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J. B. Snider

Abstract

The use of angular-scan and multi-spectral techniques for Inferring temperature profiles from ground-based radiometric observations of emission by atmospheric oxygen is discussed. A recently developed multi-frequency radiometer used to evaluate, experimentally, the two techniques is described. Temperature profiles inferred from emission measurements are compared to radiosonde profiles recorded simultaneously with the radiometric data. It is concluded that a combination of angular-scan and multi-spectral input data yields a more accurate profile recovery than other sets of input data.

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J. B. Snider

Abstract

Measurements of precipitable water vapor, cloud liquid, and cloud-base temperature made at Porto Santo Island, Madeira Archipelago, are presented. The observations were made continuously over a 1-yr period from July 1992 to June 1993. Instrumentation consisted of a 20- and 31-GHz ground-based microwave radiometer for measurement of water vapor and cloud liquid, and a 10.7-μm infrared radiometer for measurement of cloud-base temperature. A statistical summary of the data is presented for clear and cloudy conditions, considered both separately and combined. The mean and variance of precipitable water vapor are smaller during clear than during cloudy periods. Values of water vapor during the winter are 26% smaller than during summer months. The mean decorrelation time of precipitable water vapor is generally found to be less than 12 h.

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J. B. Snider and D. Rottner

Abstract

A ground-based combination microwave radiometer-satellite receiver operating at 28.5 GHz (wavelength = 1.05 cm) was employed to measure liquid water in clouds during the 1979-80 field season of the Sierra Cooperative Pilot Project (SCPP) in northern California. We report upon the ability of the instrument to detect small amounts (∼0.1 g m−3) of liquid water in non-precipitating clouds which may not be observed by other remote sensing systems. A successful cloud seeding experiment, performed after cloud liquid was detected by the instrument, is described.

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Miriam Blaskovic, Roger Davies, and J. B. Snider

Abstract

Surface measurements made at San Nicolas Island during the intensive field observation marine stratocumulus phase of the First International Satellite Cloud Climatology Progam Regional Experiment, July 1987, are analyzed to retrieve the average diurnal variation of marine stratocumulus and related surface variables. Cloud thickness and integrated liquid-water content show a clear decrease during the day from sunrise to sunset, increasing thereafter. The average liquid-water density in the cloud is closely related to the cloud thickness, decreasing as the cloud thickness decreases. The cloud-base height has a diurnal range of 150 ± 30 m, rising from sunrise till midafternoon. The cloud-top height has a similar diurnal range of 130 ± 30 m, but the main descent occurs in the late afternoon. Surface air temperature also increases at sunrise, directly in phase with the cloud-base lifting, and has a diurnal range of 2°C.

The diurnal behavior of the cloud base appears to be consistent with model-predicted uncoupling of the cloud layer and the subcloud layer as the turbulent flux of moisture is inhibited by solar heating near the cloud base. Similarly, variation in surface air temperature is consistent with the inhibition of the turbulent flux of heat between the two layers, shielding the surface from the effect of longwave cooling from the cloud top. The variation in cloud-top height, however, does not appear to he readily explainable by present diurnal models.

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J. Warner, J. F. Drake, and J. B. Snider

Abstract

Field trials have been carried out in the Boulder, Colorado area of a method of determination of cloud liquid-water distribution by inversion of brightness temperature data obtained from a pair of microwave radiometers spaced about 8 km apart and sunning in a coplanar mode through clouds located between them. In the absence of precipitation, the liquid water distribution in the cloud was retrieved with reasonable accuracy. In cases where precipitation was present, it had developed by the ice crystal process, and nearly all the liquid was in the form of rain below cloud base. For this condition the radiometer data was interpreted in terms of a distribution of rainfall rates.

