A TRAJECTORY ANALYSIS OF TROPICAL UPPER-TROPOSPHERIC MOISTURE AND CONVECTION

It is shown that the distribution of upper-tropospheric humidity (UTH) in the cloud-free Tropics can be simulated with a simple model in whi ch air expelled from moist convective regions is dried by subsidence a long its trajectory. The distribution of UTH is analyzed in the tropic al eastern Pacific using moisture data retrieved from GOES 6.7-mu m ob servations 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 e astern subtropical Pacific, convective sources lie entirely outside th e 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. Composit es of many trajectories from specific source regions show that radiati vely driven subsidence appears to control the decrease in relative hum idity away from convection. The observed UTH distribution along trajec tories is then simulated with a simple model of horizontal advection a nd 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 regi on.