Cloud Attenuation Research Articles

Cloud Transmission Estimates of UV-B Erythemal Irradiance

Two UV-Biometer 501A instruments were used to estimate global erythemal irradiance at two locations in southwest Sweden; the Earth Sciences Centre, University of Goteborg (57.69° N; 11.92° E) and the island of Nordkoster, 200 km to the north (58.83° N; 10.72° E). A semi-empirical radiative transfer model was used to calculate the global erythemally effective irradiance under clear skies. A ratio of the hourly measured to clear-sky modelled irradiance was then derived for zenith angles 35–70°. Subsequent comparisons were then made with routine measurements of sunshine duration at Goteborg and sunshine duration, cloud cover, type and height at Nordkoster. Cloud transmission of UV-B irradiance decreases with increasing solar zenith angle, with cloud attenuation being 8% stronger at Nordkoster Island for zenith angles >>;60°. Transmission also decreases with increasing cloud cover such that overcast cloud conditions reduce transmissions by an average of 75%. In addition, cloud type affects the amount of ground incident irradiant flux. Fractus cloud afforded the least UV-B transmission (0.16), while cirrus filaments afforded the most (0.95). The spatial and temporal distribution of clouds appears tobe non-random. Under conditions of 1 to 3 octas, sky cover, clouds appear to be concentrated in line with the sensor and Sun on more occasions than that expected given a random cloud distribution. The same cloud cover condition also resulted in many instances of ground incident irradiance above clear-sky values. The presence of cumuliform clouds appears to increase the likelihood of the latter phenomena.

Find the firn line! The suitability of ERS-1 and ERS-2 SAR data for the analysis of glacier facies on Icelandic icecaps

Icelandic Icecaps are sensitive indicators of climatic fluctuations. Glacier facies boundaries can be identified using remote sensing techniques; SAR data represents the first real opportunity to monitor glacier facies, without cloud attenuation at regular or inter-annual intervals. This study assesses the overall suitability of the data for the monitoring of glacier facies and in particular the firn line altitude (FLA) on Icelandic Icecaps. The results show that the firn line can be identified and located to within +/- 20-40m (vertically). These results were validated using aerial photograph and ground data. RAIDS (Rapid Information Dissemination System) processed data were found to compare favourably with other satellite data for this type of application.

GPS phase-delay measurement: Technique for the calibration and analysis in millimetre-wave radio propagation studies

Observation of phase delay from GPS satellites, followed by signal and data processing allows the quantity of water vapour in the atmosphere to be continuously monitored. At millimetre-wavelengths, water vapour has significant impact on radiowave propagation, along with rain, cloud and oxygen. Comparisons are presented of integrated precipitable water vapour measured by GPS with a co-sited radiometer and with radiosonde measurements from a nearby site. Data are also presented in which the liquid water (cloud and rain) attenuation is separated from radiometer predicted total attenuation using the measurement of water vapour by GPS. The GPS technique thus represents a new tool to study radiowave propagation, and aids in the calibration of radiometers operating in frequency bands that are sensitive to water vapour.

Measurement and modelling of cloud attenuation at millimetre wavelengths

Results are presented from a 93 GHz radiometer over a one-year period. A cloud attenuation model, which uses measurements from radiosonde profiles, has been used to provide a comparison. The effects of rain, water vapour and oxygen have been removed from the radiometer total attenuation record through the use of GPS observations and meteorological measurements at ground level. Agreement between the radiometer and cloud model is good, indicating that the cloud model is able to give an accurate prediction of attenuation due to liquid water clouds.

