Defining optimal spatial resolution for high-spectral resolution infrared sensors

The National Polar-orbiting Operational Environmental Satellite System (NPOESS) is expected to become the central element of the long-term global observing system. While the critical design and fabrication for many sensor suites have been completed, some aspects of the instrument configuration of NPOESS´s primary infrared sensor, the Cross-track Infrared Sounder (CrIS), can still be optimized. This paper considers the relationship between various specifications for the sensor spatial resolution (sampling area size) and the anticipated performance of global temperature and water vapor profile sounding, these being among the highest priority measurements NPOESS will provide. Although the trade-off between signal to noise ratio, dwell time, sampling area (SA), and other instrumental issues such as weight, power and volume are complicated and need to be carefully considered, a finer SA will theoretically enable improved sounding through scattered clouds. Preliminary analysis of high-spectral infrared measurements from Atmospheric InfRared Sounder (AIRS) (which has a 13.5 km nadir SA) on NASA´s polar-orbiting EOS-Aqua satellite shows that the percentage of its SA being cloud free is less than 20%. In addition, we demonstrate using sounding and imaging data from NPOESS´ surrogate, AIRS and MODIS, how improved SA can achieve not only the improved sounding performance for meeting threshold and objective requirements, but also reduce processing demands, simplify data assimilation and utilization, and enhance delineation of mesoscale moisture gradients.