The effect of dissolved “yellow substance” on the quantitative retrieval of chlorophyll and total suspended sediment concentrations from remote measurements of water colour

Abstract

The effect of the dissolved “yellow substance” on the quantitative retrieval of chlorophyll and suspended sediment concentrations from remote measurements of water colour, has been investigated through a sensitivity analysis applied to theoretical simulations. Two different models for yellow substance absorption, derived from experimental observations, have been used in the computation. The results obtained showed important effects, leading to the conclusion that the presence of yellow substance must be taken into careful account in the process of inferring water composition from its colour signature.     

Retrieval of atmospheric and oceanic properties from MERIS measurements: A new Case‐2 water processor for BEAM

A freely available data processor for the B asic E RS & ENVISAT ( A )ATSR and M ERIS Toolbox (BEAM) was developed to retrieve atmospheric and oceanic properties above and of Case‐2 waters from Medium Resolution Imaging Spectrometer (MERIS) Level1b data. The processor was especially designed for European coastal waters and uses MERIS Level1b Top‐Of‐Atmosphere (TOA) radiances to retrieve atmospherically corrected remote sensing reflectances at the Bottom‐Of‐Atmosphere (BOA), spectral aerosol optical thicknesses (AOT) and the concentration of three water constituents, namely chlorophyll‐a (CHL), total suspended matter (TSM) and the absorption of yellow substance at 443 nm (YEL). The retrieval is based on four separate artificial neural networks which were trained on the basis of the results of extensive radiative transfer (RT) simulations by taking various atmospheric and oceanic conditions into account. The accuracy of the retrievals was acquired by comparisons with concurrent in situ ground measurements and was published in full detail elsewhere. For the remote sensing reflectance product a mean absolute percentage error (MAPE) of 18% was derived within the spectral range 412.5–708.75 nm while the accuracy of the AOT at 550 nm in terms of MAPE was calculated to be 40%. The accuracies for CHL, TSM and YEL were derived from match‐up analysis with MAPEs of 50%, 60% and 71%, respectively.     

A multispectral remote sensing study of coastal waters off Vancouver Island

In March 1996, a multispectral aircraft survey of the coastal waters off Vancouver Island was carried out using a Compact Airborne Spectrographic Imager (CASI). This survey was combined with in situ measurements of water properties (phytoplankton composition, phytoplankton pigments, absorption spectra of phytoplankton, and concentration of dissolved organic carbon, or DOC). Comparison of the phytoplankton absorption data from this experiment with similar data from other regions shows that phytoplankton community has a significant impact on the spectral form and magnitude of absorption spectra, when normalized to unit chlorophyll-a. Concurrent measurements of in situ properties and aircraft data were obtained at eight stations. The in situ measurements of phytoplankton absorption and estimates of downwelling irradiance based on a clear-sky atmospheric-transmission model are used as inputs to a model of water-leaving irradiance. The modelled irradiances are compared with the remotely sensed values of water-leaving radiances. The observed differences between model and observation are used to evaluate the potential influence of DOC on water-leaving radiance. Practical difficulties of separating the phytoplankton signal from that of the coloured component of DOC (also known as yellow substance) are examined. Algorithms for estimation of the concentration of chlorophyll-a (the major phytoplankton pigment) can be based on their absorption or fluorescence properties. The distribution of chlorophyll-a in the study area is estimated using both these approaches, and possible causes for the observed discrepancies are discussed.     

Colour classification of coastal waters of the Ebro river plume from spectral reflectances

This paper presents a colour classification of coastal waters from features of spectral reflectance. Experimental data were collected in coastal waters of the Ebro river mouth area during two periods of non-bloom conditions and low suspended load. Examination of the reflectance spectra reveals a few significant shapes corresponding to different apparent colours and specific values of hydrological parameters. A colour classification is proposed. An empirical method and a semi-analytical method are presented in order to retrieve suspended particulate matter concentration, namely SPM, chlorophyll a concentration, chl, sediment refractive index, m_r , and absorption coefficient of yellow substance at 440 nm, a _y (440). The first method is a direct application of the colour classification. It is based on correlations found for each water type between spectral features of the first derivative of reflectance and SPM, chl, m_r , a_y (440) values. A second method uses the inversion of a three component reflectance model. Measured reflectance spectra and typical values of m_r , deduced from the colour classification, are assimilated in the model. As a result, it appears that estimates of SPM and chl are more accurate by calculating chl from the correlation technique and SPM along with a_y (440) from inversion of the reflectance model.     

