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Comparison of ground-based measured and model-derived spectral irradiances
.2. Aerosol Sci. Vol. 30, Suppl. I, pp. $835--$836, 1999 1999 Published by Elsevier Science Ltd, All rights reserved Printed in Great Britain 0021-8502/99/$ - see front matter
Pergamon
COMPARISON OF GROUND-BASED MEASURED AND MODEL-DERIVED SPECTRAL IRRADIANCES
D. Miiller, M. Wendisch, A. Keil Institute for Tropospheric Research, Permoserstr. 15, D-04318 Leipzig, Germany
KEYWORDS aerosol optical properties, airborne measurements, LACE, radiative transfer, spectral itradiance
INTRODUCTION Significant differences between the shortwave radiation measurements and the results of radiative transfer models have been extensively discussed in the literature especially in cloudy atmospheres (Pilewski and Valero, 1995) as well as under cloudless conditions (Arking et al., 1996). In order to investigate this problem we have included detailed airborne measurements of both, aerosol microphysical and optical properties into a sophisticated radiation transfer model. The model results are compared to directly-measured, ground-based spectral irradiances. The ground-based data set consists of global and diffuse spectral irradiances in the visible and near-infrared region. The measurements were performed south of Berlin (52" 10' N, 167" 07' E, 70m asl) during the Lindenberg Aerosol Characterization Experiment (LACE) from Juli 31 to August 12, 1998. Simultaneously performed aircraft measurements of atmospheric aerosol-optical parameters constitute the input for radiative transfer calculations. MEASUREMENTS A Diode Array Spectrometer (DAS) with quartz diffusor foreoptic was used to measure spectral irradiances between 500 nm and 900 nm with a spectral resolution of 2 nm at 5 sec time steps. In order to correct the non-ideal cosine response of the diffusor both, global (direct plus diffuse) and diffuse radiation was measured, the last one by using a shade mechanism. In a following step cosine corrections were separately applied to direct (global minus diffuse) and diffuse radiation. An absolute calibration of the DAS was carried out during a pre-experiment in June, 1998 supported by the Deutscher Wetterdienst (DWD) in Potsdam. Therefore, a halogen lamp, traceable to an absolute standard (maintained by the Physikalisch-Technische-Bundesanstalt (PTB)), was used. Furthermore, the cosine characteristic of the instrument was experimentally determined by aligning a halogen lamp in small steps of incident angles from a normal incidence to a horizontal position (Feister et al., 1997). Calibration checks using the sun as a radiation source were performed by concurrent measurements of global irradiance with a calibrated spectroradiometer from DWD under broken-cloudy conditions.
$835
$836
Abstracts of the 1999 European Aerosol Conference
RESULTS Examples for corresponding ground-based irradiance and airborne aerosol-optical measurements on August 01 and 10 under cloudless conditions are shown in Figure 1. Figure la. shows the vertical profile of the volume scattering coefficient measured at 700 nm and Figure lb the direct and diffuse irradiances at the surface. From the profiles of the volume scattering coefficients it becomes obvious that the troposphere on August 01 was much more polluted compared to August 10 (Fig. la). The radiative influence of the thin layer with an enhanced aerosol content in the height of 3000 m needs to be derived from subsequent calculations. The polluted air mass on August 01 leads to an increase of the proportion of the diffuse radiation which is detectable in the surface measurements (Fig. lb.). Results of corresponding radiative transfer calculations for the two cases will be presented. a. 3500 ~
.
.
.
.
3000 -~--~-'~-_-~_ . . . . . -
500
0
b.
' " ( ) 1 August
j'-
I
:--::-=-±:--
1 2 3 4 5 6 7 8 Volume scaffering coefficient [le-5/m]
1.4 ~ \ .
~
9
1
-.
0
500
01 August
2
-
550
~
600
1
10August
650 700 750 Wavelength [nm]
800
850
Figure 1: Radiative and aerosol-optical measurements on August 01 (thick lines) and August 10 (thin lines), 1998. a.: Vertical profiles of the volume scattering coefficient at 700 nm [10-~m -1] from aircraft measurements, b.: Corresponding spectra of direct and diffuse irradiance [Wm-2nm -1] at Falkenberg.
ACKNOWLEDGEMENTS U. Feister and colleagues from the DWD in Potsdam are acknowledged for their help in a friendly atmosphere during the pre-experiment.
REFERENCES Arking, A. (1996): Absorption of solar energy in the atmosphere: Descrepancy between model and observations. Science, 273, 779-782. Feister U. and R. Grewe (1997): A method for correction of cosine errors in measurements of spectral UV irradiance. Solar Energy 60, 6, 313-332. Pilewskie, P. and F.P.J. Valero (1995): Direct observations of excess solar absorption by clouds. Science, 267, 1626-1629.
