Humidity of the tropical lower stratosphere: Observations and analysis

The results of measurements of water-vapor vertical profiles in the upper troposphere and stratosphere on board a meteorological balloon with a FLASH-B optical fluorescent hygrometer (Russia) are presented. These data were obtained during two international field campaigns in West Africa (August 2006) and Central America (August 2007). Eleven high-resolution water-vapor vertical profiles measured in the course of these works make it possible to characterize the processes controlling humidity in the tropical tropopause region. Layers with increased humidity are detected in the lower stratosphere over West Africa to the level of the potential temperature 450 K. An analysis of satellite maps of the brightness temperature, balloon ozone measurements, and aerosol scattering, as well as trajectory modeling, display the relation between the observed layers with increased humidity and the phenomena of convective overshooting of the tropopause, as a result of which cold and ozone-depleted air penetrates into the lower stratosphere together with ice particles, which, rapidly sublimating, locally increase the water-vapor concentration. A comparison of the humidity data obtained in West Africa in 2006 and in Central America in 2007 reveals substantial distinctions in values and vertical structures of water vapor, both in the tropopause region and in the middle stratosphere.