An Observational and Modeling Study of the Processes Leading to Deep, Moist Convection in Complex Terrain

Alan M Blyth,Joël Van Baelen,L Jay Miller,Lindsay J Bennett,Paolo Di Girolamo,Tammy M Weckwerth,Tracy J Hertneky

doi:10.1175/mwr-d-13-00216.1

Abstract

Abstract A case study of orographic convection initiation (CI) that occurred along the eastern slopes of the Vosges Mountains in France on 6 August 2007 during the Convective and Orographically-Induced Precipitation Study (COPS) is presented. Global positioning system (GPS) receivers and two Doppler on Wheels (DOW) mobile radars sampled the preconvective and storm environments and were respectively used to retrieve three-dimensional tomographic water vapor and wind fields. These retrieved data were supplemented with temperature, moisture, and winds from radiosondes from a site in the eastern Rhine Valley. High-resolution numerical simulations with the Weather Research and Forecasting (WRF) Model were used to further investigate the physical processes leading to convective precipitation. This unique, time-varying combination of derived water vapor and winds from observations illustrated an increase in low-level moisture and convergence between upslope easterlies and downslope westerlies along the eastern slope of the Vosges Mountains. Uplift associated with these shallow, colliding boundary layer flows eventually led to the initiation of moist convection. WRF reproduced many features of the observed complicated flow, such as cyclonic (anticyclonic) flow around the southern (northern) end of the Vosges Mountains and the east-side convergent flow below the ridgeline. The WRF simulations also illustrated spatial and temporal variability in buoyancy and the removal of the lids prior to convective development. The timing and location of CI from the WRF simulations was surprisingly close to that observed.

Highlights

The Convective and Orographically-Induced Precipitation Study (COPS; Wulfmeyer et al 2008, 2011) was designed to improve the forecasting skill of convective precipitation in complex terrain using observations and modeling of its life cycle
The 6 August 2007 case study presented in this paper evaluates the first-ever combination of high-resolution dual-Doppler wind fields with small-scale moisture fields, along with high-resolution Weather Research and Forecasting (WRF) simulations, to show the combination of low-level winds, moisture, and buoyancy leading to convection initiation (CI)
The COPS field campaign was conducted in the Rhine Valley bounded on the west by the Vosges and on the east by the Black Forest Mountains

Summary

Introduction

The Convective and Orographically-Induced Precipitation Study (COPS; Wulfmeyer et al 2008, 2011) was designed to improve the forecasting skill of convective precipitation in complex terrain using observations and modeling of its life cycle. Two DOW mobile radars (Wurman et al 1997) were positioned with a north–south-oriented 30.6-km baseline on 6 August to obtain high-resolution dual-Doppler wind measurements within the Rhine Valley and along the eastern and western slopes of the Vosges and Black Forest Mountains, respectively. Both DOWs are X-band (3 cm; ;9.4 GHz) Doppler radars that are sensitive to backscattering from insects, nearby clouds, and precipitation particles. A series of sensitivity studies were performed by varying the input analyses fields and parameterization schemes and showed that the simulation results were robust, all producing similar wind, cloud, and rain fields

Overview of 6 August 2007

Convection initiation

Findings

Summary and conclusions