Convective Influence Calculations for CR-AVE and TC4

The influence of convection is of obvious importance to the tropical tropopause layer (TTL); in fact, the top of the TTL is defined as the upper limit of direct convective injection. More generally, the complexity of the TTL arises from the fact that the time scales of vertical transport due to radiative heating are comparable to convective turnover times. Convection affects both slowly varying tracers (e.g., CO) and constituents with rapid sources and sinks (e.g., water) in the TTL. It varies strongly with location in the tropics and with season. This means that TTL convective effects will depend on season and location, which is a particular concern for aircraft field campaigns that, of necessity, survey limited regions in space and time. In the boreal winter, convection hydrates the TTL by about .5 ppmv, with the global effects largely confined below about 365K. In boreal summer, warmer TTL temperatures and strong convection to higher potential temperatures, coupled with a strong nonzonal character to the circulation, yield larger effects on water vapor to higher altitudes. For CO, the convolution of convective and surface source variability yields a semiannual (and geographically complex) variation in the TTL.

This paper examines the nature of convective influence for three high altitude aircraft field campaigns addressing the TTL conducted in Costa Rica: the Pre-Ave field campaign in January 2004, the CR-AVE campaignin January 2006, and the TC-4 campaign in July-August, 2007. Convective influence was evaluated by marching parcel trajectories through geostationary IR satellite imagery, with cloud altitudes adjusted based on observed differences between IR and actual cloud heights. Because of circulation differences, convective influence during the first two campaigns differed significantly, with much more continental convective influence (from South America) during Pre-AVE in 2004 than during CR-AVE in 2006. High altitude convective influence in the latter experiment was dominated by western Pacific maritime systems. Convective influence in the boreal summer period is quite different in character. The paper will compare the TC4 period with other summer periods in order to place the TC4 results in context.