A High-Resolution Modeling Study of the 24 May 2002 Dryline Case during IHOP: Horizontal Convective Rolls and Convective Initiation

The 24 May 2002 dryline convective initiation (CI) case is studied through nested grid 1 km resolution simulations. Routine as well as special observations collected during IHOP_2002 are assimilated into the initial condition at 1800 UTC. The CI at around 2015 UTC along a section of the dryline is correctly predicted, as is the non-initiation of convection at a cold front-dryline intersection (triple point) located further north. The timing and location of predicted CI are accurate to within 20 minutes and 25 km, respectively. The general evolution of the predicted convective line up to 6 hours of model time also verifies well.

Mesoscale convergence associated with the confluent flow around the dryline is shown to produce an upward moisture bulge, while surface heating and boundary layer mixing are responsible for the general deepening of the boundary layer. These processes produce favorable conditions for convection.

Horizontal convective rolls (HCRs) develop on both sides of the dryline. The main HCRs that interact with the primary dryline convergence boundary (PDCB) are those from the west side and they are aligned at an acute angle with the dryline. They intercept the PDCB and create strong moisture convergence bands at the surface and force the PDCB into a wavy pattern. The downdrafts of HCRs and the associated surface divergence create localized maxima of surface convergence that trigger convection. The surface divergence flows also help concentrate the background vorticity and the vertical vorticity created by tilting of environmental horizontal vorticity into vortex centers or misocyclones, and such concentration is often further helped by cross-boundary shear instability. The misocyclones, however, do not in general co-locate with the maximum updrafts or the locations of convective initiation, but can help enhance surface convergence to their south and north.

Sequences of convective cells develop at the locations of persistent maximum surface convergence, then move away from the source with the mid-level winds. When the initial clouds propagate along the convergence bands that triggers them, they grow faster and become more intense. While the mesoscale convergence of dryline circulation preconditions the boundary layer by deepening the mixed layer and lifting moist air parcels to their LCL, it is the localized forcing by the HCR circulation that provides critical extra lift needed for air parcels to rise above their LFC and to develop into deep moist convection. A conceptual model summarizing the findings is proposed.