Millimeter-wave Radiometric Arctic Winter Measurements Experiment

27 February 1999 - 30 March 1999

Lead Scientist: Paul Racette

Observatory: nsa, nsa

During March, 1999, the NASA/Goddard Space Flight Center and the NOAA/Environmental Technology Laboratory conducted a joint experiment at the ARM NSA/AAO site in Barrow, Alaska. The major focus of the experiment was to determine if millimeter wavelength radiometers can measure significant changes in atmospheric water vapor during very cold and dry winter arctic conditions. Previously, success was achieved at the ARM SGP CART site in Oklahoma, by using microwave radiometer (MWR) measurements to scale radiosonde observations and to derive improved profiles. Such a procedure at the NSA/AAO could be even more beneficial because of the known bias and uncertainties of radiosonde measurements at low humidities. However, theoretical studies indicate that such a proceedure would be difficult at the NSA/AAO because of the diminished sensitivity of the MWR at low water vapor concentrations (Precipitable Water Vapor (PWV) less than 3 mm). Further theoretical studies indicate that millimeter wavelength radiometers operating around the 183 GHz water vapor line provide a greatly inhanced sensitivity, perhaps some 30 times as great.

Activity Summary

The Millimeter Wave Radiometric Arctic Winter Experiment was conducted using a large inventory of microwave and millimeter radiometers, operating from 20 to 350 GHz. The instruments were operated in the harsh arctic environment for a three week period in March 1999. In addition to the NASA and ETL radiometers, the infrastructure provided at NSA/AAO (in particular, ARM communication and housing facilities, MWRs, cloud lidars and radars, AERI, as well as once-a-day radiosondes) was of substantial benefit. During the experiment a wide range of atmospheric conditions were experienced and an excellent data set was obtained to meet the experiment's objectives. Since the experiment, significant effort has been devoted to processing and analyzing the data and developing the theoretical foundations for their application. Progress on these efforts may be divided into two subject areas - theoretical and experimental.

Theoretical: (A) A probability distribution of PWV was constructed using National Weather Service radiosondes that showed that there are a high percentage of cases in which PWV is less than 3 mm; i. e., those cases in which 183 GHz radiometry is promising. (B) A weighting function analysis was done showing the greatly inhanced sensitivity of the 183 region to water vapor at low concentrations (PWV < 3 mm); at higher concentrations (PWV > 5 mm), the sensitivity of the ARM MWR is adequate. This analysis confirms an earlier one showing that the 183 GHz channels + the existing MWR could provide complete coverage during all of the PWV conditions encountered at Barrow. (C) A comparison of contemporary absorption models showed that substantial differences existed between the models at some of the transparency channels that were operated during the experiment. Determining an adequate model will have substantial benefit to remote sensing by millimeter wavelength radiometry.

Experimental: (A) Application of calibration methods, including the "tipping calibration" method and both hot and ambient calibration reference targets, indicates that data from two completely independent radiometric systems (NASA/GSFC and ETL) were in basic agreement. However, some questions have arisen about data obtained when observing liquid nitrogen targets that require further analysis. (B) Two very cold , dry, and clear days have been analyzed and these data showed substantial variation in the millimeter wave measurements (some 25 to 30 K) while corresponding variations in the ARM MWR were less than 0.5 K. (C) Original radiosonde data and those corrected using Vaisala'a algorithm (by Barry Lesht) showed differences of 1 to 2 mm. At low vapor concentrations, these differences were 20 %, and would be difficult to correct by the MWR alone. Even larger differences were observed in comparison with National Weather Service soundings at Barrow. (D) Comparisons of brightness temperature calculations, based on radiosondes, and measurements showed differences at times of 10 K. These differences could be due to uncertainty in radiosonde data, absorption models, or both. (E) Images produced by the window channels of the scanning radiometers during cloudy conditions show the potential of millimeter radiometry for studies of arctic clouds.

Work in Progress: (A) To complete the calibration and intercomparison study and to determine the "best" set of radiometer data for retrieval analysis. (B) To refine and apply methods of deriving PWV, as well as water vapor profiles from the radiometric data. Retrievals, as applied to scaling radiosonde data, will be judged on their ability to reproduce infrared spectra measured by the ARM AERI instrument. (C) To develop and apply techniques to exploit the promising radiometric observations obtained during cloudy conditions. Such techniques will also use lidar and radar data measured by ARM instruments. (D) Based on experiences gained during the experiment, to design a millimeter wave radiometer suitable for NSA/AAO, as well as other arctic, applications.

Timeline

2000

Westwater ER, Y Han, A Gasiewski, M Klein, PE Racette, W Manning, and BM Lesht. 2000. A Comparison of Clear-Sky Emission Models with Data Taken during the 1999 Millimeter-Wave Radiometric Arctic Winter Water Vapor Experiment. In Proceedings of the Tenth ARM Science Meeting, Ed. by Nancy Burleigh, Richland, WA: U.S. Department of Energy.


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Campaign Data Sets

IOP Participant Data Source Name Final Data
Yong Han Microwave Radiometer Order Data
Joseph Michalsky Rotating Shadowband Spectroradiometer Order Data
Paul Racette Millimeter-wave Imaging Radiometer (MIR) Order Data
Ed Westwater CSR Order Data
Ed Westwater Radiometer Order Data