A model study on the influence of overshooting convection on TTL water vapour

Abstract

Abstract. Overshooting deep convection that penetrates into the Tropical Tropopause Layer (TTL) is thought to have an important role in regulating the water vapour content of this region. Yet, the net effect of such convection and the dominant mechanisms remain unclear. This study uses two idealised three-dimensional cloud-resolving model simulations to examine the influence of overshooting convection on water vapour when it penetrates into two different TTL environments, one supersaturated and the other subsaturated with respect to ice. These simulations show that the overshooting convection plays a direct role in driving the ambient environment towards ice saturation through either net moistening (subsaturated TTL) or net dehydration (supersaturated TTL). Moreover, in these cases the extent of dehydration in supersaturated conditions is greater than the moistening in subsaturated conditions. With the aid of modelled passive tracers, the relative roles of transport, mixing and ice microphysics are assessed; ultimately, ice sublimation and scavenging processes play the most important role in defining the different TTL relative humidity tendencies. In addition, significant moistening in both cases is modelled well into the subsaturated tropical lower stratosphere (up to 450 K), even though the overshooting turrets only reach approximately 420 K. It is shown that this moistening is the result of jumping cirrus, which is induced by the localised upward transport and mixing of TTL air following the collapse of the overshooting turret.Correspondence to: M. Hassim

An overview of the HIBISCUS campaign

“…In summary, the HIBISCUS and TROCCINOX water vapour and particles measurements were all suggesting moistening of the lower stratosphere by injection of ice crystals by overshooting towers over a convective land region, as observed over Northern Australia Knollenberg et al, 1993;Corti et al, 2008), and Africa (Khaykin et al, 2009), confirmed by the heavier isotopic composition of the water vapour in the stratosphere, characteristic of ice lofted from the troposphere, as observed by Hanisco et al (2007) in situ on the NASA WB-57 aircraft, or remotely by the MIPAS HDO measurements in the stratosphere (Steinwagner et al, 2010). The process, successfully simulated by Grosvenor et al (2007) and Chaboureau et al (2007), is now consistently reproduced by meso-scale non-hydrostatic models (Jensen et al, 2007;Chemel et al, 2009;Hassim et al, 2010).…”

“…The relative contributions of quasi-horizontal and vertical transport to the variation of ozone concentration in the subtropical UTLS around 10 • -20 • S were explored by Borchi et al (2005Borchi et al ( , 2007 faces as a proxy of convection. The first is provided by the MIMOSA contour advection model run every 5 K between 300 K and 700 K initialized by the 6-hourly ECMWF analyses (Hauchecorne et al, 2002), whereas the second is derived from the ECMWF vertical velocity. As shown in Fig.…”

“…Meteorological information included the European Center for Medium range Weather Forecasting (ECMWF) operational analyses (Rabier et al, 2000), the ECMWF ERA40 reanalysis (Simmons and Gibson, 2000), and the NCEP/NCAR re-analyses (Kistler et al, 2000); while atmospheric models included the Modélisation Isentrope du transport Méso-échelle de l'Ozone Stratosphérique par Advection (MIMOSA) highresolution contour advection model (Hauchecorne et al, 2002), the p-TOMCAT CTM (O'Connor et al, 2005), the Reactive Processes Ruling the Ozone Budget in the Stratosphere (REPROBUS) CTM (Lefèvre et al, 1994), a mesoscale Regional Atmospheric Modelling System (RAMS) (Pielke et al, 1992) and its Brazilian version (BRAMS) coupled with 3-D back trajectory calculations at mesoscale (Freitas et al, 2000), Garnier et al, 2004;Borchi et al, 2005). In addition, two 10 000 m 3 Zodiac light (ZL) flights were made in order to test several instruments (Table 1), in the meteorological conditions summarized in Table 2.…”

The impact of overshooting deep convection on local transport and mixing in the tropical upper troposphere/lower stratosphere (UTLS)

“…Chaboureau et al, 2007;Corti et al, 2008;Chemel et al, 2009;de Reus et al, 2009;Pommereau, 2010), mainly by direct injection of water vapour and ice that sublimates in the stratosphere. However, idealised simulations have shown the importance of prevailing TTL background conditions (Jensen et al, 2007;Hassim and Lane, 2010): by driving the humidity in the TTL towards ice saturation, overshooting convection may dehydrate or hydrate, depending on whether the TTL was supersaturated or subsaturated before. Hassim and Lane (2010) found net hydration in the stratosphere, which was subsaturated, regardless of the TTL humidity.…”

“…However, idealised simulations have shown the importance of prevailing TTL background conditions (Jensen et al, 2007;Hassim and Lane, 2010): by driving the humidity in the TTL towards ice saturation, overshooting convection may dehydrate or hydrate, depending on whether the TTL was supersaturated or subsaturated before. Hassim and Lane (2010) found net hydration in the stratosphere, which was subsaturated, regardless of the TTL humidity. At present it is unclear what the relative impacts of these localised storms are on the global scale, though observational campaigns during the early years of this millennium demonstrated a high frequency of overshooting events (Pommereau, 2010), which contrast with the generally assumed scarcity of these events.…”