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 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 ECMWF 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.