DIURNAL ASYMMETRY IN THE GERB SW FLUXES

Transcription

1 DIURNAL ASYMMETRY IN THE GERB SW FLUXES C. Bertrand 1, J. Futyan 2, A. Ipe 1, L. Gonzalez 1, N. Clerbaux 1, D. Caprion 1 1 Royal Meteorological Institute of Belgium (RMIB), Avenue Circulaire 3, B-1180 Brussels, Belgium 2 Space and Atmospheric Physics, Imperial College, London, U.K. ABSTRACT The launch of the Geostationary Earth Radiation Budget (GERB) instrument on board of Meteosat 8 allows a diurnal sampling of the Earth s Radiation Budget for the first time, providing a unique and important addition to polar orbiting measurements. However, first data from the GERB instrument exhibit systematic asymmetry in the short wave (SW) flux diurnal variation over clear ocean surfaces. Such asymmetries are not found in the Clouds and the Earth s Radiant Energy System (CERES) Angular Distribution Models (ADMs) used to convert the directional broad band GERB SW radiances to fluxes. Estimations of reflected SW flux at the top of the atmosphere computed from the Spinning Enhanced Visible and Infra Red Imager (SEVIRI) data only, and the combined use of GERB and SEVIRI data have been compared for selected ocean scenes and clear sky days. Our results indicate that a number of factors contribute to the diurnal asymmetry in the clear ocean GERB SW flux values. These include deficiencies (1) in the SEVIRI SW narrow bands to broad band conversion, (2) in the GERB SW geolocation, and (3) in the adjustment of the CERES ADMs anisotropic correction factor to account for the presence of aerosols over clear ocean scenes. 1. INTRODUCTION The Geostationary Earth Radiation Budget (GERB) (Harries et al., 2005) instrument aboard Meteosat-8 (MS-8) together with the new Spinning Enhanced Visible and Infra Red Imager (SEVIRI) (Schmetz et al., 2002) is designed to exploit the geostationary orbit to make unique measurements of the Earth s Radiation Budget (ERB) over Europe and Africa. The high time resolution possible from this orbit resolves the problem of diurnal sampling of the ERB and provides a unique and important addition to polar orbiting measurements. However, first data from the GERB instrument exhibit systematic asymmetries in the SW flux diurnal variation over clear ocean footprints. Such asymmetries are not found in the Angular Distribution Models (ADMs) (Loeb et al., 2003) built from a statistical analysis of 9 months of CERES data onboard of the Tropical Rainfall Measuring Mission (TRMM) (here after referred as CERES-TRMM ADMs) used to convert the directional broad band (BB) GERB radiances to fluxes. In this paper, we investigate the possible origin(s) of such asymmetries. This is done by analysing both GERB and GERB-like (i.e., reflected SW flux values at TOA computed from SEVIRI data only). Before

