A Trajectory Analysis of Tropical Upper-Tropospheric Moisture and Convection

Eric P. Salathé Jr. Department of Atmospheric Sciences, University of Washington, Seattle, Washington

Search for other papers by Eric P. Salathé Jr. in
Current site
Google Scholar
PubMed
Close
and
Dennis L. Hartmann Department of Atmospheric Sciences, University of Washington, Seattle, Washington

Search for other papers by Dennis L. Hartmann in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

It is shown that the distribution of upper-tropospheric humidity (UTH) in the cloud-free Tropics can be simulated with a simple model in which air expelled from moist convective regions is dried by subsidence along its trajectory. The distribution of UTH is analyzed in the tropical eastern Pacific using moisture data retrieved from GOES 6.7-μm observations during September 1992. The analysis examines the variation in moisture along horizontal trajectories derived from European Centre for Medium-Range Weather Forecasts wind analyses. Trajectory analysis is used to trace the convective sources of subtropical air. For the eastern subtropical Pacific, convective sources lie entirely outside the dry region, and are predominately in the ITCZ and over South America, with some air tracing to midlatitudes. The analysis also shows that, over large parts of the eastern subtropical Pacific, air has advected horizontally for five or more days since exiting convection. Composites of many trajectories from specific source regions show that radiatively driven subsidence appears to control the decrease in relative humidity away from convection. The observed UTH distribution along trajectories is then simulated with a simple model of horizontal advection and subsidence of an initial convective moisture profile. Finally, the monthly mean horizontal distribution of water vapor is simulated using this model of moisture transport and the computed distribution of the mean time since air at any location was in a convectively active region.

Corresponding author address: Dr. Eric P. Salathé, Department of Atmospheric Sciences, University of Washington, P.O. Box 351640, Seattle, WA 98195-1640.

Email: salathe@atmos.washington.edu

Abstract

It is shown that the distribution of upper-tropospheric humidity (UTH) in the cloud-free Tropics can be simulated with a simple model in which air expelled from moist convective regions is dried by subsidence along its trajectory. The distribution of UTH is analyzed in the tropical eastern Pacific using moisture data retrieved from GOES 6.7-μm observations during September 1992. The analysis examines the variation in moisture along horizontal trajectories derived from European Centre for Medium-Range Weather Forecasts wind analyses. Trajectory analysis is used to trace the convective sources of subtropical air. For the eastern subtropical Pacific, convective sources lie entirely outside the dry region, and are predominately in the ITCZ and over South America, with some air tracing to midlatitudes. The analysis also shows that, over large parts of the eastern subtropical Pacific, air has advected horizontally for five or more days since exiting convection. Composites of many trajectories from specific source regions show that radiatively driven subsidence appears to control the decrease in relative humidity away from convection. The observed UTH distribution along trajectories is then simulated with a simple model of horizontal advection and subsidence of an initial convective moisture profile. Finally, the monthly mean horizontal distribution of water vapor is simulated using this model of moisture transport and the computed distribution of the mean time since air at any location was in a convectively active region.

Corresponding author address: Dr. Eric P. Salathé, Department of Atmospheric Sciences, University of Washington, P.O. Box 351640, Seattle, WA 98195-1640.

Email: salathe@atmos.washington.edu

Save
  • Betts, A. K., 1990: Greenhouse warming and the tropical water budget. Bull. Amer. Meteor. Soc.,71, 1464–1465.

  • ——, and B. A. Albrect, 1987: Conserved variable analysis of the convective boundary layer thermodynamic structure of the tropical oceans. J. Atmos. Sci.,44, 83–99.

  • Chen, S. S., R. A. Houze, and B. E. Mapes, 1996: Multiscale variability of deep convection in relation to large-scale circulation in TOGA COARE. J. Atmos. Sci.,53, 1380–1409.

  • Hack, J. J., B. A. Boville, B. P. Briegleb, J. T. Kiehl, P. J. Rasch, and D. L. Williamson, 1993: Description of the NCAR Community Climate Model (CCM2). NCAR Tech. Note NCAR/TN-382+STR, 108 pp. [Available from National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307.].

  • Hartmann, D. L., and M. L. Michelsen, 1993: Large-scale effects on the regulation of tropical sea surface temperature. J. Climate,6, 2049–2062.

  • Houze, R. A., 1989a: Observed structure of mesoscale convective systems and implications for large-scale heating. Quart. J. Roy. Meteor. Soc.,115, 425–461.

  • ——, 1989b: Cloud Dynamics. Academic Press, 570 pp.

  • Lau, K. M., C. H. Sui, and W. K. Tao, 1993: A preliminary study of the tropical water cycle and its sensitivity to surface warming. Bull. Amer. Meteor. Soc.,74, 1313–1321.

  • Lin, X., and R. H. Johnson, 1996: Kinematic and thermodynamic characteristics of the flow over the western Pacific warm pool during TOGA COARE. J. Atmos Sci.,53, 695–715.

  • Picon, L., and M. Desbois, 1990: Relation between METEOSAT water vapor radiance fields and large-scale tropical circulation features. J. Climate,3, 865–876.

  • Pierrehumbert, R. T., 1995: Thermostats, radiator fins, and the local runaway greenhouse. J. Atmos. Sci.,52, 1784–1806.

  • Salathé, E. P., and D. Chesters, 1995 Variability of moisture in the upper troposphere as inferred from TOVS satellite observations and the ECMWF model analyses in 1989. J. Climate,8, 120–132.

  • Schmetz, J., and O. M. Turpeinen, 1988: Estimation of the upper tropospheric relative humidity field from METEOSAT water vapor image data. J. Appl. Meteor.,27, 889–899.

  • Sherwood, S. C., 1996: Maintenance of the free-tropospheric tropical water vapor distribution. Part I: Clear regime budget. J. Climate,9, 2903–2918.

  • Soden, B. J., and R. Fu, 1995: A satellite analysis of deep convection, upper-tropospheric humidity, and the greenhouse effect. J. Climate,8, 2333–2351.

  • ——, and J. R. Lazante, 1996: An assessment of satellite and radiosonde climatologies of upper-tropospheric water vapor. J. Climate,9, 1235–1250.

  • Sun, D.-Z., and R. S. Lindzen, 1993: Distribution of tropical tropospheric water vapor. J. Atmos. Sci.,50, 1643–1660.

  • ——, and A. H. Oort, 1995: Humidity–temperature relationships in the tropical troposphere. J. Climate,8, 1974–1987.

  • Trenberth, K. E., and J. G. Olson, 1988: An evaluation and intercomparison of global analyses from the National Meteorological Center and the European Centre for Medium Range Weather Forecasts. Bull. Amer. Meteor. Soc.,69, 1047–1057.

  • Udelhofen, P. M., and D. L. Hartmann, 1995: Influence of tropical cloud systems on the relative humidity in the upper troposphere. J. Geophys. Res.,100, 7423–7440.

  • Waliser, D. E., and C. Gautier, 1993: A satellite-derived climatology of the ITCZ. J. Climate,6, 2162–2174.

All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 424 107 10
PDF Downloads 121 35 6