Cloud Attenuation Research Articles

The Influence of the Climatic Peculiarities on the Electromagnetic Waves Attenuation in the Baltic Sea Region

The peculiarities of the climatic conditions in the Baltic Sea Region are reviewed. According to the values of rain rates measured in the Lithuanian Weather Stations with the 10- minute's integration time, a relation between the rain rate and the annual precipitation has been derived. The model for one-minute rain rate calculation on the months starting from May up to September in Lithuania has been presented. The values of the electromagnetic waves attenuation due to the rain have been determined. The cloud attenuation has been computed by using the meteorological data measured at the ground level. The semi empirical method has been used. The values of the speciflc attenuation under conditions of cloud cover have been determined. Knowledge of the electromagnetic waves attenuation due to rain and clouds is desirable while planning the communication systems, especially at frequencies of 10GHz and above. Most of the methodologies for the prediction of rain attenuation on radio waves paths require the knowledge of the rain rate R-value. Rain rate data is presented in units of length per unit time (in millimeters per hour), but in practice it is measured over intervals of typically one minute, flve minutes, hour or yearlong. In (1), it was mentioned that the integration time ¢t (time between the readings of the values of rainfall amount) is important parameter and it can change the R-value. Most of the rain attenuation prediction methods require one-minute rain rate value. One-minute rain rate (mm/h) is the rainfall for one minute (mm/min) multiplied by 60. Rainfall rate is highly variable. Therefore, the rain rate and rain attenuation are analyzed for concrete climatic conditions. However, the electromagnetic waves attenuation due to rain under the Baltic Sea Region climatic conditions has been analyzed not enough (2,3). The electromagnetic waves attenuation due the clouds, as far as we know, has been not analyzed under Lithuanian climatic conditions. The main goals of this paper were to determine the one-minute value of rain rate by using ten-minutes rainfall amount data measured in the Lithuanian Weather Stations and to estimate the values of the electromagnetic waves attenuation due to the rain and the clouds. 2. THE PECULIARITIES OF THE CLIMATIC CONDITIONS IN THE BALTIC SEA REGION The climate of Baltic Sea Region (BSR) is speciflc. BSR, being in the transition geography zone from Baltic Sea climate to Atlantic and continentals East Europe climate, may be distinguished for its variable climate. The average amount of precipitation in the BSR is 679mm. The average nebulosity in the region is 50%. The minimal nebulosity (46%) occurs in spring (4). The increase of nebulosity is going from north to south. In this paper we analyze Lithuanian climate more particularly. The climate of Lithuania is deflnable as middling cold, with the snowy winters. There are 20{30 thaw days even in mid-winter. In Lithuania, humid weathers predominate all over the year. The annual precipitation in rainy wet year is almost twice higher than in dry year. The values of annual rainfall measured in the localities of Lithuania are presented in Table 1. The showers are observed in the warm period of the year. Such climate is typical climate of the middle part of the East Europe. The climate of the west part of Lithuania (for example, of Klaipeda) is specifled as the moderate warm climate. The average temperature of the coldest month is more than i3 - C. Such type of the climate is dominating in the West Europe. Vilnius is one of the cloudiest localities of Lithuania. There are about 100 overcast days in the year.

Cloud Attenuation for Satellite Applications Over Equatorial Climate

A propagation experiment has been carried out at Penang using the SUPERBIRD-C satellite beacon. Cloud occurrences were observed during different months and it is seen that the low cloud occurrences over Penang is very significant from October to January. The cloud attenuation results that are presented, which include the testing of models, have been obtained from the data gathered over five years.

Cloud Attenuation in Millimeter Wave and Microwave Frequencies for Satellite Applications over Equatorial Climate

A propagation experiment has been carried out at Penang using the SUPERBIRD-C satellite beacon. Cloud occurrences were observed during different months and it is seen that the low cloud occurrences over Penang is very significant from October to January. The cloud attenuation results that are presented, which include the testing of models, have been obtained from the data gathered over five years. The specific attenuation of radio wave due to clouds at various frequencies 12 GHz, 20 GHz, 75 GHz, 50 GHz and 100 GHz has been estimated whereby the values varies from 0.14 dB/km at 12 GHz to 10.1 dB/km at 100 GHz.