A prediction model that combines rain attenuation and other propagation impairments along Earth-satellite paths

The rapid growth of satellite services using higher frequency bands such as the Ka-band has highlighted a need for estimating the combined effect of different propagation impairments. Many projected Ka-band services will use very small terminals and, for some, rain effects may only form a relatively small part of the total propagation link margin. It is therefore necessary to identify and predict the overall impact of every significant attenuating effect along any given path. A procedure for predicting the combined effect of rain attenuation and several other propagation impairments (at frequencies between 4 and 35 GHz) along Earth-satellite paths is presented. Where an accurate model exist for some phenomena, these have been incorporated into the prediction procedure. New models were developed, however, for rain attenuation, cloud attenuation, and low-angle fading to provide more overall accuracy, particularly at very low elevation angles (<10/spl deg/). In the absence of a detailed knowledge of the occurrence probabilities of different impairments, an empirical approach is taken in estimating their combined effects. An evaluation of the procedure is made using slant-path attenuation data that have been collected with simultaneous beacon and radiometer measurements which allow a near complete account of different impairments. Results indicate that the rain attenuation element of the model provides the best average accuracy globally between 10 and 30 GHz and that the combined procedure gives prediction accuracies comparable to uncertainties associated with the year-to-year variability of path attenuation.

Using Multiparameter Radar to Estimate the Attenuation and Water Content of Clouds

Abstract The attenuation of microwaves is caused not only by precipitation but also by clouds. Consequently, the presence of liquid cloud can affect estimates of rainfall rate computed from attenuation and reflectivity factors measured at higher frequencies typically used for spaceborne and airborne radars. Cloud attenuation also affects ground-based radar measurements of rainfall at frequencies as low as 5 GHz. This paper suggests an approach for determining the attenuation due to cloud (AC) and for estimating the cloud water content (WC) even in moderate rain by using radars operating at two frequencies with one of them capable of dual-linear (horizontal-vertical) polarization measurements. This analysis suggests that useful “instantaneous” estimates of AC and WC should be possible when an upper frequency of 13.8 GHz is used in conjunction with a lower frequency. These measurements could also be used to derive cloud attenuation statistics, potentially useful for developing techniques to help compensate ...

Implication of super-large drops in millimetre-wave radar observations of water clouds

Interpretation of radar observations of clouds employing millimetre waves may be confused by the presence of super-large drops in clouds. In the Letter, the effect of these drops on cloud attenuation (dB) and reflectivity (dBZ) is assessed, and the implications for radar system design discussed.

Artificial bubble cloud targets for acoustic remote sensing of the ocean

Theoretical calculations indicate that bubble clouds with void fraction of O(1%) and size of O(20 cm) produce large scatter due to collective oscillations. It is suggested that artificial bubble cloud targets can be used for acoustical remote sensing of ocean environment. The flow following property of such targets is excellent because the target density is quasineutral buoyant. The void fraction, bubble size distribution, and cloud geometry can be controlled. A technique using polymer solutions to stabilize bubble clouds was developed. Resonant tube measurements indicated that the acoustic velocity and attenuation of artificial clouds are similar to bubble clouds in water. This technique can be applied to the designs of underwater acoustic targets for sonar calibration, simulation of wave breaking, and neutral buoyant tracers for tracking ocean currents. The use of artificial bubble clouds targets is specially valuable for low wind condition and applications in deeper regions where bubbles generated at the surface rarely survive. Larger bubbles can also be transported into deeper regions by artificial clouds. [Work supported by ARPA.]

Direct measurements of the sound speed in the oceanic surface bubbly layer

It is well known that wave breaking events create a bubbly layer a few meters thick below the water surface. The most important characteristic of this shallow channel is its sound-speed profile. Results are presented from two recent field experiments in which the sound-speed profile was directly measured in the first few meters below the surface for frequencies in the range 5–40 kHz. Preliminary results in moderate sea states (wind speeds less than 10 m/s) indicate that significant departures from the sound speed in bubble-free water occur in a shallow [O(1 m)] layer and are intermittent in time. Two dominant time scales are present. A fast time scale associated with the surface waves, which advect the bubble clouds horizontally with respect to the buoy; and a slow time scale O(100 s). Sound-speed reductions up to 150 m/s have been observed at 50-cm depth with 10-min means well above the 5 to 15 m/s previously cited in the literature. Other results including ambient sound, acoustic attenuation, and upward looking sonar measurements of the bubble clouds will be discussed. [Work supported by ONR.]

