Marcus Johnson- April 30th

Assessment of One-Moment and Two-Moment Bulk Microphysics and Spectral Bin Microphysics Schemes using Idealized Supercell Simulations and Real Data Convective-Scale Predictions

04/30/2019

10 AM 

NWC 4140. 

Abstract:

Optimal hydrometeor parameterization and their associated processes in microphysics schemes (both spectral bin and bulk) continue to evolve as these schemes attempt to match observed hydrometeor complexity. This dissertation spans several flavors of microphysics schemes: the one-moment Unified Model (UM), the partially two-moment Thompson and Morrison, the two-moment Milbrandt-Yau (MY2) and National Severe Storms Laboratory (NSSL) with two rimed ice categories, the Predicted Particle Properties (P3) with multiple mass assumptions within its ice particle size distributions (PSDs), and the spectral bin Hebrew University Cloud Model (HUCM). Microphysical performance (including their bias documentation) is examined in idealized supercell simulations, two test cases over the Korean Peninsula (Changma front and Typhoon Sanba [2012]), and 2018 NOAA Hazardous Weather Testbed (HWT) Spring Forecast Experiment seasonal forecasts over much of the continental United States (CONUS) and four select convective line cases.

UM microphysics struggles to match observed dual-pol variables because of its one-moment parameterization of rain, specifically its rain PSD intercept parameter N₀ diagnosis. As N₀ varies inversely with rain mass, the scheme is producing too many small (large) drops in regions of too weak (intense) reflectivity. In idealized supercell simulations, the HUCM and NSSL schemes simulate larger ice crystal moments than snow, while the Thompson scheme simulates more snow mass. This is due to the aggressive cloud ice to snow conversion in the scheme, which is intended given its assumed snow PSD. Both the fully two-moment MY2 and NSSL schemes are able to simulate a local maximum of Z_DR near the forward flank edge and a gradual decrease in the direction of the deep-layer storm-relative mean wind vector, but the large, dry hail in the MY2 scheme reduces Z_DR on the edge of the supercell, while the NSSL’s Z_DR arc is less elongated compared to typical observations. The P3 scheme with two ice categories is unable to simulate a Z_DR arc and hail signature, due to the restrictive rain and ice PSD slope Λ limiters preventing larger particles. Over the 2018 NOAA HWT Spring Forecast Experiment, the Morrison scheme displays an overall storm area (Z ≥ 15 dBZ) overprediction bias for short-term forecasts, while both the Morrison and NSSL schemes overpredict areas of intense convection (Z ≥ 40 dBZ). Each BMP underpredicts light and heavy surface precipitation. The documented shortcomings and biases in this dissertation can guide NWP users to select the appropriate microphysics parameterization for their simulated storm, and assist numerical modelers in optimally tuning/constructing their microphysics scheme.