Observations & high resolution modelling of decoupled surface flows during IOP 9c of the COPS field experiment

During July & August 2007, a mesonet of 23 Automatic Weather Stations (AWS's) was deployed in the Northern part of the Black Forest Germany, as part of the UK-COPS (Convective & Orographically-induced Precipitation Study) consortium. Sites were chosen to provide surface observations in four categories relating to the region's orography. These were; Inflow valleys, Mountain Ridges, Convection Hotspot & Westerly Upslope Flow.

A case study from 20th July (IOP 9c) saw the passage of a large Mesoscale Convective System (MCS) accross the COPS region. Orographic modication of the MCS gust front by the Black Forest led to the modication of a pre-frontal convergence line along which severe Convection Initiation (CI) subsequently developed over the SE Black Forest. Wind observations from the UK-COPS Mesonet illustrate the extremely complex nature of this flow modification. We observe an apparent de-coupling of the N-S aligned valley flows from those aloft, as a result of the gust front being sheltered from the valley wind systems by steep valley gradients perpendicular to the MCS track. This paper will discuss the role of BF orography on the modication of the gust front, through comparissons between surface observations, & simulation of the event using the numerical Weather Forecasting & Research model (WRF). We will also discuss the ability of WRF to simulate the complex surface structure of this gust front, & it's role in the subsequent alteration of the MCS pre-frontal convergence line.