Experimental verification of optical backhaul links for high‐altitude platform networks: Atmospheric turbulence and downlink availability

AbstractOptical backhaul downlinks from high‐altitude platforms (HAPs) are investigated. An experiment demonstrated the advantages of optical links: a small and lightweight terminal with low power consumption was launched to the stratosphere and data transmitted down to a ground station at a rate of 1.25 Gbit/s. Owing to the chosen system parameters and the high budget margin, disturbing turbulence effects did not decrease the link performance.The scientific aspect of the experiment was to study turbulence effects in order to design future systems with higher transmission performance. On the day of the experiment, measured scintillation and wavefront distortions were minimal in the morning. The best atmospheric conditions were observed about 3 h after sunrise with a peak of the atmospheric coherence length r0 at 16 cm. An r0 of 4 cm was measured as the worst case before sunrise and later during the day. This trend could also be observed for power‐ ($ {\sigma _P^2} $) and intensity scintillation index ($ {\sigma _I^2} $). The latter $ {\sigma _I^2} $ changed from 0.28 (best case) to 1.12. For small $ {\sigma _I^2} $, a lognormal intensity probability density function was measured.Apart from the robust intensity modulation scheme with direct detection which was used for the trial, future improved systems could benefit from a coherent transmission scheme. According to the r0 measurements and further simulations on heterodyne efficiency it turned out that the aperture size can be decreased from 40 to 10 cm without any significant change in the link margin.Future stratospheric optical links between HAPs or links from platforms to satellites will not suffer from cloud blockage but it remains an issue for up/downlinks to a ground station. This can be mitigated by ground‐station diversity. Four optical ground stations in the southern part of Europe can lead to an availability of over 98%. The separation distance of the ground stations is about 900 km with a negligible correlation of cloud cover. A change of wavelength from the employed 1.55 to a wavelength around 11‐µm with minimum cloud attenuation would increase the link availability for thin clouds. Copyright © 2007 John Wiley & Sons, Ltd.

Retrieving the median droplet diameter from Ka‐ and W‐band dual‐wavelength Doppler radar measurements

In this letter, a novel method is proposed for estimating the median droplet diameter by using the Ka‐ and W‐band dual‐wavelength differences of the mean Doppler velocity and the Doppler spectrum width. The estimated median diameter together with the dual‐wavelength ratio of the radar reflectivity can be effectively used to measure and compensate the attenuation in precipitation or clouds. The estimate of the median diameter is not affected by the attenuation and does not require an accurate radar calibration. Furthermore, the vertical air flows and the turbulent broadening of the Doppler spectra have no influence on the estimate.

Degradation of Radar Reflectivity by Cloud Attenuation at Microwave Frequency

Abstract The main object of this paper is to emphasize that clouds—the nonprecipitating component of condensed atmospheric water—can produce a strong attenuation at operational microwave frequencies, although they present a low reflectivity preventing their radar detection. By way of a simple and realistic model, simulations of radar observations through warm precipitating targets are thus presented in order to quantify cloud attenuation. Simulations concern an airborne radar oriented downward and observing precipitation at four frequencies: 3, 10, 35, and 94 GHz. Two cases are first considered: a convective cell (vigorous cumulus congestus plus rain) and a stratiform one (nimbostratus plus drizzle) superimposed on the previous one. Other simulations are then performed on different types of cumulus (congestus, mediocris, and humilis) with various thicknesses characterized, in a microphysical sense, by their maximum liquid water content. Simulations confirm the low cumulus reflectivity ranging from −45 dBZ for the weakest cumulus (i.e., the humilis one) to −5 dBZ for the strongest one (i.e., the vigorous cumulus congestus). It reaches −35 dBZ for the nimbostratus cloud. On the other hand, cumulus attenuation [precisely path-integrated cloud attenuation (PICA)] is not negligible and, depending on the frequency, can be very strong: the higher the frequency, the stronger the PICA. At 3 GHz, the far less attenuated frequency, PICA for the vigorous cumulus congestus alone in the convective cell (embedded into the stratiform background) is on the order of 1.2 dB (1.5 dB) at 10 GHz, 16 dB (20 dB) at 35 GHz, and 80 dB (100 dB) at 94 GHz. For weaker cumulus, PICA is lower but, in certain cases, significant. All these results mean that it is necessary to be very careful about radar measurements if reliable information on precipitation—for example, the precipitation rate R—has to be deduced, particularly at high operational frequencies.

