Accurate Reproduction of a Mesoscale Convective System Structure Using WRF with a New Spectral Bin Microphysics

Wednesday, December 12, 2018 - 10:35
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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.

Mechanisms of formation of size distribution of ice particles and drops in stratiform and convective regions are investigated. The important role of spontaneous snow breakup and rain drop breakup as well as of environmental humidity is demonstrated.

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

The detailed comparison of polarimetric signatures calculated in simulations and derived from observations is performed.

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