2 focusing on the flux asymmetry, we first start by briefly describing the method implemented at the Royal Meteorological Institute of Belgium (RMIB) to estimate the reflected SW GERB(-like) fluxes at the 3x3 SEVIRI pixel resolution from the combined use of SEVIRI and GERB data. 2. DERIVATION OF THE REFLECTED GERB(-LIKE) SW FLUX Accurate estimation of the unfiltered reflected solar radiances, L uf, requires that the effects of the non flat spectral responses of the GERB SW channel are removed from the filtered measurements, L f. SEVIRI narrow band (NB) measurements are used to retrieve information about the spectral distribution of the observed radiation. The unfiltered GERB radiances, L uf GERB, are estimated by multiplying the filtered GERB measurements, L f GERB, by the corresponding SEVIRI unfilter factor, L uf SEVIRI/L f SEVIRI: L uf GERB = (L uf SEVIRI/L f SEVIRI). L f GERB = L uf SEVIRI. (L f GERB)/L f SEVIRI) = L uf SEVIRI. C L (1) where, L f SEVIRI is the estimate of the filtered BB GERB SW radiance measurement, estimated from the SEVIRI spectral radiances only. L uf SEVIRI is the corresponding estimate of the unfiltered BB radiance, estimated from the SEVIRI spectral radiances only. C L = L f GERB/L f SEVIRI is the pixel dependent correction factor. The BB SEVIRI filtered radiance, L f SEVIRI, and the SEVIRI unfilter factor, L uf SEVIRI/L f SEVIRI, are estimated from the SEVIRI imager through NB-to-BB conversions and convolution with the GERB point spread function (PSF). The spectral conversion is performed by the way of polynomial regression on the SEVIRI NB radiances: L SW = a + b 1 L 0.6µm + b 2 L 0.8µm + b 3 L 1.6µm + c 11 L 2 0.6µm + c 21 L 0.8µm L 0.6µm + c 22 L 2 0.8µm + c 31 L 1.6µm L 0.6µm + c 32 L 1.6µm L 0.8µm + c 33 L 2 1.6µm (2) The regression coefficients, a, b i, and c ij (with i,j= 1,3) are derived from a database of spectral radiance curves at the TOA generated by running the SBDART (Ricchiazzi et al., 1998) radiative transfer model (RTM) for a large set of Earth-atmosphere conditions. Because Lambertian surface reflectances were assumed when running the RTM, the regression coefficients in Equation 2 are neither dependent on the viewing zenith angle (VZA) nor on the relative azimuth angle (RAA) and vary only with the solar zenith angle (SZA). The error at 1σ (RMSE) introduced by the unfaltering is about 0.3% for solar radiation. Because space-borne radiometers do not measure the Earth s outgoing fluxes directly, ADMs are required to relate the SW radiance actually measured to flux at given solar angle, satellite-viewing geometry, surface and atmospheric condition. After a preliminary scene identification (i.e., surface type, cloud fraction, cloud phase, and cloud optical depth) using the SEVIRI NB measurements, the angular conversion is performed using the CERES-TRMM BB SW ADMs. Considering the appropriate CERES-TRMM ADM, the instantaneous SW flux, F SW(θ s), is computed from the directional BB radiance, L SW(θ s,θ v,φ) as follows: F SW (θ s ) = π. L uf SW (θ s,θ v,φ) / R ADM (θ s,θ v,φ) (3) where, θ s and θ v are the solar and viewing zenith angles, respectively. φ is the relative azimuth angle, and finally R ADM(θ s,θ v,φ) is the ADM dependent anisotropic correction factor (ACF) for a given scene type and viewing geometry. Note that the ADM ACFs are adjusted to avoid introducing large instantaneous flux errors or sharp flux discontinuities between ADM angular bins or scene types. Because the CERES BB radiometer footprint size on TRMM is about 10x10 km at the sub-satellite point, F SEVIRI fluxes are not retrieved at the native SEVIRI resolution (3 km at the sub-satellite point). Rather, GERB-like SEVIRI fluxes, F 3*SEVIRI, are

