Sensitivity of a regional forecast model statistical rainfall properties to resolution

We assess the representation of the statistical rainfall properties over Darwin using different regional high-resolution versions of the Australian Community Climate and Earth-System Simulator (ACCESS) model using the dual-polarization CPOL radar observation as a reference. Results from the coarser resolution (horizontal resolution of 12km, 50 height levels with a 37.5km top) show that while the model reasonably well reproduces the accumulated daily rainfall amount albeit tending to slightly overestimate it, the mean rain rate is too weak, compensated by too large a rain frequency of occurrence. In terms of intensity the model tends to perform worse in the dry regimes but improves in the wet regimes, especially during the active monsoon regime, where most of the monsoonal precipitation is produced. Probability distribution functions of daily rain rates reveal that while there are very little differences amongst regimes, marked discrepancies are found between model and observations. The model overestimates light rain rates, underestimates moderate to heavy rain rates and produces spurious very heavy rain rates that are not observed at all by the radar. The evaluation of the diurnal cycle of rainfall properties also reveals a general underestimate of rain rates compensated by an overestimated rain frequency over the whole cycle, resulting in an overestimated amplitude and a wrong timing of the diurnal cycle. In all regimes, the overestimation of total domain daily rainfall and rain frequency is found to be due to an overestimation of low rain rates produced by the model convective scheme while the underestimation of mean rain rate is due to an underestimate of high rain rates and high rain rates frequency by the model convective scheme. Using higher horizontal resolutions (4km and 1.5km) of the ACCESS model, we expect to gain further insights into the respective performances of convective parameterization and explicit convection at different model resolutions relative to the CPOL observations.