Factors contributing to bias and variability of stratiform precipitation properties with various cloud microphysics schemes for an MC3E Squall Line Case

Friday, December 14, 2018 - 16:16
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A cloud microphysics intercomparison study of a mid-latitude squall line is performed using the Weather Research & Forecasting (WRF) model at 1-km horizontal grid spacing with eight cloud microphysics schemes to understand processes and factors contributing to model bias and variability at cloud-resolving scales. Here we focus on stratiform precipitation properties. We find that most of cloud microphysics schemes underestimate total stratiform precipitation mainly due to underestimation of stratiform precipitation area. All schemes significantly underestimate the frequency of moderate stratiform rain rates (3-7 mm h-1), which might be resulted from consistently low-biased ice number concentration and mass over the 0.2-2 mm diameter range in the stratiform ice region. Simulations generally overestimate ice water content (IWC) at upper levels and produce a rapid decrease of IWC as ice particles approach the melting level, which is opposite to the trend shown by in-situ observations. This is likely the reason for the underestimation of rain water content below 3-km altitude. Stratiform precipitation area (SPA) positively correlates with convective detrainment flux but is modulated by the hydrometeor size and fall speed. Increasing the frequency of coupling with large-scale forcing from every 3 h to 1 h leads to an increase in SPA by about 25%. The variability of stratiform precipitation and area across the simulations is a factor of 2, and the large variability is primarily a result of variability in the ice mass flux, which is largely associated with differences in convective detrainment of condensate. Differences in stratiform liquid/ice microphysical processes play a secondary role.

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