Vertical velocity in oceanic convection off tropical Australia

Abstract

Time series of 1 Hz vertical velocity data collected during penetrations by research aircraft of oceanic cumulonimbus clouds near tropical Australia as part of the Equatorial Mesoscale Experiment (EMEX) are analyzed for updraft and downdraft events called cores. A core is defined whenever the vertical velocity exceeds for at least 500 m. Over 19000 km of straight and level flight legs are used in the analysis. 511 updraft cores and 253 downdraft cores are included in the data set. Core properties are summarized as distributions of average and maximum vertical velocity, diameter and mass flux in four altitude intervals between 0.2 km and 5.8 km. Distributions were approximately lognormal at all levels. Comparisons with other studies which use the same analysis technique reveal that EMEX cores have approximately the same strength, diameter, and mass flux as the cores of other oceanic areas, despite differences in the large-scale environment. Oceanic cores are much weaker than cores observed over land as part of the Thunderstorm Project. estimate of virtual temperature excess is made for the portion of the cores between 4.3 and 5.8 km using a C02 radiometer. Median observed average excesses are found to be slightly positive for both updraft and downdraft cores and much lower than what is predicted by simple parcel theory. TAMEX cores also showed this result. Examination of the variations of the statistics with height suggests a maximum of vertical velocity between 2 and 3 km; slightly lower vertical velocity is indicated at 5 km. The vertical velocity statistics are considered in the context of the environmental soundings. It is suggested that the shape of the positive area on a thermodynamic diagram, combined with water loading, can help explain the observed differences of the strength of the cores between the continents and oceans.