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Ground-based observations at millimeter (mm) and submillimeter (submm) wavelengths were collected at the Atmospheric Radiation Measurement Program site at Barrow, AK, during the Arctic winter by a new 25-channel radiometer. A weighting function analysis is presented to demonstrate the enhanced sensitivity of mm- and submm-wave (50--400 GHz) radiometers to low vapor and liquid water contents with respect to conventional instruments such as the ones operating at centimeter (cm) wavelengths (20-30 GHz). In addition, based on measurements, we carried out a quantitative analysis of mm-and submm-wavelength sensitivity, yielding improvement factors from 1.5 to 69 for precipitable water vapor (PWV) and 3 to 4 for liquid water path (LWP) when compared to 20-30 GHz radiometers. Furthermore, using a simulated data set, we evaluate the effect of hydrometeor scattering: Given the conditions occurring during the experiment, the scattering contribution is within the instrumental noise for most, but not all, of the considered channels. With the same data set, we demonstrate that in the dry conditions of the Arctic, a simple linear regression yields satisfactory results when applied on selected mm- and submm-wave channels. For a dual-channel combination, the expected accuracy is ~0.23 (0.007) mm for PWV (LWP), when using mm- and submm-wavelengths, whereas it is 0.37 (0.012) mm using cm-wave channels. When the retrieval is applied to real observations, the accuracy is found in agreement with theoretical expectations. Index Terms--Atmospheric measurements, microwave radiometry, remote sensing.

Brightness temperatures computed from five absorption models and radiosonde observations were analyzed by comparing them with measurements from three microwave radiometers at 23.8 and 31.4 GHz. Data were obtained during the Cloudiness Inter-Comparison Experiment at the U.S. Department of Energy's Atmospheric Radiation Measurement Program's (ARM) site in North-Central Oklahoma in 2003. The radiometers were calibrated using two procedures, the so-called instantaneous 'tipcal' method and an automatic self-calibration algorithm. Measurements from the radiometers were in agreement, with less than a 0.4-K rms difference during clear skies, when the instantaneous method was applied. Brightness temperatures from the radiometer and the radiosonde showed a bias difference of less than 0.69 K when the most recent absorption models were considered. Precipitable water vapor (PWV) computed from the radiometers were also compared to the PWV derived from a Global Positioning System station that operates at the ARM site. The instruments agree to within 0.1 cm in PWV retrieval. Index Terms--Global Positioning System (GPS), microwave propagation, microwave radiometry, radiometric accuracy, water vapor.

We discuss the performances of a set of four microwave water vapor radiometers operating in the 20-30-GHz band during a field experiment, with an emphasis on calibration and achievable accuracy. The field experiment was conducted at the Department of Energy's Atmospheric Radiation Measurement Program's field site in north central Oklahoma, and was focused on clear-sky water vapor measurements by both radiometers and radiosondes. A comparison between two published radiometric tip curve calibration procedures is presented, and these procedures are applied to measurements from two nearly identical instruments placed a few meters apart. Using the instantaneous tip cal method of the Environmental Technology Laboratory, the brightness temperature measurements for the two identical instruments differed by less than 0.2 K over a 24-h period. Results from reference load cryogenic tests and brightness temperature cross comparisons have shown differences within 0.7 K. In addition, we compare radiometric measurements with calculations of brightness temperature based on the Rosenkranz absorption model and radiosonde observations. During the experiment, both Vaisala-type RS80 and RS90 humidity sensors were used. Our comparisons demonstrate the improvements achieved by the new Vaisala RS90 sensors in atmospheric humidity profiling, which reduce or eliminate the 'dry bias' problem. Index Terms--Microwave radiometry, radiometric accuracy, radiometric calibration, water vapor.

Data obtained from a single-channel scanning 5-mm radiometer on a research vessel were used for the study of sea surface temperature and lower-atmospheric temperature profiles under neutral, weakly stable, and unstable conditions. Comparisons with in situ air and water temperature measurements, dropsonde temperature data, radiosonde data, and VV Ku-band side-looking radar images are presented. The technique based on 5-mm measurements determines the air-skin water (skin depth of 0.3 mm) temperature difference with an accuracy of the order of 0.1 K. Analysis of the experimental data suggests that the air-skin water temperature difference depends weakly on solar heating and wind speed V for 2.5 < V < 9 m/s.

The remote sensing of water vapor from geostationary satellites, also known as 'reflectance technique,' was used to analyze three near-IR channels, one in the 0.94 micrometer vapor band absorption domain and the rest in neighboring atmospheric windows. Water vapor in the atmosphere reduces the intensity of solar radiation. Surface reflectance and the average atmospheric water vapor transmittance of the absorption were measured using the proposed near-IR reflectance technique.

Three sets of precipitable water vapor observations over the Front Range of eastern Colorado in March 1990 are compared. The remote-sounding techniques used are emission, reflectance, and microwave. Horizontal and temporal resolutions of the various instruments account for the small variations between data sets. The reflectance and microwave readings agreed to within 0.1 cm. and those of the latter had similar correlations with the radiosonde readings. Variable terrain within the satellite footprint accounted for differences of 0.4 and 0.5 cm. between emission and microwave data.

The tipping-curve calibration method has been an important calibration technique for ground-based microwave radiometers that measure atmospheric emission at low optical depth. The method calibrates a radiometer system using data taken by the radiometer at two or more viewing angles in the atmosphere. In this method, the relationship between atmospheric opacity and viewing angle is used as a constraint for deriving the system calibration response. Because this method couples the system with radiative transfer theory and atmospheric conditions, evaluations of its performance have been difficult. In this paper, first a data-simulation approach is taken to isolate and analyze those influential factors in the calibration process and effective techniques are developed to reduce calibration uncertainties. Then, these techniques are applied to experimental data. The influential factors include radiometer antenna beam width, radiometer pointing error, mean radiating temperature error, and horizontal inhomogeneity in the atmosphere, as well as some other factors of minor importance. It is demonstrated that calibration uncertainties from these error sources can be large and unacceptable. Fortunately, it was found that by using the techniques reported here, the calibration uncertainties can be largely reduced or avoided. With the suggested corrections, the tipping calibration method can provide absolute accuracy of about or better than 0.5 K. Index Terms--Calibration, ground-based, microwave radiometers.