Vertical Velocity Characteristics of Deep Convection over Darwin, Australia

Abstract

Continuous vertical velocity measurements using a 50-MHz wind profiler located at Darwin in northern Australia during periods of active convection have been analyzed. This dataset is dominated by continental-type convection. Numerous examples of shallow, deep, and decaying convection were seen and it is shown that only the deep systems have substantial tilts to the draft structure. The most intense updrafts occur above the freezing level, but shallow convection also produces large-amplitude vertical motions. The strength of these updrafts in this dataset is very similar to other tropical, oceanic data. That observation is consistent with the idea that the magnitude of the updrafts is much less in the Tropics than for intense midlatitude convection because the convective available potential energy is distributed over a much deeper layer in the Tropics, although more intense updrafts may be present at other tropical locations, such as the Tiwi Islands north of Darwin. The size of the cores, however, is significantly greater here than with oceanic data and is similar to midlatitude results, thus supporting the suggestion that boundary layer depth is important in determining the horizontal scale. There is a net detrainment in the upward cores above the freezing level occurring at all space scales. The mass flux in intense updrafts is almost constant with height below the freezing level but is almost cancelled by downdrafts and the immediate surrounding environment. Two populations of downdrafts are seen, one a dynamical response associated with intense updrafts at all heights and a second driven by precipitation processes below the freezing level. The core size, intensity, and mass flux are all approximately lognormally distributed. It is shown that a wide range of velocity and size scales contribute to the upward mass flux.