The NASA Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) Level-2 (L2) Cloud Mask + Sounder is a new continuity product that combines the MODIS instrument’s high spatial resolution in broad spectral bands with the Atmospheric Infrared Sounder (AIRS) hyperspectral instrument’s more narrow swath absorption channels. While MODIS natively has sufficient spectral sensitivity to measure water vapor and carbon dioxide, the VIIRS instrument lacks this information. To maintain continuity of methodology, this L2 Cloud Mask product synergizes the AIRS water vapor and carbon dioxide absorption to ensure sensitivity to improve cloud detection especially at nighttime and in polar regions. This continuity product is designed to sustain the long-term records of both MODIS and VIIRS heritages.

CLDMSK_L2_MODIS-AIRS_Aqua is the shortname for the Aqua MODIS version of this product that is derived from the MODIS-VIIRS cloud mask (MVCM) continuity algorithm, which itself is based on the MODIS (MOD35) algorithm. The MODIS (from Terra and Aqua missions) and VIIRS (from SNPP and Joint Polar Satellite System (JPSS) missions) data-derived cloud mask products use the MVCM algorithm. Implemented to consistently handle MODIS and VIIRS inputs, the v2.0 Aqua MODIS Cloud Mask + Sounder collection products use calibration-adjusted NASA MODIS L1B as inputs. The nominal spatial resolution of the Aqua MODIS L2 Cloud mask is 1000 meters at nadir. The v2.0 collection starts from March 1, 2012 (2012-061).

The L2 netCDF product, acquired and processed every 5 minutes, contains the following three data groups and Science Data Set (SDS) layers in each group:

Geophysical data

• MODIS Clear Sky Confidence
• MODIS Cloud Mask and Spectral Test Results
• AIRS convolve MYD Mask
• MODIS Integer Cloud Mask
• Quality Assurance for MODIS Cloud Mask

Geolocation data

• Latitudes of pixel locations
• Longitudes of pixel locations
• Sensor azimuth angle at pixel locations
• Sensor zenith angle at pixel locations
• Solar azimuth angle at pixel locations
• Solar zenith angle at pixel locations

Scanline attributes

• Scan start time (TAI93)

Most of the SDSs are like those in the regular Aqua MODIS Cloud Mask product, but one unique SDS that deserves some explanation is “AIRS convolve MYD Mask.” This convolution is the basis for the data fusion that integrates measurements from AIRS with higher spectral resolution and MODIS with higher spatial resolution. It helps create a more comprehensive infrared radiance product than what either instrument can produce alone. Note that the reference to “MYD” is from the naming convention for Aqua MODIS.

Consult the MODIS-VIIRS Cloud Mask Product User Guide for additional information regarding the transition from the MODIS-MOD35 to the MVCM algorithm, inputs to MVCM, output files, file naming conventions, quality flags, and validation. Users are also encouraged to consult the MOD35 Algorithm Theoretical Basis Document (ATBD).