ATMOSPHERIC EFFECT ON VALIDATION OF BROADBAND SURFACE ALBEDO (SAL) PRODUCT OF CM SAF USING MAST MEASUREMENTS

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1 ATMOSPHERIC EFFECT ON VALIDATION OF BROADBAND SURFACE ALBEDO (SAL) PRODUCT OF CM SAF USING MAST MEASUREMENTS Terhikki Manninen and Aku Riihelä Finnish Meteorological Institute, P.O. Box 503, FI Helsinki, Finland Abstract Satellite Application Facilities (SAFs) are specialised development and processing centres within the EUMETSAT Applications Ground Segment. The Satellite Application Facility on Climate Monitoring (CM SAF) was initiated in order to generate and archive high quality data sets on a continuous basis for the climate monitoring and modelling (Satellite Application Facility on Climate Monitoring, User Manual Products, 2007). One of the CM-SAF products is the surface albedo product (SAL). The validation of the satellite based products has two main strategies: large areas are compared with airborne measurements instantaneously or individual pixels are compared with continuous mast measurement results during long periods, preferably covering seasonal variation. The satellite based albedo values are atmospherically corrected, but the mast measurements typically not. Since the atmosphere optical thickness depends strongly on the wavelength, the spectrum of the incoming surface radiation is not identical to that of the incoming top of atmosphere radiation. For vegetation, especially, the near infrared band reflectance is much higher than the visible band reflectance. If the visible band radiation is attenuated more than the near infrared radiation in the atmosphere, the total reflected radiation will be systematically higher. This tends to increase the measured albedo value, unless the amount of diffuse radiation is large enough to compensate for the attenuation. This happens when the sun enith angle is large. The effect is large enough to cause trouble for validation of satellite based surface albedo products. INTRODUCTION The satellite broad band surface albedo products validation is usually carried out using ground based measurements of the albedo. For temporal validation continuous pyranometer measurements are available for example, via the BSRN network (Ohmura et al. 1998; WRCP 2007) at several measurement sites. The instrumental error sources of the pyranometers have been studied for decades and are well known (Michalsky et al. 1995; Raïch et al. 2007). Typically the spectral response is good and the cosine correction is essential only when the sun enith angle is larger than 60. However, the atmospheric effect on the pyranometer measurements is given less attention, because that becomes important only when the pyranometer measurements are used for characteriing only the surface, independently of the prevailing atmospheric conditions. This is the case, for example, when pyranometer measurements are used for validation of the albedo products derived from satellite data. The atmospheric correction of satellite images has been a thoroughly studied topic for a long time. In addition, the influence of the sky radiance distribution, including the effect of aerosols, on the spectral albedo has been studied in detail (Lewis and Barnsley, 1994). The emphasis, however, has been more on the difference between the albedo values obtained in clear sky (black-sky) and diffuse sky (white-sky) conditions. Thus the atmospheric effect on the albedo from the theoretical point of view and the atmospheric effect on the reflectance observed by the satellite are well taken into account in current surface albedo products. The validation of the products based on pyranometer measurements often can be quite extensive with much attention given to leveling accuracy, nonperfect cosine response, solar enith angle and soil moisture effects (Lucht et al. 2000). Yet no correction for the

