Parameterizing Subgrid Orographic Precipitation and Surface Cover in Climate Models

Abstract

Previous development of the Pacific Northwest National Laboratory?s regional climate model has focused on representing orographic precipitation using a subgrid parameterization where subgrid variations of surface elevation are aggregated to a limited number of elevation classes. An airflow model and a thermodynamic model are used to parameterize the orographic uplift/descent as air parcels cross over mountain barriers or valleys. This paper describes further testing and evaluation of this subgrid parameterization. Building upon this modeling framework, a subgrid vegetation scheme has been developed based on statistical relationships between surface elevation and vegetation. By analyzing high-resolution elevation and vegetation data, a dominant land cover is defined for each elevation band of each model grid cell to account for the subgrid heterogeneity in vegetation. When larger lakes are present, they are distinguished from land within elevation bands and a lake model is used to simulate the thermodynamic properties. The use of the high-resolution vegetation data and the subgrid vegetation scheme has resulted in an improvement in the model?s representation of surface cover over the western United States. Simulation using the new vegetation scheme yields a 1°C cooling when compared with a simulation where vegetation was derived from a 30-min global vegetation dataset without subgrid vegetation treatment; this cooling helps to reduce the warm bias previously found in the regional climate model. A 3-yr simulation with the subgrid parameterization in the climate model is compared with observations.