Evaluation of Asian summer precipitation in different configurations of a high-resolution GCM at a range of decision-relevant spatial scales

Abstract:

We evaluate how precipitation is represented over Asia in a GCM with a grid length of 14 km during summer. Three simulations were performed, one with a convection parametrization scheme, one with convection represented explicitly by the model's dynamics, and a hybrid simulation with only shallow and mid-level convection parametrized. We evaluate the mean precipitation and the diurnal cycle of the amount, frequency and intensity of the precipitation from the simulations against satellite observations of precipitation from the CMORPH.

The precipitation in the simulations and observations is averaged over spatial scales defined by the hydrological catchment basins; these provide a natural spatial scale for performing decision-relevant analysis that is tied to the underlying physical geography of the region. By selecting basins of different sizes, we perform evaluations of the simulations as a function of the spatial scale. A new BAsin-Scale Model Assessment ToolkIt (BASMATI) is described, which facilitates this analysis.

We find that the simulations have substantial biases in their representation of the mean precipitation over Asia. There are strong positive biases in regions where there are strong interactions between the orography and the precipitation, such as the southern flank of the Tibetan Plateau. When the analysis is performed at different basin scales, the bias decreases as the spatial scales increase for all simulations, and the lowest-resolution simulation has the smallest RMSE compared to CMORPH.

From the amount, frequency and intensity analysis of the simulations, the positive mean precipitation bias over China is found to be due to too frequent precipitation for the parametrized convection simulation, and too intense precipitation for the explicit convection simulation.

The diurnal cycle of precipitation is strongly affected by the representation of convection in the simulations: parametrized convection produces a peak in precipitation too close to midday over land, whereas explicit convection produces a peak that is closer to the observed late afternoon peak. As the spatial scale of the analysis is increased, the representation of the diurnal cycle in the explicit and hybrid convection simulations improves when compared to CMORPH; this is not true for any of the parametrized simulations.