3 estimated from BB unfiltered SW radiances defined over 3x3 SEVIRI pixels boxes by averaging the BB unfiltered SW radiances of the 9 SEVIRI pixels located in each box. The F 3*SEVIRI SW fluxes are then used to enhance the GERB flux from the nominal GERB footprint resolution (50 km at nadir) to the high 3x3 SEVIRI pixel resolution. Rather than directly interpolating the filtered GERB radiances, the correction factor (CF) introduced in Equation 1 (i.e., L f GERB/L f SEVIRI) is considered. CFs computed at the GERB nominal resolution, C L(i,j), are spatially interpolated to derive correction factor at the 3x3 SEVIRI pixel resolution. In a first step, fluxes at the GERB footprint resolution are derived from the 3x3 SEVIRI pixel based flux estimates. Basically, L f SEVIRI radiances are integrated over the GERB pixel, by convolution with the GERB pixel PSF in order to determine CFs at the GERB footprint resolution, C L(i, j). F GERB (i, j) = (Σ x Σ y PSF(i, j, x, y). F 3*SEVIRI (x, y)). C L (i, j) (4) where, F GERB(i,j) is the flux at the GERB footprint resolution (i, j = 1,, m) and F 3*SEVIRI(x, y) is the flux at the 3x3 SEVIRI pixel resolution (x, y = 1,, n with n > m). PSF(i, j, x, y) is the point spread function of the GERB pixel (i, j). In the second step, the spatial resolution of the GERB fluxes is improved by use of SEVIRI high resolution information. Basically, it requires to find CFs, c H(x, y), which applied to the high resolution flux estimates allow after integration to reproduce the low resolution GERB fluxes. F GERB (i, j) = Σ x Σ y PSF(i, j, x, y). c H (x,y). F 3*SEVIRI (x,y) (5) where c H(x, y) is the CF at the high (3x3 SEVIRI pixel) resolution or resolution enhancement factor. Finally, the GERB flux at the 3x3 SEVIRI pixel resolution is given by: F GERB/3*SEVIRI (x, y) = c H (x, y). F 3*SEVIRI (x, y) (6) Because SEVIRI and GERB images are recorded at different times, a temporal matching between SEVIRI and GERB images is required prior to any comparison or combination of the 2 data types. First, SEVIRI data are interpolated (simple linear interpolation) to the GERB acquisition time and the CFs are estimated at the GERB nominal spatial resolution, C L(i, j). Then in order to produce flux estimates at the SEVIRI acquisition time, a weighted average of the CF images over a 15-minutes time interval centered on the SEVIRI acquisition time is performed. 3. DIURNAL ASYMMETRY Figure 1 displays the monthly mean diurnal evolution (from sunrise to sunset) of the computed GERB and GERB-like SEVIRI clear sky SW fluxes (dashed and dotted lines, respectively) for 9 ocean footprints selected in the SEVIRI field-of-view (3 western, 3 central and 3 eastern ocean footprints, respectively). The corresponding CERES-TRMM all wind speed clear ocean ADM (ADM 5) SW fluxes (solid lines) are also shown for comparison. As we can see, SW fluxes estimation from the SEVIRI NB radiances measurements over ocean footprints located in the western part of the SEVIRI FOV (dotted lines in the left panels 1, 4, and 7) exhibit an upward trend from sunrise to sunset. By contrast GERB-like SEVIRI SW fluxes computed over eastern ocean footprints (dotted lines in the right panels 3, 6, and 9) present a downward trend from sunrise to sunset. Such an upward/downward trend in the reflected SW flux at TOA are clearly not found in the ADM flux time series (solid lines) which are instead quite symmetric around local noon whatever the footprint location within the SEVIRI FOV may be. The magnitude of GERB-like SEVIRI fluxes computed over footprints located in the middle part of the SEVIRI FOV (dotted lines in panels 2, 5, and 8) appears to be overestimated during the morning and afternoon periods and somewhat underestimated at noon. By contrast, CERES-TRMM BB clear ocean SW ADMs indicate an increased amount of reflected SW fluxes at local noon as shown by the time evolution of the CERES-TRMM ADM 5 fluxes displayed in Figure 1. Regarding the diurnal evolution of the estimated GERB SW fluxes, the dashed lines in Figure 1 indicate that

4 the asymmetry problem is less systematic than seen for the GERB-like SEVIRI fluxes. In addition, the magnitude as well as the duration of the flux anomalies found in the GERB fluxes are reduced in comparison to those seen for the GERB-like SEVIRI fluxes. The clear sky radiance plots in Figure 2 show that the diurnal variation of the estimated BB SEVIRI SW radiances (dotted lines) does not look like that of the ADM radiances (solid lines) and the conversion to flux greatly amplifies the difference as highlighted in Figure 1. Multiplying the estimated BB unfiltered SEVIRI SW radiances by the correction factor at the high resolution (namely the resolution enhancement factor in Equations 5 and 6), c H, to correct the SEVIRI based spectral modelling by the GERB measurement allows part of the disagreement between the estimated and the ADM radiances time series to be removed (dashed vs. solid lines in Figure 2). Presumably the noise on the monthly mean GERB fluxes diurnal evolution found in panels 5 and 8 in Figure 1 is due to a slight cloud contamination in the recorded GERB filtered SW radiances which unfortunately occurs due to geolocation or matching issues. Figure 1: Comparison between the diurnal evolution (from sunrise to sunset) of the monthly mean clear sky GERB (dashed lines), GERB-like SEVIRI (dotted lines), and corresponding CERES-TRMM ADM 5 SW