Spectral optical and bio‐optical relationships in the Oslo Fjord compared with similar results from the Baltic Sea

The spectral shape of the attenuation coefficient c in Baltic waters along the Swedish coast is almost identical to the one in the Oslo Fjord, and there are also strong similarities between these two areas for the absorption coefficient a and the scattering coefficient b as functions of c. However, while the single‐scattering albedo b/c in the Oslo Fjord on an average is 62% and 78% at 425 and 525?nm, respectively, the corresponding numbers for the Gulf of Gdansk are 54% and 61%. Another characteristic difference is that particle absorption plays a much more significant role in the Gulf of Gdansk than in the Oslo Fjord. Thus some parts of the Baltic exhibit similarities with the Oslo Fjord while others display differences, and it cannot be assumed a priori that algorithms from one area will have a general validity in the other.

Algorithms for chlorophyll‐a and yellow substance as functions of colour indices, determined for the southern part of the Baltic Sea between the Pommerian Bight and the Gulf of Gdansk, are clearly not valid for the Oslo Fjord, but the result improves by changing their constants by a best‐fit procedure. We conclude that statistical algorithms for remote sensing are not generally interchangeable between the Baltic and the Oslo Fjord, and that the constants of the algorithms have to be locally tuned.     

Spectral indices for yellow canola flowers

Reproductive growth, such as by flowers, may contribute to a canopy-level signal yet there are no current indices that measure variation in flowering. This study was conducted to determine how flowers influence the overall canopy signal and what bands of light may be useful for estimating variation in flower density and leaf area index (LAI). The effects of the number of yellow flowers per unit area and LAI on canopy spectral reflectance of spring canola (Brassica napus L.) were investigated in a field study consisting of three water regimes and three fertilizer nitrogen levels near Pendleton, Oregon, USA. A band ratio of green and blue light was strongly (r² = 0.87) related to the number of yellow flowers per unit area, whereas a ratio of near-infrared and blue light was most suitable for estimating LAI during flowering. Spectral information during flowering may improve how remote sensing is used to describe plant development and reproductive capacity during the growing season.     

Remote sensing of coastal waters by airborne lidar and satellite radiometer Part 1: A model study

Abstract

Radiative transfer calculations for remote sensing of coastal waters by airborne lidar and satellite radiometer have been compared in order to answer the question, whether an airborne lidar may be used instead of in situ measurements from ships to calibrate a satellite radiometer. The radiative transfer of laserlight measuring the Raman-scattering of water molecules, the fluorescence of chlorophyll-a and the fluorescence of yellow substance or Gelbstoff is simulated by the lidar equations while the radiance to a satellite radiometer is calculated with an ocean-atmosphere model based on the matrix-operator method. Including multiple scattering in the lidar equations, an eigenvalue analysis shows that three oceanic constituents (chlorophyll-a, nonchlorophyllous particles and Gelbstoff) can be separated measuring the backscattered laserlight at three wavelengths from a height of 100 to 200m. Changes in the concentration of all three substances are detected with higher accuracy with an airborne lidar than with a radiometer even at the same height. A comparison of different algorithms indicates that the common blue-green algorithms fail in coastal waters due to the variability of several oceanic constituents, which influence the colour of sea water. In this case, algorithms using the sun-induced chlorophyll-a fluorescence at 685 nm, are superior to blue-green algorithms. Airborne lidar measurements of the chlorophyll-a fluorescence at 685 nm, normalized by the Raman-signal at 650 nm, are as good as in situ data and can be used to calibrate satellite measurements of chlorophyll-a.     

