Convergence zone development in complex terrain during the Convective and Orographically-induced Precipitation Study

An analysis of convergence boundaries was conducted using two Doppler on Wheels (DOW) mobile radars during the Convective and Orographically-induced Precipitation Study (COPS) in the complex terrain of southwestern Germany and eastern France during the summer months of 2007. A total of 24 orographically-induced boundaries were identified on 15 of the operational days, ranging in strength, duration, orientation and location.

For the case of 7 July, boundaries lasting up to 4 hours formed in the lee of the Vosges Mountains where upslope flow was observed by the radar. At the same time, an east-west oriented convergence boundary developed over the radar location, spanning the width of the Rhine Valley. This boundary persisted greater than five hours as it propagated to the south. Environmental conditions leading to the formation of this and other boundaries are not well understood. Even with the large suite of instruments deployed during COPS, observations which could aid in understanding the development and evolution of these boundaries in the Rhine Valley were limited. Therefore, high resolution numerical model simulations are being employed.

Nested model simulations of 1 km resolution for the inner domain using the Weather Research and Forecasting Model (WRF) were run for select cases where convergence boundaries formed when weak synoptic forcing was observed over the region. Preliminary results show similarities in the location, orientation and evolution of these boundaries compared to the observations. For the 7 July case, WRF is able to reproduce the upslope boundaries in the lee of the Vosges as well as the southward propagating boundary within the Rhine Valley. Simulations will also be performed on days with similar synoptic forcing and no observed convergence boundaries, where organization of boundaries in the simulated convergence field would not be expected. This combination of observational and model data aims to assist in determining the necessary conditions leading to orographically-induced convergence zone development in this complex terrain region.