5 fluxes (solid lines). The comparison is given for April 2004 and for 9 ocean footprints (3x3 SEVIRI pixel resolution) located within the SEVIRI FOV. It is worth pointing out that in addition to the adjustment of the ADM ACF applied prior to perform the radiance to flux conversion (Equation 3), the CERES algorithm accounts for an additional correction not originally implemented in the RMIB GERB SEVIRI processing (RGSP). This correction (e.g., ADM normalization factor) allows to remove the bias introduced in the mean flux when linear interpolation is used to adjust the ADM ACF whereas the actual ADM radiance varies nonlinearly within an angular bin (Loeb et al., 2003). Moreover, because the anisotropy of clear ocean scenes depends on aerosol optical depth (AOD), the CERES algorithm also includes an aerosol correction when estimating the SW flux from the measured directional radiance over clear ocean. The TOA SW flux estimates over clear ocean scene is thus calculated as follows: πl(,, ) ( ) θ s θ v φ F θ s = + δ ( ; θ, θ, φ) th F ADM ws s v ( ; ) ADM( ; θ s, θ v, φ) R ws L R ws th R ( ws; L ADM) (7) where R ADM(ws; θ s, θ v, φ) is determined from the wind speed dependent CERES-TRMM clear ocean SW ADMs. R th (ws; L) and R th (ws; L ADM) are theoretical ACF inferred from the measured and ADM interpolated SW radiances, respectively. They are determined by comparing L(θ s, θ v, φ) and L ADM(θ s, θ v, φ) with look-uptables (LUT) of theoretical SW radiances stratified by AOD.

6 Figure 2: Same as Figure 1 but for the monthly mean clear sky directional BB SW radiances. Panels B in Figure 3 indicate that by contrast to the ADM and the reference no aerosols contaminated theoretical SW radiances (as retrieved from the LUTs of theoretical SW radiances used in the ADM aerosol correction process Equation 7 -hereafter referred as reference NACT SW radiances-) which behave more or less similarly throughout the day (solid line vs. solid line with circles), the estimated BB unfiltered SEVIRI SW radiances overestimate the ADM radiance in the afternoon (morning) over western (eastern) ocean footprints while underestimate the ADM radiance in the vicinity of the local noon over middle ocean footprint (see the time evolution of the solid lines with X in panels B). This is further illustrated in panel C by the level of the aerosol contamination. While the AOD values inferred from the ADM radiances (solid lines) do not vary much through the day for a given footprint, AOD values estimated from the BB unfiltered SW SEVIRI radiances (solid lines with X) show a distinct diurnal cycle pattern, which seems to be an artefact. The fictitious sinusoidal shape of the AOD diurnal cycle appears to be a function of the sine of the relative azimuth angle (see dotted lines in panels A in Figure 3).

7 The theoretical SW radiances from which the theoretical ACFs are estimated have been simulated by running the DISORT model (Stammes, 1988) assuming the Hess et al. (1998) maritime tropical aerosol model and using an aerosol phase function based on Mie computation. In the other hand, the regression coefficients used in Equation 2 were derived from SBDART spectral radiance curves assuming all the default SBDART aerosol models and considering an Henyey-Greestein (HG) phase function for the boundary layer aerosol. For aerosol models, the aerosol scattering phase function changes substantially according to aerosol type and representation. Therefore if the wrong aerosol model (phase function) is used to estimate the level of radiance contamination by aerosol this could introduce a diurnally (scattering angle) dependent error. As shown in panels B in Figure 3, the estimated SEVIRI BB radiances behave very similarly to the theoretical SBDART radiances from which the regression coefficients for use in Equation 2 were derived (solid lines with X vs. solid lines with diamonds). From the total number of 600 different RTM numerical simulations carried out to generate our spectral radiances data base, 150 were devoted to ``clear ocean scenes (including aerosol). The average of the 150 clear ocean theoretical BB SBDART SW radiances corresponding to the viewing conditions defined in panels A is given in Figure 3 for illustration (solid lines with diamonds). Because of the apparent dependence of our estimated clear ocean BB unfiltered SEVIRI SW radiance on the SBDART RTM, their conversion to fluxes by use of the CERES-TRMM clear ocean SW ADMs leads to the generation of a diurnal asymmetry or overestimation in the magnitude of the reflected SW in the GERB-like SEVIRI fluxes (see panels D in Figure 3).