Absorption characteristics of ocean water in the Arabian Sea during winter bloom from in situ measurements and Oceansat 2 OCM and MODIS data

In the northern Arabian Sea, blooms usually occur during the northeast monsoon (November–January) and inter-monsoon (February–April) periods. After death, these phytoplankton blooms produce massive subsurface zones of low dissolved oxygen levels that have a major impact on the ocean water ecosystem. Many studies have been done to identify the bloom in this region, but those on the optical properties of bloom water are scarce. The present study emphasizes the optical properties (inherent) of the bloom water in the study region using in situ and satellite data. The total absorption coefficient of ocean water was measured from in situ radiance data collected in the northern Arabian Sea from the Sagar Sampada cruise (SS-286) during March 2011. The same data were also derived from the top-of-atmosphere radiance and remote sensing reflectance of the Oceansat 2 Ocean Colour Monitor (OCM-2) and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) sensors, respectively. A comparison between measured (in situ) and retrieved total absorption coefficients from OCM-2 was made. The measured and retrieved absorption coefficients are in good agreement. Root mean square errors between measured and retrieved absorption coefficients are 0.018 m^?1, 0.026 m^?1, and 0.034 m^?1 for 490 nm, 510 nm, and 555 nm, respectively. An inter-comparison of total absorption properties retrieved from OCM-2 and MODIS data in the region of one degree radius around the stations was also made. A fairly good match was observed on 10, 14, and 16 March 2011 (coefficient of determination, R² = 0.75, 0.87, and 0.62, respectively) for the blue band (490 nm) and (R² = 0.77, 0.79, and 0.71, respectively) the green band (555 nm). The study demonstrates the potential of using remote-sensing optical data for identifying bloom waters.     

An algorithm for the retrieval of suspended sediment concentrations in the Irish Sea from SeaWiFS ocean colour satellite imagery

A study was conducted in the Irish Sea with the aim of deriving an algorithm for the retrieval of suspended sediment concentrations from ocean colour imagery obtained from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). In situ observations of the diffuse attenuation coefficient, K_d , and irradiance reflectance, R, were obtained at wavelengths coincident with the SeaWiFS visible wavebands using a Profiling Reflectance Radiometer (PRR600, Biospherical Instruments Inc., San Diego). Results showed that surface reflectance at 665 nm (R₆₆₅ ), rather than variations in the intrinsic colour of the ocean (using colour ratios), was the most widely applicable method of obtaining suspended sediment concentrations from ocean colour imagery in this region. The derived algorithm enabled the estimation of mineral suspended sediment (MSS) concentrations from ocean colour in the Irish Sea, accurate to within 1 mg l^?1 (see equation below). Furthermore, the application of this algorithm to a SeaWiFS image of the Irish Sea accurately reproduced known regions of high turbidity with realistic MSS concentrations.

MSS=0.0441R ² ₆₆₅ + 1.1392R ₆₆₅ + 1.7459

(R ²=0.9105, n=124, RMS error=0.907)

Specific absorption and scattering coefficients were derived for all optically active in-water constituents, namely yellow substance (YS), mineral suspended sediments (MSS) and phytoplankton pigments (C). An optical model based on the empirically derived absorption and scattering coefficients reproduced the observed relationship between MSS and R ₆₆₅. Model results highlighted the relative insensitivity of reflectance at 665 nm to variations in the concentrations of other in-water constituents, suggesting that the algorithm may be applicable to the Irish Sea throughout the year.     

Sea surface colour in the field of biological oceanography

Abstract

Variations of marine surface optical properties (generally grouped under the term ‘sea surface colour’) are due to dissolved and suspended materials, with different absorption and scattering characteristics, present in sea water. Remote assessments of sea surface colour, therefore, can be used to determine the presence and abundance of water constituents such as biological pigments, suspended sediments or other products of organic matter degradation (the so-called yellow substance). In open sea waters, the pigments due to biological activities, and particularly phytoplankton chlorophyll-like pigments, are the main contributors to surface colour. Hence, observations in the visible spectrum can provide synoptic and repetitive information on parameters linked to biological production and patchiness, or bio-geo-chemical cycles in general. Since water constituents act as tracers of various marine processes, bio-optical patterns on the sea surface can also provide indications about the relationships existing between forcing mechanisms and biological response in the marine environment. These capabilities render optical remote sensing an invaluable tool in the field of biological oceanography, although atmospheric processes and signal ambiguities in the water column may pose severe limitations on this technique. The feasibility and potential of passive remote sensing in the visible spectrum have been demonstrated primarily by the Coastal Zone Color Scanner (CZCS) experiment. Important results of this experiment have been reported in the study of coastal phenomena, sediment transport, frisheries, upwelling, climatic events, and factors controlling the distribution, growth and fate of phytoplankton. On these latter topics, indications of a strong coupling between dynamical and bio-optical conditions of the marine environment are emerging from the analysis of CZCS image series, for open ocean, near-coastal and enclosed basin conditions. Examples of such studies, covering regions of both the North Atlantic and North Pacific Oceans and of the Mediterranean Sea, provide clues on the promises of large-scale sea surface colour assessments in the field of biological oceanography.     

