Precipitating convection in a mountain region of moderate topography is investigated, with particular emphasis on its initiation in response to boundary-layer and mid-and upper-tropospheric forcing mechanisms. The data used in the study are from COPS (Convective and Orographically-induced Precipitation Study) that took place in southwestern Germany and eastern France in the summer of 2007. It is found that the initiation of precipitating convection can be roughly classified as being due to either: (i) surface heating and low-level flow convergence; (ii) surface heating and moisture supply overcoming convective inhibition during latent and/or potential instability; or (iii) mid-tropospheric dynamical processes due to mesoscale convergence lines and forced mean vertical motion. These phenomena have to be adequately represented in models in order to improve quantitative precipitation forecast. Selected COPS cases are analysed and classified into these initiation categories. Although only a subset of COPS data (mainly radiosondes, surface weather stations, radar and satellite data) are used here, it is shown that convective systems are captured in considerable detail by sensor synergy. Convergence lines were observed by Doppler radar in the location where deep convection is triggered several hours later. The results suggest that in many situations, observations of the location and timing of convergence lines will facilitate the nowcasting of convection. Further on, forecasting of the initiation of convection is significantly complicated if advection of potentially convective air masses over changing terrain features plays a major role. The passage of a frontal structure over the Vosges-Rhine valley-Black Forest orography was accompanied by an intermediate suppression of convection over the wide Rhine valley. Further downstream, an intensification of convection was observed over the Black Forest due to differential surface heating, a convergence line, and the flow generated by a gust front.

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Sea breezes and low-level jet (LLJ) streams are studied in the region between the Eastern Castilla-La Mancha and the Mediterranean coast of southeastern Spain. The simplified concept of two-dimensional terrain and meteorological conditions explains the spatially and diurnally varying wind system consistently. The changes, as a function of time, height and distance to the coast, of temperature, pressure and wind as well as of certain observed phenomena, such as LLJs and strongly baroclinic zones, result from the response of the mesoscale flow to differential heating. Wind changes are generally found to be consistent with mesoscale changes in geostrophic winds as estimated from station pressure measurements. The LLJs mark the penetration depth of the sea breeze during daytime, where a baroclinic zone develops at the transition between continental air masses and marine air masses, typically at a distance of 150 km from the coast. The analysis is based on aerological data and ground-based measurements from the European Field Experiment in a Desertification Threatened Area (EFEDA) experiments in 1991 and 1994 as well as on 3-hourly data from three synoptic stations of the Spanish meteorological network for April-October in 1991 and 1994. After corrections were made for instrument errors and atmospheric tides, diurnal pressure variations document vertically integrated mass fluxes perpendicular to the coast. Amplitudes of diurnal pressure and temperature changes scatter significantly and are largest in midsummer. They are weakly correlated and pressure amplitudes remain large in spring and autumn when diurnal temperature changes are reduced. With about 6 h delay to the regular development of onshore winds at the coast during daytime, pronounced wind peaks are observed over the eastern plateau at Albacete and Barrax in the late afternoon. Case studies show that they are accompanied by development of baroclinically driven LLJs, whereas winds at Tomelloso, 220-km inland, show less pronounced diurnal changes and are beyond the normal penetration distance of sea breezes.