Data from the Convection and Precipitation/Electrification (CaPE) project were combined with numerical simulations to examine horizontal convective rolls. Four primary topics were addressed: (i) the ...environmental conditions necessary for sustaining rolls, (ii) horizontal thermodynamic variability within the convective boundary layer (CBL) due to rolls, (iii) thunderstorm initiation by rolls and (iv) the cause of along-roll periodicities. Both observations and numerical model simulations suggested that a moderate surface heat flux and some vertical wind shear was necessary for roll existence. Unlike previous studies, however, it was shown that rolls occurred within very low shear conditions ($\sim2\times10\sp{-3}$ s$\sp{-1}$. In addition, the low-level (i.e., $\sim$200 m) shear seemed to be more important than the CBL shear in roll sustenance. The horizontal potential temperature variations caused by rolls were 0.5 K while mixing ratio variations of 1.5-2.5 g kg$\sp{-1}$ were observed at all heights throughout the CBL. The observed cloud base heights, measured with photogrammetry, were produced from the highest moisture values which occurred within the roll updraft regions. Since the roll updrafts contained the moisture values which were most representative of cloud base, it is likely that measurements from within roll updrafts are most representative of the potential for deep convection. It was documented that thunderstorms were initiated by rolls in Florida. Detailed comparisons were made between the days on which thunderstorms were initiated by rolls to days on which rolls existed but did not produce thunderstorms. No apparent differences were identified between the two types of days in any of the parameters examined. Along-roll irregularities were often apparent along the cloud streets, as well as within the CBL motions. The observed periodicities within the CBL had the same wavelength along adjacent roll updraft regions and were continuous in the cross-roll direction. The most likely source of these features was found to be shearing instabilities. A significant amount of knowledge about rolls has been obtained from analyzing the CaPE dataset although many assumptions and simplifications were required. These restrictions of the CaPE dataset, as well as plans for a field project during the summer of 1995 chiefly designed to sample rolls and waves, are presented.
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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