Numerical Simulations and Sensitivity of a Mesoscale Convective System Structure Using WRF with a New Bin Microphysics

A modified Fast Spectral Bin Microphysics scheme (FSBM-new) included into the Weather Research and Forecasting Model (WRF) is used to simulate a mesoscale convective system observed during the Midlatitude Continental Convective Clouds Experiment (MC3E). In contrast to the current FSBM, FSBM-new describes dense ice hydrometeor using graupel or hail on either 33 or 43 mass bins that allows simulation of hail of several cm in diameter. FSBM-new includes a spontaneous breakup of rain drops and snow. Nucleation of cloud droplets is described using a new analytical approach allowing calculation of supersaturation maximum at cloud base. Release of aerosols into the atmosphere by droplet evaporation is taken into account. Detailed melting is included with calculation of liquid water fraction within snow, graupel/hail. Simulations are performed at different aerosol concentrations.

It is shown that allowing hail particles of diameters exceeding 1 cm leads to intensification of convection in squall line and to an increase in the radar reflectivity, resulting in good agreement between the simulated and observed squall line structures. The results stress the dominating role of hail in microphysical processes in deep convective areas within mesoscale convective systems. In contrast, if graupel particles are used to represent high density hydrometeors in convective areas, the radar reflectivity in the convective updraft is substantially lower and the convective-to-stratiform areas ratio diverge from those seen in observations.

The structure and precipitation of MC3E to aerosols, to melting in the boundary layer, as well as to aerosol return by droplet evaporation is investigated.

In parallel to the FSBM-new a new SBM-full was developed and included into WRF. In addition to hydrometeor types in the FSBM-new, the SBM-full includes ice crystals and two hydrometeor types (graupel and hail) to describe ice microphysics.

Results obtained using the FSBM-new and new SBM full are compared. The model includes polarimetric radar operator allowing calculation of radar variables which helps greatly to analyze the microphysics of mesoscale convective system. In particular, it is shown that high concentration of ice particles above the homogeneous freezing level is caused by small aerosols in the boundary layer via process of in-cloud nucleation.