Cloud Observations Day and Night from the ISS using the Near InfraRed Airglow Camera (NIRAC)

Abstract:

The Near Infrared Airglow Camera (NIRAC) is a space based infrared imager deployed to the International Space Station in May 2019. The camera bandpass is 1.5 to 1.72 microns (H band), which is ideal for reflective imagery of cloud and surface features using illumination from the bright OH Meinel airglow emission. NIRAC has a 23° x 23° FOV, sweeping out a 160 km swath from the ISS, with a native spatial resolution of ~85 meters. The camera utilizes a state-of-the-art 4 megapixel (2K x 2K, 18 micron pixel) Teledyne H2RG detector and custom optical assembly to obtain high spatial resolution, high-performance space-based imagery. The focal plane array has a long-wavelength cutoff of 1.72 microns, which precludes the need for cold optics and reduces cooling requirements (to ~ 180 K). The camera is fed by a custom optical telescope which compensates for orbital motion of the ISS during ~1.5s integration time and creates equal-area ground pixels. This allows smear-free, high-spatial resolution imaging from a moving platform.

While NIRAC obtains images of clouds during daytime and moonlit conditions, it has the unique ability to observe clouds at night, even in the absence of moonlight, using ambient airglow. NIRAC nominally acquires images at 10s cadence in both its daytime, short-exposure mode and nighttime, long-exposure mode. Sequential images overlap by about 50%, providing multiple views of the same scene. Tomographic analysis of overlapping images at NIRAC’s 85m native pixel resolution allows determination of cloud heights within a scene to ~300-800 m accuracy, depending on the number of overlapping images. Daytime imagery and nighttime imagery under brighter illumination conditions (partial moon, bright airglow) can also be used to evaluate ice and water cloud features by albedo. Cloud heights in sunlit and moonlit scenes can be verified via analysis of cloud shadows. In this presentation we show examples of cloud cover and cloud characteristics, including comparisons to VIIRS DNB imagery, particularly for moon-down periods where the primary illumination source is airglow. NIRAC has flown over several large tropical storms, with excellent views into the eyewall; cloud height maps can be derived for these types of scenes. The cloud height algorithm and the results from its application will also be discussed.