2 atmospheric optical thickness (AOT) on the measured irradiation value of the pyranometer is so far included. The problem in taking into account the atmospheric effects in continuous broadband measurements is that both the atmosphere optical thickness and the target spectra are strongly wavelength dependent. Spectrometers can measure relatively quickly the whole bandwidth to which the pyranometers are sensitive. However, the amount of data is so large for one spectrum that continuous measurements are not normally carried out all year round. Thus,ideal atmospheric correction of the mast data is often not possible. In this paper, we simulate the atmospheric contribution to the surface albedo that would be obtained by pyranometer measurements on the ground using diverse land cover type reflectance spectra, various sun enith angle values and typical aerosol optical thickness values as input information. The direct and diffuse irradiance values are obtained using the top of atmosphere solar spectral irradiance (ASTM Standard G173-03) and the SPCTRAL2 model (Bird and Riordan, 1986) for estimation of the diffuse irradiance. In addition, a robust regression formula is derived for the relationship of the simulated measured surface albedo and the corresponding black-sky albedo. MATERIALS In order to analye the atmospheric effect on the measured surface albedo 87 individual spectra of diverse land cover types were chosen from the USGS Spectral lab data base: 10 of grass, 19 of forest, 5 of crop, 6 lichen, 4 of minerals, 18 man-made materials, 4 water, 8 snow/ice and 13 mixtures of rock etc (Clark et al. 2007). Aerosol optical thickness measured at Cabauw in clear days during January September 2007 were available at four wavelengths (440nm, 675nm, 870nm and 1020 nm). Values in two wavelengths are shown as a function of the sun enith angle in Figure 1. Obviously the variation of the values is large for all angles, so that the effect of the AOT and the sun enith angle can be studied separately. Also the largest AOT values are very high, so that the data set is able to demonstrate the effect of aerosols very well. AOT nm 870 nm Sun enith angle HDegreesL Figure 1. The measured aerosol optical thickness (AOT) values measured at Cabauw during January September 2007 versus the sun enith angle (Holben et al. 1998) in wavelengths 440 nm and 870 nm. METHODS The broadband surface albedo α bb is the ratio of the total reflected radiation and the incoming radiation. In practice only the shortwave part of the spectrum is taken into account. Pyranometers measure usually the bandwidth 305 nm 2800 nm. The reflected short wave radiation R sw is related to the short wave irradiance I sw the spectral albedo α( and reflectance r( via

3 α bb = R I sw sw = α( I( dλ = I( dλ r( BRDF( θ, θ, ϕ, ϕ, I( dλ I( dλ. (1) Where the bidirectional reflectance distribution function BRDF(θ,θ,ϕ,ϕ, determines how large fraction of the reflected radiation coming from direction (θ,θ) is reflected to the direction (ϕ,ϕ), where θ and ϕ are the sun enith and aimuth angles and θ and ϕ are the enith and aimuth angles of the viewing direction. The BRDF is not typically a strong function of the wavelength, but slightly different values are often obtained for the visible and near infrared wavelengths. If there were no atmosphere the irradiance to be used in Eq. 1 would be directly the top of atmosphere solar spectral irradiance I 0 ( (ASTM Standard G ). Then the broad band albedo would be the black-sky albedo α 0bb. When the atmosphere is taken into account both the direct and diffuse components of the irradiance have to be taken into account. Surface albedo values measured at graing incidence angles are typically very noisy and values measured at sun enith angles larger than 70 are usually discarded from analysis. When largest sun enith angles (>70 ) are excluded the attenuation by the atmosphere can quite accurately be taken into account by assuming that the light propagates in the atmosphere along a straight path. Then the direct solar irradiation at the surface per unit area I dir ( is related to the irradiation at the top of the atmosphere I 0 ( by (Bird and Riordan 1986) I dir ( τ a ( / ) ( = I 0 ( exp (2) where θ is the sun enith angle and τ a ( is the aerosol optical depth (AOT), which depends on the wavelength according to (Bird and Riordan 1986) τ α n a ( = β nλ (3) When the AOT is measured in several wavelengths (at least two) the coefficients α n and β n are coefficients can be determined with regression to the measured AOT values and corresponding wavelengths. Unfortunately the estimation of the diffuse irradiation is much more complex. As the motivation for this work is to derive atmospheric correction for mast measurements to be used for validation of satellite based albedo estimates, only clear sky cases are of interest. In addition, the fewer parameters are required as input for the model the more suited it will be for atmospheric correction purposes of all kinds of existing mast measurements. Also the amount of data produced by continuous mast measurements in various fixed sites is considerable. Thus it is considered sufficient to use a relatively simple but informative model for the diffuse irradiation estimation. The model SPCTRAL2 (Bird and Riordan 1986) was chosen for that reason. The diffuse irradiance of SPCTRAL2 consists of three components: 1) the Rayleigh scattering, 2) the aerosol scattering component and 3) the component that accounts for multiple reflection of irradiance between the ground and the air. The model takes into account the oone, water vapour and mixed gas absorption and the attenuation caused by aerosols separately. The absorption gaps of the oone and water vapour are mainly situated at longer wavelengths, so that the exact amount of oone and water vapour is not assumed to be crucial for the total broad band irradiance. Therefore in the simulations the oone and water vapour contributions are taken to equal those of the standard atmosphere (ASTM Standard G173-03). The simulated broadband albedo that would be measured by a pyranometer is then obtained using the sum of the modelled direct and diffuse radiation for the irradiance in Eq. 1. The required reflectance spectra are directly derived from the USGS Spectroscopy Lab database (Clark et al. 2007). Then only the BRDF is needed to obtain the surface albedo estimate.

