Advances in a Microphysics Parameterization to Predict Supercooled Liquid Water and Application to Aircraft Icing

Aerosols influence cloud and precipitation development in complex ways due to myriad feedbacks at a variety of scales from individual clouds through entire storm systems. This thesis describes the implementation, testing, and results of a newly-modified bulk microphysical parameterization with explicit cloud droplet nucleation and ice activation by aerosols (Chapter 2). Furthermore, in order to simulate properly the well known aerosol indirect effects, the explicitly-predicted cloud droplet and ice radiative effective size had to be fully coupled with the radiation parameterization. Since this connection did not previously exist within the Weather Research and Forecasting (WRF) model, the methodology to link these two parameterizations is detailed in Chapter 3. Subsequent evaluation of 28 daily WRF convection-permitting forecasts using the coupled cloud-radiation system resulted in sensible cloud-radiation indirect effects and modest improvements in simulated infrared brightness temperature, amount of solar radiation reaching the ground, and surface temperature. However, it also uncovered the fact that WRF run at this grid spacing (~4 km) generally under-predicted aerial coverage and depth of clouds, particularly in the mid-troposphere at temperatures conducive to ice accreting on aircraft. Further evidence of the model predicting insufficient clouds at typical aircraft icing temperatures of -5 to -20°C was found by comparing aircraft measurements of liquid water content (LWC), median volume diameter (MVD), and temperature against model results from a decade-long WRF simulation (Chapter 4). In general, WRF correctly represented the typical characteristics of LWC and MVD stratified by temperature that was found in aircraft data that were collected during icing events. Also, in a case study analysis, the model correctly predicted the occurrence of aircraft icing between 63 and 84% of the time for a nine-hour duration. Ultimately, however, this research indicates a need to consider sub-grid scale cloud production in WRF and to take into account the activation of aerosols as cloud condensation nuclei within unresolved eddies