Impact of land convection on temperature diurnal variation in the tropical lower stratosphere inferred from COSMIC GPS radio occultations

Abstract

Following recent studies evidencing the influence of deep convection on the chemical composition and thermal structure of the tropical lower stratosphere, we explore its impact on the temperature diurnal variation in the upper troposphere and lower stratosphere using the high-resolution COSMIC GPS radio-occultation temperature measurements spanning from 2006 through 2011. The temperature in the lowermost stratosphere over land during summer displays a marked diurnal cycle characterized by an afternoon cooling. This diurnal cycle is shown collocated with most intense land convective areas observed by the Tropical Rainfall Measurement Mission (TRMM) precipitation radar and in phase with the maximum overshooting occurrence frequency in late afternoon. Two processes potentially responsible for that are identified: (i) non-migrating tides, whose physical nature is internal gravity waves, and (ii) local cross-tropopause mass transport of adiabatically cooled air by overshooting turrets. Although both processes can contribute, only the lofting of adiabatically cooled air is well captured by models, making it difficult to characterize the contribution of non-migrating tides. The impact of deep convection on the temperature diurnal cycle is found larger in the southern tropics, suggesting more vigorous convection over clean rain forest continents than desert areas and polluted continents in the northern tropics

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

“…To explain this feature, Liu and Zipser (2009) suggested two hypotheses: (1) in situ ice formation when deep convection generates gravity waves that lift and cool the tropopause (Potter and Holton, 1995;Sherwood and Dessler, 2001) leading to the dehydration of the TTL in the late afternoon, a process known as "freeze and dry"; and (2) "ice geysers" that can directly inject ice crystals formed in the adiabatically cooled core of the overshoot turrets potentially hydrating the TTL after being sublimated (Corti et al, 2008;Khaykin et al, 2009). Both hypotheses result in a cooling of the TTL, which is consistent with the results of Khaykin et al (2013). Following Liu and Zipser (2009), we investigate the diurnal H 2 O variability in the tropics using an extended and improved MLS data set described in the following section.…”

“…The positive D-N, however, is consistent with the diurnal temperature cycle as presented by Khaykin et al (2013), and attributed to non-migrating tides and convective updraught of adiabatically cooled air, of maximum amplitude in the LS. H 2 O potentially turns into ice with the afternoon temperature drop, and then sublimates the next morning when the temperature rises.…”

“…The larger amplitude of the cooling over Amazonia and central Africa would imply a much more intense convection over clean rainforest areas than the aerosol-rich northern continental troposphere. As proposed by Khaykin et al (2013), the larger aerosol concentration in the northern tropics might reduce the convective available potential energy (CAPE). This idea was first suggested by Rosenfeld et al (2008), who developed a conceptual model to address the question of the relationship between aerosols, cloud microphysics, and radiative properties.…”

“…At this level, the temperature is lower in DJF over the equatorial South America and Africa, the maritime continent, and western Pacific than in JJA, where less cooling is observed above the most intense convective areas. Khaykin et al (2013) estimated the diurnal temperature cycle from the COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) satellite's GPS radio occultation measurements. At the MLS sampling time, the temperature measured by the COSMIC had not reached its maximal amplitude but did show its premises, with a ∼ 0.2 K cooling (warming) at 13:30 LT (01:30 LT), in agreement both in sign and magnitude with the temperature measured by MLS.…”

Evidence of horizontal and vertical transport of water in the Southern Hemisphere tropical tropopause layer (TTL) from high-resolution balloon observations

“…In contrast to that, Randel et al (2015) found that enhanced tropospheric convection within Asian and North American monsoons leads to reduced stratospheric water vapour. While upscaling the overshooting events and quantifying their net effect on water vapour remains a difficult task, various space-borne observations suggest that the most vigorous convection, capable of direct transport of material into the lower stratosphere, is mostly restricted to the tropical land regions (Liu and Zipser, 2005;Ricaud et al, 2009;Iwasaki et al, 2010;Bergman et al, 2012;Khaykin et al, 2013a;Carminati et al, 2014).…”