Water Vapor Attenuation Research Articles

Quality Assessment and Application Scenario Analysis of AGRI Land Aerosol Product from the Geostationary Satellite Fengyun-4B in China.

The Advanced Geostationary Radiation Imager (AGRI) sensor on board the geostationary satellite Fengyun-4B (FY-4B) is capable of capturing particles in different phases in the atmospheric environment and acquiring aerosol observation data with high spatial and temporal resolution. To understand the quality of the Land Aerosol (LDA) product of AGRI and its application prospects, we conducted a comprehensive evaluation of the AGRI LDA AOD. Using the 550 nm AGRI LDA AOD (550 nm) of nearly 1 year (1 October 2022 to 30 September 2023) to compare with the Aerosol Robotic Network (AERONET), MODIS MAIAC, and Himawari-9/AHI AODs. Results show the erratic algorithmic performance of AGRI LDA AOD, the correlation coefficient (R), mean error (Bias), root mean square error (RMSE), and the percentage of data with errors falling within the expected error envelope of ±(0.05+0.15×AODAERONET) (within EE15) of the LDA AOD dataset are 0.55, 0.328, 0.533, and 34%, respectively. The LDA AOD appears to be overestimated easily in the southern and western regions of China and performs poorly in the offshore areas, with an R of 0.43, a Bias of 0.334, a larger RMSE of 0.597, and a global climate observing system fraction (GCOSF) percentage of 15% compared to the inland areas (R = 0.60, Bias = 0.163, RMSE = 0.509, GCOSF = 17%). Future improvements should focus on surface reflectance calculation, water vapor attenuation, and more suitable aerosol model selection to improve the algorithm's accuracy.

Radiometric Measurements of Atmospheric Attenuation Over a Tropical Location

AbstractThe information of atmospheric attenuation at Ka‐band is important during rain and clear air condition. In this paper, Ka‐band attenuation data obtained from radiometric measurements at Kolkata, India (22°034′E, 88°022′N) have been presented. The role of water vapor is examined in determining the behavior of atmospheric attenuation during and without rain condition at 22.24, 26.24, and 31.4 GHz frequencies. It has been observed that at lower rain rate (<5 mm/hr), attenuation at 22.24 GHz is the highest among three frequencies due to water vapor absorption at this frequency. Millimeter Wave Propagation (MPM) model has been used to observe the contribution of rain and water vapor in determining atmospheric attenuation for different rain rates. It has been noted that at 22.24 GHz, water vapor attenuation is comparable to the rain attenuation up to 5 mm/hr. The measured attenuation from radiometer has been compared with ITU‐R model‐generated data and both the attenuation data match reasonably well up to rain rate 20 mm/hr. The attenuation obtained from radiometric measurements shows saturation with increasing rain rate due to the darkening effect. The ITU‐R frequency scaling model has been used to compare the measured and calculated attenuation at 31.4 GHz. This study gives some useful information about the radiometric measurements of atmospheric attenuation at a tropical location that shows that not only rain rate but also water vapor is an important cause of attenuation at Ka‐band.

Method for retrieval of atmospheric water vapor using OH airglow for correction of astronomical observations

Context. Water vapor in the atmosphere undergoes quick spatial and temporal variations. This has a serious impact on ground-based astronomical observations from the visible band to the infrared band resulting from water vapor attenuation and emission. Aims. We seek to show how the sky spectrum of an astronomical observation can be used to determine the amount of precipitable water vapor (PWV) along the line of sight toward the science target. Methods. In this work, we discuss a method to retrieve the PWV from the OH(8-3) band airglow spectrum. We analyzed the influences of the pressure and temperature of the atmosphere and the different water vapor vertical distributions on the PWV retrieval method in detail. Meanwhile, the accuracy of the method was analyzed via Monte Carlo simulations. To further verify the method of PWV retrieval, we carried out cross comparisons between the PWV retrieved from OH airglow and PWV from the standard star spectra of UVES using equivalent widths of telluric absorption lines observed from 2000 to 2016 at Cerro Paranal in Chile. Results. The Monte Carlo tests and the comparison between the two different methods prove the availability the PWV retrieval method from OH airglow. These results show that using OH airglow spectra in astronomical observations, PWVs along the same line of sight as the astronomical observations can be retrieved in real time. Conclusions. We provide a quick and economical method for retrieving the water vapor along the same line of sight of astronomical observation in the real time. This is especially helpful to correcting the effect of water vapor on astronomical observations.

