Parallelization of a Subgrid Orographic Precipitation Scheme in an MM5-Based Regional Climate Model

Regional Climate Models (RCMs) are practical downscaling tools to yield regional climate information for assessing the impacts of climate variability and change. The Pacific Northwest National Laboratory (PNNL) RCM, based on the Penn State/NCAR Mesoscale Model (MM5), features a novel subgrid treatment of orographic precipitation for coupling climate, hydrologic, and ecologic processes at the watershed scale. The parameterization aggregates subgrid variations of surface topography into a finite number of surface elevation bands. An airflow model and a thermodynamic model are used to parameterize the orographic uplift/descent as air parcels cross over mountain barriers or valleys. The parameterization has significant performance advantages over nesting to achieve comparable resolution of climate information; however, previous implementations of the subgrid scheme required significant modification to the host MM5 model, prohibiting its incorporation within the NCAR-supported community version of MM5. With this effort, software engineering challenges have been addressed to incorporate, parallelize, and load-balance the PNNL subgrid scheme with minimum changes to MM5. The result is an efficient, maintainable tool for regional climate simulation and a step forward in the development of an MM5based community regional climate model.