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Yong Han and E. R. Westwater

Abstract

A technique is presented for deriving tropospheric water vapor and cloud liquid water, as well as temperature, from a suite of ground-based sensors. Included in the suite are a dual-channel microwave radiometer, a ceilometer, a radio acoustic sounding system (RASS), and conventional surface meteorological instruments. A linear statistical inversion algorithm, combined with a data classification technique, is applied to retrieve water vapor and cloud liquid water profiles. The linear statistical inversion algorithm is also applied to derive temperature profiles from RASS virtual temperature measurements and surface meteorological parameters. A physical retrieval algorithm is then applied to retrieve integrated water vapor and liquid water. Finally, these two algorithms are coupled in a two-step iteration process. The technique is evaluated by comparing retrieved quantities with radiosonde measurements and by comparing this technique with the traditional technique based solely on dual-channel microwave radiometric measurements. Significant improvement is achieved in retrieving dominant structures in the water vapor profile when liquid clouds are present. This evaluation also predicts significant improvement in measuring integrated liquid water, but lack of ground truth prevented experimental verification.

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E. R. Westwater and N. C. Grody

Abstract

Remote sensing of vertical temperature profiles by combined satellite- and surface-based microwave radiometers is evaluated. For both clear and cloudy conditions, realistic simulations of retrieval accuracy for three climatologies with highly variable temperature structure suggest that rms retrieval accuracies from 1.0 to 2.0 K can be obtained from the surface to 300 mb using the combined passive measurements. When the height of the tropopause, as measured by ground-based radars, is used in a retrieval algorithm, simulations show that retrievals are improved over a broad altitude range, and that retrieval accuracies better than ∼2 K rms from the surface to 100 mb can be achieved.

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W. B. Sweezy and E. R. Westwater

Abstract

Four methods of estimating the height of the tropopause with VHF radar are compared and evaluated using data from a wind-profiling radar located at Platteville, Colorado, approximately 50 km north of Denver. An empirically derived method determines the tropopause height from the strength and persistence of the reflections. A second, theoretically based, method uses Fresnel scattering model estimates of the temperature gradient to determine the tropopause height. A third method identifies specular reflections associated with the tropopause by comparing vertical with oblique reflections. The final method combines the theoretically based method with a consensus set method for determining the most consistent estimates. The methods are compared by taking differences between the estimates and the tropopause heights determined from radiosonde data using the WMO definition of the tropopause. Each method is evaluated by calculating rms differences from the radiosonde determinations and by the number of differences that exceed an acceptable magnitude.

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B. B. Stankov, E. R. Westwater, and E. E. Gossard

Abstract

A method is presented to obtain a high-vertical-resolution humidity profile if the location and strength of only a few significant segments of the humidity gradient profile are known. The method is based on a previously developed statistical inversion technique coupled with moisture gradient information derived from wind profiler and the radiosonde temperature measurements. An existing retrieval algorithm uses an independent historical radiosonde-derived dataset and data from a two-channel microwave radiometer, standard surface meteorological instruments, and a lidar ceilometer. In this study, the possibility of constraining the statistical retrieval using measurements of significant moisture gradients derived from wind profiler signals and radiosonde temperature observations is investigated. An example is given to illustrate the method: on 26 May 1994 the 449-MHz wind profiler/RASS at Erie, Colorado, detected a strong humidity gradient at 4.9 km MSL. A statistical inversion algorithm constrained to the radar-measured gradient at 4.9 km was used to estimate the moisture profile. Results from this example show that an improvement in retrieved humidity profiles in particular, in the strength and location of a shallow layer, can be obtained if only significant radar-sensed humidity gradient information is added to other ground-based remote sensing measurements.

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Y. Han, E. R. Westwater, and R. A. Ferrare

Abstract

A two-stage retrieval technique is presented for deriving water vapor profiles from data provided by a Raman lidar, a microwave radiometer, a radio acoustic sounding system, and surface in situ instruments. In the first stage, a Kalman filtering algorithm is applied to derive water vapor profiles using surface in situ and current and past Raman measurements. In the second stage, a statistical inversion technique is applied to combine the Kalman retrieval with radiometric and climatological data. This retrieval method is tested using data collected during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment II experiment. The method is demonstrated to provide accurate profiles at altitudes above which the Raman lidar technique is limited.

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M. T. Decker, E. R. Westwater, and F. O. Guiraud

Abstract

Profiles of atmospheric temperature and water vapor derived from ground-based microwave radiometric measurements are compared with concurrent rawinsonde profiles including both clear and cloudy cases. Accuracies of the temperature profiles including the cloudy cases are quite close to predicted accuracies. Mean virtual temperatures between commonly used pressure levels are also compared and resulting rms accuracies are 1.1, 1.6, 2.0 and 2.8°C for the 1000–850, 850–700, 700–500 and 500–300 mb layers, respectively. The microwave technique is potentially useful in applications requiring high time resolution or in data-sparse regions of the oceans that might be covered by an ocean data buoy system.

