Sensitivity of numerical simulations of a mesoscale convective system to double-moment representation of hydrometeors in bulk microphysical parameterization

Microphysical parameterization (MP) is an important source of uncertainty in the numerical prediction of mesoscale convective systems (MCSs). The influence of different moment representations of warm-rain and ice hydrometeors in bulk MP schemes on the numerical simulations of an MCS over the Southern Great Plains during the Midlatitude Continental Convective Clouds Experiment has been investigated using the Weather Research and Forecasting (WRF) model. It is found that the simulated structure, life cycle, cloud coverage, and precipitation of the convective system as well as its associated cold pools are sensitive to three selected MP schemes, namely, the WRF single-moment 6-class (WSM6), WRF double-moment 6-class (WDM6), and Morrison double-moment (MORR) schemes. Compared with observations, the WRF simulation with WDM6 can produce the structure and length of the MCS very well, while WSM6 produces a less organized convection structure with a short lifetime. Both simulations heavily underestimate the precipitation amount, the height of the radar echo top, and stratiform cloud fractions. With MORR, the model performs well in predicting the lifetime, cloud coverage, echo top, and precipitation amount of the convection. The importance of including double-moment representation of warm-rain and ice hydrometeors is evident, as it has strong influence on convective cloud properties through the diabatic heating/cooling rates, which determine the hydrometeor drag and cold pool characteristics.

Additional experiments indicate that replacing graupel with hail in the MORR scheme improves the prediction of the convective structure, especially in the convective core region. Results imply that ice hydrometeors play an important role in determining convective structure.