Tracking upper tropospheric water vapor radiances: A satellite perspective

Hourly observations of 6.7 mu m "water vapor" radiances from geostationary satellites are used in conjunction with an objective pattern-tracking algorithm to trace upper tropospheric water vapor features from sequential images. Analysis of measurements covering the 3 month period of June-August 1987 illustrates a close relationship between the upper tropospheric moisture and the tropical circulation. Over humid tropical regions the movement of water vapor patterns reveals a diverging upper,level flow away from centers of deep convection. Likewise, arid subtropical regions are characterized by converging upper level water vapor patterns indicating sinking air. The evolution of upper tropospheric moisture is also explored from a Lagrangian perspective by considering the change in moisture of a pattern as it is tracked from one image to the next. This analysis reveals that the clear-sky upper troposphere in both tropical and subtropical regions becomes increasingly drier with time reflecting the impact of large-scale subsidence in drying the troposphere; When separated according to cloud amount, water vapor patterns associated with clouds are observed to dry substantially slower than those without clouds, presumably due to the evaporation of cloud condensate. Trajectories of upper tropospheric moisture are also constructed by iteratively tracking water vapor patterns from successive satellite images. The trajectories reveal two distinct paths for water vapor patterns entering the dry subsidence region of the subtropical South Pacific. One path highlights the southward propagation of convective outflow from the tropics while the other path reflects the injection of moisture from eastward propagating subtropical disturbances. The existence of two distinct source regions suggests that a complete picture of the processes regulating the driest and most radiatively transparent regions of the subtropics will require an understanding of both types of convective systems.