Convection initiation and evolution on 6 August 2007 during COPS

The Convective and Orographically-induced Precipitation Study (COPS) occurred during the summer of 2007 in southwestern Germany and eastern France. COPS was aimed at advancing the quality of forecasts of orographically-induced convective precipitation with the use of 4-D observations and modeling of its life cycle. One of the components of improving quantitative precipitation forecasting skill is better understanding of convection initiation and evolution processes.

On 6 August 2007 a frontal system with a preceding trough approached the COPS low-mountain region. In advance of the front, convection initiation occurred on the eastern slopes of the Vosges Mountains. The storms strengthened and merged as they propagated eastward into the Rhine Valley. They produced an outflow boundary which triggered secondary initiation in the Rhine Valley.

Dual-Doppler observations were obtained from two Doppler on Wheels (DOW) mobile radars. These systems provided clear-air measurements prior to convection forming in the Vosges Mountains, as well as observations of the precipitation initiation and evolution. Prior to convection initiation, the low-level winds were south-southwesterly throughout the Rhine Valley with an upslope component generating low-level convergence along the eastern slope of the Vosges Mountains. Additional information about the pre-convective environment is being assessed with the use of surface stations, soundings and GPS moisture retrievals.

WRF simulations using a nested grid with an inner domain of 0.7-km resolution produced the south-southwesterly Rhine Valley and upslope flows, similar to the observations. Convection initiation in the model occurred at a similar time and location as the observations. Numerous sensitivity studies were performed by varying the input analysis fields and the parameterization schemes. These simulations were quite robust in that they all produced similar wind, cloud and rain fields. They also compare favorably with the sounding, surface station and lidar data. This provides confidence in using the WRF simulations to enhance the observations and improve understanding of the important processes for convection initiation and evolution on this day.