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

Abstract In Part I of this paper, the timing and location of convective initiation along a dryline on 24 May 2002 were accurately predicted, using a large 1-km-resolution nested grid. A detailed analysis of the convective initiation processes, which involve the interaction of the dryline with horizontal convective rolls, is presented here. Horizontal convective rolls (HCRs) with aspect ratios (the ratio of roll spacing to depth) between 3 and 7 develop in the model on both sides of the dryline, with those on the west side being more intense and their updrafts reaching several meters per second. 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 play an important role in creating localized maxima of surface convergence that trigger convection. The downward transport of westerly, southwesterly, or northwesterly momentum by the HCR downdrafts creates asymmetric surface divergence patterns that modulate the exact location of maximum convergence. Most of the HCRs have a partially cellular structure at their mature stage. The surface divergence flows help concentrate the background vertical vorticity and the 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 collocate with the maximum updrafts and, therefore, the points 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 midlevel winds. When the initial clouds propagate along the convergence bands that trigger 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 determines the exact locations of convective initiation. A conceptual model summarizing the findings is proposed.