Characteristics of Convection during DYNAMO Inferred from Large-scale Heat and Moisture Budget Analyses

A primary component of the observing system in the CINDY2011-DYNAMO-AMIE field campaign was an enhanced atmospheric sounding network comprised of two sounding quadrilaterals, one north and one south of the equator over the central Indian Ocean. With the quality control of the upper-air soundings near completion, we use this sounding dataset along with satellite-derived products to create largely model-independent gridded analyses over the DYNAMO experimental domain. These gridded analyses are then used to compute the large-scale heat and moisture budgets over this region for the period from 1 October to 30 November 2011, during which time two prominent MJOs passed through the sounding network.

Using the gridded analysis product, surface precipitation is computed from the vertically integrated moisture (Q2) budget along with estimates of surface evaporation over the sounding arrays from the Woods Hole OAFlux product (adjusted as needed based on comparisons with DYNAMO Revelle fluxes at the Revelle location). Time series of budget-derived rainfall over the Northern Sounding Array (NSA) show strong modulation associated with the MJOs. In contrast, rainfall variability over the Southern Sounding Array (SSA) is characterized by more persistent yet briefer rainfall episodes likely related to ITCZ convection. Excellent agreement between budget-derived and TRMM 3B42 rainfall time series over the sounding arrays gives us confidence that the enhanced sounding network accurately captured the large-scale vertical motion signal associated with the MJOs.

Time series of the apparent heat source Q1 and moisture sink Q2 indicate periods of shallow heating and moistening (negative Q2) during the pre-active MJO phases in the first halves of October and November. For both MJOs, these fields suggest a transition from shallow to deep convection, followed by a top-heavy heating structure indicative of stratiform precipitation, consistent with the generally accepted paradigm for MJO convection (Mapes et al. 2006). The transition from shallow to deep convection was more rapid during the November MJO.

Smaller vertical separation of mean Q1 and Q2 peaks over the NSA compared to the SSA suggests a higher percentage of stratiform precipitation north of the equator where the MJO signal was more prominent. This supposition will be examined using independent measurements but is consistent with the finding by Lin et al. (2004) of a greater stratiform rain fraction within MJO convection than the global tropical mean.