Invigoration of cumulus cloud fields by mesoscale ascent



Large-eddy simulations of trade-wind cumuli impinging on an idealized island ridge are conducted to investigate the impact of mesoscale ascent on the morphology and vigour of cumulus convection. The simulations develop realistic cloud fields that invigorate when forced to ascend the high terrain despite being trapped beneath a sinking trade-wind inversion. Two upstream cloud regimes are considered, the first non-precipitating and the second lightly precipitating. Focus is placed on the precipitating case, where the island gives rise to a dramatic (15-fold) precipitation enhancement compared to the upstream ocean. This arises from (i) an increased concentration of buoyant updraughts, (ii) the ‘lapse-rate’ mechanism, wherein saturated parcels gain buoyancy as they ascend alongside dry parcels with different adiabatic lapse rates, and (iii) the ‘cloud-size’ mechanism, wherein the fractional entrainment rate decreases due to an increase in the mean horizontal cloud size. Although the first two mechanisms have received previous attention, the third is novel and potentially important. The broader clouds within the ascending airstream possess less dilute and more buoyant inner core regions that ascend faster through the cloud layer. Moreover, the increased liquid-water supplies and longer residence times of raindrops within these clouds lead to a sharp enhancement in the precipitation efficiency. The island cloud broadening is favoured by the presence of broad water-vapour anomalies within the impinging airstream that are mechanically lifted to saturation, along with basic energetic constraints that support wider, less dilute clouds in areas of rapid forced ascent. Copyright © 2012 Royal Meteorological Society