A robust automated technique for operational calibration of ceilometers using the integrated backscatter from totally attenuating liquid clouds

Abstract. A simple and robust method for calibrating ceilometers has been tested in an operational environment, demonstrating that the calibrations are stable to better than ±5 % over a period of a year. The method relies on using the integrated backscatter (B) from liquid clouds that totally extinguish the ceilometer signal; B is inversely proportional to the lidar ratio (S) of the backscatter to the extinction for cloud droplets. The calibration technique involves scaling the observed backscatter so that B matches the predicted value for S of 18.8±0.8 sr for cloud droplets, at ceilometer wavelengths. For accurate calibration, care must be taken to only use profiles where the range correction is implemented and to exclude any profiles having targets with different values of S, such as drizzle drops and aerosol particles, profiles that do not totally extinguish the ceilometer signal, profiles with low cloud bases that saturate the receiver, and any profiles for which the window transmission or the lidar pulse energy falls below 90 %. A range-dependent multiple-scattering correction that depends on the ceilometer optics should also be applied to the profile. For ceilometers operating at around 910 nm wavelength, a simple correction for water vapour attenuation is applied to the signal using the vapour profiles from a forecast analysis. For a generic ceilometer in the UK the 90 d running mean of the calibration coefficient over a period of 20 months is constant to within 3 % with no detectable annual cycle, thus confirming the validity of the humidity and multiple-scattering correction. For Gibraltar, where cloud cover is less prevalent than in the UK, the 90 d running mean calibration coefficient was constant to within 4 %. The more sensitive ceilometer model operating at 1064 nm is unaffected by water vapour attenuation but is more prone to saturation in liquid clouds; such profiles can be recognised and rejected and, despite the more restricted sample of cloud profiles, a robust calibration is readily achieved. In the UK, the running mean 90 d calibration coefficients varied by about 4 % over a period of 1 year. The consistency of profiles observed by nine pairs of co-located ceilometers in the UK Met Office network operating at around 910 and 1064 nm provided independent validation of the calibration technique. In all cases, if quantitative and reliable backscatter observations are to be obtained it is essential to keep the window clean. This may be a challenge in dusty locations. EUMETNET is currently networking 700 European ceilometers so they can provide ceilometer profiles in near real time to European weather forecast centres and has adopted the cloud calibration technique described in this paper for ceilometers with a wavelength of around 910 nm.

Attenuation Induced by Water Vapor Along Earth-Space Links: Selecting the Most Appropriate Prediction Method

This communication presents an improved approximate method for the estimation of water vapor attenuation along Earth-space slant paths, over the 1–350-GHz frequency range. Evaluated against a large set of radiosonde data, the proposed method provides more accurate results than those delivered by the current alternative approximation methods included in Annex 2 of recommendation ITU-R P.676-10. This result is important as it sheds light on which approximate water vapor attenuation model should be selected for predictions along Earth-space slant paths.

Improving the Accuracy in Predicting Water-Vapor Attenuation at Millimeter-Wave for Earth-Space Applications

This contribution focuses on improving the accuracy of the simplified method currently recommended by the ITU-R for the prediction of water-vapor attenuation at millimeter-wave on Earth-space links (Annex 2, paragraph 2.3 of recommendation ITU-R P.676-10), which receives as input only the local integrated water-vapor content $V$ . The main improvement to such model originates from taking into account the dependence of the water-vapor mass absorption coefficient $a_{V}$ on the reference site altitude, which is investigated by taking advantage of an extensive set of radiosonde observations (RAOBS) collected in several sites worldwide and characterized by high accuracy and reliability. Tested against attenuation estimates obtained from the mass absorption models coupled with the mentioned RAOBS data, the model’s prediction accuracy turns out to improve significantly with respect to the current recommendation and to be less dependent both on the operational frequency (20–100 GHz range) and on the considered site.

