6.2      Mechanisms improving tropical rainfall diurnal cycle in convection-permitting WRF.

Argüeso, D., Department of Physics, University of the Balearic Islands, Spain, A. Di Luca, Climate Change Research Centre, University of New South Wales, Australia, and R. Romero, Department of Physics, University of the Balearic Islands, Spain

The Maritime Continent is a major active convective centre and one of the most challenging regions in terms of atmospheric modelling. Rainfall in the Maritime Continent is driven by both large-scale phenomena such as the Madden-Julian Oscillation and ENSO, and fine-scale processes such as land-sea breeze and tropical convection. In combination, they produce very particular land and ocean precipitation regimes that models consistently fail to reproduce, regardless of their dynamic formulation and across spatial resolutions.

 Climate simulations at convection-permitting scales are now becoming possible. Instead of an increment in spatial resolution, these experiments constitute a conceptual advance since they no longer rely on many of the assumptions of convective schemes. Here, we use the Weather Research and Forecasting model operating at multiple resolutions (32, 16, 8, 4 and 2km) and running independently from each other to determine the role of the spatial resolution and the representation of convection on the realism of precipitation in Maritime Continent. We show better simulating precipitation requires both explicit deep convection and high-resolution.

Both explicit convection and parameterized shallow convection produce precipitation at the right time of the day because they do not transform CAPE as readily as the deep convection scheme. Explicitly resolving deep convection has implications for the lower atmosphere mixing, where the model creates a moister layer as compared to the parameterized case. This dramatically modifies temperature and humidity profiles creating a more unstable atmosphere. The convective representation also changes the structure of clouds and explicit convection produce shallower clouds that modify the rainfall patterns. As such, the precipitation onset is delayed to better match observations. In spite of generating a better diurnal cycle, there are features that remain a challenge even at the kilometre scale, such as the land/ocean distribution of total precipitation amounts. Overall, these results show how microphysics and the planetary boundary layer take on particular importance in the simulation of tropical convective precipitation in very high-resolution experiments.