Numerical prediction of atmospheric mixed layer variations over the Gulf Coast region during NOAA/ARL-JSU meteorological field experiment summer 2009—sensitivity to vertical resolution and parameterization of surface and boundary layer processes

Prediction of the mixed layer characteristics is important for generating the atmospheric dispersion characteristics using air quality models. The height of the mixed layer indicates the depth of the atmosphere over which the emissions could be diluted. It is known that the mixed layer height has diurnal variation and is influenced by local topographical and land use features. The present study aims to understand the utilization of the different parameterization schemes of the planetary boundary layer (PBL) and surface processes in the prediction of the characteristics of the PBL. For this purpose a high resolution mesoscale atmospheric model was used to predict the three dimensional atmospheric features over the Gulf coast region during 16-19 June 2009, when special observations were collected as a part of NOAA/ARL JSU/TLGVRC meteorological field experiment.

ARW (Advanced Research WRF) model was designed to have nested two-way interactive three domains with 36, 12 and 4 km resolutions, with the inner most domain covering the entire Gulf Coast region. The initial and boundary conditions were provided from NCEP FNL data available at 1 degree interval and the boundary conditions were updated at every 6 hours. The model was integrated for 48 hours starting from 00 UTC of each day starting from 15 June up to 00 UTC of 18 June 2009. Six sensitivity experiments were performed with the choice of thermal diffusion, NOAH-LSM and RUC schemes for surface processes and YSU and MRF schemes for PBL. Separately, three experiments were performed with three different vertical resolutions (i.e.) 27, 42 and 63 vertical levels.

As a part of joint NOAA-ARL and JSU-TLGVRC Meteorological Field Experiment Summer 2009, special radiosonde observations were collected at 5 times of 1400, 1600, 1800, 2000 and 2200 UTC for four consecutive days from 16 to 19 June, 2009 at the two locations, normal to the Mississippi Gulf Coast, of Harrison County School (30.5N,89.1W) and Wiggins Airport (30.8N,89.13W). The main objective of this field experiment was to understand the characteristics of the planetary boundary layer (PBL) over the Gulf Coast region.

The model output from the different experiments was analyzed and the thermodynamic fields corresponding to the locations of Harrison County High School and Wiggins Airport were retrieved. The vertical distributions of temperature and humidity fields along with wind variations were compared with the observations collected at these two locations during the field experiment. Specifically the characteristics of the mixed layer as obtained from the different sensitivity experiments were compared with the observations to assess the relative importance of the combination of surface and PBL schemes. The model results indicate distinct variations between different predictions. The results indicate that the PBL processes play a significant role as compared to the surface processes. Of all the sensitivity experiments, the combination of YSU scheme for PBL and RUC scheme for surface processes produce the best prediction. In general, the decrease of the temperature with height capped by a small inversion layer and then gradual decrease of temperature in the free atmosphere could be well simulated. Sensitivity experiments with respect to vertical resolution show improvement in the prediction of vertical thermodynamic structure with increased vertical resolution below 800hPa level.