Lifting Condensation Level and Its Relation to Convective Cloud Base

摘要

This thesis explores the relationship between the lifting condensation level (LCL) and the height and time of convective cloud formation. Data collected from several different sources during the BLX83 field program is utilized. Sounding data is used to infer the mean structure of the LCL in the boundary layer. It is found that the lowest LCLs are usually within the lowest, or surface, portions of the convective boundary layer. High resolution cloud base data is obtained from lidar measurements of the boundary layer. These are compared to LCLs calculated using surface data from a PAM (Portable Automatic Mesonet) station. The initial formation of cumulus clouds is below, and earlier than, would be predicted based on the PAM data. The spatial variability of the surface LCL is inferred from near surface flights made by the NCAR Queen Air aircraft. It is seen that by combining the very lowest LCLs obtained by aircraft with the very highest thermals measured by lidar, a more accurate estimate of the time of initial cloud formation can be made. In addition, a decoupling of the surface-based LCL and cloud base is seen to occur later in the day. This is thought to be a function of greater, and more effective, entrainment into individual, surface-based thermals as the day wears on. (Author)