4 The BRDF values were obtained using the methods developed for visible and near infrared bands of satellite instruments (Roujean et al., 1992; Wu et al., 1995). The spectra was split in two at 750 nm and the visible bands BRDF was applied to the shorter wavelengths and the near infrared BRDF to the longer wavelength part. In fact, to analye the atmospheric effect it is sufficient that the BRDF is typical sie, but it does not have to be in high precision the true one related to each target spectrum. RESULTS The aerosol optical thickness affects the direct and diffuse components of the irradiance the opposite way. The larger the AOT is the smaller is the amount of diffuse radiation reaching the surface, but the larger is the diffuse component. The effect is most evident at the shortest wavelengths. Also the increase of the sun enith angle corresponds to decrease of the direct irradiance and increase of the fraction of diffuse irradiance. The calculated broad band black-sky albedo is shown in Figure 2 as a function of the simulated measured broad band surface albedo for all studied land cover types and AOT values. Although the effect of the atmosphere is not large in the standard atmosphere (ASTM Standard G173-03) the whole range of AOT values measured at Cabauw cause a drastic variation in the simulated values. Without atmospheric correction this kind of data would be problematic for validation of satellite products. Taking a subset of just grass spectra does not help the situation much (Figure 2). The difference between the simulated measured surface albedo and the corresponding black-sky albedo depends on the sun enith angle, overestimations being more common at small sun enith angle values, but the variation range is about the same for all angles (Figure 3). However, one has to take into account that the simulated result corresponds to an ideal pyranometer. The cosine correction is largest at large sun enith angle values and typically such, that would lead to overestimation of the albedo. A typical cosine correction would shift the error points of Figure 3 corresponding to the sun enith angle value 70 a few percent upwards. Due to the complex nature of the diffuse radiation component and the varying shapes of the diverse land cover type spectra it is not possible to derive a simple analytic relationship between the black-sky surface albedo and the simulated measured surface albedo. Therefore an empirical regression of the Figure 2. The black-sky albedo versus the corresponding simulated surface albedo measurement for all 87 land cover spectra (left) and all 9 grass (right) for all 2533 AOT values measured at Cabauw in January July 2007 (Holben, 1998).

5 Figure 3. The black-sky albedo versus the corresponding simulated surface albedo measurement for all 87 land cover spectra used in all studied atmospheres for sun enith angle values 30 (left) and 70 (right). form α bb (1 + ) was sought for them. The parameters included were the AOT values at two wavelengths (τ 440 and τ 870 ), the sun enith angle (θ ) and the direct (I dir ) and diffuse (I diff ) irradiance and the mathematical form of their appearance is sought to resemble expressions in the diffuse radiation components (Bird and Riordan, 1986). The obtained relationship is ˆ α 0bb ( 1 exp( τ )) ( 1 exp( τ )) I ( 1 exp( τ )) dir 440 = α bb 1 + c1 + c2 + c3 + c4 I 2 diff 1 α bb (4) The regression parameter values are given in Table 1 and the coefficient of determination for the regression was 0.999, when all AOT values and all spectra were included in the regression. The individual correction terms equal ero, when the AOT=0, because then τ 440 =0, τ 870 =0 and I diff =0, so that the estimate approaches the black-sky value properly. The black-sky albedo estimation formula is applied to the studied land cover spectra, and it turned out that the absolute difference between the estimate and the true value was quite small (Figure 4). The estimation accuracy decreases when the sun enith angle exceeds 60. The mean and the relative mean accuracy of the estimated black-sky albedo are shown in Figure 5. Obviously the atmospherically corrected albedo estimate is on the average within of the black-sky value, whereas the uncorrected albedo deviates roughly 0.01 of it. However, more important is that the atmospheric correction removes large errors so that the deviation from the true value is in 90 % of the Table 1. The values of the regression parameters for the empirical relationship between the simulated measured broadband surface albedo and the corresponding broadband black-sky albedo. Regression parameter Value c c c c

