Balloon-borne Measurements of the Vertical Propagation of Thermal IR Radiance in a Tropical Atmosphere

Overview. Radiometric measurements of the upwelling 11 micron flux from the ocean surface through a tropical atmospheric boundary layer have been made from an aerostat platform to test a radiative transfer model prediction of the outgoing sea surface radiance. The radiance measurements were made for the spectral bandpass 900 cm-1 to 980 cm-1 using an infrared radiometer having an instrument precision of 0.002°C and an absolute accuracy of 0.1°C. The radiometer was mounted beneath an aerostat tethered to the USCG ship Windward Sentry, and flown over the ocean near Cape Canaveral, Florida in late October, 1989. Other atmospheric measurements collected simultaneously near the location of the radiometer included dry and wet bulb temperatures and total pressure. Sea surface and air surface temperatures were measured from the ship's stern. Two profiles of radiance are examined in this presentation. The first profile was obtained in tropical atmospheric conditions; the sampling technique involved a slow (45 minutes) vertical ascension of the aerostat to a maximum tether altitude of about 2300 feet. Radiometric measurements for the second profile were made a few days later in atmospheric conditions almost 10 degrees cooler than the first day. Standard line-by-line radiative transfer calculations of the outgoing flux were carried out using the actual values of air temperature, water vapor content and atmospheric pressure measured for each day. The observed and calculated profiles, which will be presented during the poster session, show very close agreement (to within a few percent). The results show that there is no significant error in present empirical expressions for the water vapor continuum absorption (for the 900-980 cm-1 spectral range) which are based on the laboratory measurements given by Burch* (1970) and Burch and Alt** (1984). This conclusion is shown to depend on the selection of appropriate weighting functions to characterize the mean water vapor partial pressure and temperature over the observing path.