Research supported by NOAA CPO’s Climate Variability and Predictability (CVP) program has been accepted for publication in the Journal of Atmospheric Science. The paper by Li et al., "The sensitivity of simulated shallow cumulus convection and cold pools to microphysics," explores how two separate microphysical schemes (the Thompson and Morrison schemes) used in nested Weather Research and Forecasting (WRF) model simulations affect the generation of precipitation and evaporation in the model.
The authors find that while both schemes produce similar mesoscale variability, the Morrison simulation produced larger cloud cover and liquid water path (LWP) in conjunction with a larger number of strong cold pools with more intense shallow convection. Neither scheme was ideal for simulating precipitating shallow marine cumuli, so additional studies exploring other parameterization schemes were encouraged.
The sensitivity of nested-WRF simulations of precipitating shallow marine cumuli and cold pools to microphysical parameterization is examined. The simulations differ only in their use of two widely-used double-moment rain microphysical schemes: the Thompson and Morrison schemes. Both simulations produce similar mesoscale variability, with the Thompson scheme producing more weak cold pools, and the Morrison scheme producing more strong cold pools that are associated with more intense shallow convection. The most robust difference is that the cloud cover and LWP are significantly larger within the Morrison simulation than the Thompson simulation. One-dimensional kinematic simulations confirm that dynamical feedbacks do not mask the impact of microphysics. These also help elucidate that a slower autoconversion process along with a stronger accretion process explain the Morrison scheme’s higher cloud fraction for similar rain mixing ratio. Differences in the raindrop terminal fall speed parameters explain the higher evaporation rate of the Thompson scheme at moderate surface rainrates. Given the implications of the cloud cover differences for the radiative forcing of the expansive trade-wind regime, the microphysical scheme should be considered carefully when simulating precipitating shallow marine cumulus.
To access the paper, visit: http://journals.ametsoc.org/doi/pdf/10.1175/JAS-D-14-0241.1