8 Figure 3: Diurnal evolution of the reflected clear sky SW flux as estimated from SEVIRI NB radiances measurements only. The numbers in brackets above each top panels indicate the footprint location within the SEVIRI FOV as given in Figure 1, and the associated dates provide the selected day. In panels D, ``A c. (dashed lines) refers to SW flux computed with the ADM aerosols correction whereas ``A+N c. (solid lines) are for fluxes computed with both the aerosol correction and the ADM normalization factor, and finally, ``n.c. are for fluxes computed without any ADM s corrections. For each of the 3 selected ocean footprints in Figure 3, panels A in Figure 4 display the time evolution of the SW correction factor at the high resolution, c H. The corrected BB SEVIRI SW radiances are given in panels B together with the estimated BB unfiltered SEVIRI BB SW radiances and the ADM radiances for the viewing condition given in Figure 3 (panels A). Clearly, correcting the SEVIRI based spectral modelling by GERB measurement allows part of the disagreement between the estimated and the ADM SW radiances to be removed. This is especially evident in the central panel B in Figure 4 where the corrected radiances fit the ADM radiances perfectly from sunrise to local noon. However due to a decrease in the geolocation accuracy of the GERB SW filtered measurements in the vicinity of sunrise and sunset and in the vicinity of clouds, the SW correction factor can be contaminated by numerical noise at some slots. These erroneous c H values (e.g. from 9:00 to 10:30 UTC in the left panel A in Figure 4) affect therefore the corrected SW radiances values and thus the estimation of the reflected GERB SW flux at TOA. Also interesting to note in the left panel B in Figure 4 is the fact that, after the first period of overestimation, the corrected radiances (solid line with star) become lower than the ADM radiances (solid line) but also lower than the reference theoretical NACT DISORT BB SW radiances (solid line with circles) for a time, which is not at all the case for the estimated BB SEVIRI SW radiances from the SEVIRI data only (solid line with X). Such underestimation may also be responsible for some of the asymmetry problem found in the diurnal evolution of GERB fluxes because for the ADM aerosol correction scheme as implemented in the CERES data processing algorithm to be activated requires that an aerosol contamination is detected in both the ADM radiance and the measured/estimated SW radiance. The adoption of the newly estimated GERB SW spectral response function in the RGSP will reduce the occurrence of such underestimation in the corrected GERB SW radiances. Finally, because of the non linearity introduced in the GERB SW flux computation by the implementation of the ADM aerosol correction and the ADM normalization to a lesser extent (Equation 7), estimating the GERB flux by multiplying the ADM corrected SEVIRI SW flux with the SW correction factor differs substantially from a radiance to flux conversion using the corrected SEVIRI SW radiances (solid lines vs. solid with stars lines in panels C in Figure 4).

9 Figure 4: Same as Figure 3 but for the unfiltered high resolution GERB SW radiances. Note that rather than providing the aerosol contamination as done in Figure 3, the figure displays the time evolution of the high resolution SW correction factor. 4. CONCLUSIONS Because the available SEVIRI spectral information in the solar spectrum above clear ocean surface essentially reduces to its 0.6 µm spectral channel measurement, the SEVIRI NB-to-BB conversion does not perform well. The estimated SEVIRI BB radiances behave very similarly to the theoretical BB SW radiances from which the regression coefficients used in the spectral conversion were derived. Due to the deficiencies in the SEVIRI spectral conversion, converting the SEVIRI BB SW radiances to fluxes using the CERES- TRMM SW ADMs causes asymmetry in the diurnal evolution of the computed SW fluxes. Although the application of CERES-TRMM SW ADMs includes a correction which adjusts the anisotropy of clear ocean scenes to account for the presence of aerosols, this ADM correction does not allow the diurnal asymmetry in the GERB-like SEVIRI SW fluxes to be removed. Deficiencies in the SEVIRI spectral conversion makes the ADM aerosol correction sensitive to more than just the presence of aerosols. Therefore, applying this correction to the SEVIRI BB SW radiances yields a large retrieved AOD diurnal cycle, which is an artifact. Because of the fictitious aerosol contamination detected in the SEVIRI SW radiances the ADM aerosol correction fails to remove the diurnal asymmetry in the GERB-like SEVIRI SW flux values. Correcting the SEVIRI based spectral modelling by the GERB SW measurement allows part of the disagreement existing between the estimated SEVIRI BB SW radiances and the ADMs radiances to be removed. However, due to the GERB geolocation issues, the SW correction factor at the high 3x3 SEVIRI pixel resolution can be contaminated by numerical noise which unfortunately impacts on the computed SW flux value. Note that some asymmetry in the GERB SW flux values over clear ocean scenes can also simply be introduced by a slight undetected cloud contamination in the recorded GERB SW radiance.

