Balloon-borne observations of water vapor and ozone in the tropical upper
troposphere and lower stratosphere
H. Vömel
Cooperative Institute for Research in Environmental Sciences, University
of Colorado at Boulder
S. J. Oltmans, B. J. Johnson
Climate Monitoring and Diagnostics Laboratory, National Oceanic and Atmospheric
Administration, Boulder, Colorado
F. Hasebe
Ibaraki University, Ibaraki, Japan
M. Shiotani, M. Fujiwara
Hokkaido University, Sapporo, Japan
N. Nishi
Kyoto University, Kyoto, Japan
M. Agama, J. Cornejo, F. Paredes, H. Enriquez
Instituto Nacionál de Meteorología y Hidrología, Ecuador
Abstract. Balloon borne observations of frost-point temperature and ozone
in the equatorial western, central and eastern Pacific as well as over
equatorial eastern Brazil provide a highly accurate data set of water vapor
across the tropical tropopause. Data were obtained at San Cristóbal,
Galapagos, Ecuador (0.9°S, 89.6°W), during the late northern winter
and the late northern summer in 1998 and 1999 and at Juazeiro do Norte,
Brazil (7.2°S, 39.3°W), in February and November 1997. Earlier
data in the western Pacific region in March 1993 were reanalyzed to extend
the scope of the observations.
The data show three different circumstances in which saturation or
supersaturation occurs and imply different mechanisms for dehydration at
the tropical tropopause: 1) convective dehydration, 2) slow-ascent dehydration,
and 3) large-scale wave-driven dehydration. Furthermore, air which crosses
the tropical tropopause in the late northern summer may be dehydrated further
during late northern fall, as the average tropical tropopause rises and
cools. Not all soundings show dehydration and there are clear differences
in the frequency and depth of saturation in different regions and seasons.
The tropopause transition region can be identified in accurate measurements
of relative humidity, even under conditions where ozone observations are
ambiguous. Deep convection plays an important role in setting up this transition
region, which is then subject to large-scale wave activity and wave breaking
at the tropopause or mid-latitude intrusions. High relative humidities
over regions of strong subsidence show that descending motion in the troposphere
is limited to levels below the transition region.