Cloud Morphology Over Three Indian Tropical Stations for Earth Space Communication

Microwave and millimeter wave frequency bands are in demand for requirement of larger bandwidth for various applications of radio systems. In future in India too, microwave and millimeter wave frequencies will be in use very extensively for radio communication purposes and remote sensing applications. But, the attenuation due to cloud as well as thermal noise associated with could in millimeter wave and microwave frequency bands are of great concern to system engineers and radio researchers. Both cloud attenuation and cloud noise temperature lead to degradation in the performance of microwave communication and radar propagation. The effects of rain on radio wave are more than cloud but the occurrence of cloud is more than rain. In some parts of India cloud occurs for weeks together. The cloud morphology particularly in relation to radio wave propagation over different geographical region of India is therefore very essential and important. In view of this, systematic studies on cloud occurrence morphology over different geographical locations in India have been undertaken. In recent past some results on cloud occurrences over different stations in India have been reported. In this paper, cloud characteristics and cloud attenuation over three more stations are presented. Based on low level cloud coverage observations, cloud occurrences frequency over Mumbai (19.07°N, 72.50°E), Nagpur (21.06°N, 79.03°E) and Ahmadabad (23.04°N, 72.38°E) situated in Indian tropical west coast, central plain and semi arid region of western India in different months during daytime and nighttime has been deduced. The low-level cloud over Mumbai, Nagpur and Ahamedabad has been found to occur for many days and nights and particularly in the months of June, July, August and September. The attenuation of radio wave due to clouds at various frequencies ∼10, 18, 32, 44 and 70 GHz over the aforesaid three stations also been deduced.

Cloud Height, Cloud Temperature and Cloud Attenuation in Microwave and Millimeterwave Frequency Bands over Indian Tropical East Coast

Based on radar RHI (Range height indicator) measurements, cloud height has been deduced during the worst months (July–August) over Kolkata. Such cloud height results have been utilized to estimate cloud temperature. The attenuation of radiowave due to cloud in various probability levels has been determined in millimeter wave and microwave frequency bands. Such results on different probability levels are useful for satellite communication and remote sensing application over the aforesaid station in tropical India.

Validation and Improvement of Satellite-Derived Surface Solar Radiation over the Northwestern Pacific Ocean

The purpose of this study is to validate and improve satellite-derived downward surface shortwave radiation (DSSR) over the northwestern Pacific Ocean using abundant in situ data. The DSSR derivation model used here assumes that the reduction of solar radiation by clouds is proportional to the product of satellite-measured albedo and a cloud attenuation coefficient. DSSR is calculated from Geostationary Meteorological Satellite-5/Visible Infrared Spin-Scan Radiometer data in 0.05° × 0.05° grids. The authors first compare the satellite DSSR derived with a cloud attenuation coefficient table determined in past research with in situ values. Although the hourly satellite DSSR agrees well with land in situ values in Japan, it has a bias of +13∼+34 W/m2 over the ocean and the bias is especially large in the low latitudes. The authors then improve the coefficient table using the ocean in situ data. Usage of the new table successfully reduces the bias of the satellite DSSR over the ocean. The cloud attenuation coefficient for low-albedo cases over the ocean needs to be larger in the low latitudes than past research has indicated. Daily and hourly DSSR can be evaluated from the satellite data with RMS errors of 11–14% and 30–33%, respectively, over a wide region of the ocean by this model. It is also shown that the cloud attenuation coefficient over land needs to be smaller than over the ocean because the effect of the radiation reflected by the land surface cannot be ignored.

