Numerical Simulation of the Interaction between the Sea-Breeze Front and Horizontal Convective Rolls. Part I: Offshore Ambient Flow

Abstract

A three-dimensional, cloud-resolving model is used to investigate the interaction between the sea-breeze circulation and boundary layer roll convection. Horizontal convective rolls (HCRs) develop over land in response to strong daytime surface heating and tend to become aligned parallel to the vertical wind shear vector, whereas the land?sea heating contrast causes the formation of the sea-breeze front (SBF) along the coastline. The ability of HCRs to modulate the along-frontal structure of the SBF is examined, complementing and extending previous observational and numerical studies. Three simulations are discussed, the first two demonstrating that the model can produce both phenomena independently. The third is initialized with offshore mean flow and vertical shear perpendicular to the coastline, and results in a sharply defined, inland-propagating SBF that encounters HCRs aligned perpendicular to it. Before the interaction takes place, the SBF is nearly two-dimensional and devoid of along-frontal variability. Its subsequent encounter with the HCRs, however, causes enhanced (suppressed) convection at frontal locations where HCR roll updrafts (downdrafts) intersect. The suppressing effect of the roll downdrafts seems particularly striking. The interaction as it relates to vertical and horizontal motion, vorticity, and the cloud field are discussed. In future work, a similar simulation with HCRs oriented parallel to the SBF will be analyzed. These results provide further evidence that HCRs can play an important role in the initiation and modulation of convection along a sea-breeze front.