Moisture Distributions in Tropical Cold Pools From Equatorial Indian Ocean Observations and Cloud-Resolving Simulations

Abstract

The near-surface characteristics of approximately 300 convective cold pools over the equatorial Indian Ocean are studied using surface meteorological variables from two equatorial Indian Ocean sites, radar imagery, and constrained cloud-resolving simulations. The observed temperature drop at cold pool onset is typically accompanied by a drying and a decrease in moist static energy, signifying air transport from above the boundary layer through precipitation-induced downdrafts. The decrease in the surface water vapor mixing ratio is more pronounced for stronger temperature drops. Composites reveal a slight enhancement in moisture coincident with a slight enhancement in temperature prior to the cold pool frontal temperature drop. The slight enhancements occur prior to a gust of increased surface winds, suggesting that the immediate cause is wind convergence. A statistical analysis combined with a focus on selected case studies is consistent with a view that the strongest cold pools occur in intermediate column water vapor paths with drier midtropospheres. Such conditions are more likely to occur during convectively suppressed phases of the Madden-Julian Oscillation, when cold pool mesoscale organization facilitates the ability of cumulus congestus to reach the middle troposphere. Cold pools thus help explain why tropical cumulus congestus are common. Cloud-resolving simulations capture realistic rain rates and surface wind changes (and thereby surface fluxes). The evolution in the model near-surface moisture field is unrealistic, however, with an erroneous moisture enhancement inside the cold pool edge that is attributed to rain evaporation. This supports a further focus on the model representation of cold pool frontal dynamics and mixing.