The Millimeter-wave Imaging Radiometer (MIR) was a passive airborne cross-track scanning radiometer developed by NASA. MIR measured brightness temperature across nine channels and was used for clouds, precipitation, and atmospheric water vapor studies. MIR operated near and around the following frequencies: 89, 150, 183, 220, and 325 GHz. It had an angular swath of 100 degrees and a sampling frequency of 3 seconds. MIR typically operated on NASA ER-2 aircraft before its retirement.
MIR was installed on the front end of the right superpod of the ER-2 aircraft. MIR has a 3-dB beamwidth of about 3.5 degrees at all frequency channels and covers an angular swath of +/- 50 degrees with respect to nadir. In every scanning cycle of about 3 sec in duration, it views two external calibration targets; one of them is heated to a temperature of 330 K and another remains at the ambient temperature that, at the cruising altitude of the ER-2 aircraft, is about 240 K. The temperatures of these calibration targets are closely monitored to within +/- 0.1 K. The temperature sensitivity of the six low-frequency channels (220 GHz) is on the order of 0.4 K and the calibration accuracy is better than +/- 2 K in the brightness temperature range of 240-300 K.
The measurement accuracy below 240 K is somewhat uncertain; based on the calibration studies in the laboratory, the accuracy near the liquid nitrogen temperature of 77 K is estimated to be +/- 3 K. The three high-frequency channels near 325 GHz are quite noisy and relatively unstable; the temperature sensitivity is on the order of +/- 8 K, and the calibration accuracy is estimated to be about +/- 5 K. The weighting functions for these three channels are quite similar to the corresponding channels at 183.3 GHz. Consequently, very little additional information is obtained in water vapor profiling by including the measurements from these channels in the retrieval process. For measurements over storms or ice clouds, these high frequency channels would be quite useful, if the noise level could be substantially reduced.
At the ER-2 aircraft cruising altitude of about 20 km, the footprint at nadir is about 1 km. The speed of the aircraft is about 200 m/sec. With a scanning cycle of about 3 seconds, the MIR will produce contiguous images at all nine channels with a ground swath of about 42 km.
MIR responds predominantly to atmospheric parameters like water vapor, clouds, and precipitation. Therefore, the major emphasis has been placed on the measurements of these parameters. The instrument is also sensitive to snow and sea ice, but its potential has not been fully explored in these areas. Passive microwave measurements of snow and sea ice have mostly been made at the frequencies near 19 and 37 GHz. MIR could enhance and improve the accuracy of these measurements.
Since its first flight in May 1992, MIR has participated in a number of field experiments, notably the Tropical Ocean Global Atmosphere / Coupled Ocean Atmosphere Response Experiment (TOGA/COARE), Convection and Moisture Experiment (CAMEX) 1 and CAMEX-2, Special Sensor Microwave Humidity Sounder (SSM/T-2) validation/calibration, SUbsonic aircraft: Contrail & Clouds Effects Special Study (SUCCESS), and Lidar Atmospheric Sensing Experiment (LASE) validation. More than 300 hours of MIR data were collected from these experiments, and over 90% of the collected data are of excellent quality. The only corrupted data set acquired in September 1995 is caused by interference in the LASE operation. Data of good quality have been used for studies of water vapor, clouds, and precipitation.
About six levels of water vapor mixing ratio between the surface and 10-km altitude can be derived from the MIR measurements. The vertical resolution is poor because of the generally broad weighting functions associated with the passive instrument. However, the cross-track scanning capability of the instrument could provide three-dimensional distribution of water vapor over the ocean areas. Retrieval of water vapor profiles can be made under clear-sky or moderately cloudy conditions.
Microwave radiometric measurements of rain are in general more effectively made at frequencies below 90 GHz, which are outside the range of MIR capability. However, it has been demonstrated by several studies that storm-associated scattering signatures at frequencies greater than 85 GHz are useful in rain rate estimation, especially over the land surfaces. Therefore, the MIR measurements will complement the measurements at low frequencies (by the Advanced Microwave Precipitation Radiometer [AMPR]) in this regard. In addition, the measurements by the MIR in the frequency range of 89-325 GHz will give a phenomenological description of the hydrometers above the freezing level.
MIR measurements are sensitive to moderate liquid clouds as well as intense ice clouds. A by-product in water vapor profiling from the MIR measurements is cloud liquid water. It will be interesting to compare the cloud liquid water estimated by the MIR with that derived from the low-frequency measurements. For ice clouds, only the very intense ones can be detected because the 325 GHz channels are noisy.
100 degrees
3 seconds
Lower Stratosphere
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