A three-dimensional, cloud resolving model is used to investigate the interaction between the sea-breeze circulation and boundary layer convection. Horizontal convective rolls (HCRs) develop over land in response to strong daytime surface heating and tend to align themselves 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, complimenting and extending previous observational and numerical studies.
Four 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 updrafts (downdrafts) intersect. The suppressing effect of the rolls seems particularly striking. The interaction as it relates to vertical and horizontal motion, vorticity, and the cloud field are discussed.
In the fourth simulation, the model is initialized with mean flow and vertical wind shear oriented parallel to the coast. The result, contrasted with the offshore simulation, is a more diffuse front that propagates inland to interact with well defined HCRs. In this simulation the front periodically encounters entire HCR updrafts and downdrafts aligned parallel to the SBF. Animation of this temporal modulation of the SBF by roll convection is quite striking. The evolution of the cloud field is discussed in depth. The study suggests that the orientation of synoptic flow during a convective sea-breeze event may play a key role in both the timing and positioning of convective initiation along the front.