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C. W. Fairall, J. E. Hare, and J. B. Snider

Abstract

As part of the First International Satellite Cloud Climatology Regional Experiment (FIRE), a surface meteorology and shortwave/longwave irradiance station was operated in a marine stratocumulus regime on the northwest tip of San Nicolas island off the coast of Southern California. Measurements were taken from March through October 1987, including a FIRE Intensive Field Operation (IFO) held in July. Algorithms were developed to use the longwave irradiance data to estimate fractional cloudiness and to use the shortwave irradiance to estimate cloud albedo and integrated cloud liquid water content. Cloud base height is estimated from computations of the lifting condensation level. The algorithms are tested against direct measurements made during the IFO; a 30% adjustment was made to the liquid water parameterization. The algorithms are then applied to the entire database. The stratocumulus clouds over the island are found to have a cloud base height of about 400 m, an integrated liquid water content of 75 gm−2, a fractional cloudiness of 0.95, and an albedo of 0.55. Integrated liquid water content rarely exceeds 350 g m−2 and albedo rarely exceeds 0.90 for stratocumulus clouds. Over the summer months, the average cloud fraction shows a maximum at sunrise of 0.74 and a minimum at sunset of 0.41. Over the same period, the average cloud albedo shows a maximum of 0.61 at sunrise and a minimum of 0.31 a few hours after local noon (although the estimate is more uncertain because of the extreme solar zenith angle). The use of joint frequency distributions of fractional cloudiness with solar transmittance or cloud base height to classify cloud types appears to be useful.

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Mark Heggli, Robert M. Rauber, and J. B. Snider

Abstract

The dual-channel microwave radiometer is evaluated in regard to the measurement of integrated water vapor and supercooled liquid water. The study includes comparisons of integrated vapor content measured by the radiometer and rawinsondes as part of the Sierra Cooperative Pilot Project in the central Sierra Nevada. In addition, two radiometers with virtually identical characteristics were brought together on the Colorado Orographic Seeding Experiment at Steamboat Springs, Colorado, in order to study the stability and comparability of integrated vapor and liquid measurements.

Comparison of vapor measurements by the radiometer and by rawinsonde yielded a correlation coefficient of 0.94, and a rms difference of 0.08 cm. There were no significant differences between results for paired data gathered in storms in the presence of supercooled liquid water and for paired data gathered on clear days under more ideal conditions. The collocated-radiometer experiment showed slightly closer agreement than did the comparisons of radiometer data with rawinsondes. Comparisons of paired data from the vapor channel yielded a correlation coefficient of 0.95 with a rms difference of 0.05 cm, while the liquid water channel data yielded a correlation coefficient of 0.99 with a rms difference of 0.02 mm.

The study lends further credence to other theoretical estimations of the accuracy of the radiometer measurement, in that the measured values of integrated vapor are probably within 15% of truth. Measurements of supercooled liquid water are reproducible and very stable.

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J. B. Snider, F. O. Guiraud, and D. C. Hogg

Abstract

We report on observations of liquid water in clouds made by two independent ground-based microwave instruments. One system is a dual-frequency (20.6, 31.65 GHz) microwave radiometer designed to measure emission from the precipitable water vapor and from liquid in the zenith direction; we refer to this as System 1. The other system is a combination receiver-radiometer that utilizes absorption of a 28 GHz signal from the COMSTAR satellite to measure the liquid content of clouds; we refer to this as System 2. Comparative measurements on liquid-bearing clouds in Colorado are given.

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Kenneth Sassen, Robert M. Rauber, and J. B. Snider

Abstract

The temporal and spatial distribution of cloud liquid water in a winter storm from the 1983 Utah/NOAA Cooperative Weather Modification Program is characterized using remote sensing observations. The remote sensors, located at a mountain-base site near Beaver, Utah, consisted of a dual-channel microwave radiometer operated in an azimuthal scanning mode, and a polarization lidar and Ku-band radar both operated in the vertically pointing mode. The cloud system was associated with the passage of a weak cold front and produced only light snowfall across the barrier network of precipitation gages. Although the amounts of supercooled water detected radiometrically varied considerably during the storm, liquid water depths were consistently enhanced in the direction of the barrier. The spatial distribution of liquid water was observed to undergo a transition from a primarily orographic distribution to a more area-wide pattern immediately behind the front, and then became convective as the storm dissipated. A new method of analysis applied to the scanning microwave radiometer measurement appears promising for relating liquid water concentrations with the local topography. It is suggested that the near real-time availability of the measurements could lead to improvements in cloud seeding strategies.

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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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