Effect of dry large-scale vertical motions on initial MJO convective onset

Abstract

Anomalies of eastward propagating large-scale vertical motion with ~30 day variability at Addu City,Maldives, move into the Indian Ocean from thewest and are implicated inMadden-Julian Oscillation (MJO) convective onset. Using ground-based radar and large-scale forcing data derived froma sounding array, typical profiles of environmental heating, moisture sink, vertical motion, moisture advection, and Eulerian moisture tendency are computed for periods prior to those during which deep convection is prevalent and those during whichmoderately deep cumulonimbi do not forminto deep clouds. Convection with 3–7 kmtops is ubiquitous but present in greater numbers when tropospheric moistening occurs below 600 hPa. Vertical eddy convergence of moisture in shallow to moderately deep clouds is likely responsible for moistening during a 3–7day long transition period between suppressed and activeMJO conditions, althoughmoistening via evaporation of cloud condensate detrained into the environment of such clouds may also be important. Reduction in large-scale subsidence, associated with a vertical velocity structure that travels with a dry eastward propagating zonal wavenumbers 1–1.5 structure in zonal wind, drives a steepening of the lapse rate below 700 hPa, which supports an increase in moderately deep moist convection. As the moderately deep cumulonimbi moisten the lower troposphere,more deep convection develops, which itselfmoistens the upper troposphere. Reduction in large-scale subsidence associated with the eastward propagating feature reinforces the upper tropospheric moistening, helping to then rapidlymake the environment conducive to formation of large stratiform precipitation regions, whose heating is critical for MJO maintenance.