Sensitivity of forecast rainfall in a texas convective system to soil moisture and convective parameterization

The impact of soil moisture on the forecast of a small-scale convective sys tem, and sensitivity of results to the convective parameterization used, ar e investigated through Era Model simulations (run in an operational-like se tting) of a convective system occurring on 27 May 1997 in Texas. The event was influenced by a southwestward-propagating gravity wave from early morni ng convection in Arkansas that intersected a slow-moving cold front, releas ing extreme conditional static instability. Isolated heavy rainfall, over 1 00 mm, occurred in some regions. A control simulation with 22-km horizontal resolution reasonably simulated the event, even though mesoscale influences such as the gravity wave import ant to this event are often poorly captured by numerical models. A series o f sensitivity tests were performed to examine the impact of soil moisture o n the simulations. Two different convective parameterizations were used for the tests. Although domain average precipitation is found to generally var y in a straightforward way with soil moisture, peak precipitation in the re gions of intense convection shows more complex behavior. Sensitivity of pre cipitation amounts to soil moisture differs significantly among runs having different convective parameterizations. For instance, with the Kain-Fritsc h convective scheme, relatively dry soil is found to result in stronger con vective outflows that converge with stronger ambient flow to greatly enhanc e the precipitation in the region where heaviest rainfall occurs. With the Betts-Miller-Janjic scheme, drier soil generally results in less precipitat ion than in the control run, although some enhancement in peak amount does occur within a narrow range of drying. The differences between the peak qua ntitative precipitation forecasts in the runs is primarily due to the inclu sion of a convective downdraft in the Kain-Fritsch parameterization, and it s impact on secondary convective development. Additional sensitivity tests find limited impact from prescribed vegetation coverage. A final sensitivity test shows that precipitation amounts are ev en more strongly affected by the vertical resolution of the data used to in itialize the shallow bur moist boundary layer than by variations in the soi l moisture or vegetation fraction.