How Do Microphysical Processes Influence Precipitation Variability and Extremes?

Cloud permitting model simulations using the Model for Prediction Across Scales-Atmosphere (MPAS-A) are used to examine the processes that control precipitation variability and extremes. The high resolution (4km grid spacing) region of the global model simulations covers the equatorial Indian Ocean, with a 32 km grid spacing elsewhere. The November 2011 episode of MJO is selected as a case study. It is shown that (i) microphysical processes regulate precipitable water statistics and the shape of the well-known non-linear relationship between precipitation and precipitable water and (ii) precipitable water values that occur most frequently and those corresponding to the steep part of the non-linear curve contribute disproportionately to the precipitation variability. Therefore changes in microphysical parameters that shift the column precipitable water statistics relative to the steep part of curve even slightly have large impacts on precipitation variability and extremes. Understanding the mechanisms behind the large sensitivity of the non-linear relationship to microphysical processes may provide rationales for using precipitation variability and extremes and the non-linear relationship to constrain microphysical parameters in climate models.