Comparison of Ground-Based Millimeter-Wave Observations During RHUBC I

Cimini, D., CETEMPS - Dipartimento di Fisica





Radiation Processes

Radiative Processes

Cimini D, F Nasir, ER Westwater, VH Payne, DD Turner, EJ Mlawer, ML Exner, and MP Cadeddu. 2009. "Comparison of Ground-Based Millimeter-Wave Observations and Simulations in the Arctic Winter." IEEE Transactions on Geoscience and Remote Sensing, 47(9), 10.1109/tgrs.2009.2020743.


Tb residuals (simulated minus observed) for (blue) GSR, (red) GVR, and (green) MP-183A versus channels’ spectral displacement from line center (183.31 GHz). The dashed line indicates calculated Tb differences associated with realistic sensor humidity uncertainty affecting the sounding in an opposite way at lower and upper levels (5% drier above and wetter below a reference level fixed arbitrarily to 3 km).


Tb residuals (simulated minus observed) for (blue) GSR, (red) GVR, and (green) MP-183A versus channels’ spectral displacement from line center (183.31 GHz). The dashed line indicates calculated Tb differences associated with realistic sensor humidity uncertainty affecting the sounding in an opposite way at lower and upper levels (5% drier above and wetter below a reference level fixed arbitrarily to 3 km).

In the winter of 2007 the Radiative Heating in Underexplored Bands Campaign (RHUBC) was conducted in Barrow, Alaska. The purpose of the campaign was to obtain accurate measurements of radiance in the far-infrared region of the electromagnetic spectrum. Water vapor absorption in this spectral region affects radiative heating and cooling of the upper troposphere; however, due to the strong water vapor absorption, this spectral region is largely opaque, and accurate measurements are difficult to obtain. RHUBC was conducted in extremely dry Arctic conditions (PWV < 5 mm); at these low PWV amounts, the far-infrared does become semi-transparent, allowing the spectroscopy of the band to be critically examined. One important aspect of the campaign was therefore the accurate retrieval of water vapor. For this purpose, three millimeter-wave radiometers operating in the 183.31-GHz region of the spectrum were deployed: the Ground-Based Scanning Radiometer (GSR) developed by the Center for Environmental Technology of the University of Colorado, the G-Band Vapor Radiometer Profiler (GVRP) built by Radiometrics Corporation, and the G-band Vapor Radiometer built by Prosensing, Inc. The last two radiometers were developed under a U.S. Department of Energy Small Business Innovation Research (SBIR) grant, and they are currently operated by the Atmospheric Radiation Measurement (ARM) Climate Research Facility. The instruments operated in cold and dry conditions for the duration of the experiment with air temperature varying from -38° C to -19°C and integrated water vapor ranging from 1–3 mm. The sensitivity to water vapor of millimeter-wave radiometers operating in the 183.31-GHz spectral region is about 30 times higher than the sensitivity of traditional microwave radiometers operating in the 20-30 GHz region of the electromagnetic spectrum.

The three radiometers have different hardware designs and are calibrated with different techniques. Observations collected from the three instruments were compared with each other and with forward model simulations. The instruments showed good agreement with each other in within the expected uncertainty. From the comparison the authors showed that, given the high level of consistency among the various instruments, the expected accuracy of water vapor retrieval is 2–4%. Increased frequency in radiosonde launches during the campaign provided the authors with enough data to compare the measurements with model simulations. A total of 38 clear-sky cases were obtained. Results showed that all instruments agreed with the model in within +/- 2 K. An interesting result from the authors showed a frequency dependency of the residuals that was consistent with the presence of a height-dependent bias in the radiosonde soundings (Figure 1).

Millimeter-wave radiometers are becoming important instruments for the retrieval of small amounts of water vapor, especially in the Polar region. The RHUBC campaign provided an excellent opportunity to evaluate three millimeter-wave radiometers operating side by side in February and March 2007. Evaluation of the radiometers’ performance is important to ensure the quality of the retrievals and the overall uncertainty in the measurements. The authors showed that the instruments operated by the ARM Facility provide reliable performance, and they are suitable for retrievals of low amounts of water vapor in the Arctic. Following the successful deployment during RHUBC, one of the millimeter-wave radiometers, the GVRP, was employed in the second RHUBC campaign in Chile in the fall of 2009. Water vapor amounts in this location were even lower than in the Arctic, well below 1 mm.