Analysis of the MPAS Convective-Permitting Physics Suite in the Tropics

Convection-permitting climate models (CPMs) offer the potential to speed up our understanding of climate variations and impact of climate change at cloud-resolving horizontal scales (less than 4 km). Major limitations in using CPMs are twofold: 1) CPMs remain computationally expensive to run for long simulations; and 2) physics parameterizations developed for low horizontal resolution climate models, in particular convective parameterizations, do not perform as well at cloud-resolving scales.

One approach to advance regional CP climate modeling is through the use of global variable-resolution meshes as the ones available with the Model for Prediction Across Scales (MPAS). These meshes allow CP climate modeling inside a locally refined mesh embedded within a global coarser domain, bypassing the need for nesting and nudging techniques at the edges of the computational domain. Also, variable-resolution meshes spanning between nonhydrostatic and hydrostatic scales are shown to be ideal tools to evaluate the horizontal dependence of parameterized convective and grid-scale moist processes.

To date, the convective-permitting (CP) physics suite implemented in MPAS has been extensively tested over the Continental United States (CONUS) for spring convection 10-day forecasts with a global variable resolution mesh that varies from 3 km over CONUS to 15 km elsewhere. The performance of the physics suite at CP scales outside of mid-latitude numerical weather prediction (NWP) forecasts remains mostly unknown. The CP suite includes the scale aware convection parameterization of Grell and Freitas (2014), and the cloud microphysics scheme developed by Thompson and Eidhammer (2014).

As we plan to transition MPAS to a near-global convection-permitting model for seasonal time-scale predictions, an improved understanding of the interactions between convective, grid-scale, and radiation processes parameterized with the CP suite in the tropics and at longer time-scales is needed. Using the 15 km-3 km variable-resolution mesh now centered over the Western Pacific Ocean, we compare outputs of a one-month simulation of our CP suite against satellite data over the most refined region of the mesh. Our results focus on the distribution of cloudiness, its optical properties, and its impact on the surface and top-of-the-atmosphere radiation budgets.

References:

Grell, G.A., and S.R. Freitas, 2014: A scale and aerosol aware stochastic convection parameterization for weather and air quality modeling. Atmos. Chem. Phys., 14, 5233-5250.

Thompson, G., and T. Eidhammer, 2014: A study of aerosol impacts on clouds and precipitation development in a large winter cyclone. J. Atmos. Sci., 71,3636-3658.