The MODIS Cloud Product combines infrared emission and solar reflectance techniques to determine both physical and radiative cloud properties. Daily global Level 2 data are provided. Cloud particle phase (ice vs. liquid water clouds, cloud particle effective radius, and cloud optical thickness are retrieved at a spatial resolution of 1km (nadir) during daytime only using MODIS visible, near-infrared, shortwave-infrared and mid-wave infrared channel reflectances as well as several IR cloud products. Cloud top temperature, height, effective emissivity, IR thermodynamic phase, and cloud fraction are produced by infrared-only retrieval methods both day and night at 5x5 1km pixel resolution. The cloud top products are also produced at the native 1km resolution beginning with Collection 6. The MODIS Cloud Product includes cirrus reflectance in the visible at 1km pixel resolution; the product was intended for removing cirrus scattering effects from land surface reflectance products, however the product is no longer maintained by NASA. There are two sets of MODIS Cloud Product files: MOD06_L2, containing data collected from the Terra platform, and MYD06_L2, containing data collected from the Aqua platform. Since the cloud algorithms are essentially the same for both Terra and Aqua MODIS, the use of MODxx in the following text is understood to mean the algorithm for either instrument.
Research and Application
A thorough description of global cloudiness and associated properties is an essential goal of the MODIS mission for two reasons. First, clouds play a critical role in the radiative balance and water cycle of the Earth and must be accurately described to properly assess climate and potential climate change. In addition, the presence or absence of clouds must be determined accurately to successfully retrieve clear-sky atmospheric and surface geophysical parameters. For many of these retrievals, cloud contamination of an otherwise clear pixel (e.g., partly cloudy pixel coverage, thin sub-visible cirrus, etc.) represents significant contamination.
Data Set Evolution
Cloud Top Properties. The determination of cloud top properties requires the use of MODIS bands 28, 29 and 31-36 (See MODIS Spectral Bands), along with the cloud-mask product (MOD35) to screen for clouds. In addition, global model analyses of surface temperature and pressure, profiles of temperature and moisture, and blended SST analyses are required in the calculation of cloud forcing as a function of atmospheric pressure and emissivity. The infrared cloud phase algorithm requires MODIS bands 28, 29 and 31. The validation of cloud top heights has been conducted through comparisons with CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) data and other lidar estimates using aircraft observations. Cloud emissivity has been compared to lidar-determined values of cloud optical thickness.
Cloud Optical Properties. The cloud optical properties algorithm requires the MOD02 calibrated multispectral reflectances, in addition to the cloud top parameters within MOD06 and the IR cloud phase parameter. Other ancillary requirements include MODIS product MOD43 (surface reflectance) and the global model analyses and profiles described above. Look-up tables for spectral cloud-top reflectance use light scattering properties of severely roughened aggregate columns for ice clouds (in C6) and the well-known Mie theory for liquid water clouds. The validation and quality control of these products is performed primarily using in situ measurements obtained during field campaigns as well as the MODIS Airborne Simulator (MAS) instrument.
Spatial/Temporal Characteristics: Resolutions of 1 km or 5 km/once or twice per day (varies with parameter)
Key Science Uses: Cloud processes and parameterization, climate modeling validation, climate data records, increasing accuracy of other MODIS geophysical retrievals
Key Geophysical Parameters: Cloud particle phase (two algorithms), effective particle size, optical thickness, water path, visible cirrus reflectance, and cloud top temperature, pressure, emissivity, and height
Processing Level: 2
Product Type: Standard, at-launch
Maximum File Size: 160 MB
File Frequency: 288/day
Primary Data Format: HDF-EOS