Adaptation of ground-based radiometers for outgoing radiative flux measurements from Ultra Long Duration Balloons

Top of Atmosphere (TOA) radiative fluxes are critical to our understanding of the physics of the present climate and its variability. Hence, various NASA programs have measured the shortwave (0.2–4 μm) fluxes reflected by the Earth-atmosphere system, and the longwave(4–100 μm) fluxes emitted for almost 40 years. Outgoing flux can be measured directly from satellites using wide field of view radiometers with a footprint of a few thousand kilometers. In order to provide fluxes more commensurate with the increasingly fine spatial resolution of climate models, the Cloud and Earth's Radiant Energy System (CERES) instruments on Terra and Aqua are based entirely on narrow field of view scanners which measure directional radiances. Therefore, we have to rely on empirical angular directional models to convert radiance to flux; the conversion is the principle source of uncertainty in instantaneous fluxes estimated from Earth radiation budget satellites. Unlike other retrieved variables, in situ validation of outgoing fluxes has been nearly impossible for lack of platforms that operate for long periods at TOA. The NASA Ultra Long Duration Balloon (ULDB) program opens a new avenue for direct measurement of the Earth radiation budget near TOA. A radiative transfer model shows that the difference in the outgoing flux between ULDB flight altitude and TOA are less than 1 W/m2 . Therefore, it provides a unique platform to validate the satellite retrieved TOA radiative fluxes. Since the ambient environment at ULDB flight altitude is very different from the ground, the performances of the ground-based radiometers in the ULDB flight environment must be assessed. A series of thermal- vacuum chamber tests have been done for Epply Precision Infrared Radiometer, Kipp and Zonen CG4, CM21, and CM22. The performance of these instruments under low-pressure and low-temperature conditions will be presented. Uncertainties of using these standard commercial instrumentation to measure outgoing fluxes at the ULDB flight altitude will also be addressed.