The initiation of moist convection at the dryline: Forecasting issues froma case study perspective

The processes that force the initiation of deep convection along the drylin e are inferred from special mesoscale observations obtained during the 1991 Central Oklahoma Profiler Studies project, the Verification of the Origins of Rotation in Tornadoes Experiment 1994 (VORTEX-94), and the VORTEX-95 he ld projects. Observations from aircraft, mobile CLASS soundings, and mobile mesonets define the fields of airflow, absolute humidity, and virtual temp erature in the boundary layer across the dryline on the 15 May 1991, 7 June 1994, and 6 May 1995 case days. Film and video cloud images obtained by ti me-lapse cameras on the NOAA P-3 are used to reconstruct the mesoscale dist ribution of cumulus clouds by photogrammetric methods, permitting inference s concerning the environmental conditions accompanying cloud formation or s uppression. The results of the present study confirm the classical notion that the dryl ine is a favored zone for cumulus cloud formation. The combined cloud distr ibutions for the three cases are approximately Gaussian, suggesting a peak expected cloud frequency 15 km east of the dryline. Deep mesoscale moisture convergence is inferred in cloudy regions, with either subsidence or a lac k of deep convergence in cloud-free regions. The results document the modul ating effect of vertical wind shear and elevated dry layers in combination with the depth and strength of mesoscale updrafts on convective initiation, supporting the notion that moist boundary layer air parcels must be lifted to their lifted condensation level and level of free convection prior to l eaving rile mesoscale updraft to form deep convection. By relaxing the over ly restrictive assumptions of parcel theory, it is suggested that a modific ation of proximity soundings to account for mesoscale lift and westerly win d shear effects can improve the diagnosis of the mesoscale dryline environm ent and the prediction of convective initiation at the dryline. Conversely, proximity environmental soundings, taken by themselves with consideration of CAFE and convective inhibition values according to parcel theory but neg lecting vertical boundary layer circulations, are found to have less progno stic value than is conventionally assumed.