Version 2 data of the National Science Foundation's Ultraviolet Radiation Monitoring Network: South Pole

Spectral ultraviolet (UV) and visible irradiance has been measured at the South Pole between 1991 and 2003 by a SUV‐100 spectroradiometer, which is part of the U.S. National Science Foundation's UV Monitoring Network. Here we present a new data edition, labeled “Version 2.” The new version was corrected for wavelength shift errors and deviations of the spectroradiometer from the ideal cosine response. A comprehensive uncertainty budget of the new data set was established. Below 400 nm the expanded standard uncertainty (coverage factor 2) varies between 4.6 and 7.2%, depending on wavelength and sky condition. The uncertainty of biologically relevant UV irradiances is approximately 6%. Compared to the previously published data set, Version 2 UV data are higher by 5–14%, depending on wavelength, solar zenith angle (SZA), and year of observation. By comparing Version 2 data with results of a radiative transfer model, the good consistency and homogeneity of the new data set were confirmed. The data set is used to establish a UV climatology for the South Pole, focusing on the effects of aerosols, clouds, and total column ozone. Clouds are predominantly optically thin; 71% of all clouds have an optical depth between 0 and 1. The average attenuation of UV irradiance at 345 nm by clouds is less than 5% and no attenuations greater than 23% were observed. Attenuation by homogeneous clouds is generally larger in the visible than in the UV. The wavelength dependence of cloud attenuation is quantitatively explained with the wavelength‐dependent radiance distribution on top of clouds and the incidence‐angle dependence of cloud transmittance. Largest radiation levels occur in late November and early December when low stratospheric ozone amounts coincide with relatively small SZAs. Owing to the large effect of the “ozone hole,” short‐ and long‐term variability of UV during the austral spring is very high. When the ozone hole disappears, DNA‐damaging irradiance can decrease by more than a factor of two within 2 days. Typical summer UV index values range between 2 and 3.5 and vary by ±30% (±1σ) between different years. Linear regression analyses did not indicate statistically significant UV trends owing to the large year‐to‐year variability and the fact that the network was established only after the first occurrence of the ozone hole. Current measurements therefore document variability on an elevated level.

Effect of Fog and Clouds on the Image Quality in Millimeter Communications

The effect of fog and clouds in millimeter communication is discussed, and the attenuation caused by fog and clouds is reviewed. Signal-to-noise ratio (SNR) of image is derived using relating models of fog and clouds attenuation. According to the relation of image quality and its signal-to-noise ratio, the system behavior is forecasted theoretically. It is shown that the signal-to-noise ratio of receiver at certain transmitter power is inverse with radio wave frequency, from about 70dB at 10GHz to 48dB for fog and 49dB for clouds. The image quality of received signal at certain transmitter power is inverse with radio wave frequency, from about 7 grade at 10GHz to 5.27 grade for fog and 5.37 grade for clouds. The above calculated results are consistent with experimental results.

Moderation of Cloud Reduction of UV in the Antarctic Due to High Surface Albedo

To gauge the impact of clouds on erythemal (sunburn causing) UV irradiances under different surface albedo conditions, UV measurements from two Antarctic sites (McMurdo and South Pole Stations) and a midlatitude site (Lauder, New Zealand) are examined. The surface albedo at South Pole remains high throughout the year, at McMurdo it has a strong annual cycle, and at Lauder it is low throughout the year. The measurements at each site are divided into clear and cloudy subsets and are compared with modeled clear-sky irradiances to assess the attenuation of UV by clouds. A radiative transfer model is also used to interpret the observations. Results show increasing attenuation of UV with increasing cloud optical depth, but a high surface albedo can moderate this attenuation as a result of multiple scattering between the surface and cloud base. This effect is of particular importance at high latitudes where snow may be present during the summer months. There is also a tendency toward greater cloud attenuation with increasing solar zenith angle.

