Chapter Two - The Interpretation Problem of Satellite Water Vapor Imagery

Abstract

This chapter describes the information content of radiances measured by satellites in water vapor (WV) channels and illustrates the approach for interpreting imagery gray shades. Instruments aboard meteorological satellites measure infrared radiation in several wavelength ranges over which radiation is significantly absorbed and reradiated by water in its gaseous, liquid, or ice crystal form. Such wavelength ranges are referred to as water vapor bands and the wavelengths where they are centered are referred to as WV channels. Within the WV channels, infrared (thermal) radiation is emitted by solid objects, such as cloud elements, precipitation, and the surface of the Earth. These channels are sensitive to the profiles of both the temperature and the humidity. WV in typical concentrations is semitransparent to the radiation. Therefore, the brightness temperature measured by the satellite is a “net” temperature of some layer of moisture, not the temperature of any single surface or level. Interpretation of the WV image gray shades is intended to shed light on the distribution of moisture and temperature in the atmospheric column that contributed to a specific radiation measurement. A water vapor image is considered to represent a “moisture terrain,” in which the light shades are areas where low-level moisture extends upward to high altitudes, and the dark shades are areas where the high-tropospheric dry air extends downward to the lowest levels. This topographic representation is the most appropriate approach for interpreting WV imagery jointly with PV fields for monitoring tropopause foldings.