Pergamon
COMPARISON OF GROUND-BASED MEASURED AND MODEL-DERIVED SPECTRAL IRRADIANCES
D. Miiller, M. Wendisch, A. Keil Institute for Tropospheric Research, Permoserstr. 15, D-04318 Leipzig, Germany
KEYWORDS aerosol optical properties, airborne measurements, LACE, radiative transfer, spectral itradiance
INTRODUCTION Significant differences between the shortwave radiation measurements and the results of radiative transfer models have been extensively discussed in the literature especially in cloudy atmospheres (Pilewski and Valero, 1995) as well as under cloudless conditions (Arking et al., 1996). In order to investigate this problem we have included detailed airborne measurements of both, aerosol microphysical and optical properties into a sophisticated radiation transfer model. The model results are compared to directly-measured, ground-based spectral irradiances. The ground-based data set consists of global and diffuse spectral irradiances in the visible and near-infrared region. The measurements were performed south of Berlin (52" 10' N, 167" 07' E, 70m asl) during the Lindenberg Aerosol Characterization Experiment (LACE) from Juli 31 to August 12, 1998. Simultaneously performed aircraft measurements of atmospheric aerosol-optical parameters constitute the input for radiative transfer calculations. MEASUREMENTS A Diode Array Spectrometer (DAS) with quartz diffusor foreoptic was used to measure spectral irradiances between 500 nm and 900 nm with a spectral resolution of 2 nm at 5 sec time steps. In order to correct the non-ideal cosine response of the diffusor both, global (direct plus diffuse) and diffuse radiation was measured, the last one by using a shade mechanism. In a following step cosine corrections were separately applied to direct (global minus diffuse) and diffuse radiation. An absolute calibration of the DAS was carried out during a pre-experiment in June, 1998 supported by the Deutscher Wetterdienst (DWD) in Potsdam. Therefore, a halogen lamp, traceable to an absolute standard (maintained by the Physikalisch-Technische-Bundesanstalt (PTB)), was used. Furthermore, the cosine characteristic of the instrument was experimentally determined by aligning a halogen lamp in small steps of incident angles from a normal incidence to a horizontal position (Feister et al., 1997). Calibration checks using the sun as a radiation source were performed by concurrent measurements of global irradiance with a calibrated spectroradiometer from DWD under broken-cloudy conditions.
$835
$836
Abstracts of the 1999 European Aerosol Conference
RESULTS Examples for corresponding ground-based irradiance and airborne aerosol-optical measurements on August 01 and 10 under cloudless conditions are shown in Figure 1. Figure la. shows the vertical profile of the volume scattering coefficient measured at 700 nm and Figure lb the direct and diffuse irradiances at the surface. From the profiles of the volume scattering coefficients it becomes obvious that the troposphere on August 01 was much more polluted compared to August 10 (Fig. la). The radiative influence of the thin layer with an enhanced aerosol content in the height of 3000 m needs to be derived from subsequent calculations. The polluted air mass on August 01 leads to an increase of the proportion of the diffuse radiation which is detectable in the surface measurements (Fig. lb.). Results of corresponding radiative transfer calculations for the two cases will be presented. a. 3500 ~
.
.
.
.
3000 -~--~-'~-_-~_ . . . . . -
500
0
b.
' " ( ) 1 August
j'-
I
:--::-=-±:--
1 2 3 4 5 6 7 8 Volume scaffering coefficient [le-5/m]
1.4 ~ \ .
~
9
1
-.
0
500
01 August
2
-
550
~
600
1
10August
650 700 750 Wavelength [nm]
800
850
Figure 1: Radiative and aerosol-optical measurements on August 01 (thick lines) and August 10 (thin lines), 1998. a.: Vertical profiles of the volume scattering coefficient at 700 nm [10-~m -1] from aircraft measurements, b.: Corresponding spectra of direct and diffuse irradiance [Wm-2nm -1] at Falkenberg.
ACKNOWLEDGEMENTS U. Feister and colleagues from the DWD in Potsdam are acknowledged for their help in a friendly atmosphere during the pre-experiment.
REFERENCES Arking, A. (1996): Absorption of solar energy in the atmosphere: Descrepancy between model and observations. Science, 273, 779-782. Feister U. and R. Grewe (1997): A method for correction of cosine errors in measurements of spectral UV irradiance. Solar Energy 60, 6, 313-332. Pilewskie, P. and F.P.J. Valero (1995): Direct observations of excess solar absorption by clouds. Science, 267, 1626-1629.