Effects of clouds on the surface erythemal UV-B irradiance at northern midlatitudes: estimation from the observations taken at Belsk, Poland (1999–2001)

Results of 2-year measurements at Belsk, Poland, have been analyzed to investigate the effects of clouds on ultraviolet (UV) radiation. Data were collected using a Solar Light UV-biometer 501A (erythemally weighed UV irradiances), Kipp&Zonnen CM11 pyranometer (total solar radiation), and SCI-TEC Brewer Spectrophotometer (total ozone, aerosol and thin cloud optical depth). Hourly reports on total cloud cover and cloud type were also used in this study. It is found that overcast conditions prevail over central Poland and the sky is covered mainly by low-level clouds. The averaged cloud properties derived from visual observations correspond with those estimated from satellite measurements. An empirical relationship has been postulated for the cloud reduction factor of the erythemally weighted UV radiation as a function of the cloud reduction factor for total solar radiation. The relation may be used to reconstruct the past UV-time series. It is established that information of cloud amount and type to estimate the UV-B attenuation provides a substantial uncertainty. Stronger attenuation of the UV radiation by real clouds was found than that estimated by a radiative transfer model when examining results of the cloud optical depth at 320 nm and the cloud attenuation of the UV radiation measurements. The reduction of the erythemally weighted UV irradiance by thin clouds seems to be independent of the cloud type. It is reported that 1.4% of UV-B irradiance measurements were cloud enhanced in the snowless period of the year. The maximum UV-B enhancement of ∼20% greater than an equivalent clear-sky value was found. Numerous enhancements occurred when high-, mid-, and low- (broken Cu) clouds appeared simultaneously. Similar range of the observed enhancements and cloud composition as in previous cloud enhancement studies (over low latitudinal regions) suggest that common mechanisms may govern this phenomenon in low- and mid-latitudinal regions.

Role of aerosols on the Mediterranean solar radiation

The shortwave radiation constitutes a major driving force for the oceanic thermohaline circulation and must be accurately parameterized. An extensive comparison between the monthly mean solar radiation measured at numerous meteorological stations along the Mediterranean coast and the estimated solar radiation from a widely used bulk formula leads to two important conclusions: first, there is systematic overestimation of the shortwave radiation from the bulk formula at the station sites of about 25 W m−2 (averaged over a 30 year period), and second, there is significant interannual variability in the observed solar radiation that cannot be explained either by the variability in the available data for the cloudiness or by the absorption due to water vapor content in the atmospheric column. As the overestimation occurs during summer cloud‐free months, we assume that cloud attenuation is adequately parameterized and the discrepancy between the observations and the estimates comes from the formula for clear‐sky insolation. A correction to the clear‐sky insolation formula is attempted on the basis of recent satellite data on aerosol optical thickness index. Results from our analysis indicate that aerosols may provide an explanation for both the observed weaker shortwave radiation and its interannual variations. This implies that a different parameterization scheme must be sought for the aerosol attenuation in the shortwave radiation formula.

One-year cloud attenuation results at 50 GHz

A propagation experiment has been carried out in Madrid using the ITALSAT F1 satellite beacon at 50 GHz. The cloud attenuation results that are presented, which include the testing of models, have been obtained from data gathered over one year.

