Tropical Atlantic convection as revealed by ozone and relative humidity measurements

摘要

Properties of deep convection over the tropical central Atlantic are analyzed in light of ozone as a quasi-conservative quantity on the convective timescale. Multiple years of measurements of ozone from aircraft, shipboard and balloon platforms reveal that the mean ozone mixing ratio near 250 hPa, close in time and distance to the convective outflow at that pressure, is 7 ppb higher than at sea surface and marine boundary layer (MBL). The process that increases the ozone mixing ratio in the convective outflow is shown to be lateral entrainment of higher value ozone mixing ratio originating from the subsiding branch of the Hadley Cell. Ozone and humidity soundings obtained from cruise campaigns over the same region are used to identify the preferred or most pronounced levels of entrainment, which appear to be near 700 hPa and from 500 to 400 hPa, as indicated by layers of simultaneous drying and enhanced ozone mixing ratio in otherwise smooth profiles. There are also indications of convective detrainment at around 600 hPa and 300 hPa, which may correspond to shallow and deep convection, respectively. A simple model is used to estimate the ratio of the bulk entrainment mass flux (Φ2) between 900 and 400 hPa to the convective mass flux from the MBL below (Φ1). The ratio is calculated, on the basis of climatological ozone measurements, to be Φ2/Φ1 = 0.50. Thus the bulk outflow is 50% larger than the lateral mass flux in the MBL. The relative humidity over ice (RHi) of air at the convective outflow is centered at RHi = 110%, with a considerable range from a low near 40% to a high near 150%.