SHALLOW CONVECTION DURING THE MJO IN THE TWP

Abstract:

This study investigates the role of shallow convection during the pre-onset of the Madden-Julian Oscillation (MJO) over the Tropical Western Pacific. Ten years of observations from a vertically pointing cloud radar, soundings, and other instruments from the ARM site at Manus Island are used. Shallow-cloud moistening is estimated from these observations. Episodes of large-scale convection at Manus are classified into MJO and non-MJO events. The characteristics leading to the onset of both types of events in the radar-observed shallow precipitating and non-precipitating clouds, sounding-observed humidity, and sub-grid moisture and temperature tendencies from reanalysis are compared. Deepening of positive low-tropospheric moisture anomalies is characteristic during the pre-onset of MJO events but not during the pre-onset of non-MJO events. This MJO feature is accompanied by a tilt of positive anomalies in diabatic heating. There are positive anomalies in shallow-cloud abundance and rainfall during both types of events, but these are slightly larger during MJO events. Shallow convection thrive during convectively suppressed conditions, which are absent during non-MJO events. However, shallow cloud moistening is smaller during MJO events due the slightly higher rainfall rates. In average, moistening from shallow clouds is about one tenth of the total column moistening (1 mm/day) during the pre-onset of the MJO. The differences in humidity between MJO and non-MJO events might be explained by the differences in the dynamic fields. This suggests that shallow convection do not play a leading role during the moistening phase of the MJO.