Revisiting the dynamics of sea/gust fronts and their role in triggering deep convection

The merge of opposing sea-breeze fronts across islands can sometimes trigger the onset of deep convective bursts with both the timing and strength of the burst depending on the speed of the front across the island land surface. Theories proposed in the past for the propagation of density currents are highly idealized and may not apply quantitatively to sea or gust fronts propagating in conditions of non-uniform stratification and with inputs of heat from the surface. We have therefore conducted a series of tests ranging from highly idealized to more realistic, to ascertain the applicability of these theories. In the simplest configuration with a neutrally stable cold layer of air being released into a neutrally stable warmer layer (with a strong capping inversion above the entire layer) the current speed satisfies Benjamin's formula V = 0.5 sqrt(g'H), where g' is the reduced gravity g* dT/T and H= initial depth of cold layer. In the second set of experiments with initially stratified layers and an initial temperature impulse applied to the center (a hot spot), the best fit to data is given by V= sqrt(g'h) where h is current depth (estimated as the mean height at which the flow inside the current reverses direction). This is in agreement with Shin et al. 2004 for their deep ambient case. In the case of a diurnally heated island, cool maritime air from outside the island is fed along the ground to the head of the density current while the undisturbed air over the island continues to heat up. This temporally increasing horizontal temperature difference across the head causes the current to accelerate as it moves to the center of the island. For small island the instantaneous current velocity is again close to sqrt(g'h). However, for larger islands the current reaches a maximum speed independent of g'h.