Composite Views of Tropical Convective Evolution Dependence On Aggregation Using a Gross Moist Stability Framework

Abstract:

Tropical convection spans a large variety of organizations and this may strongly modulate radiative forcing, moisture distribution, and large-scale circulations. Advances in understanding of interactions among convection and its environment will provide new insights in how the organization process could be reconsidered in contemporary global circulation model parameterizations. Gross moist stability (GMS) acts as a useful estimate of atmospheric stability and is highly associated with structures of large-scale circulation and moisture. Based on GMS, this study aims to explore the convective evolution dependence on convective aggregation over tropical oceans.

5-year multiple gridded datasets of a 3-hourly resolution, including satellites and reanalysis, are used to identify convective events in 5⁰x5⁰ boxes spanning over the deep oceanic tropics (10⁰S-10⁰N). Convective events are determined when observed box-averaged precipitation maximum exceeds 5 mm/d and is centered in a 4-day time window. The degree of aggregation of an event is then estimated at the precipitation maximum by Simple Convective Aggregation Index (SCAI) which is based on cold cloud objects defined by connected IR pixels (BT<240K) in the box. Variables are composited and analyzed in both GSM and temporal space from identified convective events.

SCAI serves as a reliable indicator of aggregation in the 5⁰x5⁰ spatial scale. Given the same box-averaged precipitation intensity, a smaller SCAI accords with larger size and a smaller number of cloud objects. A more aggregated status shows (1) a drier environment with a larger horizontal gradient of moisture and (2) a more bottom-heavy structure of large-scale upwelling. According to the concept of GMS, these two features may contribute to negative GMS, destabilizing the atmosphere by importing moist static energy. Reliability of reanalysis in representing moist dynamics is examined by estimating GMS and related components from reanalysis and field campaigns. A rapid intensification of precipitation prior to precipitation maximum is found in events of a more aggregated status. This phenomenon is associated with more negative GMS explained by a derived relationship between GMS, precipitation and its tendency, suggesting a potential role of aggregation in the convective evolution.