Evaluating the Influence of Ice Microphysics on an Idealized Simulation of Orographic Precipitation

Abstract:

This study aims to understand the impacts on surface precipitation and mesoscale flow structures associated with ice and mixed-phase microphysical processes. Experiments are conducted in the NCAR Cloud Model 1 (CM1) using an idealized moist stable flow interacting with a Gaussian bell-shaped mountain. The control simulation uses a liquid-only (Kessler) scheme, while ice microphysics experiments are performed using two parameterizations available in CM1 (NASA-Goddard version of the Lin, Farley, Orville (LFO) scheme and the Morrison (MOR) scheme), which both contain three ice species: cloud ice, snow, and graupel/hail. LFO simulations produce flow structures that are comparable to the control run, but generate less precipitation. MOR simulations produce a completely different flow structure, exhibiting laminar flow downstream of the mountain while the LFO and control simulation produce a breaking wave and downslope windstorm. This results in a “double-peaked” precipitation distribution in MOR, with warm-rain processes seemingly dominating the first peak and melting of ice species contributing to the secondary peak. A change in the rimed ice species results in systematic differences in amount and location of precipitation received on the mountain slope. Overall, the choice of microphysics parameterization has a larger impact on the dynamical features and surface precipitation rates than the choice of rimed ice species (graupel vs. hail). These results were similarly found in simulations with different initial conditions. This presentation will focus on the microphysical processes leading to the substantial differences between the LFO and MOR experiments.