Intrinsic transition mechanism of shallow convection in the wintertime extratropics

Abstract We examined the evolution of shallow convective structures in the atmospheric boundary layer in response to changes in the basic state. Idealized numerical experiments were designed to reflect a typical winter monsoonal environment with prescribed sea surface temperatures (SSTs) varying from 273 to 287 K. The results show that in the high-SST experiments, enhanced surface heat flux intensified convection, and the associated momentum flux reduced the vertical wind shear, leading to scattered convective cells dominating the model domain, after the short-time dominance of horizontal convective rolls. In contrast, in low-SST experiments, small surface heat flux resulted in weak convection, maintaining strong vertical wind shear, and horizontal convective rolls initially dominated. Even in such cases, the convection structure gradually transitioned into scattered convective cells. This transition is governed by convectively induced vertical momentum transport, which effectively reduces vertical wind shear, thereby altering the preferable cloud structure. The above transition in convective structure was not affected by horizontal grid spacing of the model but affected by the lowest model level in the boundary layer.