Approximate Method for Estimating Gaseous Loss at Very Low Angles Along Paths of Finite Length and Earth-Space Paths

This paper presents an approximate method for estimating gaseous absorption at very low elevation angles along paths of finite length or Earth-space paths, taking into account both the effects of the Earth curvature and refraction. An exponential atmosphere is assumed. Gaseous absorption is evaluated by breaking its integral representation into a number of smaller pieces that are evaluated in closed form by the leading term of their asymptotic expansions; there is no need for numerical integration. The new method extends the validity of Annex 2 of Recommendation ITU-R P.676-10 to path elevation angles below 5°. Comparisons with the line-by-line method of the same Recommendation, using radiosonde data as input, show good to very good agreement at 20, 30, and 350 GHz for an Arctic location, a tropical area in Australia, and two midlatitude locations-one in the Rocky Mountains and the other closer to sea level. Some limitations of the approximate method over very high mountains, where surface pressure is lower than the 780-hPa reference of the water vapor attenuation model of Annex 2, are also discussed.

Rain Attenuation Effects on 2.6 GHz WiMAX Networks Deployment in Ghana

WiMAX communication systems operating at 2.6 G frequencies are used for broadband multimedia and internet based services. At these frequencies, the signal will be affected by various propagation impairments such as rain attenuation, cloud attenuation, tropospheric scintillation, ionospheric scintillation, water vapour attenuation, and rain and ice depolarization. Among all the pro-pagation impairments, rain attenuation is the most important and critical parameter. In this research, rain attenuation is calculated at KNUST, Kumasi using ITU-R rain attenuation model. The preliminary results of the work will be used to calculate the attenuation experimentally and comparison can be made, which helps to develop a new rain attenuation model at 2.6 G bands. Rain attenuation is an important aspect of signal propagation above 2.6 GHz frequency. The attenuation time series generation from point rain rate measurement is crucial due to unavailability of actual signal measurements. In this research, a simple and realistic approach has been demonstrated for better estimation of rain attenuation using WiMAX-band signal propagation data and ground rain rate measurements in Ghana. The ITU-R model of rain attenuation has been modified by incorporating an effective slant path model. The effective slant path has been estimated and modeled in terms of a power-law relationship of rain rate data of 2007-2008. The methodology has been validated with the measured data of 2014. Comparison with ITU-R and GMET clearly demonstrates the improved predictability of the proposed model at the present tropical location.

Ultrafast broadband terahertz waveform measurement utilizing ultraviolet plasma photoemission

This paper reviews the recent progress in the development of ultrafast broadband terahertz waveform measurement utilizing ultraviolet photoemission from laser-induced gases. We present the theoretical study and experimental investigation on ultrafast dynamics of terahertz-enhanced photoemission in various gas species. By using two-color laser field to coherently control the electron drift velocity and subsequent terahertz-enhanced photoemission, we develop an “all-optical” broadband terahertz waveform measurement method, which, by encoding terahertz pulse information into plasma photoemission, is capable of remote operation due to the minimized water vapor attenuation and unlimited optical signal collection. Broadband terahertz waveform measurement at a distance of 10 m is demonstrated.

Utilizing the MODIS 1.38 μm channel for cirrus cloud optical thickness retrievals: Algorithm and retrieval uncertainties

The cloud products from the Moderate Resolution Imaging Spectroradiometers (MODIS) on Terra and Aqua have been widely used within the atmospheric research community. The retrieval algorithms, however, oftentimes have difficulty detecting and retrieving thin cirrus, due to sensitivities to surface reflectance. Conversely, the 1.38 μm channel, located within a strong water vapor absorption band, is quite useful for detecting thin cirrus clouds since the signal from the surface can be blocked or substantially attenuated by the absorption of atmospheric water vapor below cirrus. This channel, however, suffers from nonnegligible attenuation due to the water vapor located above and within the cloud layer. Here we provide details of a new technique pairing the 1.38 μm and 1.24 μm channels to estimate the above/in‐cloud water vapor attenuation and to subsequently retrieve thin cirrus optical thickness (τ) from attenuation‐corrected 1.38 μm reflectance measurements. In selected oceanic cases, this approach is found to increase cirrus retrievals by up to 38% over MOD06. For these cases, baseline 1.38 μm retrieval uncertainties are estimated to be between 15 and 20% for moderately thick cirrus (τ > 1), with the largest error source being the unknown cloud effective particle radius, which is not retrieved with the described technique. Uncertainties increase to around 90% for the thinnest clouds (τ < 0.5) where instrument and surface uncertainties dominate.

