Further improvements to US Warm-season diurnal cycle of precipitation in very high-resolution GFDL HiRAM: Nesting and Microphysics

Most global models have an incorrect diurnal cycle of warm-season continental precipitation which peaks near noon instead of the observed late-afternoon or evening peak. Much of the problem arises from the convection parameterization, which is nevertheless essential for a correctly simulated tropical circulation. One solution is to push the global model to successively higher resolution to limit the need for parameterized convection. We present multi-year climate simulations using the global convection-permitting GFDL High-Resolution Atmosphere Model (HiRAM) with two-way nesting over the CONUS to reduce the model grid-cell-width to 8 km. Doing so reduces the dependence on parameterized convection, and moves the peak central US precipitation into the late evening, without degrading the simulated general circulation. Further CONUS-wide improvement in the diurnal cycle is found by the use of a fast saturated adjustment to sub-cycle microphysical processes, which greatly reduces the dependence the parameterized convection. It is also demonstrated that at higher resolution we are better able to simulate propagating storms over the central US.