Changes in Sub-daily Precipitation Extremes in the Super-parameterized Version of NCAR’s Community Atmosphere Model

Abstract:

Future projections of the extremes have been mostly assessed using output of general circulation models (GCMs), and most of the conclusions on precipitation extremes have been based on daily averages. The robustness of most GCM simulations is limited by the daily scale, because the convective cloud systems are not explicitly resolved and the diurnal cycle of precipitation is generally not well simulated by the GCMs. However, both temperature and precipitation are modulated by diurnal cycles, and heavy precipitations are commonly related to mesoscale systems which have life time less than a day.

In this study, super-parameterization (SP) was applied to enhance the performance of Community Atmosphere Model (CAM) at mesoscale and sub-daily scales. Unlike the conventional GCMs, the SP version employs an embedded cloud resolving model on each model grid. It provides us an opportunity to assess the sub-daily extremes on the global scale. Two 10-years simulations were performed using the present sea-surface temperature (SST) and perturbed SST in RCP8.5 scenario respectively. In addition, the simulations were repeated using the traditional CAM for comparison. SP-CAM simulates more reasonable diurnal cycles and closer match of heavy precipitations to observations than CAM in present climate. Moderate precipitation extremes are defined by the 99^th percentile of the 3-hourly precipitation rate. The changes in sub-daily precipitation extremes agree with that of daily extremes in terms of trends both globally and regionally. But the sub-daily extremes have larger changes than daily extremes in general, since the dynamic and thermodynamic factors have different impacts on the precipitation with different intensities. The change of SPCAM sub-daily precipitation extreme is 6.2%/K on the global scale and can be quite different regionally. Clausius-Clapeyron (CC) relation, although is instructive on global scale, does not predict the exact change of precipitation extremes regionally, since it is only based on thermodynamics.