The structure and evolution of the dryline and surrounding boundary layer on 22 May 2002 during IHOP

During the afternoon and evening of 22 May 2002, dense multi-platform high-resolution data were collected over a small portion of the Oklahoma and Texas panhandles as part of the International H2O (IHOP) project. Using over-determined multiple Doppler radar syntheses in concert with an innovative Lagrangian analysis technique, the full 3-dimensional kinematic and thermodynamic structure of the dryline and surrounding boundary layer (BL) was obtained for a nearly 2 hour period. These analyses reveal the dryline as a strong (2-4 g kg-1 km-1) gradient of water vapor mixing ratio, which separated a shallow moist internal boundary layer to its east from a deep, dry CBL to its west.

The dryline was located within a persistent virtual potential temperature gradient, which solenoidally forced a thermally direct secondary circulation. The analyses show substantial along-line variability in the frontogenetical forcing that produces and extends the dryline gradient downstream. The secondary dryline-normal circulation was enhanced in regions of strong frontogenesis, increasing moisture lift within the updraft collocated with the dryline. The secondary circulation also enhanced top-down entrainment of dry air in the downdraft on the downstream (moist) side of the dryline. The boundary layer surrounding the dryline exhibited a complex, evolving organization consisting of varying combinations of open dry convective cells and horizontal convective rolls (HCRs). A field of updrafts and vortices were observed to move northward along and interact with the dryline to produce small-scale waves. The resulting deepening of moisture on the northern flank and drying on the southern flank of a vortex may be potentially important to convection initiation.