6 Figure 4. The difference between the simulated estimate of a measured surface albedo obtained using Eq. 4 and the corrresponding simulated black-sky albedo for the 87 diverse land cover type spectra used in all studied atmospheres and for the sun enith angle values 0, 30, 50 and 70. The opacity of the points is related to the frequency of the value. cases smaller than in absolute units and the relative error is smaller than 8 %. For the uncorrected albedo the corresponding deviations are 0.03 and 11%. The largest difference between the simulated measured albedo and the corresponding black-sky value was at nadir 0.12, which was reduced to by the atmospheric correction. Although large sun enith angle values (~70 ) are typically problematic for measurements (for cosine response for example), the atmospheric effect is non-negligible also when the sun elevation is high. Albedo error q Mean q SunenithangleHDegreesL Relative albedo error q Mean q Sun enith angle HDegreesL Figure 5. The deviation (left) and relative deviation (right) of the simulated measured albedo (dashed curves) and the regression based estimated albedo (solid curves) from the corresponding black-sky value. The mean, 0.1 quantil and 0.9 quantile values are shown. The curves are based on all 87 spectra and all 2533 AOT values.

7 Albedo SEVIRI AVHRR Ground Julian Day Figure 6. The weekly mean black-sky surface albedo product (SAL) of CM-SAF derived from satellite based AVHRR and SEVIRI instruments in spring 2007 at Cabauw. The corresponding ground measurement values are shown without (continuous curve) and with minimum and maximum (error bars) atmospheric correction due to observed AOT values during January September 2007 at Cabauw (Figure 1, Holben et al. 1998). All albedo values are normalied to sun enith angle 60. The atmospheric correction presented requires besides the normal pyranometer measurements (global irradiance and reflected radiation) only AOT values for at least two wavelengths to determine the spectral behaviour (Eq. 3). The direct solar irradiation can be calculated from the sun enith angle and the solar constant, when the AOT is known (Eq. 2). Then the diffuse component is the difference of the measured global irradiance and the direct solar irradiance. Typically aerosol measurements do not exist for each albedo value measured at the ground. Thus usually only the range of the probable atmospheric correction can be estimated. The effect of the uncertainty of the atmospheric contribution to the ground measurements turne out to be large enough to cause trouble for the validation of satellite products of surface albedo (Figure 6) for the test data available. The regression parameters presented here were derived to be applicable to various kinds of land cover spectra. For permanent measurement sites one could improve the accuracy if the spectra used in the simulations were adapted to be typical of the site. Also the sun enith angle effect could be better taken into account, if the regression parameters were derived separately for all angles. CONCLUSIONS The effect of the atmosphere on the measured broadband surface albedo can be marked and depends on the sun enith angle. When the aerosol optical thickness (AOT) value is known in two wavelengths (440 nm and 870 nm) the pyranometer measurements can be corrected to remove the atmospheric component from the surface albedo. The correction is important also for small sun enith angle values. ACKNOWLEDGEMENTS The work was financially supported by EUMETSAT in the project Satellite Application Facility on Climate Monitoring. The authors thank Prof. Gerrit de Leeuw (FMI) for the Cabauw AOT data and Dr. Fred Bosveld (KNMI) for the Cabauw surface albedo data. REFERENCES ASTM Standard G173-03, Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on a 37 Tilted Surface.

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