The interaction of a surface cold front with a prefontal thermodynamically well-mixed boundary layer

The idealised two-dimensional numerical model of Reeder and Smith (1987) is extended to investigate the dynamics of a surface cold front, initially at sea, as it propagates onshore and encounters a thermodynamically well-mixed boundary layer resulting from diurnal heating. The effect of heating on the transverse frontal circulation is shown to be locally profound. An intense prefrontal updraft develops in the late afternoon, accompanied by rapid unbalanced in situ frontogenesis and an Increase in the speed of frontal propagation. When heating is excluded, the magnitude of the low-level cross-front wind component is everywhere less than the speed of propagation of the front, whereas when diabatic heating is added the low-level cross-front wind component immediately behind the front becomes greater than the frontal speed. It is shown that these results cannot be interpreted simply as a superposition of the frontal circulation and a sea-breeze, nor can they be attributed to symmetric instability. Comparison of the model with observations of Australian summertime cold fronts shows substantial agreement. Implications for the circumstances under which fronts develop gravity-current-like features are discussed.