Observations of the development of convection through a series of stable layers during the Convective Storm Initiation Project

This article describes the development of convection through a series of stable layers on 20 July 2004 during the pilot project of the Convective Storm Initiation Project (CSIP). Convective clouds developed in south central England as a result of diurnal heating. They formed near the south coast and grew gradually as they moved northwards, eventually producing light precipitation. Several stable layers and associated dry layers (or lids) were observed by sequential radiosondes launched every two hours. Most of these lids were also detected by the Chilbolton L-band radar and with a wind profiler as prominent layers of Bragg echo. One particular lid at 820 mb (1.7 km) inhibited the early development of the convection for about an hour, delaying the production of precipitation. The uppermost lid at 650 mb stopped all convection from intensifying in the CSIP domain. Estimates of the growth of convection through the lids were sensitive to the values of temperature and humidity chosen to represent the ascending parcels. The development of the thermals in the boundary layer was best explained by using the surface-layer average values. Parcels would not have been sufficiently buoyant to reach the lifting condensation level (LCL) unless these values were used. However, such parcels in the absence of mixing would then have been too buoyant to be capped by the main lid at 1.7 km, as was observed by the radars. In fact the development of the cumulus clouds above the LCL is best represented by using 950-1000-mb average thermodynamic values. The results highlight the importance of predicting the altitude, strength and persistence of such lids in order to forecast the intensity, location and timing of convective precipitation more accurately. Copyright © 2008 Royal Meteorological Society.