STRATOSPHERIC WATER-VAPOR MEASUREMENT IN THE TROPICAL ZONE BY MEANS OF A FROST POINT HYGROMETER ON BOARD LONG-DURATION BALLOONS

Two long-duration balloon flights were performed during austral summer in the southern hemisphere tropical zone, between 23-degrees-S and 10-degrees-S, for stratospheric water vapor measurement by means of a frost point hygrometer. The observations show a mixing ratio minimum of about 3.5 ppmv near 50 hPa that corroborates the existence of the hygropause in low-latitude areas. Also observed was the approach of saturation conditions near a particularly cold tropopause in the highly convective area around New Hebrides, where cloud top temperatures as low as -84-degrees-C were seen with GMS satellite data.