Numerical simulations of mesoscale convective systems: techniques for comparison with observations and a high resolution analysis of flow patterns in the convective region and transition zone of a simulated mcs

Abstract

Comparison between numerical simulations and observations is useful for improving understanding of weather phenomena, such as mesoscale convective systems (MCSs). The Bow Echo and Mesoscale Convective Experiment (BAMEX) provides a multi-sensor, high-resolution dataset that can be used for such comparisons. In Chapter 2 of this work, airborne radar observations taken during the 9-10 June 2003 MCS observed during BAMEX are compared against numerical simulations performed using the Weather Research and Forecasting (WRF) model. Comparisons are carried out using the equivalent radar reflectivity factor and 3-D wind fields derived from quad-Doppler radial velocity measurements from two research aircraft using the well-documented contoured frequency by altitude (CFAD) technique to compare vertical variability between the model and the observations. In addition, in order to quantify horizontal variability, a contoured frequency by distance diagram (CFDD) and average altitude per bin diagram (AABD) are introduced, which display frequency and altitude (respectively) of a variable at a given horizontal distance from a fixed location in the domain. Using these statistical techniques, a quantitative comparison of the observed and simulated rear inflow jet (RIJ) and other kinematic features of the MCS is carried out at varying times in the MCS evolution to assess the level of agreement of the storm kinematics and morphology. The new statistical analysis techniques show that the RIJ remains elevated in both the simulation and the observations, as well as structural differences in the pre-storm environment between the simulation and observations. Chapter 3 investigates the mechanism(s) by which downdrafts at the rear of the convective precipitation and in the transition zone affect the maintenance and evolution of a simulated “early nocturnal” (approximately one hour after sunset) MCS. Additionally, the impact of melting in the transition zone on the MCS will be examined as well as the fine scale structure of the convective region and transition zone. While the storm was not elevated during the time period of this study, transition zone downdrafts were found to still affect the convective updrafts in addition to their immediate environment, including the horizontal velocity and temperature fields. The convective updraft was found to be highly unsteady, frequently changing between one or two coherent primary updrafts and a series of smaller adjacent updraft cores. Subsidence enhanced by entrainment occurred around these cores, producing downdrafts near the leading edge of the convective precipitation. In areas where precipitation was occurring, the melting layer divided a downdraft-dominated regime below it from an updraft-dominated regime above it. The downdrafts, initially induced by melting, were enhanced by evaporative cooling caused by precipitation falling into the dry rear inflow jet beneath the melting layer. Thus, melting of precipitation reinforced the cold pool and helped to maintain the MCS. Trajectory analysis showed that air parcels within the core of the rear inflow jet, which contained low θe air, could be ingested into the convective updraft cores, possibly reducing the buoyancy of the air within the updraft