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J. R. Wang
,
J. Zhan
, and
P. Racette

Abstract

Radiometric measurements were made by a millimeter-wave imaging radiometer (MIR) at the frequencies of 89, 150, 183.3 ± 1, 183.3 ± 3, 183.3 ± 7, and 220 GHz aboard the NASA ER-2 aircraft at an altitude of about 20 km over two rainstorms: one in the western Pacific Ocean on 19 January 1993 and another in southern Florida on 5 October 1993. These measurements were complemented by nearly simultaneous observations by other sensors aboard the same aircraft and another aircraft flying along the same path. Analysis of data from these measurements, aided by radiative transfer and radar reflectivity calculations of hydrometeor profiles, which are generated by a general cloud ensemble model, demonstrates the utility of these frequencies for studying the structure of frozen hydrometeors associated with storms. Particular emphasis is placed on the three water vapor channels near 183.3 GHz. Results show that the radiometric signatures measured by these channels over the storm-associated scattering media bear a certain resemblance to those previously observed over a clear and fairly dry atmosphere with a cold ocean background. Both of these atmospheric conditions are characterized by a small amount of water vapor above a cold background. Radiative transfer calculations were made at these water vapor channels for a number of relative humidity profiles characterizing dry atmospheres over an ocean surface. The results are compared with the measurements to infer some characteristics of the environment near the scattering media. Furthermore, radiometric signatures from these channels display unique features for towering deep convective cells that could be used to identify the presence of such cells in storms.

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J. R. Wang
,
J. D. Spinhirne
,
P. Racette
,
L. A. Chang
, and
W. Hart

Abstract

Simultaneous measurements with the millimeter-wave imaging radiometer (MIR), cloud lidar system (CLS), and the MODIS airborne simulator (MAS) were made aboard the NASA ER-2 aircraft over the western Pacific Ocean on 17–18 January 1993. These measurements were used to study the effects of clouds on water vapor profile retrievals based on millimeter-wave radiometer measurements. The CLS backscatter measurements (at 0.532 and 1.064 μm) provided information on the heights and a detailed structure of cloud layers; the types of clouds could be positively identified. All 12 MAS channels (0.6–13 μm) essentially respond to all types of clouds, while the six MIR channels (89–220 GHz) show little sensitivity to cirrus clouds. The radiances from the 12-μm and 0.875-μm channels of the MAS and the 89-GHz channel of the MIR were used to gauge the performance of the retrieval of water vapor profiles from the MIR observations under cloudy conditions. It was found that, for cirrus and absorptive (liquid) clouds, better than 80% of the retrieval was convergent when one of the three criteria was satisfied; that is, the radiance at 0.875 μm is less than 100 W cm−3 sr−1, or the brightness at 12 μm is greater than 260 K, or brightness at 89 GHz is less than 270 K (equivalent to cloud liquid water of less than 0.04 g cm−2). The range of these radiances for convergent retrieval increases markedly when the condition for convergent retrieval was somewhat relaxed. The algorithm of water vapor profiling from the MIR measurements could not perform adequately over the areas of storm-related clouds that scatter radiation at millimeter wavelengths.

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P. Racette
,
R. F. Adler
,
J. R. Wang
,
A. J. Gasiewski
,
D. M. Jakson
, and
D. S. Zacharias

Abstract

A six-channel airborne total-power Millimeter-wave Imaging Radiometer (MIR) was recently built to provide measurements of atmospheric water vapor, clouds, and precipitation. The instrument is a cross-track scanner that has a 3-dB beamwidth of 3.5° and an angular swath of 100°. It measures radiation at the frequencies of 89, 150, 183.3 ± 1, 183.3 ± 3, 183.3 ± 7, and 220 GHz. The inclusion of the 220-GHz receiver makes this instrument unique; no other instrument has made atmospheric radiation measurements using this combination of frequencies. The temperature sensitivities ΔT, based on the actual flight data with a 6.8-ms integration time, are found to be 0.44, 0.44, 1.31, 1.30. 1.02, and 1.07 K. The instrument has two external calibration loads maintained at the temperatures of 330 and 250 K (the ambient temperature at an aircraft altitude of 20 km). These calibration load temperatures are monitored precisely so that the radiometric measurements of the instrument could be made to better than 1 K of accuracy in the brightness temperature range of 240–300 K. Measurements made with a calibration target emmersed in liquid nitrogen indicate a measurement accuracy of 2–4 K for brightness temperatures below 100 K. The instrument has flown successfully aboard the National Aeronautics and Space Administration (NASA) ER-2 aircraft for more than 130 h. This paper is an overview of the system design, calibration, and measurement capabilities.

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J. R. Wang
,
S. H. Melfi
,
P. Racette
,
D. N. Whitemen
,
L. A. Chang
,
R. A. Ferrare
,
K. D. Evans
, and
F. J. Schmidlin

Abstract

Simultaneous measurements of atmospheric water vapor were made by the Millimeter-wave Imaging Radiometer (MIR), Raman lidar, and rawinsondes. Two types of rawinsonde sensor packages (AIR and Vaisala) were carried by the same balloon. The measured water vapor profiles from Raman lidar, and the Vaisala and AIR sondes were used in the radiative transfer calculations. The calculated brightness temperatures were compared with those measured from the MIR at all six frequencies (89, 150, 183.3 ± 1, 183.3 ±3, 183.3 ±7, and 220 GHz). The results show that the MIR-measured brightness temperatures agree well (within ±K) with those calculated from the Raman lidar and Vaisala measurements. The brightness temperatures calculated from the AIR sondes differ from the MIR measurements by as much as 10 K, which can be attributed to low sensitivity of the AIR sondes at relative humidity less than 20%. Both calculated and the MIR-measured brightness temperatures were also used to retrieve water vapor profiles. These retrieved profiles were compared with those measured by the Raman lidar and rawinsondes. The results of these comparisons suggest that the MIR can measure the brightness of a target to an accuracy of at most ±K and is capable of retrieving useful water vapor profiles.

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