Cumulus Moistening, the Diurnal Cycle, and Initiation of the Madden–Julian Oscillation during DYNAMO

Abstract:

Atmospheric soundings, cloud radar, and air-sea flux measurements collected during DYNAMO (Dynamics of the MJO) are employed to study the initiation of the Madden–Julian Oscillation (MJO) over the tropical Indian Ocean. Findings indicate that moistening of the troposphere during MJO initiation occurs in two stages: low-to-midlevel moistening in the suppressed phase (i.e., prior to the onset of deep convection), followed by deep-column moistening during the early active phase. The first stage of moistening occurs as the drying by large-scale subsidence and horizontal advection wane and convective clouds increase in areal coverage and deepen from shallow to congestus. Analysis of the suppressed phase reveals the striking degree to which the moistening characteristic of this period is accomplished by the diurnal cycle of cumulus clouds. This diurnal cycle is driven by the cycle in sea surface temperature and air-sea fluxes linked to shallow oceanic diurnal warm layers. This coupled diurnal cycle, and the associated afternoon peak in convective cloud depth, cloud areal coverage, and cumulus moistening, likely drives more vigorous overall moistening than would occur without this diurnal cycle. A conspicuous finding pertaining to the suppressed phase convection is the prominence of mesoscale cloud organization (i.e., open cells and horizontal convective rolls), which is hypothesized to enhance convective instability in localized patches, thereby promoting deeper, more vigorous cumulus moistening than would otherwise occur.