Impact of tropical land convection on the water vapour budget in the tropical tropopause layer
Abstract. The tropical deep overshooting convection is known to be most intense above continental areas such as South America, Africa, and the maritime continent. However, its impact on the tropical tropopause layer (TTL) at global scale remains debated. In our analysis, we use the 8-year Microwave Limb Sounder (MLS) water vapour (H2O), cloud ice-water content (IWC), and temperature data sets from 2005 to date, to highlight the interplays between these parameters and their role in the water vapour variability in the TTL, and separately in the northern and southern tropics. In the tropical upper troposphere (177 hPa), continents, including the maritime continent, present the night-time (01:30 local time, LT) peak in the water vapour mixing ratio characteristic of the H2O diurnal cycle above tropical land. The western Pacific region, governed by the tropical oceanic diurnal cycle, has a daytime maximum (13:30 LT). In the TTL (100 hPa) and tropical lower stratosphere (56 hPa), South America and Africa differ from the maritime continent and western Pacific displaying a daytime maximum of H2O. In addition, the relative amplitude between day and night is found to be systematically higher by 5–10% in the southern tropical upper troposphere and 1–3% in the TTL than in the northern tropics during their respective summer, indicative of a larger impact of the convection on H2O in the southern tropics. Using a regional-scale approach, we investigate how mechanisms linked to the H2O variability differ in function of the geography. In summary, the MLS water vapour and cloud ice-water observations demonstrate a clear contribution to the TTL moistening by ice crystals overshooting over tropical land regions. The process is found to be much more effective in the southern tropics. Deep convection is responsible for the diurnal temperature variability in the same geographical areas in the lowermost stratosphere, which in turn drives the variability of H2O.
