Atmospheric Chemistry and Physics Discussions
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10.5194/acpd-7-6655-2007
http://www.atmos-chem-phys-discuss.net/7/6655/2007/
http://www.atmos-chem-phys-discuss.net/7/6655/2007/acpd-7-6655-2007.html
http://www.atmos-chem-phys-discuss.net/7/6655/2007/acpd-7-6655-2007.pdf
6655
6685
2007-05-16
The CO<sub>2</sub> tracer clock for the Tropical Tropopause Layer
S. Park
R. Jiménez
B. C. Daube
L. Pfister
T. J. Conway
E. W. Gottlieb
V. Y. Chow
D. J. Curran
D. M. Matross
A. Bright
E. L. Atlas
T. P. Bui
R.-S. Gao
C. H. Twohy
S. C. Wofsy
swofsy@deas.harvard.edu
Department of Earth and Planetary Sciences and the Division of Engineering and Applies Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
NASA, Ames Research Center, Moffett Field, California 94035, USA
NOAA, Earth System Research Laboratory, Boulder, Colorado 80305, USA
University of Miami, Rosenstiel School of Marine and Atmospheric Science, Miami, Florida 33149, USA
NOAA Aeronomy Laboratory, Boulder, Colorado 80303, USA
Oregon State University, College of Oceanic and Atmospheric Science, Corvallis, Oregon 97331, USA
now at: Department of Environmental Science Policy and Management, University of California, Berkeley, California 94720, USA
Observations of CO<sub>2</sub> were made in the upper
troposphere and lower stratosphere in the deep tropics in order to determine
the patterns of large-scale vertical transport and age of air in the
Tropical Tropopause Layer (TTL). Flights aboard the NASA WB-57F aircraft
over Central America and adjacent ocean areas took place in January and
February, 2004 (Pre-AURA Validation Experiment, Pre-AVE) and 2006 (Costa
Rice AVE, CR-AVE), and for the same flight dates of 2006, aboard the Proteus
aircraft from the surface to 15 km over Darwin, Australia (Tropical Warm
Pool International Cloud Experiment , TWP-ICE). The data demonstrate that
the TTL is composed of two layers with distinctive features: (1) the lower
TTL, 350–360 K (potential temperature (θ); approximately
12–14 km), is subject to inputs of convective outflows, as indicated by
layers of variable CO<sub>2</sub> concentrations, with air parcels of zero age
distributed throughout the layer; (2) the upper TTL, from θ=
~360 K to ~390 K (14–18 km), ascends slowly and ages uniformly,
as shown by a linear decline in CO<sub>2</sub> mixing ratio tightly correlated
with altitude, associated with increasing age. This division is confirmed by
ensemble trajectory analysis. The CO<sub>2</sub> concentration at the level of
360 K was 380.0(±0.2) ppmv, indistinguishable from surface site values
in the Intertropical Convergence Zone (ITCZ) for the flight dates. Values
declined with altitude to 379.2(±0.2) ppmv at 390 K, implying that air
in the upper TTL monotonically ages while ascending. In combination with the
winter slope of the CO<sub>2</sub> seasonal cycle (+10.8±0.4 ppmv/yr), the
vertical gradient of 0.78 (±0.09) ppmv gives a mean age of
26(±3) days for the air at 390 K and a mean ascent rate of 1.5(±0.3) mm s<sup>−1</sup>. The TTL near 360 K in the Southern Hemisphere over
Australia is very close in CO<sub>2</sub> composition to the TTL in the Northern
Hemisphere over Costa Rica, with strong contrasts emerging at lower
altitudes (<360 K). Both Pre-AVE and CR-AVE CO<sub>2</sub> observed unexpected
input from deep convection over Amazônia deep into the TTL. The CO<sub>2</sub>
data confirm the operation of a highly accurate tracer clock in the TTL that
provides a direct measure of the ascent rate of the TTL and of the age of
air entering the stratosphere.
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