Influence of the Orography of West-Central European Low Mountain Ranges on the Intensity of Deep Moist Convection

Abstract

<p>Deep moist convection comes in many different forms and degrees of organization while producing a wide spectrum of severe weather. It is long known that orography can influence the appearance of deep moist convection. In this work, which is based on the bachelor thesis of the first author, we examine whether and to what extent orography influences the intensity of deep moist convection. Therefore, two case studies with typical single cell environments (weak shear and little synoptic forcing) are simulated using COSMO-DE, a numerical weather prediction model provided by the German Weather Service (Deutscher Wetterdienst, DWD). The simulations were performed with a grid spacing of 2.8 km with a temporal resolution of 15 min. In both cases deep moist convection occurred over western Germany, Belgium and The Netherlands over low mountain ranges (below 1000 m) and over adjacent lowlands. In order to investigate the influence of the orography on the simulated deep moist convective cells, maximum updraft speed and maximum precipitation rate over the cells’ lifetime was analyzed. The intensity of the convective cells located over the low mountain ranges is compared to that of the convective cells located over the lowlands. The results show that convective cells over the mountain ranges differ to convective cells over the lowlands. However, these differences are small, if an additional initiation mechanism on the larger scale such as a convergence line is in place. In contrast, major intensity differences occur if there is no such larger scale initiation mechanism. In this case, the deep moist convection is far more intense over the low mountain ranges compared to the lowlands, which provides an important result for nowcasting and forecasting of deep moist convection in these regions and to assess whether convective cells might reach severe weather criteria or not. In addition to these results of the first author's bachelor thesis, recent simulations performed with the Cloud Model 1 (CM1) of the National Center for Atmospheric Research (NCAR) are also presented. Idealized simulations of the two cases with a higher spatio-temporal resolution were examined with regard to updraft speeds and precipitation rates of the simulated cells.</p>