Measurements of upper tropospheric and lower stratospheric water substance content using ARES instrumentation

Water plays a central role in the greenhouse effect in the Earth´s atmosphere. The quantity of water substance (water vapor plus condensed water) in the upper troposphere and lower stratosphere (altitudes of approximately 6-18 km), is especially important to the modeling of the global climate. Of particular interest is the water substance in the vicinity of the tropopause. Empirical data concerning the mean water substance content and its spatial and temporal variability through this range of altitudes are extremely sparse; in fact, the available data are inadequate for meaningful comparison with the predictions of global climate models. The ARES program, which employs a WB-57F aircraft that can fly at a wide range of altitudes as high as 19 km, provides a powerful new resource for measuring the atmospheric water substance content through the critical range of interest. The primary instrument on board the ARES aircraft is the imaging spectrometer, which collects data in the 2-6.3 μm wavelength region. The volume density of the atmospheric water substance is determined as a function of altitude by measuring the absorption of solar or lunar irradiance at several altitude increments from 6 km to 18 km. The use of the WB-57F aircraft for these data collections allows for the measurement of geographic and seasonal variations in the atmospheric water content. Upcoming flights will examine these variations and will include an instrument upgrade package that includes an ozone meter to determine the altitude of the tropopause, as well as Lyman-α and chilled mirror hygrometers for independent verification of the water vapor densities measured by the ARES spectrophotometric data. This paper presents the results of a preliminary set of measurements obtained in May and December 1994, and describes plans for upcoming flights