28 Invigoration of cumulus cloud fields by mesoscale ascent

Monday, 20 August 2012
Priest Creek AB (The Steamboat Grand)
Daniel J. Kirshbaum, McGill University, Montreal, QC, Canada; and A. L. M. Grant

Although the basic role of mesoscale ascent (from mountains, gravity waves, sea breezes, etc.) on the initiation of atmospheric convection is well established, little attention has been devoted to the detailed internal response of clouds embedded within these flows. To this end, large-eddy simulations of precipitating trade-wind cumuli impinging on an idealized island ridge are conducted to investigate the impact of forced ascent on the cloud morphology, internal dynamics, and microphysics. Despite being trapped beneath a sinking trade-wind inversion, the simulated island clouds are more numerous, vigorous, and liquid-rich than those over the open ocean. This invigoration results in part from a sharp increase in horizontal cloud size over the island, which reduces the dilution of the buoyant convective cores by the entrainment of environmental air. The increased coverage and precipitation efficiency of the island clouds increases the mean precipitation rate 20-fold compared to that over the upstream ocean. The island cloud broadening is favoured by the presence of broad water-vapour anomalies within the impinging airstream that are forcibly lifted to saturation, along with basic energetic constraints that support wider, less dilute clouds in areas of rapid ascent. Radar and in situ aircraft observations over the mountainous Caribbean island of Dominica from the recent DOMEX field campaign are presented to reinforce these conclusions.
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