Convective Initiation ahead of the Sea-Breeze Front

Abstract

Abstract In earlier work, a three-dimensional cloud model was used to simulate the interaction between the sea-breeze front (SBF) and front-parallel horizontal convective rolls (HCRs), resulting in the SBF systematically encountering roll updrafts and downdrafts as it progressed inland. Interestingly, deep convection was spawned above an HCR updraft ahead of the SBF as the front approached, well before the inevitable front–roll merger. Ostensibly, both the sea-breeze and roll circulations were required for deep convection to be present in this case at all because convection was entirely absent when either phenomenon was removed. Further analysis reveals why both circulations were necessary yet not sufficient for the excitation of deep convection in this case. The sea-breeze circulation (SBC) made its upstream (inland) environment more favorable for convection by bringing about persistent if gentle lifting over an extended region stretching well ahead of the SBF. This persistent ascent established a moist and cool tongue of air, manifested by a visible and/or subvisible cloud feature termed the cloud shelf emanating ahead of the front. Though this lifting moistened and destabilized the environment, the roll’s direct and indirect effects on this moist tongue were also required. The former consisted of a moisture plume lofted by the roll updraft, and the latter consisted of obstacle effect gravity waves generated as the roll drafts penetrated through the top of the boundary layer, into the SBC-associated offshore flow farther aloft. These provided the missing spark, which led to rapid growth of cumulus above the roll updraft, drawing first from air located above the boundary layer. Once established, deep convection above the roll updraft modulated cloudiness above the approaching SBF, at first suppressing it but subsequently assuring its reestablishment and eventual growth into deep convection, again prior to the front–roll merger. This resulted from the influence of gravity waves excited owing to heating and cooling within the roll cloud.