The influence of spatial variability of boundary-layer moisture on tropical continental squall lines

Previous work has described how rainfall from several storms showed persistent patterns with a length-scale of the order of 10-20 km, and suggested that a land-atmosphere feedback was responsible. As a first step in examining this mechanism, a two-dimensional model is used to simulate the response of a squall line to idealized variability of boundary-layer moisture. Ensembles of simulations are used so as to better account for the stochastic element of convection. Extra boundary-layer humidity results in increased rainfall slightly downshear of the location of the anomalous humidity. The rainfall increase tended to be largest at the upshear edge of the rainfall anomaly. This was explained in terms of the horizontal gradient of humidity resulting in the growth of an especially intense cell just within the patch, at the expense of a weaker cell immediately before the patch. The largest rainfall increases occur when the humidity anomaly is some 10 km or so long, which is consistent with this length-scale matching the scale of the convective cells, so that a single cell prospered at the expense of two adjacent cells, one each on the upshear and downshear sides. A humidity anomaly resulted in greater spatial variability of rainfall, which was consistent with the feedback mechanism previously proposed to explain rainfall persistence at convective length-scales.