The Role of Convective Self‐Aggregation in Extreme Instantaneous Versus Daily Precipitation

Abstract

Abstract The impacts of convective self‐aggregation on extreme precipitation and updraft velocity are investigated by using the Weather Research and Forecasting Model in the idealized setting of radiative‐convective equilibrium with a 3‐km horizontal resolution. Aggregated and unaggregated states are achieved by conducting simulations with fully interactive and fixed radiation, respectively. We find that convective self‐aggregation has a negligible impact on extreme instantaneous precipitation but weakens the extreme updrafts and condensation, indicating a negative dynamical contribution from aggregation to extreme instantaneous precipitation. However, this is balanced by higher precipitation efficiency to maintain the same extreme instantaneous precipitation. This balance occurs because updrafts decrease mainly above the freezing level, suppressing graupel production. As graupel has a longer residence time than rain, less graupel formation with aggregation implies enhanced instantaneous local precipitation efficiency. Peak updraft velocity scales with the vertical integral of buoyancy, measured with respect to the local prestorm environment. This local environment is warmer and moister at middle and high levels when convection is aggregated compared to when it is unaggregated, reducing the buoyancy and updraft velocity. Unlike extreme instantaneous precipitation, extreme daily precipitation is stronger in aggregated states, as self‐aggregation localizes and sustains updrafts and condensation in relatively fixed locations. Our results imply that extreme instantaneous precipitation is more sensitive to microphysical processes while extreme daily precipitation is more linked to the degree of aggregation.

Keywords

• extreme precipitation
• self‐aggregation
• radiative‐convective equilibrium
• convective updraft
• microphysics
• precipitation efficiency