Over the past decade, fine-scale observational studies have revealed that atmospheric boundaries (e.g., sea-breezes, convergence zones, drylines) can possess a large amount of along-line variation. These studies have focused on the relationship between these along-line variations to the presence of horizontal convective rolls (HCRs) within the planetary boundary layer. The results suggest that, besides producing along-line undulations, areas of enhanced ascent occur where HCR updrafts intersect with the boundaries. Further, convective clouds and/or storms develop in the region of enhanced ascent.

With the increase in computational resources over the past decade, numerical modeling studies are beginning to investigate the interaction between convective rolls and atmospheric boundaries. Recent studies have demonstrated that high-resolution numerical simulations of the sea breeze can reproduce many of the observed phenomena (e.g., sea-breeze boundary, HCRs). Further, these simulations demonstrate how the interaction between HCRs and boundaries plays an important role in the formation and evolution of convective clouds along the sea breeze. Numerical studies of drylines interacting with boundary layer circulations have been reported, in which horizontal resolution is adequate to resolve boundary layer circulations with wavelengths as small as ~ 5-10 km. However, these simulations use multiple nested domains in which the limited spatial and temporal use of high-resolution domains does not permit the resolving of the complete CBL and HCR circulation evolution.

This presentation will discuss results of high-resolution simulations of an idealized dryline environment. The use of a single domain with 1 km horizontal grid spacing, combined with accurate advection numerics and minimized numerical filtering, allows the explicit resolution of large HCR circulations and their daytime evolution. The control simulations exhibit HCR development within the convective boundary layer across the entire domain. The rolls are oriented in the direction of the mean PBL wind and evolve from bands into a random structure by afternoon. The HCRs in the western domain half having stronger and deeper vertical circulations compared to the eastern domain half. By 7 hours into the control simulation a north-south oriented dryline develops near the center of the simulation domain with the HCR circulations from both sides intersecting the boundary at multiple locations. The interaction of the intense western HCR circulations with the dryline boundary appears responsible for creating a considerable amount of along-line variation. This is due to two different mechanisms. The first is the HCRs downward transport of higher momentum air generating along-dryline differences in horizontal wind speed. The second is the convergence of moisture under the ascending branches of the HCRs. The alternating regions of increased (decreased) moisture within the western CBL assist in producing a westward (eastward) shift in the dryline location.

Many shallow convective clouds develop along and to the west of the dryline by 7.5 hours into the simulation (1930 UTC). The clouds are located over the western HCRs as well as at the intersection points of the western HCRs and the dryline. Over the next half-hour, several deep convective clouds develop along the dryline near HCR-dryline intersections and propagate toward the northeast. Additional results from trajectory and sounding analyses as well as sensitivity studies will be discussed.