10 Finally, because of the non linearity introduced in the GERB SW flux computation by the implementation of the ADM aerosol correction, it appears essential to estimate the clear ocean SW flux from the corrected SEVIRI BB SW radiances rather than from the corrected SEVIRI SW flux and to adopt the newly estimated GERB SW spectral response function in the RGSP in order to reduce the occurrence of diurnal asymmetry in the estimated GERB SW fluxes. 5. ACKNOWLEDGEMENTS This study was supported by the PRODEX Program (contract PRODEX-8 no.15162/01/nl/sfe (IC), Belgian State, Prime Minister s Office, Federal Office for Scientific, Technical and Cultural Affairs). 6. REFERENCES HARRIES, J.E., RUSSELL, J.E., KELLOCK, S., HANAFIN, J.A., MATTHEWS, G., WRIGLEY, R., LAST, A., MUELLER, J., RUFUS, J., BRINDLEY, H., FUTYAN, J., MOSSAVATI, R., ASHMALL, J., SAWYER, E., PARKER, D., CALDWELL, M., ALLEN, P.M., SMITH, A., BATES, M.J., COAN, B., STEWART, B.C., LEPINE, D.R., CORNWALL, L.A., CORNEY, D.R., RICKETTS, M.J., DRUMMOND, D., SMART, D., CUTLER, R., DEWITTE, S., CLERBAUX, N., GONZALEZ, L., IPE, A., BERTRAND, C., JOUKOFF, A., CROMMELYNCK, D., NELMS, N., LLEWELLYN-JONES, D.T., BUTCHER, G., SMITH, G.L., SZEWCZYK, Z.P., MLYNCZAK, P.E., SLINGO, A, ALLAN, R.P., and RINGER, M, (2005) The Geostationary Earth Radiation Budget experiment (GERB). Bulletin of the American Meteorological Society, 86, pp HESS, M., P. KOEPKE, and SCHULT, I (1998) Optical properties of aerosols and clouds: The software package OPAC. Bulletin of the American Meteorological Society, 79, pp LOEB, N., SMITH, N.M., KATO, S., MILLER, W.F., GUPTA, S.K., MINNIS, P., and WIELICKI, B.A., (2003) Angular distribution models for top-of-atmosphere radiative flux estimation from the Clouds and the Earth s radiative flux estimation from the Clouds and the Earth s Radiant Energy System instrument on the Tropical Rainfall Measuring Mission Satellite. Part I: Methodology. Journal of Applied Meteorology, 42, pp RICCIAZZI, P., YANG, S., GAUTIER, C., and SOWLE, D., (1998) SBDART: a research and teaching software tool for plane-parallel radiative transfer in Earth s atmosphere. Bulletin of the American Meteorological Society, 79, pp SCHMETZ, J., PILI, P., TJEMKES, S., JUST, D., KERKMANN, J., ROTA, S., and RATIER, A., (2002) An introduction to Meteosat Second Generation (MSG). Bulletin of the American Meteorological Society, 83, pp STAMMES, K., TSAY, S-C., WISCOMBE, W., and JAYAWEERA, K. (1988) Numerically stable algorithm for discrete-ordinate-method radiative transfer in multiple scattering and emitting layered media. Appl. Opt., 24, pp