Invigoration of shallow cumuli by mesoscale ascent

Simulated fields of trade-wind convection impinging on an idealized island ridgeSimulated fields of trade-wind convection impinging on an idealized island ridge

These images show simulated fields of trade-wind convection impinging on an idealized island ridge with a height of 500 m. Conditions for these cases are derived from field campaigns (BOMEX and RICO) over the western Atlantic Ocean. The ocean is depicted in blue, the island in green, and the cloud field (the 0.1 g/kg isosurface) in gray. The clouds clearly become more vigorous and deeper over the island ridge. Note also that individual cloud complexes are apparent with much larger sizes than any oceanic clouds. These larger clouds are the main precipitation producers over the island.

Cumulus cloud fields are typically studied in idealized environments that are horizontally homogenous over large areas. In reality, however, these clouds fields typically exhibit substantial variability on the mesoscale (scales of 1-1000 km), which appears to violate the assumption of horizontal homogeneity. Rather than forming in random locations, clouds often cluster in specific regions. This variability may be associated with feedbacks from the clouds onto the larger-scale flow (e.g., cold pools, gravity waves) and/or external forcings imposed on the cloud field. This study investigates the latter mechanism, specifically the impact of mountainous islands on preexisting cloud fields. This is carried out through an analysis of radar, rain-gauge, and aircraft observations over the Caribbean island of Dominica and through idealized large-eddy simulations. The observations document an intense enhancement in cloud coverage and precipitation over Dominica, which is large in virtually all synoptic environments. The simulations provide a physical basis for interpreting the observations. They reveal two mechanisms for the large enhancement in cloud vigour: (1) an increase in cloud buoyancy as moist air ascends alongside dry air with different adiabatic lapse rates and (2) an increase in the mean size of cumulus clouds, which weakens the fractional entrainment of environmental air. Although both mechanisms increase the buoyancy and vertical velocity of convective cores, the latter is potentially more important due to the increase in cloud liquid water, which stimulates faster accretional growth of precipitation particles. For more information see the following references:

  • Kirshbaum, D. J. and A. L. M. Grant, 2012: Invigoration of cumulus cloud fields by mesoscale ascent. Q. J. R. Meteorol. Soc., in press, DOI: 10.1002/qj.1954
  • Smith, R. B., J. R. Minder, A. D. Nugent, T. Storelvmo, D. J. Kirshbaum, R. Warren, N. Lareau, P. Palany, A. James, and J. French, 2012: Orographic precipitation in the tropics: the Dominica Experiment. Bull. Amer. Meteor. Soc., 93, 1567-1579.
  • Kirshbaum, D. J. and R. B. Smith, 2009: Orographic precipitation in the tropics: large-eddy simulations and theory. J. Atmos. Sci., 66: 2559-2578.

About the author

Dr. Kirshbaum is an Assistant Professor in the Atmospheric and Oceanic Sciences Department at McGill. His research focuses on the mechanisms and predictability of atmospheric convection and other mesoscale phenomena.