A five channel chlorophyll concentration algorithm applied to SeaWiFS data processed by SeaDAS in coastal waters

Chlorophyll-a concentration derived from the Sea-viewing Wide Fieldof-view Sensor (SeaWiFS) after applying the current SeaWiFS Data Analysis System (SeaDAS) processing tools appears to be higher than reality in coastal areas, particularly from late summer to early spring when optical properties of water are dominated by yellow substances and suspended matter. As a complement to the SeaWiFS standard procedure addressing clear water, empirical algorithms can bring immediate progress for observing the coastal domain. This paper proposes to modify the SeaWiFS Ocean Colour 4 band algorithm (OC4) by including the 412 and 555 channels. The effect of the suspended matter on the ratios used as inputs in OC4 is revealed by the 555 channel whereas the atmospheric over-correction and the absorption by yellow substances are related to the 412 channel. Based on a dataset located in the English Channel and on the continental shelf of the Bay of Biscay, a parametrization of the relationship between the OC4 ratio and the 412 and 555 bands has been empirically proposed for different chlorophyll concentrations. Application of a lookup table, relating triplets (OC4 band ratio, 412 and 555 bands) to chlorophyll-a concentration, provides realistic concentration maps likely to satisfy the needs of researchers involved in environmental surveying or fishery management.     

SeaWiFS data interpretation in a coastal area in the Bay of Biscay

The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) instrument, launched onboard the Orbimage 2 satellite, is composed of an optical scanner with eight channels that are used to interpret the ocean colour, and has been operational since September 1997. SeaWiFS data were received by the Dundee Satellite Receiving Station and processed by the Plymouth Marine Laboratory with a slight time-lag. In situ measurements of reflectance, salinity, seston and chlorophyll a were analysed during the Biomet surveys to gain a better knowledge of the dynamics of the Gironde turbid plume during this period. The results showed logarithmic relationships between the SeaWiFS normalized water-leaving radiances (n L w ) at 490 and 555 nm (n L w (490) and n L w (555)), and the suspended particle matter concentrations. The relationship between the n L w (555) radiances and these concentrations is used to map the coastal terrigenous turbidities. But the chlorophyll a concentrations calculated from SeaWiFS are overestimated in the turbid waters. The n L w (490)/n L w (555) ratio decreases with increasing turbidity and with increasing chlorophyll a concentration. To distinguish the chlorophyll a in turbid waters, the n L w (490) radiances are calculated from n L w (555) considering only the effect of terrigenous turbidity. Then, the n L w (490) SeaWiFS image is compared to the calculated n L w (490), to reveal the 'negative' areas caused by the chlorophyll a and yellow substance absorption.     

On the use of satellite thermal data for determining evapotranspiration in partially vegetated areas

Abstract

The use of satellite data for determining daily evapotranspiration over completely vegetated areas has been analysed by different authors. However, if the soil surface is partially covered by vegetation the problem becomes more complicated, because in this case the value of the temperature measured by the satellite varies with the viewing geometry. In this work we have studied the influence of the satellite observation angle for crops with different cover degree, field geometry and ground emissivity. For this purpose we have used the simplified relation proposed by Jackson et al.LET _d=R_nd?B(T₃?T _a)iwhere ET ₄ and R _nd are the daily values of evapotranspiration and net radiation respectively, (T ₃?T _a)i is the difference between crop surface and air temperatures measured at midday, B is a semi-empirical parameter, and L is the latent heat of vaporization of water. And we have concluded that the imposition of a single value of B is unacceptable and specific values must be chosen according to certain broad categories of the viewing geometry, whose number will depend on the geometry of each crop.     

