Current Status of Urban Modeling in the Community Weather Research and Forecast (WRF) Model

Mesoscale numerical weather prediction (NWP) models in connection with increasing capacities of computers in the last few years have considerably increased the spatial (vertical and horizontal) resolution, and some NWP models are executed with a grid-spacing of 0.5-1 km. At such fine scales, the role of urban landuse in local and regional weather needs to be reasonably represented in these models and it is important for NWP models to capture effects of urban on wind, temperature, and humidity in the boundary layer. Not only these boundary layer weather variables influence people's daily life in the urban region, but also they are important input for air dispersion and quality models, which will benefit from improved prediction of the urban meteorological conditions. A major effort at NCAR has been undertaken in the last two years to develop urban models for the communityWeather Research and Forecast (WRF) Model.

Two different approaches were tested in WRF: 1) simply modifying the values of albedo, roughness length, soil thermal properties, and evaporation for the urban landuse in the Noah land surface model (LSM), and 2) coupling a more complex single-layer urban canopy model with the Noah LSM, which considers the 2-D geometry of building and roads to represent the radiation trapping and wind shear in the urban canopy. In this paper, we report recent progress in urban landuse modeling for WRF and recent applications of the WRF/Urban coupled model for driving Computation Fluid Dynamic models. We will especially address the following challenging issues: 1) the degree of complexity of urban modeling that is appropriate for NWP models, 2) the specification of urban landuse characteristic, 3) the initialization of state variables (such as temperature profiles in building roof, wall, and road) required by urban canopy models.