On the Role of Shallow Clouds during the pre-onset of MJO over Manus

Description

Relatively successful model simulations of the Madden-Julian Oscillation suggest a crucial role of shallow convection during the pre-onset stage of this phenomenon. However, it is unknown if shallow convection is important during the MJO in a particular sense compared to other perturbations. This issue is investigated here using long term observations over Manus (approximately from 2001 to 2011). Observations from a millimeter vertically pointing radar and soundings are primarily used. The cloud and moisture evolution from composites of MJO episodes and other large-scale perturbations (non-MJOs) are compared. Results show that shallow precipitating (not-precipitating) clouds are more (less) abundant during the pre-onset of MJOs compared to those during non-MJOs. During the pre-onset of MJOs, rainfall from shallow clouds starts increasing before to that from deeper clouds. In contrast, during the pre-onset of non-MJOs, rainfall from all convective clouds starts increasing at a similar timing. These characteristics of shallow clouds during MJOs occur at the same time that moisture anomaly at the low level develop, however analogous moisture anomalies at this level during non-MJOs are hardly observed. Therefore, these results suggest an important role of shallow clouds during the pre-onset of the MJO over the Manus.