C-Band Attenuation by Tropical Rainfall in Darwin, Australia, Using Climatologically TunedZe-RRelations

The probability matching method (PMM) is used as a basis for estimating attenuation in tropical rains near Darwin, Australia. PMM provides a climatological relationship between measured radar reflectivity and rain rate, which includes the effects of rain and cloud attenuation. When the radar sample is representative, PMM estimates the rainfall without bias. When the data are stratified for greater than average rates, the method no longer compensates for the higher attenuation and the radar rainfall estimates are biased low. The uncompensated attenuation is used to estimate the climatological attenuation coefficient. The method is applicable to any wavelength. The two-way attenuation coefficient was found to be 0.0085 dB km−1 (mm h−1)−1.08 for the tropical rains and associated clouds in Darwin for the first 2 months of the year for horizontally polarized radiation at 5.63 GHz. This unusually large value is discussed. The risks of making real-time corrections for attenuation are also treated.

Derivation of total ozone abundance and cloud effects from spectral irradiance measurements

We describe a method to infer total ozone abundance and effective cloud transmission from global (diffuse plus direct) spectral irradiance measurements taken at the Earth's surface. The derivation of total ozone abundance relies on the comparison of measured irradiance ratios at two wavelengths in the UV part of the spectrum with a synthetic chart of this ratio computed for a variety of ozone abundances. One of these wavelengths should be appreciably absorbed by ozone (e.g., 305 nm) compared with the other one (e.g., 340 nm). This synthetic ratio (and therefore also the inferred total ozone abundance) is insensitive to the value of the surface albedo used in the model computations. Comparison with independent in situ and remote (from ground and space) determinations of total ozone abundance shows that measurements of global irradiances provide a reliable means of inferring the total column ozone amount for clear as well as cloudy sky conditions. Computer simulations are used to demonstrate that the ozone abunance inferred from global irradiance measurements is quite insensitive to cloud effects, whereas the use of the scattered irradiance only or the zenith sky intensity (measured routinely in the Dobson network on overcast days) requires substantial corrections for cloud effects. Effective cloud transmission is estimated from the data by comparing the measured irradiance at a wavelength where ozone absorption is minimal (e.g., 350 nm) to the clear-sky value. Irradiances generated by a plane-parallel radiation model as a function of cloud optical thickness are used to estimate an equivalent stratified cloud optical depth. These estimates of cloud transmission and optical depth are sensitive to ground reflection, implying that the accurate determination of cloud attenuation requires precise knowledge of the surface albedo.

New prediction method of cloud attenuation

A new method for cloud attenuation calculation on an earth–satellite link is presented. The method uses temperature and humidity profiles as input parameters. The method has been applied to radiosonde measurements and to the gridded data of the numerical analyses of ECMWF.

The main results of cloud and rain remote sensing by multichannel microwave radiometry

The basic results have been considered for multiwave remote sounding of the troposphere with clouds and rain from ground-based station in the zenith direction. The radio wave scattering by rain drops as well as variation of vapour content in the atmosphere during measurements are taken into account when processing of the experimental data. A separation of the complete attenuation in clouds with rain has been made over three components: due to vapour, cloud and rain. A relation is considered of millimeter and centimeter wave attenuation in clouds with rain between each other and with the rain intensity. A behaviour of the relation of attenuation structural functions is explained. The Diagnostic problems of millimeter wave attenuation are considered.

Statistics of clear-air attenuation on satellite links at 20 and 30 GHz

The letter reports on results obtained from a radiometer measurement campaign to study clear-air attenuation phenomena at 20 and 30 GHz. Statistics for cloud and water vapour attenuation for a one-year period are presented.

Cloud attenuation at millimeter wavelengths

Total atmospheric attenuation under conditions of complete and partial cloud cover was measured at frequencies of 15 and 35 GHz in the Boston area. The attenuations were actually inferred from extinction measurements using the Sun as a source. Measurements were made at 29 elevation angles from 1 degrees to 20 degrees , and the angular dependence of the attenuation was examined. For most cloud conditions the attenuation was found to be proportional to the slant path distance through the absorbing atmosphere. For elevation angles above about 8 degrees , a flat Earth approximation is valid and the slant path distance is proportional to the cosecant of the elevation angle. For low elevation angles the slant path distance is a function of the effective earth radius and the effective height of the attenuating atmosphere, in addition to the elevation angle. A statistical technique for determining the radius and height is described. A zenith attenuation was extrapolated from each set of data. The humidity and frequency dependence of the attenuation was examined. An algorithm for estimating total atmospheric attenuation as a function of elevation angle, frequency, and surface absolute humidity was derived. >

