This study is used to introduce a conditional sampling technique associated with a heuristic model of the convective boundary layer (CBL) based upon top-down, bottom-up physical processes. A new concept of a distribution of neutral buoyancy levels associated with those two classes of physical processes is included in the CBL model. Analysis of surface, aircraft, and sounding data from June 6, 1987, during FIFE showed two periods of drying in an otherwise moistening fair weather CBL with 7 to 9 ms-1 southerly winds. One period was during the so-called morning boundary layer transition, when the mixed layer was growing rapidly and the other during quasi-steady state afternoon conditions. Similar drying periods were observed on June 1 and 4. On June 6, however, morning and afternoon National Center for Atmospheric Research King Air missions were specifically designed to investigate time variations in the daytime CBL. The conditional sampling technique and the CBL model are used to investigate the composition of the moisture flux and flux divergence.

We conclude that the drying effect of strong surface thermals carrying upward moving moist air was substantially decreased by overturning thermals and that compensating downdrafts were the dominant process associated with the net positive flux divergence responsible for part of the observed drying during the morning transition. The remainder was inferred to be due to a combination of vertical and horizontal advection. By afternoon drying due to compensating downdrafts had ceased and was due entirely to advection mitigated by turbulent processes which were moistening the mixed layer. The drying and inversion rise were coupled phenomena that were due to moist surface thermals which penetrated a weak lower inversion into an elevated mixed layer (EML). The EML was left from the previous day, and was well below a much stronger capping inversion. This penetrative convection into the EML suddenly removed moisture from the lower part of the mixed layer, moistened the region of the EML, and dried a growing mixed layer. ¿ American Geophysical Union 1995