Impact of ice and snow fall speed computations on the structure of mesoscale convective systems in spectral and bulk microphysical schemes

Summertime mesoscale convective systems (MCSs) in the central and northern Plains provide a substantial portion of the annual rainfall to that region, and can be classified as the following (Gallus et al. 2008): linear (bow echoes, squall lines with leading stratiform rainfall (LS), lines with trailing stratiform (TS) rainfall, lines with parallel stratiform rainfall (PS) or lines with no stratiform rainfall), nonlinear or cellular (individual cells, cluster of cells or broken squall lines). Unique to the group of MCSs above are the LS and TS systems, which have stratiform rainfall located behind or in front of the main updraft region, respectively, as a result of different kinematic and microphysical processes associated with both systems. These kinematic and microphysical processes can be influenced by the fall speeds of ice and snow particles. Spectral (bin) microphysical schemes, hardly used today in operational and research settings because of their computational expense, are considered to better represent the structure of convective systems partly because of their ability to predict interactions between as many as 100 particle sizes and allow for as many fall speeds for a given volume of air. This is unlike commonly used bulk microphysical schemes, which, to reduce the number of computations, do not allow for such particle size interactions and uses one mass-weighted fall speed for a given volume meaning that all particles unrealistically begin falling at the same time when their speed exceeds that of the cloud updraft.

The hypothesis is that the bin approach to representing ice and snow fall speeds is an improvement over the bulk approach due to the presence of smaller particles that can be advected away from the main updraft region, and should improve the structure of LS and TS systems. The bin approach should improve the quality of quantitative precipitation forecasts (QPF) of rainfall for the LS and TS systems. To evaluate this, Weather Research and Forecasting (WRF) Advanced Research WRF (ARW) simulations will be performed using a bin and a bulk microphysical scheme. Ice and snow fall speed values in the bin scheme will be modified to resemble those from a bulk scheme, while the bulk scheme fall speed values will be modified to resemble those from a bin scheme. It is believed that ice and snow fall speeds are the most important causes for any differences between the schemes and should be a main focus for future studies.