Three-dimensional numerical modeling of the sea breeze circulation and its interaction with horizontal convective rolls occurring in the convective boundary layer.

摘要

A three-dimensional numerical model is used to simulate the evolution of the sea breeze circulation and its interaction with the boundary layer. Two basic flow regimes are examined with emphasis on how the evolving sea breeze front interacts with roll vortices pre-existing in the boundary layer and how this interaction relates to the initiation of convection.;In the first experiment, the model is initialized with mean flow directed offshore, perpendicular to the coastline. The sea breeze circulation pushes inland slowly due to the opposing flow. Its front, however, is sharply defined. In the undisturbed environment ahead of the sea breeze circulation, horizontal convective rolls develop. The rolls are oriented perpendicular to the coastline, aligned with the mean flow at low levels. When the sea breeze front propagates far enough inland to encounter the rolls, the result is an intiguing three-dimensional structure. The front develops points of enhanced vertical motion which are spatially periodic having a wavelength corresponding to that of the convective rolls. Vertical motion is enhanced where the front intersects roll updrafts and suppressed where it encounters roll downdrafts. Points of enhanced convection are responsible for the deepest cumuli located alone, the front.;A second experiment is initialized with mean now parallel to the coastline. Since this now does not oppose its propagation, the sea breeze front surges inland more quickly. The front is less sharply defined but still can easily be identified. As in the offshore experiment, convective rolls develop out ahead of the marine circulation, though this is where the similarity ends. As the front reaches well-developed roll circulations, frontal lifting is enhanced and suppressed in a temporally periodic fashion. When the front approaches the axis of a roll updraft, frontal lifting and roll lifting merge. The resulting enhancement in vertical motion triggers the production of a cloud which propagates rearward with respect to the front. Additional storms are produced periodically corresponding to the merger of the sea breeze front with the updraft axis of subsequent rolls.;The results of this research can be extended to a situation in which the sea breeze front interacts with convective rolls oriented at some angle to the coastline. These events will encompass some combination of the parallel and perpendicular regimes and thus exhibit both spatial and temporal variability.