Time Scales of the Shallow-to-Deep Convective Transition at MJO Initiation

This study investigates the timing and underlying processes of shallow to deep convection transition (SDT) at the onset of 74 MJO events over the Central Indian Ocean using 16-yr Tropical Rainfall Measuring Mission (TRMM) data. SDT time scales, as well as shallow, deep, and decaying convective periods, are identified by convective cloudtop brightness temperature (infrared), which has been shown to be a better proxy than radar echo-top (18 dBZ) height for such purposes. It is found that SDT exhibits a broad spectrum from two days to more than two weeks with the median values around 7-9 days. Approximately half of the SDT spectrum falls into the 10-15 day time scale, which is consistent with the “discharge-recharge” mechanism. This finding underpins the previously established important role of midtropospheric moistening by congestus. On the other hand, SDT takes less than one week in ~25% of the MJOs sampled, including two strong MJOs observed during the Dynamics of the MJO (DYNAMO) field campaign. In these short SDT periods, previous DYNAMO studies suggest that moistening by large-scale mechanisms (e.g., uplift) dominates the local convective moistening. Statistical analysis shows that SDT is negatively correlated with the time scale of shallow convection prior to SDT and slightly correlated with the pre-onset environments such as SST, winds, and mid-tropospheric humidity. It is interesting that SDT has no correlation with the magnitude and duration of individual MJOs. Future work will examine the importance of various moistening mechanisms (e.g., discharge-recharge vs. large-scale forcing) as a function of SDT time scale (e.g., rapid vs. gradual).