Evaporation and Cold Pools Beneath Trade Cumulus Clouds

Abstract

The sources of air and water vapor in the tropical trade‐wind Atlantic subcloud boundary layer (SBL) are analyzed using in situ measurements of temperature, specific humidity, and stable water isotopologues (HDO and H ₂ ¹⁸ O) from shipboard observations in January‐February 2020. The coldest cold pools had a higher maximum D and coincided with precipitation. The hydrogen isotope ratio (D) of water vapor was greater within 9 (64% of 14) cold pools than in their surroundings. The strongest D change at the arrival (or front) of a cold pool was ‰ (−72‰ to −66‰). The isotopic composition, potential temperature, and specific humidity can be explained by evaporation of hydrometeors in convective downdrafts. In these cases, evaporation of enriched hydrometeors dominates over depletion of atmospheric vapor by removal of enriched precipitation. Mixtures of ambient subcloud air with 20% mass fraction of air, having water vapor with the isotope composition of the hydrometeors ( ¹⁸ O _L  = +0.7‰; D _L  = +15.4‰), explain the observed enrichment of vapor in the cold pools ( ¹⁸ O = −9.6‰; D = −66.6‰). As the strength of a cold pool decays, the isotopically enriched vapor from the downdrafts is diluted by surface and entrainment fluxes. Isotopes of water vapor in the wakes dilute ∼30× faster than the residence time of water vapor in the subcloud layer, suggesting the cold pool wakes are shallow and originate from concentrated evaporative downdrafts.