Version 2 total ozone mapping spectrometer ultraviolet algorithm: problems and enhancements

Satellite instruments provide global maps of surface UV irradiance by combining backscattered radiance measurements with radiative transfer models. The accuracy of the models is limited by uncertainties in input parameters representing the atmosphere and the Earth's surface. To reduce these uncertainties, we have made enhancements to the currently operational TOMS surface UV irradiance algorithm (Version 1) by including the effects of diurnal variations of cloudiness, an improved treatment of snow/ice, and a preliminary aerosol correction. We compare results of the version 1 TOMS UV algorithm and the proposed version. We evaluate different approaches for improved treatment for average cloud attenuation within a satellite pixel, with and without snow/ ice on the ground. In addition to treating cloud transmission based only on the measurements at the local time of the TOMS observations, the results from other satellites and weather assimilation models can be used to estimate atmospheric UV irradiance transmission throughout the day. A new method is proposed to obtain a more realistic treatment of the effects from snow-covered terrain. The method is based on an empirical relation between UV reflectivity and measured snow depth. The new method reduces the bias between the TOMS UV estimations and ground-based UV measurements for snow periods. We also briefly discuss the complex problem of estimating surface UV radiation in presence of UV-absorbing aerosols. The improved (Version 2) algorithm can be applied to reprocess the existing TOMS UV irradiance and exposure estimates (since November 1978) and to future satellite sensors (e.g., GOME-2, OMI on EOS/Aura, and Triana/EPIC).

Use of bubble cloud attenuation measurements to determine the bubble size distribution and comparison with backscatter measurements

This paper presents the method of using bubble cloud attenuation measurements to determine the bubble size distribution, taking into account the effect of time-dependent bubble scatter and absorption. It then shows that the results of attenuation measurements due to bubble clouds (generated by a ship’s wake) agree with backscatter measurements made at a much higher frequency.

Cloud Characteristics and Cloud Attenuation in Millimeter Wave and Microwave Frequency Bands for Satellite and Remote Sensing Applications over a Northern Indian Tropical Station

Microwave and millimeter wave frequency bands are in demand for requirement of more channels in radio communication systems. It has also been recognized that microwave and millimeterwave frequency radiometers on board satellites as promising tools for remote sensing. The frequency more than 10 GHz is affected by rain and cloud. Though the effects of rain on radiowave is more than cloud but the occurence of cloud is more than rain. Cloud has been found to occur for weeks together over this part of the world. It is therefore essential to study cloud morphology over different geographical region. In this paper, an attempt has been made to the cloud occurrences over an Indian tropical station, Delhi (28.35°N, 77.12°E) observed during different months and daytime and nighttime. It is seen that low clouds occurrence over Delhi is very significant and particularly during July, August and September. The specific attenuation of radiowave due to clouds at various frequencies 10 GHz, 20 GHz, 50 GHz and 100 GHz has been deduced. The specific attenuation of radio wave due to cloud at 10 GHz varies from 0.0608 dB/km to 0.1190 dB/km while at 100 GHz the specific attenuation varies from 6.8460 dB/km to 11.9810 dB/km

Cloud attenuation modelling for SHF and EHF applications

AbstractTropospheric propagation impairments that affect earth–space communication signals increase in severity with the increase of frequency. Introduction of satellite services using higher frequency bands such as Ka‐ and V‐band requires the characterization of propagation factors that are normally considered negligible at lower frequency bands. Cloud attenuation is considered one such factor. Clouds are present during a large fraction of an average year and cloud attenuation, together with gaseous absorption, will determine the system performance under non‐rainy conditions. A cloud attenuation model that has global applicability is presented in this paper. The model is based on average properties of four cloud types and their occurrence probabilities. The occurrence probabilities are derived from long‐term observations of cloud cover carried out by several thousand meteorological stations throughout the world. Predictions made with the model are compared with cloud attenuation data gathered using Ka‐band beacon signals and radiometers. Copyright © 2001 John Wiley & Sons, Ltd.

Millimeter-Wave Attenuation Due to Fog and Clouds

The parameters of gamma drop size distribution model of fog and clouds are derived based on the liquid water content and optical visibility, the attenuation are calculated and discussed with this model and empirical relations of the liquid water content and the visibility or other parameters of fog and clouds. A new empirical formula to estimate fog and clouds attenuation is presented based on the Reyleigh absorption approximation, which is more accurate in general and can be used in wider frequency and temperature range than other empirical formulas.