Evidence of horizontal and vertical transport of water in the Southern Hemisphere tropical tropopause layer (TTL) from high-resolution balloon observations
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-...
| Main Authors: | , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-09-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/16/12273/2016/acp-16-12273-2016.pdf |
| Summary: | 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 Single-Particle
Lagrangian Integrated Trajectory) model.<br><br>
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.<br><br>
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.<br><br>
The 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. |
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| ISSN: | 1680-7316 1680-7324 |