Sensitivity Studies on Cloud Measurement Using Oxygen A- and B-band for the Deep Space Climate Observatory

The Earth Poly-Chromatic Imaging Camera (EPIC) on board the Deep Space Climate Observatory will have 10 channels with spectral resolutions from 1 to 3 nm to measure aerosols, clouds, ozone, SO2, vegetation and Earth's radiation budget on the dayside of Earth from the Earth's L1 Lagrangian point, a stable gravity-neutral point 1.5 million km away from Earth. EPIC is in refurbishing stage to optimize original design toward a future mission. Among the EPIC channels, one pair of O2 A-band channels (on-line and off-line) and one pair of O2 B-band channels have been chosen to measure cloud fraction and cloud top height at a nominal spatial resolution of approximately 10-km. A set of sensitivity studies has been performed for these two pairs of channels using the Line-by-Line Radiative Transfer Model, the HIgh-resolution TRANsmission molecular absorption database (HITRAN) 2008 and the up-to-date sensor specifications. This paper will present the sensitivity study results which include the measurement sensitivities to cloud top height, cloud fraction, optical depth and phase, the sensor filter center position and wavelength shift due to the Angles of Incidence, and the sensitivities to surface properties, atmospheric temperature and moisture profile. Given the sensor filter nominal spectral resolution and signal-to-noise ratio, O2 A-band provides greater sensitivity to cloud top height change than O2 B-band. However, over vegetation A-band has much higher surface returns and is much more sensitive to atmospheric temperature than B-band. Using the radiance ratio of on-line and off-line channels and combining information from both bands will lead to high-quality measurements of cloud top height and cloud fraction and will help the retrievals of aerosols, SO2, ozone, vegetation and Earth's radiation from other channels in high temporal resolution for weather and climate studies.