Neural Networks Based Colour Measuring for Process Monitoring and Control in Multicoloured Newspaper Printing

This paper presents a neural networks based method and a system for colour measurements on printed halftone multicoloured pictures and halftone multi-coloured bars in newspapers. The measured values, called a colour vector, are used by the operator controlling the printing process to make appropriate ink feed adjustments to compensate for colour deviations of the picture being measured from the desired print. By the colour vector concept, we mean the CMY or CMYK (cyan, magenta, yellow, and black) vector, which lives in the three- or four-dimensional space of printing inks. Two factors contribute to values of the vector components, namely the percentage of the area covered by cyan, magenta, yellow and black inks (tonal values) and ink densities. Values of the colour vector components increase if tonal values or ink densities rise, and vice versa. If some reference values of the colour vector components are set from a desired print, then after an appropriate calibration, the colour vector measured on an actual halftone multicoloured area directly shows how much the operator needs to raise or lower the cyan, magenta, yellow and black ink densities to compensate for colour deviation from the desired print. The 18 months experience of the use of the system in the printing shop witnesses its usefulness through the improved quality of multicoloured pictures, the reduced consumption of inks and, therefore, less severe problems of smearing and printing through.     

利用MODIS数据估计中国黄东海区域测深参数

目的 现有关于漫衰减系数的研究大多是在490 nm波段建立反演模型,且未将相关研究与机载激光雷达测深能力建立联系,本文尝试获取测深参数532 nm漫衰减系数K_d(532)和透明度SD(Secchi disk depth),为机载双色激光雷达测深能力的评估和飞行方案的制定提供了重要技术参数。方法 首先分析了测深参数532 nm漫衰减系数K_d(532)和透明度SD对于评估机载双色激光系统测深能力的重要性。利用2003年春季中国黄东海区域的实测光学数据,对现有的漫衰减系数反演模式进行改进,建立了K_d(532)和K_d(490)=的线性关系以及SD和K_d(532)的幂函数关系。结果 根据2003年春季MODIS的490 nm漫衰减系数产品和上述函数关系获取了K_d(532)和SD参数。结论 本文获取测深参数K_d(532)和SD的方法能够用来有效评估该区域机载激光雷达的测深能力,准确性和精度依赖于实测光学数据的精度、分布和数量以及MODIS的K_d(490)产品的准确性。     

Statistical model development and estimation of suspended particulate matter concentrations with Landsat 8 OLI images of Dongting Lake,China

Suspended particulate matter (SPM) is a dominant water constituent of case-II waters, and SPM concentration (C_SPM) is a key parameter describing water quality. This study, using Landsat 8 Operational Land Imager (OLI) images, aimed to develop the C_SPM retrieval models and further to estimate the C_SPM values of Dongting Lake. One Landsat 8 OLI image and 53 C_SPM measurements were employed to calibrate Landsat 8-based C_SPM retrieval models. The C_SPM values derived from coincident Landsat 8 OLI and Moderate Resolution Imaging Spectroradiometer (MODIS) images were compared to validate calibrated Landsat 8-based C_SPM models. After the best stable Landsat 8-based C_SPM retrieval model was further validated using an independent Landsat 8 OLI image and its coincident C_SPM measurements, it was applied to four Landsat 8 OLI images to retrieve the C_SPM values in the South and East Dongting Lake. Model calibration results showed that two exponential models of the red band explained 61% (estimated standard error (SE) = 7.96 mg l^–1) and 67% (SE = 6.79 mg l^–1) of the variation of C_SPM; two exponential models of the red:panchromatic band ratio obtained 81% (SE = 5.48 mg l^–1) and 77% (SE = 4.96 mg l^–1) fitting accuracy; and four exponential and quadratic models of the infrared band explained 72–83% of the variation of C_SPM (SE = 5.18–5.52 mg l^–1). By comparing the MODIS- and Landsat 8-based C_SPM values, an exponential model of the Landsat 8 OLI red band (C_SPM = 1.1034 × exp(23.61 × R)) obtained the best consistent C_SPM estimations with the MODIS-based model (r = 0.98, p < 0.01), and its further validation result using an independent Landsat 8 OLI image showed a significantly strong correlation between the measured and estimated C_SPM values at a significance level of 0.05 (r = 0.91, p < 0.05). The C_SPM spatiotemporal distribution derived from four Landsat 8 images revealed a clear spatial distribution pattern of C_SPM in the South and East Dongting Lake, which was caused by natural and anthropogenic factors together. This study confirmed the potential of Landsat 8 OLI images in retrieving C_SPM and provided a foundation for retrieving the spatial distribution of C_SPM accurately from this new data source in Dongting Lake.     

