Impact of tropical land convection on the water vapour budget in the tropical tropopause layer

F Carminati,P Ricaud,E Rivière,S Khaykin,J Warner,J.-L Attié,J.-P Pommereau

doi:10.5194/acp-14-6195-2014

Abstract

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.

Highlights

The tropical tropopause layer (TTL), the transition layer sharing upper tropospheric (UT) and lower stratospheric (LS) characteristics, is the gateway for troposphere to stratosphere transport (TST), and plays a key role in the global composition and circulation of the stratosphere (Holton et al, 1995; Fueglistaler et al, 2009)
TST processes responsible for the upward motion of air masses impacting on the water budget are (1) the slow ascent (300 m month−1) due to radiative heating associated with horizontal advection, known as “cold trap” (Holton and Gettelman, 2001; Gettelman et al, 2002; Fueglistaler et al, 2004), (2) fast overshooting updraughts followed by detrainment referred to as “freeze and dry” process (Brewer, 1949; Sherwood and Dessler, 2000, 2001, 2003; Dessler, 2002), and (3) the fast and direct injection by “geyser-like” overshoots (Knollenberg et al, 1993; Corti et al, 2008; Khaykin et al, 2009) that can penetrate into the LS
We focus on restricted areas of the northern tropical and the southern tropical South America, Africa, the maritime continent, and western Pacific

Summary

Introduction

The tropical tropopause layer (TTL), the transition layer sharing upper tropospheric (UT) and lower stratospheric (LS) characteristics, is the gateway for troposphere to stratosphere transport (TST), and plays a key role in the global composition and circulation of the stratosphere (Holton et al, 1995; Fueglistaler et al, 2009). F. Carminati et al.: Impact of tropical land convection on the water vapour budget in the TTL pointed out that most vigorous convections occur over continental tropical areas where overshooting precipitation features (OPFs) are more frequent (Alcala and Dessler, 2002; Liu and Zipser, 2005). A significant contribution of continental convection to the chemical composition of the LS has been reported by Ricaud et al (2007, 2009) from Odin-SMR (sub-millimetre radiometer) satellite observations. They showed a higher mixing ratio of tropospheric trace gases (N2O and CH4) in the TTL during the southern summer. The results of Ricaud et al (2007, 2009) are consistent with the cleansing of the aerosols in the LS seen by Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) during the same season (Vernier et al, 2011)

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