A New High Spatial Resolution Cloud Detection Algorithm Based on the NASA Continuity MODIS/VIIRS Cloud Mask

With the launch of the Visible Infrared Imaging Radiometer Suite (VIIRS) in 2011 the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) Cloud Mask needed an update to account for the different spectral characteristics of the new sensor. For this reason, the NASA Continuity MODIS/VIIRS Cloud Mask (MVCM) was developed, which allows to produce a consistent record of cloud coverage leveraging only the spectral bands common to both the MODIS and VIIRS instruments. The algorithm uses a series of spectral tests in a fuzzy logic approach, each with assigned low and high confidence thresholds. Specific sets of tests are run on a given pixel based on surface type such as land, water, coast, desert, snow or ice, and solar illumination, i.e., day or night. Each individual test that belongs to the sets describing the current scene returns a clear sky confidence value and eventually gets combined with all the other test outcomes to create the overall clear sky confidence as reported in the final output.

For this work we modified the existing MVCM to ingest both VIIRS M-bands (750m spatial resolution) and I-bands (375m spatial resolution) and created a new set of tests, while maintaining those from the original algorithm that were still relevant for the I-bands. We then fine-tuned the old thresholds and defined additional ones to account for the new tests. As opposed to MODIS, that has high-resolution bands only in the visible, VIIRS I-bands extend from the visible (0.64µm) to the infrared window (11µm), allowing for multiple spectral tests for cloud detection to be performed during both daytime and nighttime.

Here we show how we use VIIRS high-spatial resolution bands (I-bands) to better detect fine structures with the High-Resolution MVCM, and touch on some of the improvements to the algorithm since the first early results were presented last year. Moreover, we will show some case studies to highlight the benefits of using the higher spatial resolution and discuss on the impact of detecting small scale structures on the global cloud coverage.

The high-resolution cloud mask is released alongside the existing one as an additional NASA operational product. This will help building an unprecedented multi-decadal record across multiple platforms from early 2012 with S-NPP and well into the 2030s and beyond with NOAA-20, NOAA-21 already flying, and the future JPSS-4 and JPSS-3 currently scheduled for 2027 and 2032, respectively.