Validation of the TUV module in CWRF using USDA UV-B network observations

Ultraviolet (UV) radiation is the source energy for tropospheric photolysis processes, while harmful for living organism of the earth. It is thus necessary to incorporate UV radiation for an integrated earth modeling system to predict interactions between climate, chemistry and ecosystem processed. The widely-used NCAR TUV (Tropospheric Ultraviolet and Visible) radiation model has been coupled with the state-of-the-art mesoscale CWRF (Climate extension of the Weather Research and Forecasting model) to predict the UV dependence of local climate conditions and its impacts on air quality and crop growth. The original TUV v4.2 has been significantly improved by (1) replacing the core radiation transfer solver, DISORT v 1.1 with the latest v2.0beta; (2) adding a new aerosol scheme based on the Shettle (1989); (3) receding the entire model to follow the CWRF F90 standard with dynamic memory allocation and modular design; and (4) developing a flexible interface for coupling with CWRF. Given the lack of detailed cloud information in observations, this study focuses on validation of the TUV module in a standalone mode against the USDA UV-B data under clear-sky conditions. To facilitate this, a cloud detection scheme based on Long and Ackerman (2000) is incorporated to distinguish clear versus cloudy sky conditions from the UV-B observations. The model input includes in situ measurements of the column ozone and total aerosol optical depth; TOMS retrievals of the column ozone (in case missing in situ) and climatologically surface reflectivity; and the NARR (North American Regional Analysis) meteorological conditions. The TUV results agree well with the UV-B measurements at 7 narrow spectral bands (300, 305, 311, 317, 325, 332, 368 nm).