The MODIS Precipitable Water product consists of column water vapor amounts. During the daytime, a near-infrared algorithm is applied over clear land areas of the globe and above clouds over both land and ocean. Over clear ocean areas, water vapor estimates are provided over the extended glint area. An infrared algorithm for deriving atmospheric profiles is also applied both day and night for Level-2. There are two MODIS Water Vapor data product files: MOD05_L2, containing data collected from the Terra platform; and MYD05_L2, containing data collected from the Aqua platform.
The Level-2 data are generated at the 1-km spatial resolution of the MODIS instrument using the near-infrared algorithm during the day, and at 5 km x 5 km pixel resolution both day and night using the infrared algorithm, when at least nine FOVs are cloud-free. The infrared-derived precipitable water vapor is generated as one component of the Atmospheric Profile product (MOD07), and simply added to MOD05 for convenience. The solar retrieval algorithm relies on observations of water vapor attenuation of reflected solar radiation in the near-infrared MODIS channels so that the product is produced only over areas where there is a reflective surface in the near IR.
The solar-column water vapor parameter is derived from the attenuation by water vapor of near-IR solar radiation. Techniques are used that employ ratios of water vapor-absorbing channels (17, 18, and 19) with the atmospheric window channels (2 and 5). The ratios partially remove the effects of variation of surface reflectance with wavelength and result in the atmospheric water vapor transmittance. The column water vapor amounts are derived from the transmittance, based on theoretical radiative transfer calculations and using look-up-table procedures. MODIS is the first space-based instrument to use near-IR bands together with the traditional IR bands to retrieve total precipitable water. Experience in this retrieval is based on the airborne visible/infrared imaging spectrometer (AVIRIS) instrument aboard the NASA Earth Resources-2 (ER-2) aircraft. Atmospheric water vapor should be determined with an accuracy of 5-10%.
The thermal column water vapor parameter is derived by integrating the moisture profile through the atmospheric column. Other split-window methods also exist for determining water vapor. The split-window techniques use the difference in water vapor absorption that exists between channel 31 (11 µm) and channel 32 (12 µm). Data validation is conducted by comparing these data with water vapor measurements from the National Oceanic and Atmospheric Agency (NOAA) National Weather Service (NWS) radiosonde network, ground-based upward-looking microwave radiometers, a ground-based GPS network, and from the AErosol RObotic NETwork (AERONET) ground-based sunphotometer network.
The near-infrared total column precipitable water is very sensitive to boundary layer water vapor since it is derived from attenuation of reflected solar light from the surface. This data product is essential to understanding the hydrological cycle, aerosol properties, aerosol-cloud interactions, energy budget, and climate. Of particular interest is the collection of water vapor data above cirrus cloudiness, which has important applications to climate studies. "