JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, D18112, doi:10.1029/2005JD006639, 2006

Tropopause and hygropause variability over the equatorial Indian Ocean during February and March 1999

A. R. MacKenzie
Environmental Science Department, Lancaster University, Lancaster, UK
C. Schiller
Institute for Stratospheric Research, Forschungszentrum Jülich GmbH, Julich, Germany
T. Peter
Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology, Zurich, Switzerland
A. Adriani
Institute for Atmospheric Physics, Consiglio Nazionale delle Ricerche, Rome, Italy
J. Beuermann
Institute for Stratospheric Research, Forschungszentrum Jülich GmbH, Julich, Germany
O. Bujok
Institute for Stratospheric Research, Forschungszentrum Jülich GmbH, Julich, Germany
F. Cairo
Institute for Atmospheric Physics, Consiglio Nazionale delle Ricerche, Rome, Italy
T. Corti
Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology, Zurich, Switzerland
G. DiDonfrancesco
Ente per le Nuove Tecnologie, l'Energia, el'Ambiente, Casaccia, Rome, Italy
I. Gensch
Institute for Stratospheric Research, Forschungszentrum Jülich GmbH, Julich, Germany
C. Kiemle
Arbeitsgruppe Lidar, Institut für Physik der Atmosphäre, Deutches Zentrum für Luft- und Raumfahrt Oberpfaffenhofen, Wessling, Germany
M. Krämer
Institute for Stratospheric Research, Forschungszentrum Jülich GmbH, Julich, Germany
C. Kröger
Environmental Science Department, Lancaster University, Lancaster, UK
S. Merkulov
Central Aerological Observatory, Dolgoprudny, Russia
A. Oulanovsky
Central Aerological Observatory, Dolgoprudny, Russia
F. Ravegnani
Luigi Foschini Institute, Consiglio Nazionale delle Ricerche, Bologna, Italy
S. Rohs
Institute for Stratospheric Research, Forschungszentrum Jülich GmbH, Julich, Germany
V. Rudakov
Central Aerological Observatory, Dolgoprudny, Russia
P. Salter
Met Office, Exeter, UK
V. Santacesaria
Airborne Platform for Earth Observation, Comitato di Gestione, Florence, Italy
L. Stefanutti
Airborne Platform for Earth Observation, Comitato di Gestione, Florence, Italy
V. Yushkov
Central Aerological Observatory, Dolgoprudny, Russia

Abstract
Measurements of temperature, water vapor, total water, ozone, and cloud properties were made above the western equatorial Indian Ocean in February and March 1999. The cold-point tropopause was at a mean pressure-altitude of 17 km, equivalent to a potential temperature of 380 K, and had a mean temperature of 190 K. Total water mixing ratios at the hygropause varied between 1.4 and 4.1 ppmv. The mean saturation water vapor mixing ratio at the cold point was 3.0 ppmv. This does not accurately represent the mean of the measured total water mixing ratios because the air was unsaturated at the cold point for about 40% of the measurements. As well as unsaturation at the cold point, saturation was observed above the cold point on almost 30% of the profiles. In such profiles the air was saturated with respect to water ice but was free of clouds (i.e., backscatter ratio <2) at potential temperatures more than 5 K above the tropopause and hygropause. Individual profiles show a great deal of variability in the potential temperatures of the cold point and hygropause. We attribute this to short timescale and space-scale perturbations superimposed on the seasonal cycle. There is neither a clear and consistent “setting” of the tropopause and hygropause to the same altitude by dehydration processes nor a clear and consistent separation of tropopause and hygropause by the Brewer-Dobson circulation. Similarly, neither the tropopause nor the hygropause provides a location where conditions consistently approach those implied by a simple “tropopause freeze drying” or “stratospheric fountain” hypothesis.

Received 7 September 2005; accepted 23 May 2006; published 30 September 2006.