James R. Wang
NASA Goddard Space Flight Center
Over the years passive microwave measurements of geophysical parameters like snow, sea ice, soil moisture, vegetation, precipitation, clouds, and water vapor have been made at frequencies below 200 GHz. In fact, satellite microwave radiometric measurements of these parameters for large-scale geophysical applications have been limited to frequencies < 100 GHz. The usefulness of radiometric measurements at these frequencies have been demonstrated time and again by orbiting sensors like ESMR (Electronically Scanned Microwave Radiometer), SMMR (Scanning Multichannel Microwave Radiometer), and SSM/I (Special Sensor Microwave/Imager). At frequencies > 100 GHz, the first special sensor microwave water vapor sounder (SSM/T2) with five frequency channels at 91, 150, 183.3±1, 183.3±3, and 183.3±7 GHz was placed in orbit aboard the DMSP F11 satellite in late November 1991 (Falcone et al., 1992). The SSM/T2 sensor provides new information on water vapor profiles, clouds and precipitation distributions on global scale. However, the potential applications of radiometric measurements at frequencies above 200 GHz have not been fully explored. Based on theoretical calculations reported in the literature (Evans and Stephens, 1994), the ice particles in the cirrus clouds should exhibit detectable signatures at these high frequencies.
The measurements of cirrus clouds are important in studying the climate change. It has been shown from model calculations that the cloud feedback on climate change depends on the particle size and asymmetry parameter of the ice clouds (Stephens et al., 1990). According to these authors, the sign of cirrus cloud ice water-temperature feedback varies from positive to negative depending on the specific choice of particle size. The predictions of the cirrus impact on the climate system are limited because of our lack of understanding between the size and shape of ice crystals and the gross radiative properties of the cirrus clouds. Adequate measurements of these cirrus microphysical properties and specific research directed towards these issues are clearly needed.
II. Current Status of Technology/Instrument
An airborne Millimeter-wave Imaging Radiometer (MIR) was built jointly by NASA Goddard Space Flight Center and Georgia Institute of Technology and first flown successfully in May 1992. The instrument was installed on the NASA ER-2 aircraft and provided six frequency channels of measurements at 89, 150, 183.3±1, 183.3±3, 183.3±7, and 220 GHz. It has proven to be very reliable by making more than 200 hours of measurements in various missions over the past three years. In the fall of 1993 three more channels near another strong water vapor line at 325 GHz were added to the instrument. Useful data were acquired from these new channels during September-October of 1993 over the east coast of the continental U.S., but they were more noisy compared to those from the low-frequency channels. After these flights in 1993, the 325 GHz channels stopped functioning properly and are now currently under repair.
During one deployment in early 1993, MIR measurements were made concurrently with those of the MODIS Airborne Simulator (MAS) and the Cloud Lidar System (CLS) aboard the same ER-2 aircraft. Both MAS and CLS provided measurements over high cloud regions that could be compared with the MIR data to assess the radiometric sensitivity to cirrus clouds at the MIR frequencies. It was found that the brightness temperature depression observed at the highest frequency channel of 220 GHz was mostly due to the enhancement of water vapor in the cloudy regions. After the effect of water vapor was removed, there was a small residual temperature depression < 1 K that appeared to correlate with the increase in optical depth derived from the measurements of MAS and CLS. However, the results were not conclusive that cirrus clouds were detected at 220 GHz. Measurements at higher frequencies are required to positively infer the properties of cirrus clouds.
III. Application Areas
Microwave radiometry at high frequencies has the following potential applications. First, the submillimeter-wave measurements of cirrus clouds could provide information (i.e., size distribution of ice particles) that is important in climate modeling and cannot be obtained from measurements at visible and infrared regions alone. Next, the sensor technology developed around the water vapor line of 325 GHz could be used in the geostationary satellite application that could perform the function of the current polar-orbiting SSM/T2 with a respectable spatial resolution. This is due to the fact that the strengths of the two water vapor lines at 183.3 GHz and 325 GHz are about the same, and water vapor profiling from the radiometric measurements around these lines should be quite comparable. The advantage of using 325 GHz is that the same angular resolution would be nearly a factor of two better compared to a 183 GHz system with the same aperture because of a shorter wavelength.
Another potential application is for the study of frozen hydrometeors associated with rain storms. It has been known for some time that the radiometric measurements at frequencies > 90 GHz over rain storms reflect strong signatures of scattering by frozen hydrometeors above the freezing level. MIR has made a number of measurements over such storm systems in the past three years. These new measurements generally confirm the past findings. These measurements also showed that the scattering is stronger at higher frequencies, at least up to 325 GHz. The research in this area is only at the stage of infancy, and a radiometer capable of measuring radiation at frequencies beyond 325 GHz is needed.
IV. Recommendation for Further Activity.
Microwave radiometry at frequencies above 200 GHz has the potential of providing microphysical properties of cirrus clouds and structure of frozen hydrometeors associated with storm systems, which are the key parameters critical for understanding the dynamics of global climate system and hydrological cycle. The technology of building reliable radiometers at these millimeter and submillimeter wavelengths is yet to be fully realized. The current airborne or spaceborne millimeter-wave radiometers operate reliably at frequencies 220 GHz. A three-channel radiometer operating near 325 GHz, included as a component of MIR, worked only for a brief period. There is need to enhance the development of submillimeter-wave technology for remote sensing applications. A plan to upgrade the 325 GHz channels in the MIR has been developed. An additional channel near 430 GHz for the MIR has also been considered.
Evans, K. F., and G. L. Stephens (1994), Microwave radiative transfer through clouds composed of realistically shaped ice crystals. Part II: remote sensing of ice clouds. Submitted to J. Atmos. Sci.
Falcone, V. J., M. K. Griffin, R. G. Isaacs, J. D. Pickle, J. F. Morrissey, A. J. Jackson, A.
Bussey, R. Kakar, J. Wang, P. Racette, D. J. Boucher, B. H. Thomas, and A. M. Kishi (1992), SSM/T-2 Calibration and Validation Data Analysis. PL-TR-92-2293, Environmental Research Papers, No. 1111, Phillips Laboratory, Hanscom Air Force Base, MA 01731-5000.
Stephens, G. L., S. Tsay, P. W. Stackhouse, Jr., and P. J. Flatau (1990), The relevance of the microphysical and radiative properties of cirrus clouds to climate and climate feedback. J.
Atmos. Sci., 47, 1742-1753.