Development of the off-line clear sky radiation flux by using the operational Eta code: applications for photolysis rate calculations

The chemical production of tropospheric O3 is dependent on non-linear photochemical reactions involving precursor species of NOx and volatile organic compounds (VOCs). The photochemical reaction rates are dependent on the intensity of incoming radiation which in turn is modulated by the prevalent cloudiness. The accurate representation of three-dimensional perturbation of the clear-sky photolysis rates is thus critical for accurate simulation of the production and distribution of tropospheric O3. In this study we explore the development of a new technique for calculating the below cloud attenuation of photolysis rates in the Eta-CMAQ air quality forecasting system that couples the radiation fields calculated in Eta with the photolysis rate specification in CMAQ. In this scheme, the below cloud attenuation of clear-sky photolysis rates is assumed to be proportional to the ratio of the short-wave radiation flux reaching the surface to that of its clear sky value. An off-line clear sky radiation module was developed by using the operational 12-km Eta code for incorporation in PREMAQ, an interface preprocessor that casts Eta-derived meteorological fields to form required compatible input of CMAQ. This off-line clear-sky radiation module is intended for the calculation of attenuation factor when clear sky radiation is not available in the archived meteorological data.

Preliminary results show this off-line product is consistent with the on-line product in terms of timing as well as magnitude. Detailed comparison of the off-line and on-line calculations will be presented. The ability of this off-line module to calculate 3-D clear sky fields also provides opportunities to extend the technique to specify 3-D photolysis fields in a manner consistent with the simulated meteorological fields including potential photolysis enhancement above clouds due to reflection as well within cloud photolysis variations. The resulting effects of these changes on computed photolysis rates will also be discussed.

Disclaimer: The research presented here was performed under the Memorandum of Understanding between the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and under agreement number DW13921548. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views.