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E. R. Westwater, J. B. Snider, and A. V. Carlson

Abstract

A one-week experiment was conducted to evaluate a dual-frequency microwave radiometer for recovering low altitude temperature profiles; the two-channel radiometer operated at 53.5 and 54.5 GHz. Meteorological support included radiosondes, helicopters, and an instrumented 150 m tower. Statistical inversion of 13 radiometer angular scan data sets resulted in an average rms error of 2.0 K up to 3 km for the microwave system. Significant features of thermal inversion structure were recovered. A continuous set of fixed-angle brightness observations correlated well with temperatures measured on the tower.

The statistical inversion method and the Backus-Gilbert method were applied to the analysis of the accuracy and the spatial resolution of the ground-based system. Model calculations were performed to estimate the effects of departures from horizontal stratification and of significant time variation in temperature structure during an elevation scan.

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E. R. Westwater, Y. Han, V. G. Irisov, V. Leuskiy, E. N. Kadygrov, and S. A. Viazankin

Abstract

Two techniques for deriving low-altitude temperature profiles were evaluated in an experiment conducted from November 1996 to January 1997 at the Boulder Atmospheric Observatory (BAO). The first used a scanning, single wavelength, 5-mm (60 GHz) microwave radiometer to measure vertical temperature profiles. Two radiometers were operated simultaneously; one used a discrete scan, the other scanned continuously. The second technique was a Radio Acoustic Sounding System (RASS) that operated at 915 MHz. Typically, radiometric profiles were produced every 15 min; those from RASS were 5-min segments taken every hour. Ground truth for the experiment was available from in situ measurements at the BAO. The BAO has an instrumented 300-m tower with 5-min measurements of temperature and relative humidity available at the surface and at altitudes of 10, 50, 100, 200, and 300 m. The tower measurements were occasionally supplemented with radiosonde releases and with hand-held meteorological measurements taken on the tower elevator.

The differences between the radiometers and the tower sensors were about 1°C rms. The accuracy using an in situ temperature measurement at the radiometer height as a predictor was also evaluated; at the 200- and 300-m levels, only about 4°C rms accuracies resulted. During the experiment, the RASS occasionally experienced radio frequency interference; to eliminate these effects, a quality-control algorithm for the RASS system was developed and evaluated. In addition, an experiment was held in September 1996 at the Department of Energy’s Atmospheric Radiation Program Southern Great Plains site in north central Oklahoma. For this experiment, evaluations of a scanning 5-mm radiometer relative to 3-hourly radiosondes are presented. Quality control on the radiosondes was provided by comparisons with independent in situ surface and 60-m tower observations. The agreement between the radiometric profiles and the quality-controlled radiosondes was better than 1°C up to 800 m. Plans for future deployments of these instruments are discussed.

In addition to the in situ comparisons, theoretical analyses of the scanning radiometer systems were also conducted. The effects of angular resolution of the current system, noise level, prediction from in situ measurements, and vertical resolution were examined.

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B-C. Gao, A. F. H. Goetz, Ed R. Westwater, J. E. Conel, and R. O. Green

Abstract

Remote sensing of tropospheric water vapor profiles from current geostationary weather satellites is made using a few broadband infrared (IR) channels in the 6–13-µm region. Uncertainties greater than 20% exist in derived water vapor values just above the surface from the IR emission measurements. In this paper, we propose three near-IR channels, one within the 0.94-µm water vapor hand absorption region, and the other two in nearby atmospheric windows, for remote sensing of precipitable water vapor over land areas, excluding lakes and rivers, during daytime from future geostationary satellite platforms. The physical principles are as follows. The reflectance of most surface targets varies approximately linearly with wavelength near 1 µm. The solar radiation on the sun-surface-sensor ray path is attenuated by atmospheric water vapor. The ratio of the radiance from the absorption channel with the radiances from the two window channels removes the surface reflectance effects and yields approximately the mean atmospheric water vapor transmittance of the absorption channel. The integrated water vapor amount from ground to space can be obtained with a precision of better than 5% from the mean transmittance. Because surface reflectances vary slowly with time, temporal variation of precipitable water vapor can be determined reliably. High spatial resolution, precipitable water vapor images are derived from spectral data collected by the Airborne Visible-Infrared Imaging Spectrometer, which measures solar radiation reflected by the surface in the 0.4–2.5-µm region in 10-nm channels and has a ground instantaneous field of view of 20 m from its platform on an ER-2 aircraft at 20 km. The proposed near-IR reflectance technique would complement the IR emission techniques for remote sensing of water vapor profiles from geostationary satellite platforms, especially in the boundary layer where most of the water vapor is located.

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E. R. Westwater, M. T. Decker, A. Zachs, and K. S. Gage

Abstract

This paper describes the results of a three-week experiment in which ground-based microwave radiometricmeasurements were combined with VHF radar measurements of tropopause height to yield vertical temperature profiles. Several algorithms to derive tropopause height are presented and their results are comparedwith radiosondes. The best of the algorithms yields radar versus radiosonde rms differences of 0.65 km.By the use of the combined radar-radiometric method, improvements were obtained in rms temperatureaccuracy of as much as 2.0 K rms over the pure radiometric technique.

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