Propagation predictions for low-elevation, low-availability satellite systems

The letter discusses the problem of predicting propagation losses occurring for more than 0.1% of the time, for satellite links with low elevation angles in the 20/30 GHz band. Results using simple algorithms taking into account the effects of cloud and rain attenuation, gaseous absorption and scintillation are presented.

Semiempirical model for cloud attenuation prediction

This letter presents a semiempirical model for the estimation of annual cumulative statistics of cloud attenuation. The statistics are predicted from ground-based humidity and temperature measurements. Some of the model parameters have been derived from radiometer measurements carried out at 20 and 30GHz.

Effects of atmosphere on radio imaging for target detection

The predictions of atmospheric effects on millimeter wave radio imaging of launch vehicles can be obtained by theoretical extrapolation of measured data from propagation studies for millimeter wave satellite communication systems. Attenuation exceedance statistics versus percentage of time have been estimated for both relatively wet and dry temperate climates. Attenuation problems are minimal at 10 GHz, but increase with frequency. Excessive rain attenuation is expected at 30 GHz for a small fraction of time. Cloud attenuation becomes also significant at 100 GHz. Water vapor in clear air can often cause more than 10 dB attenuation at 300 GHz. By use of the reciprocity theorem the effects of angle‐of‐arrival fluctuations on beam spot resolution have been found to be negligible for a space‐based telescope detecting Earth‐launched vehicles. Beam coverage area averaging can reduce cloud‐induced signal scintillations to less than 1 dB. These statements are valid except at very low elevation angles approaching tangent to the Earth, where the dominant problem is excessive signal attenuation.

Measurements of cloud contribution to rain millimeter wave attenuation by using a radar and radiometers

The measurement technique of cloud contribution to rain attenuation and the equipment consisted of the coherent pulse Doppler radar at wavelength λ=3.2 cm, the radiometers at λ=0.4; 0.8 and 1.35 cm and apparatus for signal recording and processing is described. The results of such measurements are given. The Doppler spectrum of the rain backscattered radar signal was used for determination of rain drop size distribution height profile then rain attenuation was calculated and cloud attenuation was determined as the difference between the total attenuation measured by using the radiometers and rain attenuation. The results of this work gave possibility to improve the known rain model of P.Misme for prediction of rain attenuation statistics for Earth-satellite links at millimeter wave.

Predicting Daily Insolation with Hourly Cloud Height and Coverage

Solar radiation information is used in crop growth, boundary layer, entomological and plant pathological models, and in determining the potential use of active and passive solar energy systems. Yet solar radiation is among the least measured meteorological variables. A semi-physical model based on standard meteorological data was developed to estimate solar radiation received at the earth's surface. The radiation model includes the effects of Rayleigh scattering, absorption by water vapor and permanent gases, and absorption and scattering by aerosols and clouds. Cloud attenuation is accounted for by assigning transmission coefficients based on cloud height and amount. The cloud transmission coefficients for various heights and coverages were derived empirically from hourly observations of solar radiation in conjunction with corresponding cloud observations at West Lafayette, Indiana. The model was tested with independent data from West Lafayette and Indianapolis, Madison, WI, Omaha, NE, Columbia, MO, Nashville, TN, Seattle, WA, Los Angeles, CA, Phoenix, AZ, Lake Charles, LA, Miami, FL, and Sterling, VA. For each of these locations a 16% random sample of days was drawn within each of the 12 months in a year for testing the model. Excellent agreement between predicted and observed radiation values was obtained for all stations tested. Mean absolute errors ranged from 1.05 to 1.80 MJ m−2 day−1 and root-mean-square errors ranged from 1.31 to 2.32 MJ m−2 day−1. The model's performance judged by relative error was found to be independent of season and cloud amount for all locations tested.