Numerical Simulation of the Interaction between the Dryline and Horizontal Convective Rolls

摘要

The results of high-resolution simulations of an idealized dryline environment are discussed. The use of a single high-resolution domain, combined with accurate advection numerics and minimized numerical filtering, allows the explicit resolution of large horizontal convective roll (HCR) circulations and their daytime evolution. The horizontal convective rolls are oriented in the direction of the lower planetary boundary layer (PEL) wind shear. By midafternoon a north-south-oriented dryline develops near the center of the simulation domain with the PBL circulations from both sides intersecting the dryline at multiple locations. West of the dryline, the HCR bands evolve into open convective cell (OCC) structures having stronger and deeper vertical circulations compared to the OCCs and HCRs to the east. The OCCs and HCRs east of the dryline impact the dryline and convective cloud location by modulating the low-level moisture and upslope easterly flow. The interaction between OCC and HCR circulations and the dryline appears primarily responsible for creating a considerable amount of along-line variation in the dryline characteristics. Many shallow convective clouds develop along and west of the dryline over the OCC and HCR updrafts as well as OCC-dryline and HCR-dryline intersection points. The shallow convective clouds evolve into deep convective clouds where OCCs and HCRs to the east intersect the dryline near the same location. When the cumulus clouds move to the east of the dryline and remain over an OCC/HCR updraft, the persistent low-level lifting permits the convective updraft to overcome the cap east of the dryline and directly lift near-surface moisture to its level of free convection.