Ground-based Remote Sensing Of The Atmosphere By Combined Passive And Active Sensors

Introduction Over the past ten years, ground-based microwave radiometers have been extensively used to measure temperature, water vapor, and cloud liquid. With careful design, these radiometers are capable of providing data during nearly all-weather conditions from unattended instruments; temporal resolutions of 1 to 30 sec are common. For some geophysical purposes, the pathintegrated quantities directly derivable from radiometric data are immediately useful. Such quantities include path attenuation, precipitable water vapor, integrated cloud liquid, and the wet component of electrical path length. However, for problems in which vertical profiles of atmospheric parameters are needed, microwave radiometers are still useful, but primarily as components of an integrated system. Ground-based infrared Fourier transform interferometers are also showing potential for deriving profiles of temperature and water vapor from the surface to cloud base. To extend profiles above cloud base, the infrared system must also be combined with data from other measurement systems. Because of wide variety of spectral responses of microwave and infrared radiometers to temperature, water vapor, and clouds, their combination can provide a valuable source of useful information. In general, however, to provide high vertical resolution profiles, active sensors, such as lidar or radar, are required. Even for these sensors, usually only a restricted portion of a desired profile can be measured. To derive a complete profile, data from a variety of sensors may be required. For example, a combination of dual-channel radiometers, a lidar, and a Radio Acoustic Sounding System (RASS), has provided useful moisture data for the monitoring of aircraft icing conditions. Infrared radiometers will add useful information to this combination. In this paper, we present examples of data from radiometers, first as stand-alone systems and then as components of integrated systems. The passive systems include microwave radiometers, P R T J cloud sensing radiometers, and Fourier transform infrared interferometers. The active components of the integrated system that we consider are lidar ceilometers, centimeter-wavelength radars, and RASS.