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

Abstract

Abstract. High-resolution in situ balloon measurements of water vapour, aerosol, methane and temperature in the upper tropical tropopause layer (TTL) and lower stratosphere are used to evaluate the processes affecting the stratospheric water budget: horizontal transport (in-mixing) and hydration by cross-tropopause overshooting updrafts. The obtained in situ evidence of these phenomena are analysed using satellite observations by Aura MLS (Microwave Limb Sounder) and CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) together with trajectory and transport modelling performed using CLaMS (Chemical Lagrangian Model of the Stratosphere) and HYSPLIT (Hybrid SingleParticle Lagrangian Integrated Trajectory) model.Balloon soundings were conducted during March 2012 in Bauru, Brazil (22.3 • S) in the frame of the TRO-Pico campaign for studying the impact of convective overshooting on the stratospheric water budget. The balloon payloads included two stratospheric hygrometers: FLASH-B (Fluorescence Lyman-Alpha Stratospheric Hygrometer for Balloon) and Pico-SDLA instrument as well as COBALD (Compact Optical Backscatter Aerosol Detector) sondes, complemented by Vaisala RS92 radiosondes. Water vapour vertical profiles obtained independently by the two stratospheric hygrometers are in excellent agreement, ensuring credibility of the vertical structures observed.A signature of in-mixing is inferred from a series of vertical profiles, showing coincident enhancements in water vapour (of up to 0.5 ppmv) and aerosol at the 425 K (18.5 km) level. Trajectory analysis unambiguously links these features to intrusions from the Southern Hemisphere extratropical stratosphere, containing more water and aerosol, as demonstrated by MLS and CALIPSO global observations. The in-mixing is successfully reproduced by CLaMS simulations, showing a relatively moist filament extending to 20 • S. A signature of local cross-tropopause transport of water is observed in a particular sounding, performed on a convective day and revealing water vapour enhancements of up to 0.6 ppmv as high as the 404 K (17.8 km) level. These are shown to originate from convective overshoots upwind detected by an S-band weather radar operating locally in Bauru.Published by Copernicus Publications on behalf of the European Geosciences Union. S. M. Khaykin et al.: Evidence of horizontal and vertical transport of waterThe accurate in situ observations uncover two independent moisture pathways into the tropical lower stratosphere, which are hardly detectable by space-borne sounders. We argue that the moistening by horizontal transport is limited by the weak meridional gradients of water, whereas the fast convective cross-tropopause transport, largely missed by global models, can have a substantial effect, at least at a regional scale.

Accuracy of Balloon Trajectory Forecasts in the Lower Stratosphere

“…The largest errors are however localized in space and time, as they occured in episodes, over regions where wind observations are exremely sparse (Indian Ocean, eastern Pacific Ocean). This is not incompatible with previous case studies which convincingly showed, using complementary observations of water vapor in particular, trajectory calculations which provided insight on injection and transport of water vapor in the UTLS ( [14,15], and B. Legras and S. Bucci, personal communication). Our results nonetheless emphasize the need for continued efforts to better understand processes and model error in the UTLS near the Equator, and to pay particular attention to uncertainties in trajectory calculations in process studies.…”

Convective hydration in the tropical tropopause layer during the StratoClim aircraft campaign: pathway of an observed hydration patch

“…The target convective overshoots and the moistened TTL were simulated using the non-hydrostatic numerical research model, Meso-NH (Lac et al, 2018). For a fine-scale analysis, the simulation uses about 400 million grid points with horizontal grid spacing of 2.5 km.…”

Identifying Source Regions and the Distribution of Cross‐Tropopause Convective Outflow Over North America During the Warm Season

“…Cross-tropopause convection provides a means of rapidly transporting boundary layer air into the lower stratosphere, thus bypassing the dominant global-scale troposphere-to-stratosphere transport mechanism associated with the Brewer-Dobson circulation in which air ascends slowly across the tropical tropopause and then moves poleward and descends. Observational and modeling studies indicate that convective overshooting moistens the lower stratosphere through the lofting of ice crystals into the stratosphere, which then subsequently sublimate (Corti et al, 2008;de Reus et al, 2009;Hanisco et al, 2007;Hassim & Lane, 2010;Hegglin et al, 2004;Homeyer, 2014;Homeyer et al, 2017;Iwasaki et al, 2010;Iwasaki et al, 2012;Khaykin et al, 2009;Khaykin et al, 2016;Phoenix et al, 2017;Poulida et al, 1996;Randel et al, 2012;Ray et al, 2004;Sang et al, 2018;Sargent et al, 2014;Sayres et al, 2010;Smith et al, 2017;Wang et al, 2011;Weinstock et al, 2007). Analysis of water and its heavy isotopologue, HDO, indicates that up to 45% of water vapor in the NAMA may be attributed to convective transport of water-predominantly as ice-into the UTLS, though the convective source may not be entirely local (Hanisco et al, 2007;Randel et al, 2012).…”