Shortwave Infrared and Day-Night Band VIIRS Polar Winds
The properties of winds from visible/near-infrared/thermal infrared satellite imagers – speed, direction, and altitude – are derived by tracking clouds in data from the Visible and Infrared Imaging Radiometer Suite (VIIRS), the Moderate Resolution Imaging Spectroradiometer (MODIS), and the Advanced Very High Resolution Radiometer (AVHRR), and by tracking water vapor with MODIS. However, polar clouds are notoriously difficult to detect and characterize with satellite imagers because of the similarities between their temperature and reflectance properties and those of the underlying snow and ice surface. Ubiquitous lower-tropospheric temperature inversions in winter and nearly isothermal temperature profiles in summer result in a very small temperature contrast between low, stratiform clouds – the most common cloud type over much of the Arctic Ocean – and the surface. In visible imagery, clouds and snow/ice are similarly bright, again resulting in very low contrast. This lack of contrast means that there are fewer good features to track, yielding fewer and/or lower quality wind vectors.
In the shortwave infrared (SWIR) portion of the spectrum the scattering properties of liquid-phase clouds and snow/ice are significantly different. Clouds are much brighter than the underlying snow or ice surface and the contrast between low clouds and the surface is large in SWIR bands around 1.6, 2.2, and 3.7 μm. This fact that has been exploited in polar cloud detection algorithms at least since the early 1990s. AVHRR, MODIS, and VIIRS all have bands at 1.6 and 3.7 μm; MODIS and VIIRS also have bands at 2.1-2.2 μm. In theory, SWIR data will provide more good features for cloud tracking and atmospheric motion vector derivation in the presence of sunlight (daytime), especially for liquid clouds over snow and ice. The VIIRS day-night band (DNB) provides another unique source of spectral information: reflected radiation at night in the presence of moonlight.
Here we report on the advantages of utilizing these new spectral bands to extend the current VIIRS polar winds products. Tests using a MODIS SWIR band have demonstrated that additional wind information can be obtained, particularly lower in the atmosphere. The DNB has not previously been employed for winds, though it is currently being successfully employed for sea ice motion.