Influence of Absorption by Environmental Water Vapor on Radiation Transfer in WildLand Fires

The attenuation of radiation transfer from wildland flames to fuel by environmental water vapor is investigated. Emission is tracked from points on an idealized flame to locations along the fuel bed while accounting for absorption by environmental water vapor in the intervening medium. The Spectral Line Weighted-sum-of-gray-gases approach was employed for treating the spectral nature of the radiation. The flame and fuel bed for the simulations are modeled two-dimensionally with the flame being one-tenth as long as the fuel bed. Flame heights of 1 and 10 m were explored, and both vertical and angled flames were studied. Simulated flame temperatures of 1000 K and 1500 K were investigated. The study reveals that the effect of absorption of flame radiation by environmental water vapor is quite modest locally. For the worst-case scenario of 100% relative humidity, water vapor was found to reduce the incident radiant flux at the base of a vertically oriented flame at 1000 K flame by 9% for a 1 m flame and 16% for a 10 m flame. Radiation from the angled flame (30 deg from the vertical) experiences less attenuation from water vapor than the vertical flame. Further, local attenuation of the hotter flame (1500 K) from environmental water vapor is higher than for the 1000 K flame. The relative effect of the water vapor attenuation increases with distance from the flame base.

Use of shadows to retrieve water vapor in hazy atmospheres

Techniques aimed at retrieving water vapor from satellite data of reflected near-infrared solar radiation have progressed significantly in recent years. These techniques rely on observation of water vapor attenuation of near-infrared solar radiation reflected by the Earth's surface. Ratios of measured radiances at wavelengths inside and outside water vapor absorbing channels are used for retrieval purposes. These ratios partially remove the dependence of surface reflectance on wavelength and are used to retrieve the total column water vapor amount. Hazy atmospheric conditions, however, introduce errors into this widely used technique. A new method based on radiance differences between clear and nearby shadowed surfaces, combined with ratios between water vapor absorbing and window regions, is presented that improves water vapor retrievals under hazy atmospheric conditions. Radiative transfer simulations are used to demonstrate the advantage offered by this technique.

A Feasibility Study for Simultaneous Estimates of Water Vapor and Precipitation Parameters Using a Three-Frequency Radar

Abstract The radar return powers from a three-frequency radar, with center frequency at 22.235 GHz and upper and lower frequencies chosen with equal water vapor absorption coefficients, can be used to estimate water vapor density and parameters of the precipitation. A linear combination of differential measurements between the center and lower frequencies on one hand and the upper and lower frequencies on the other provide an estimate of differential water vapor absorption. The coupling between the precipitation and water vapor estimates is generally weak but increases with bandwidth and the amount of non-Rayleigh scattering of the hydrometeors. The coupling leads to biases in the estimates of water vapor absorption that depend primarily on the phase state and the median mass diameter of the hydrometeors. For a down-looking radar, path-averaged estimates of water vapor absorption are possible under rain-free as well as raining conditions by using the surface returns at the three frequencies. Simulations of the water vapor attenuation retrieval show that the largest source of error typically arises from the variance in the measured radar return powers. Although the error can be mitigated by a combination of a high pulse repetition frequency, pulse compression, and averaging in range and time, the radar receiver must be stable over the averaging period. For fractional bandwidths of 20% or less, the potential exists for simultaneous measurements at the three frequencies with a single antenna and transceiver, thereby significantly reducing the cost and mass of the system.

Observation of Water Vapor Attenuation at Ku Band in Tropical Region

The measurement results of clear sky attenuation on an earth-satellite path at frequency Ku band in Laos are described. The measurement results show that diurnal clear sky noise vary with respect to humidity characteristics, which is a significant value in the early morning while low at daytime. The mean difference in variation is about 0.7 dB.

