm to dual-polarimetric data obtained from the NCAR S-PolKa radar. The organization of these hydrometeors relative to airflow through MCSs is determined by simultaneously examining Doppler-radar-observed air motions and PID data. In convective cores, moderate rain occurs within the updraft core, where the rapidly rising air prevents hydrometeors from growing significantly. The heaviest rain and narrow, shallow regions of graupel/rimed aggregates are located just downstream of the updraft core, where the convective downdraft is likely located. Wet aggregates are located slightly further downstream from the updraft core in a narrow layer just below the 0°C layer, where the vertical velocities are likely less intense. The upper-levels of the convective core, where there is a lot of turbulence, are dominated by dry aggregates. Small ice crystals are located along the cloud edges. Within the stratiform region the rain intensity systematically decreases with distance from the center of the storm. Descending from cloud top small ice crystals, dry aggregates, and wet aggregates are sequentially layered in a manner consistent with the gradual gravitational setting observed in the upper portions of the stratiform region. Additionally, pockets of graupel/rimed aggregates are occasionally observed just above the wet aggregate layer. It is suggested that these graupel/rimed aggregates could result from localized wind-shear-induced turbulence, previous convective cells, and/or small, embedded convective cells. While previous studies have found evidence of these spatial hydrometeor patterns, this dissertation is the first to analyze Doppler-radar-observe air motions simultaneously with the PID data and show that these are patterns are systematically organized with respect to the mesoscale circulation of MCSs. Thus, this work builds upon a 50 year tradition of using the latest radar technology to advance our understanding of the fundamental nature of tropical oceanic MCS. While the PID is traditionally interpreted as an indication of the dominant hydrometeor type within a volume of air sampled by a radar, this dissertation takes advantage of the fact that the frozen hydrometeors identified by the PID methodology can be interpreted in terms of the microphysical processes producing the ice particles. Using this microphysical interpretation of the PID and constraining simulations to have the same mesoscale circulations as observations, the second part of this dissertation investigates whether numerical simulations can replicate the microphysical patterns observed in the S-PolKa data in a manner that is consistent with previous theoretical and laboratory studies. These simulations used three routinely available microphysical parameterizations. The simulated mesoscale airflow patterns were free to interact with the model microphysics, but the air motions were constrained to observations via assimilation of the S-PolKa radial velocity data. Broadly speaking, the simulated ice microphysical patterns were consistent with each other, with radar observations, and with previous theories and laboratory studies. All suggest that the ice microphysical processes in the midlevel inflow region are characterized by deposition anywhere above the 0°C level where upward vertical velocity is present, aggregation at and above the 0°C level, riming near the 0°C level, and melting at and below the 0°C level. Despite these broad similarities, the simulated ice microphysical patterns substantially differed in details from observations and previous theoretical and laboratory studies. Each simulation was characterized by riming and aggregation occurring over too deep of a layer and riming occurred too frequently. Additionally, details of the simulated ice microphysical patterns always differed among the parameterizations; no two parameterizations consistently produced similar details in every ice process considered. These discrepancies likely factored into creating substantial reflectivity differences among the parametrizations and with observations, which suggests that reliable consistent simulations will not be achieved until the parameterized representation of microphysical processes is improved. As a whole, this dissertation advances our understanding of the fundamental nature of tropical oceanic MCSs and provides important insights into the relationship between the dynamical and microphysical patterns within these storms from an observational and modeling perspective.