Vertical velocity characteristics of oceanic convection

Abstract

Oceanic cumulonimbus updraft and downdraft events observed in the Western Pacific during the TAMEX program by NOAA P-3 research aircraft are analyzed and discussed. TAMEX drafts and cores are comparable in size and strength with those measued in GATE and hurricanes but much weaker than those measured in continental thunderstorms. The median core updraft was less than 3 m s-1, implying a time scale for ascent from cloud base to the freezing level of about 35 min. The microphysical implications are consistent with the hypothesis that the oceanic convection is dominated by warm rain coalescence proceses and that a large fractional rainout occurs below the freezing level. The rapid reduction of cloud water and radar reflectivity above the freezing level, as well as observations of abundant ice particles in all but the strongest updraft cores at temperatures just below 0°C, implies a rapid conversion of cloud water and rain to ice and graupel as the air ascends through the freezing level. The majority of the strongest downdrafts possess positive temperature perturbations (probably as a result of mixing with nearby updraft air) with the negative buoyancy being sustained by large amounts of rainwater. -from Authors