Water Vapor Studies using
Aircraft Remote Sensing Instruments

Several aircraft sensors were developed by NASA in the mid 1980's to verify data from new satellite sensors and to collect unique datasets which would serve to justify future space-based instruments on low-Earth and geostationary observation platforms. In 1985, the Multispectral Atmospheric Mapping Sensor (MAMS) was developed and flown to verify small-scale water vapor features observed in Visible Infrared Spin Scan Radiometer (VISSR) Atmospheric Sounder (VAS) imagery aboard the Geosynchronous Operational Environmental Satellites (GOES). This aircraft sensor provided an opportunity to independently verify single-pixel variations observed in the VAS channels. This verification continued for several years providing useful correlative measurements. More recently, NASA developed the Moderate Resolution Imaging Spectrometer (MODIS) Airborne Simulator (MAS) to provide preliminary data and to underfly the EOS (Earth Observing System) MODIS instrument to be launched in the late 1990's. While MAS provides unique spectral channels in which to study atmospheric moisture variations, MAMS also provides significant moisture data, particularly in the upper troposphere. Because of this MAMS will continue to play an important role in the study of atmospheric moisture variability.

MAMS is a multispectral scanner which measures reflected radiation from the Earth's surface and clouds in eight visible/near-infrared bands, and thermal emission from the Earth's surface, clouds, and atmospheric constituents (primarily water vapor) in four infrared bands. The 5.0 mRa aperture of MAMS produces an instantaneous field-of-view (IFOV) resolution of 100 m at nadir from the nominal ER-2 altitude of 20 km. The width of the entire cross path field-of-view scanned by the sensor is 37 km, thereby providing detailed resolution of atmospheric and surface features across the swath width and along the aircraft flight track. For clouds and thunderstorm features the IFOV decreases with increasing cloud height by a factor of (Z-20)/20, where Z is the cloud height in kilometers.

The MAMS 6.5 micrometer channel has been used to map variations in upper tropospheric water vapor associated with a variety of atmospheric disturbances. The upper tropospheric water vapor imagery from VAS and the new GOES imager and sounder is very useful in the study of upper-level dynamics of mid-latitude weather systems. This is readily apparent in video "loops" of this from the satellite channel, which show smooth flowing patterns associated with large-scale weather disturbances. Changes in the brightness of the water vapor features are related to the vertical distribution of water vapor in the middle and upper troposphere, the integrated water vapor amount, and to a lesser degree the temperature profile. In addition, water vapor imagery can be used to discern small-scale variability of high clouds (particularly cirrus) and clear air atmospheric water vapor fields. In particular, MAMS water vapor imagery has been used to map clear air moisture variations in a number of different applications including lee wave situations.

The split-window channels from MAMS are similar to those from the Advanced Very High Resolution Radiometer (AVHRR), VAS, and GOES-8/9/10 imager and sounder. The 11 micrometer channels of MAMS and VAS are very similar, while those of AVHRR and the GOES-8/9/10 imager and sounder are narrower and shifted toward shorter wavelengths. The 12 micrometer channel of AVHRR is positioned near 11.8 micrometer with a bandwidth about twice that of MAMS and VAS (which are centered at longer wavelengths). The GOES-8/9/10 imager and sounder 12 micrometer channels are also narrow when compared to AVHRR. One of the sounder 12 micrometer channels and the imager 12 micrometer channel are centered near 12.0 micrometer, while the other sounder channel is near 12.7 micrometers. These 12 micrometer channels measure upwelling radiation where water vapor and other constituent absorption (particularly, by the Q-branch of CO2 at 12.63 micrometers) are more significant. The spectral differences of the 12 micrometer channels produce small differences in brightness temperatures for VAS and MAMS, but somewhat larger differences between AVHRR and MAMS (or VAS).

Last updated on: November 2, 1999