Tracer measurements in the tropical tropopause layer during the AMMA/SCOUT-O3 aircraft campaign 1Institute for Atmospheric and Environmental Sciences, Goethe University Frankfurt, Germany
2Istituto Nazionale di Ottica Applicata, Florence, Italy
3Central Aerological Observatory, Dolgoprudny, Russia
4CNR Institute of Atmospheric Science and Climate, Bologna, Italy
*now at: Department of Physics, University of Wuppertal, Germany
Received: 29 September 2009 – Published in Atmos. Chem. Phys. Discuss.: 25 November 2009 Abstract. We present airborne in situ measurements made during the AMMA (African
Monsoon Multidisciplinary Analysis)/SCOUT-O3 campaign between 31 July and
17 August 2006 on board the M55 Geophysica aircraft, based in Ouagadougou,
Burkina Faso. CO2 and N2O were measured with the High
Altitude Gas Analyzer (HAGAR), CO was measured with the Cryogenically
Operated Laser Diode (COLD) instrument, and O3 with the Fast Ozone
Revised: 02 April 2010 – Accepted: 08 April 2010 – Published: 19 April 2010
We analyse the data obtained during five local flights to study the dominant
transport processes controlling the tropical tropopause layer (TTL, here
~350–375 K) and lower stratosphere above West-Africa: deep convection up
to the level of main convective outflow, overshooting of deep convection, and
horizontal inmixing across the subtropical tropopause. Besides, we examine
the morphology of the stratospheric subtropical barrier.
Except for the flight of 13 August, distinct minima in CO2 mixing
ratios indicate convective outflow of boundary layer air in the TTL. The
CO2 profiles show that the level of main convective outflow was
mostly located at potential temperatures between 350 and 360 K, and for
11 August reached up to 370 K.
While the CO2 minima indicate quite significant convective influence,
the O3 profiles suggest that the observed convective signatures were
mostly not fresh, but of older origin (several days or more). When compared
with the mean O3 profile measured during a previous campaign over
Darwin in November 2005, the O3 minimum at the main convective
outflow level was less pronounced over Ouagadougou. Furthermore O3
mixing ratios were much higher throughout the whole TTL and, unlike over
Darwin, rarely showed low values observed in the regional boundary layer.
Signatures of irreversible mixing following overshooting of convective air
were scarce in the tracer data. Some small signatures indicative of this
process were found in CO2 profiles between 390 and 410 K during the
flights of 4 and 8 August, and in CO data at 410 K on 7 August.
However, the absence of expected corresponding signatures in other tracer
data makes this evidence inconclusive, and overall there is little indication
from the observations that overshooting convection has a profound impact on
gas-phase tracer TTL composition during AMMA.
We find the amount of photochemically aged air isentropically mixed into the
TTL across the subtropical tropopause to be not significant. Using the
N2O observations we estimate the fraction of aged extratropical
stratospheric air in the TTL to be 0.0±0.1 up to 370 K during the local
flights. Above the TTL this fraction increases to 0.3±0.1 at 390 K.
The subtropical barrier, as indicated by the slope of the correlation
between N2O and O3 between 415 and 490 K, does not appear as
a sharp border between the tropics and extratropics, but rather as a gradual
transition region between 10° N and 25° N where isentropic
mixing between these two regions may occur.
Citation: Homan, C. D., Volk, C. M., Kuhn, A. C., Werner, A., Baehr, J., Viciani, S., Ulanovski, A., and Ravegnani, F.: Tracer measurements in the tropical tropopause layer during the AMMA/SCOUT-O3 aircraft campaign, Atmos. Chem. Phys., 10, 3615-3627, doi:10.5194/acp-10-3615-2010, 2010.