The boundary layer cumulus formation process near a cold frontal-dryline intersection on 24 May 2002 during IHOP

One focus of the recent International Water Vapor (H2O) Project (IHOP-2002) was to learn how airflow, water vapor, and temperature in the boundary layer (BL) control deep, moist convection initiation (CI). High-resolution, time-spaced 3-D multi-Doppler boundary layer radar wind analyses were obtained every 3 min for about 2 hours across a dryline, a cold front, and their “triple-point” intersection during the IHOP mission of 24 May 2002. Proceeding from dense in-situ BL observations propagated along radar-derived airflow trajectories via a new Lagrangian objective analysis technique, cumulus cloud volumes are derived from gridded output fields of water vapor content, virtual potential temperature, and saturation point. Retrieved cloud areas are in good agreement with GOES-8 visible satellite imagery. The combined analyses are used to determine the mesoscale boundary layer processes that govern storm suppression and the timing and location of cumulus clouds on 24 May.

The Lagrangian and radar wind analyses support a “parcel continuity principle” for cumulus formation, which requires that rising moist air parcels achieve their lifting condensation level (LCL) before being moved out of the updraft by the horizontal airflow. Cumuli form above penetrative updrafts in the elevated residual layer (ERL) overlying the moist BL east of the triple point on 24 May, but remain capped by a convective inhibition (CIN)-bearing layer above the ERL. Other forced cumuli develop in strong vertical motion within the nose of the cold front that occludes the dryline. Dropsonde data suggest a convergence line about 80 km east of the triple point where deep lifting of BL moisture and locally reduced CIN together support CI.