Water vapor retrievals using Moderate Resolution Imaging Spectroradiometer (MODIS) near‐infrared channels

At present, two Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on board the NASA Terra and Aqua Spacecraft platforms are operational for global remote sensing of the land, ocean, and atmosphere. In this paper, we describe an algorithm for water vapor derivations using several MODIS near‐IR channels. The derivations are made over areas that have reflective surfaces in the near‐IR, such as clear land areas, clouds, and oceanic areas with Sun glint. The algorithm relies on observations of water vapor attenuation of near‐IR solar radiation reflected by surfaces and clouds. Techniques employing ratios of water vapor absorbing channels centered near 0.905, 0.936, and 0.940 μm with atmospheric window channels at 0.865 and 1.24 μm are used. The ratios partially remove the effects of variation of surface reflectance with wavelengths and result in the atmospheric water vapor transmittances. The column water vapor amounts are derived from the transmittances based on theoretical calculations and using lookup table procedures. Typical errors in the derived water vapor values are in the range between 5% and 10%. The daily “pixel‐based” near‐IR water vapor product, which is a standard MODIS level 2 data product, at the 1‐km spatial resolution of the MODIS instrument, and the daily, 8‐day, and monthly near‐IR water vapor products, which are standard MODIS level 3 products, at a 1° by 1° latitude‐longitude grid globally are now routinely produced at a NASA computing facility. We present samples of water vapor images and comparisons to ground‐based measurements by microwave radiometers.

Error Characteristics of the Atmospheric Correction Algorithms Used in Retrieval of Sea Surface Temperatures from Infrared Satellite Measurements: Global and Regional Aspects

Abstract A database of cotemporal, collocated satellite and in situ observations is used to examine the association between the brightness temperature differences measured by the thermal infrared channels (T45) of the Advanced Very High Resolution Radiometer (AVHRR) and water vapor estimates (ω) derived from the Special Sensor Microwave Imager (SSM/I). This channel difference is used to estimate the atmospheric correction (due mostly to water vapor absorption) in sea surface temperature (SST) algorithms. The association between T45 and ω is found to be greatest for tropical latitudes; for mid- and high latitudes, the association is best during summer. However, the association tends to decrease toward mid- and higher latitudes during other periods. SST residual errors (satellite − buoy) show a negative mean in the Tropics, suggesting undercorrection for water vapor attenuation in the Tropics. This underestimation is explicitly shown for SST residuals in the high-water-vapor regimes of the Arabian Sea. In m...

Evaluation of bistatic intersystem interference due to scattering by hydrometeors on tropical paths

Evaluation of bistatic transmission loss cumulative distribution is very useful in assessing the effect of interference due to hydrometeor scatter between the communication links operating at the same frequency. Out of the many factors that could be responsible for the intersystem interference between the microwave communication systems, this paper presents the result of computation of intersystem interference resulting from the hydrometeor scatter on tropical paths. Interference is computed in terms of the cumulative distribution of transmission loss. The effect of varying common volume formed by the intersection of the antenna beams on the transmission loss is investigated. Results show that at frequencies higher than 10 GHz, for antenna separation longer than 100 km, common volume will be in the ice region, leading to a higher interference level at the interfered terminal. Also, results obtained show that because total path attenuation at 30 GHz is lower than at 20 GHz (this is due to the decrease in water vapour attenuation in the 22.2-30 GHz window), transmission loss tends to be higher at 20 GHz than at 30 GHz. In addition, increasing the antenna gain of the interfering station will result in the increasing interference level at the interfered station. Evaluation of the effective transmission loss shows that this parameter gives a better assessment of interference on the tropical propagation paths noted for high-intensity convective precipitation. In this case, the effective transmission loss has been evaluated in terms of the joint occurrence of additional rain attenuation on the wanted path, and the cumulative distribution of transmission loss on the intersecting paths.

