Cold Pools and Entrainment Dominate Surface Air Temperature in the Central Indian Ocean

Cold pools are responsible for most of the air temperature variability observed from a ship over the central Indian Ocean. We identify 215 cold pools from 62 days of 1-minute ship time series from DYNAMO, as temperature drops of at least 0.7 C over 20 minutes plus time that temperature decreases monotonically. Daily surface air temperature is anticorrelated with the frequency of cold pools at r=–0.62. The distribution of outgoing longwave radiation (OLR) and surface temperature are negatively skewed. The lowest quartile of OLR (<198 W/m² has 82 (38%) of the cold pools detected, while top quartile (OLR>260 W/m²) has only 30 (14%) of the cold pools. Cold pool strength, measured by the temperature drop, is uncorrelated to OLR. Surface fluxes increase modestly within cold pools, mostly due to a brief gust at the cold pool front.

Saturated downdrafts--cooled and moistened to their wet bulb potential temperature by rain evaporation--are responsible for cold pools, rather than boundary layer air moistened by evaporation in situ. Downdrafts, surface fluxes, and entrainment from the stratified shallow cumulus layer are all required to explain the observed boundary layer potential temperature and specific humidity. Saturated downdrafts are drier and colder than air saturated over the sea surface temperature, and have comparable humidity to dry air entrained from the cumulus layer. Entrainment is diagnosed using radiosonde observations and time series of temperature and surface fluxes of cold pools and their recoveries.