Satellite-Derived Cold Pool Activity, Convection and Moisture Evolution During MJO Initiation in the Indian Ocean

Through use of the CPC MORPHing (CMORPH) rainfall product, large spatial-scale rainfall anomalies associated with Madden - Julian Oscillation (MJO) events are traced in space and time extending from initiation in the Indian Ocean to termination in the Pacific Ocean. Focusing on the anomalies associated with events initiating in the Indian Ocean, and utilizing all available TRMM overpasses geo-located with the spatial envelope of the rainfall anomalies, a temporal evolution (10-20 days relative to passage of large-scale envelope) of the probabilities for non-raining scenes, shallow precipitating convection, deep unorganized and organized convection (comprising these larger-scale rainfall envelopes) is derived. Overall, non-raining scenes, unorganized deep and organized deep convective cloud ensembles are modulated in frequency relative to the passage of the large-scale rainfall envelope; in contrast, populations of shallow raining clouds (2 - 4 km in depth) exhibit much less temporal variability. Instead of a conceptual view that entails a transition from shallow rain to deeper systems, these results suggest that there is an addition of deeper systems to large fields of shallow raining clouds at the expense of non-raining scenes. This notion of addition instead of transition may help to explain the long time scale for deeper convection, since shallow convection fields are continually replenishing moisture and instability despite the stabilizing effects of deep convection. An additional analysis of these various convective states relative to background moisture and a novel measure of mesoscale cold pool activity (deduced from the QuikSCAT satellite) is performed. While deep convective states are typically associated with elevated mid-tropospheric relative humidity or column-integrated water vapor, it is shown that substantial cold pool activity can help offset the negative effects of a drier atmosphere, thus leading to enhanced probabilities of deep convection in a decreased-moisture background. This finding is discussed within the context of the initial appearance of deep convection during MJO initiation, given that the moisture content of the Indian Ocean is typically decreased relative to other portions of the tropical oceans in which deep convection is found.