A comparison of TOVS ocean skin and surface air temperatures with other data sets

A data set of geophysical fields has been processed at the Goddard Laboratory for Atmospheres from Tiros operational vertical sounder (TOVS) high‐resolution infrared sounder 2 (HIRS2) and microwave sounding unit (MSU) radiance observations. The retrieval system is the physically based Pathfinder Path A methodology. The data set currently covers the period 1985–1995. We compare monthly means of TOVS skin temperature Ts and surface air temperature Ta to the Tropical Ocean Global Atmosphere (TOGA) buoy observations, National Center for Environmental Prediction (NCEP) Ts and the Comprehensive Ocean Atmosphere Data Set (COADS) Ts and Ta for the time period 1987–1989. We also compare the Ta data to the European Center for Medium‐Range Weather Forecasts (ECMWF) 2 m temperature. The TOVS Ts has a mean cold bias of ≤0.5 K relative to NCEP Ts, while the Ta has a mean cold bias of ≤1.0 K in the tropics relative to both ECMWF and COADS Ta. The bias in the TOVS Ta is largely due to the diurnal sampling of NOAA 10 observations. The bias in the Ts has additional contributions from a possible cool skin effect as well as retrieval errors associated with cloud clearing and accounting for water vapor attenuation and reflected solar radiation. Because of the holistic retrieval of TOVS Ts and Ta there is a near cancellation of biases due to diurnal sampling and cloud clearing, so that the derived Ts‐Ta have higher accuracy than the individual Ts and Ta and give a more realistic representation of the evolution of known features of the ocean surface circulation than does the COADS. These results indicate that TOVS Ts and Ta may be suitable for climate variability studies involving air‐sea interactions. The data set is available on a daily, 5‐daily, or monthly mean basis with a spatial resolution of 1° by 1°.

Insolation over the western Mediterranean Sea: A comparison of direct measurements and Reed's formula

Direct measurements of solar radiation at sea were carried out in the Western Mediterranean Sea during several cruises in the years 1989–1994. The data collected were used to perform a test on the empirical formula proposed by Reed [1977]. The agreement between measured and empirical data is noteworthy, taking into account the coarseness of the formula. Nevertheless, Reed's estimates show a bias of about 5 W m−2. In order to investigate the causes of this bias, the data were analyzed according to the time of year and the cloud cover index. The results reveal that the computed insolation is overestimated under clear sky conditions and underestimated for cloudy sky. The disagreement of clear sky predictions can be ascribed to a regional misevaluation of both aerosols and water vapor attenuation. The inadequacy of the cloud cover index is indicated to explain the discrepancy of the estimates under cloudy sky. In any case, the mean difference between observations and formula is very slight, and this supports the use of this type of formula to compute long time average insolation over the sea.

Correcting infrared satellite estimates of sea surface temperature for atmospheric water vapor attenuation

A new satellite sea surface temperature (SST) algorithm is developed that uses nearly coincident measurements from the microwave special sensor microwave imager (SSM/I) to correct for atmospheric moisture attenuation of the infrared signal from the advanced very high resolution radiometer (AVHRR). This new SST algorithm is applied to AVHRR imagery from the South Pacific and Norwegian seas, which are then compared with simultaneous in situ (ship based) measurements of both skin and bulk SST. In addition, an SST algorithm using a quadratic product of the difference between the two AVHRR thermal infrared channels is compared with the in situ measurements. While the quadratic formulation provides a considerable improvement over the older cross product (CPSST) and multichannel (MCSST) algorithms, the SSM/I corrected SST (called the water vapor or WVSST) shows overall smaller errors when compared to both the skin and bulk in situ SST observations. Applied to individual AVHRR images, the WVSST reveals an SST difference pattern (CPSST‐WVSST) similar in shape to the water vapor structure while the CPSST‐quadratic SST difference appears unrelated in pattern to the nearly coincident water vapor pattern. An application of the WVSST to week‐long composites of global area coverage (GAC) AVHRR data demonstrates again the manner in which the WVSST corrects the AVHRR for atmospheric moisture attenuation. By comparison the quadratic SST method underestimates the SST corrections in the lower latitudes and overestimates the SST in the higher latitudes. Correlations between the AVHRR thermal channel differences and the SSM/I water vapor demonstrate the inability of the channel difference to represent water vapor in the midlatitude and high latitudes during summer. Compared against drifting buoy data the WVSST and the quadratic SST both exhibit the same general behavior with relatively small differences with the buoy temperatures.