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- 25 -<br />

Broadband Cloud Albedo from MODIS<br />

Anja Hünerbein, Rene Preusker, Jürgen Fischer<br />

Institut für Weltraumwissenschaften, Freie Universität Berlin, Germany<br />

Carl-Heinrich Becker Weg 6-10, D-12165 Berlin, e-mail: anja.huenerbein@wew.fu-berlin.de<br />

1. Introducti<strong>on</strong><br />

One of the significant comp<strong>on</strong>ents of Earth’s energy balance<br />

is the cloud-reflecti<strong>on</strong> of incoming solar radiati<strong>on</strong> back to<br />

space. The radiative budged is modified by the comp<strong>on</strong>ents<br />

of the Earth-atmosphere system such as clouds, aerosols and<br />

the surface. Accurate global observati<strong>on</strong> of top of the<br />

atmosphere (ToA) radiative flux is crucial for improving<br />

global climate models and for the understanding of climate<br />

processes. An algorithm of the estimati<strong>on</strong> of broadband<br />

albedo above clouds is developed for the Moderate<br />

Resoluti<strong>on</strong> Imaging Spectroradiometer (MODIS) King et al.<br />

(1992).<br />

2. Materials and Methods<br />

Basic idea is to use a forward model to get the upward flux<br />

at ToA and a inverse model to get a relati<strong>on</strong> between the<br />

measured MODIS radiance and the simulated upward flux.<br />

The forward model is based <strong>on</strong> a radiative transfer model<br />

(MOMO) Fell and Fischer (2001) and for the inverse model<br />

a neural network is used.<br />

Besides the physical properties of the cloud like<br />

stratificati<strong>on</strong>, particle radius, cloud height and thickness the<br />

ToA albedo depends <strong>on</strong> several additi<strong>on</strong>al parameters such<br />

as ground reflecti<strong>on</strong>, aerosol and water vapour. The radiative<br />

transfer model MOMO is used for the radiative properties to<br />

receive upward radiance at ToA. The model assumes a plan<br />

parallel atmosphere, however any vertical inhomogeneity<br />

and media of any optical thickness as well as any spectral<br />

resoluti<strong>on</strong> can be c<strong>on</strong>sidered. The simulati<strong>on</strong>s are<br />

m<strong>on</strong>ochromatic. The spectrum (200-3700nm) of the solar is<br />

divided in 183 individual narrow bands. The simulated<br />

m<strong>on</strong>ochromatic upward radiance is integrated over<br />

wavelength and weighted with incoming radiance at ToA to<br />

receive the upward flux. Arbitrary chosen cases are<br />

simulated to represent natural variability of the atmosphere.<br />

Thereafter the model is used to simulate the radiance in the<br />

short-wave MODIS channel for each defined case. The<br />

simulated dataset is used for neural network training. The<br />

input parameters are the spherical angles, surface type, water<br />

vapour and the radiance of the MODIS channels. The<br />

artificial neural network estimates the upward flux at ToA<br />

<strong>on</strong> a pixel by pixel basis.<br />

3. Results<br />

Clouds and Earth’s Radiant Energy System (CERES) NASA<br />

(2004) also <strong>on</strong>board the Terra platform is chosen to compare<br />

the results of the developed algorithm. CERES data is<br />

processed by the ERBE-like Inversi<strong>on</strong> Subsystem to provide<br />

estimates of radiant flux at the ToA. The spatial resoluti<strong>on</strong><br />

of CERES is 20 km, while MODIS has a 1km resoluti<strong>on</strong>.<br />

That is why CERES images like figure1 has smoother<br />

pattern than MODIS’ images. A close look is given in figure<br />

2, where <strong>on</strong>e cross secti<strong>on</strong> is shown. MODIS has a higher<br />

variability than CERES, but the values are in the same<br />

range. Comparis<strong>on</strong> has shown the results being reas<strong>on</strong>able.<br />

This method allows a rapid processing of satellite data and<br />

will be incorporated into the near-real-time processing at the<br />

Frei Universität Berlin.<br />

Figure 1. On the left sight: neural network derived cloud<br />

albedo [W/m²] for MODIS and <strong>on</strong> the left sight: upward<br />

short-wave radiance [W/m²] from CERES for the<br />

16.06.2002 at 9.32 am.<br />

Figure 2. Cross secti<strong>on</strong> from figure 1 for latitudes 54°,<br />

red line is upward short-wave radiance [W/m²] of CERES,<br />

black line is cloud albedo [W/m²] from MODIS<br />

References<br />

Fell, F., and J. Fischer, Numerical simulati<strong>on</strong> of the light<br />

field in the atmosphere-ocean system using the matrixoperator<br />

method, J. Quant. Spectroscop. Radiat.<br />

Transfer, Vol. 69, pp. 351-388, 2001<br />

King, M. D., Y. J. Kaufman, W.P. Menzel and D. Tanré,<br />

Remote sensing of cloud, aerosol, and water vapour<br />

properties from the Moderate Resoluti<strong>on</strong> Imaging<br />

Spectroradiometer (MODIS), IEEE Trans. Geosci.<br />

Remote. Sensing, Vol.30, 1992<br />

NASA, Langley Atmospheric Science Data Hierarchical<br />

Data Format Web site, URL =<br />

http://eosweb/HBDOC/hdf.html, 2004

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