An evaluation of MM5 surface fluxes and mixing depths during the Nashville Southern Oxidants Studies

Understanding how and where tropospheric ozone forms requires detailed observations and conceptual models of both the chemistry and meteorology of the troposphere, especially the atmospheric boundary layer (ABL). During the 1995 and 1999 Southern Oxidants Studies (SOS95, SOS99) the NOAA Environmental Technology Laboratory (ETL) deployed an array of in situ and remote sensing systems in and surrounding the Nashville metropolitan area. We compared the observed fluxes of heat and moisture with forecast values derived using the Blackadar ABL parameterization embedded in the Pennsylvania State University (PSU), National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5). We found that the MM5 surface latent heat flux was 200 W m-2 higher than the observed values in the majority of cases we examined. When we reduced the MM5 latent heat fluxes to bring them into agreement with the observations, the model's ABL depth grew unrealistically.

Our tests suggest that errors exist in the atmospheric radiation and surface energy balance parameterizations used in MM5. The extremely high latent heat fluxes compensate for a systematic overestimation of the net surface radiation. We found that the errors in the net surface radiation can be attributed to a 100 W m-2 error in MM5 predicted clear-sky solar irradiance. In our presentation we will show a term by term evaluation of MM5's surface energy balance with observations gathered during SOS 95 and SOS 99.