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.