Measurement of Low Amounts of Precipitable Water Vapor Using Ground-Based Millimeterwave Radiometry

Abstract

-2 -Extremely dry conditions characterized by amounts of precipitable water vapor (PWV) as low as 1-2 mm commonly occur in high-latitude regions during the winter months. While such dry atmospheres carry only a few percent of the latent heat energy compared to tropical atmospheres, the effects of low vapor amounts on the polar radiation budget -both directly through modulation of longwave radiation and indirectly through the formation of clouds -are considerable. Accurate measurements of precipitable water vapor (PWV) during such dry conditions are needed to improve polar radiation models for use in understanding and predicting change in the climatically sensitive polar regions. To this end, the strong water vapor absorption line at 183.3 10 GHz provides a unique means of measuring low amounts of PWV. Weighting h c t i o n analysis, forward model calculations based upon a 7-year radiosonde dataset, and retrieval simulations consistently predict that radiometric measurements made using several millimeter-wavelength (MMW) channels near the 183 GHz line, together with established microwave (MW) measurements at the 22.235 GHz water vapor line and -3 1 GHz atmospheric absorption window can be used to determine within 5% uncertainty the full range of PWV expected in the Arctic. This unique collective capability stands in spite of accuracy limitations stemming from uncertainties due to the sensitivity of the vertical distribution of temperature and water vapor at MMW channels.In this study the potential of MMW radiometry using the 183 GHz line for measuring low amounts of PWV is demonstrated both theoretically and experimentally. The study uses data obtained during March 1999 as part of an experiment conducted at the Department of Energy's Cloud and Radiation Testbed (CART) near Barrow, Alaska. Several radiometers from both NOAA and NASA were deployed during the experiment to provide the first combined MMW and MW ground-based data set during dry arctic conditions. Single-channel retrievals of PWV were performed using the MW and MMW data. Discrepancies in the retrieved values were found to be consistent with differences observed between measured brightness temperatures (TBs) and forward-modeled TBs based on concurrent radiosonde profiles. These discrepancies are greater than can be explained by measurement error alone and are attributed to absorption model uncertainty. We discuss here the measurements, retrieval technique, and line model discrepancies along with difficulties and potential of MMWMW PWV measurement.

183-GHz Radiometer Handbook - November 2006

“…Figure 3 shows the nonlinear response of the GVR to water vapor as well as the saturation of the channels close to the line center. These results are consistent with those of [3] who estimated the sensitivity to PWV at these frequencies to be approximately 30 times higher than at the frequencies of the MWR for PWV less than 2.5 mm. The 183.31 ± 14 GHz channel sensitivity to liquid water path is about 3.5 times greater than the sensitivity of the 31.4-GHz channel of the MWR [2].…”

“…(3) Keeping into account the Mylar loss factor L = 1.0116, the final brightness temperature is computed as:…”

A Dual-Frequency Method of Eliminating Liquid Water Radiation to Remotely Sense Cloudy Atmosphere by Ground-Based Microwave Radiometer

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Ground-based microwave radiometer is the main device to remotely sense atmosphere passively which can detect the water vapor density, temperature, integral water vapor, etc.. For measurement in very dry conditions, typically in Arctic, several high frequency channels around water vapor line are added, and a 1-D variational retrieval technique has been developed [16][17][18]. Radiometer profiling during dynamic weather conditions is discussed and shows that the accuracy of radiometer retrievals is similar to radiosonde soundings when used for numerical weather prediction [19].…”

Millimeter-wave Radiometer for High Sensitivity Water Vapor Profiling in Arid Regions

“…A ground-based instrument was also constructed in the late 90s and was operated jointly by ETL and NASA in March 1999 in Barrow, Alaska to compare the ability of 22 GHz and 183 GHz radiometers to measure water vapor in winter arctic conditions [3]. More recently, the Ground-based Scanning Radiometer [4] participated in a late winter Intensive Observation Period (IOP) at the DOE North Slope of Alaska ARM site in 2004.…”

“…However, the dependence of brightness temperatures on precipitable water vapor and liquid water is linear only in a limited region, and a nonlinear retrieval algorithm is needed. An additional layer of complexity is added by the fact that the radiometer response saturates at a PWV of ~ 5 mm [4] for the most sensitive channels. The absorption line centered at 183.3 GHz has been extensively used from satellites [5] and aircraft [6], however very few measurements ( [7], [4]) have been reported from the ground.…”