Most urban modeling studies ignore urban effects on convection and precipita-tion. The few 3-D efforts that produced urban precipitation and/or convection all omitted urban building barrier effects. Some also omitted microphysical effects or produced con-vection as an undesired by-product of UHI simulations. The current study thus uses Pro-ject Atlanta data and MM5 to investigate an UHI initiated convection case over Atlanta, Georgia. The specific hypothesis tested is that urban-induced convergence zones initiate convective precipitation.

Current simulations use quadruple nested grids (81, 27, 9 and 3 km resolutions), 30 vertical layers (10 below 1 km), minimum full sigma level of 46 m, 25 USGS land-use classes, topographic heights from USGS 30 s data, SSTs from GDAS (resolution of 2.5x 2.5 degrees), model analyses (at 2.5x2.5 degree resolution) from NCEP/NCAR for initial and boundary conditions, explicit ice scheme of Dudhia, convective parameterization in Domains 1 and 2 by Kain-Fritsch, force-restore surface temperature, and Gayno-Seaman 1.5 order TKE scheme.

Results show significant precipitation only from a quasi-stationary front south of Atlanta and from UHI urban induced convergence around the edges of Atlanta. When Atlanta roughness lengths, thermal properties, and building barrier was removed from the simulations, the second area of precipitation was eliminated, thus showing its urban ori-gin. Results to date show good correlation between the surface UHI and convergence areas. They show urban precipitation, however, somewhat northeast of the observations, about 3 h too early, but around the correct magnitude. Future simulations will focus on correcting these differences by further refinements of input urban surface parameters.