Comparison of measured and satellite-derived spectral diffuse attenuation coefficients for the Arabian Sea

We present here the results of our study comparing the spectral diffuse attenuation coefficients K _d(λ) measured in the Arabian Sea with those derived from the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) using three algorithms, of which two are empirical-data-driven and one is semi-analytical. The measurements were carried out in all water types and the mean values of the measured spectral K _d(λ) are 0.105, 0.092, 0.077, 0.082, 0.110 and 0.490 m^?1 for wavelength λ at 412, 443, 490, 510, 555 and 670 nm, respectively. This profile corresponds to a chlorophyll value of about 1 mg m^?3. The maximum values of the measured K _d correspond to waters with chlorophyll of about 8 mg m^?3. Though the satellite-derived K _d(λ) are found to be overestimated in all bands, we have observed good correlations between the measured and satellite-derived values in all bands, and excluding the band at 670 nm, the mean absolute percent deviations are observed to be less than 50% in all bands. The performance of the data-driven empirical methods was found to be consistent in all the bands, except at the red band of 670 nm, which is uncorrelated with the measured values and has large errors. The performances of the empirical methods depend on the accuracy of the band ratios of the retrieved remote sensing reflectance. Though the performance of the semi-analytical algorithm is found to be spectrally varying, with large positive bias observed in the blue regions, this algorithm is recommended for hyperspectral applications. The performance of the semi-analytical algorithm could be improved by having a robust algorithm to accurately derive spectral inherent optical properties of absorption and backscattering coefficients from the satellite data.     

Modelling of post-rain attenuation of microwaves at 22·235 GHz in relation to vertical transport of water vapour

Abstract

Attenuation of microwaves (up to 30 GHz) around the water vapour absorption line on 22·235 GHz tends to persist even after the stoppage of rain during the monsoon season. A theoretical model has been developed for the post-rain residual absorption of radio waves at 22·235 GHz (λ = 1·349 cm) by considering the vertical transport of water vapour in the region of the rain event. The theoretical estimate agrees fairly well with that obtained experimentally during different post-rain events, recorded by a radiometer, operating at the Institute of Radio Physics and Electronics, Calcutta University on a round-the-clock basis.     

Applicability of the PROSPECT model for Norway spruce needles

The potential applicability of the leaf radiative transfer model PROSPECT (version 3.01) was tested for Norway spruce (Picea abies (L.) Karst.) needles collected from stress resistant and resilient trees. Direct comparison of the measured and simulated leaf optical properties between 450–1000 nm revealed the requirement to recalibrate the PROSPECT chlorophyll and dry matter specific absorption coefficients k _ab(λ) and k _m(λ). The subsequent validation of the modified PROSPECT (version 3.01.S) showed close agreement with the spectral measurements of all three needle age‐classes tested; the root mean square error (RMSE) of all reflectance (ρ) values within the interval of 450–1000 nm was equal to 1.74%, for transmittance (τ) it was 1.53% and for absorbance (α) it was 2.91%. The total chlorophyll concentration, dry matter content, and leaf water content were simultaneously retrieved by a constrained inversion of the original PROSPECT 3.01 and the adjusted PROSPECT 3.01.S. The chlorophyll concentration estimated by inversion of both model versions was similar, but the inversion accuracy of the dry matter and water content was significantly improved. Decreases in RMSE from 0.0079 g cm^?2 to 0.0019 g cm^?2 for dry matter and from 0.0019 cm to 0.0006 cm for leaf water content proved the improved performance of the recalibrated PROSPECT version 3.01.S.     

Accuracy of in-flight calibration of the water-vapour channel of a satellite radiometer by radiance calculation

Abstract

The accuracy of a calibration of a water-vapour channel of a radiometer on board a satellite by using a radiative transfer code with radiosonde profiles was examined with the aid of the calibrated WV channel data of the Visible and Thermal Infrared Radiometer (VTIR) on the MOS-1 satellite. It has been verified that the assumption that a linear extrapolation for the relative humidity in the upper troposphere was reasonable for estimating the radiances in the water-vapour channel by a comparison of the brightness temperatures of the channel measured by the calibrated VTIR with those calculated by the radiative transfer code. An error in water-vapour distribution or radiosonde data considerably affected the estimated brightness temperature by the radiance calculation in the water vapour absorption band. The accuracy of the calibration of the water-vapour channel by the radiance calculation was with the root-mean-square (rms) temperature difference of 3–96°C.