Franz H. Berger

Evaporation over a heterogeneous land surface - The EVA-GRIPS project

The representation of subgrid-scale surface heterogeneities in numerical weather and climate models has been a challenging problem for more than a decade. The Evaporation at Grid and Pixel Scale (EVA-GRIPS) project adds to the numerous studies on vegetation-atmosphere interaction processes through a comprehensive field campaign and through simulation studies with land surface schemes and mesoscale models. The mixture of surface types in the test area in eastern Germany is typical for larger parts of northern Central Europe. The spatial scale considered corresponds to the grid scale of a regional atmospheric weather prediction or climate model and to the pixel scale of satellite images. Area-averaged fluxes derived from point measurements, scintillometer measurements, and a helicopter-borne turbulence probe were widely consistent with respect to the sensible heat flux. The latent heat flux from the scintillometer measurements is systematically higher than the eddy covariance data. Fluxes derived from numerical simulations proved the so-called mosaic approach to be an appropriate parameterization for subgrid heterogeneity.

The influence of land surface parameters on energy flux densities derived from remote sensing data

Knowledge of the vegetation properties surface reflectance, Normalised Difference Vegetation Index (NDVI) and Leaf Area Index (LAI) are essential for the determination of the heat and water fluxes between terrestrial ecosystems and the atmosphere. Remote sensing data can be used to derive spatial estimates of the required surface properties. The determination of land surface parameters and their influence on radiant and energy flux densities is investigated with data of different remote sensing systems. Sensitivity studies show the importance of correctly derived land surface properties to estimate the key quantity of the hydrological cycle, the evapotranspiration (L.E), most exactly. In addition to variable parameters like LAI or NDVI there are also parameters which are can not be inferred from satellite data but needed for the Penman-Monteith approach. Fixed values are assumed for these variables because they have little influence on L.E. Data of Landsat-7 ETM+ and NOAA-16 AVHRR are used to show results in different spatial resolution. The satellite derived results are compared with ground truth data provided by the Observatory Lindenberg of the German Weather Service.

Über den „Taucheffekt” bei der Lichtmessung über und unter Wasser

Das Verhaltnis der mesbaren Intensitaten in einem Lichtmesgerat flacher Bauart dicht unter und uber Wasser wird „Taucheffekt” genannt. Dieses Verhaltnis weicht von der wahren Relation der Lichtintensitaten innerhalb weiter Grenzen ab. Entsprechend den verschiedenen optisch-geometrischen Bedingungen bei Opal-, Klarglas- und Mattscheiben gibt es drei verschiedenartige Taucheffekte.

Inference of the climatic efficiency of clouds from satellite measurements

Abstract The influence of clouds over the North Sea on the radiation field and on climate is investigated by analysing satellite measurements. The main interest is on high clouds due to their ambivalent behaviour in the radiation field. To quantify the influence of clouds on climate, the cloud-climate efficiency is introduced. The cloud-climate efficiency allows us to estimate the gain or respectively loss of energy of the earth/atmosphere system in the presence of a cloud, which can be specified by a cloud classification. The spatial integration of the cloud-climate efficiencies results in the cloud forcing defined by the difference of the radiative fluxes within a clear sky and a cloudy satellite image pixel. The first step is an accurate detailed cloud classification based on the maximum likelihood method. The method developed for National Oceanograph and Atmospheric Administration Advanced Very High Resolution Radiometer (NOAA AVHRR)data and Meteosat data can be used to discriminate 24 clouds, especia...

Validating MODIS land surface reflectance products using ground-measured reflectance spectra – a case study in semi-arid grassland in Inner Mongolia, China

Validation of Moderate-Resolution Imaging Spectroradiometer (MODIS) land surface reflectance products is important to effective utilization of such products for earth systems science. Ground-based measurements are normally utilized for such validation. However, the major scale mismatch between the ground ‘point’ measurement and MODIS resolution (500 m and 1 km) makes direct comparison infeasible over many land surface types. In this paper, an indirect comparison between ground ‘point’ measurements and MODIS land surface products via high-resolution remotely sensed imagery (Landsat Thematic Mapper/TM) was utilized in semi-arid grassland of Inner Mongolia in summer 2005, where ground measurements are relatively sparse in comparison with other locations around the world. Within the validation, the TM reflectance imagery was first calibrated by the ground ‘point’ measurements, and then aggregated to MODIS data resolution for determination of their accuracy. Besides common direct spectral band comparison of reflectance between TM and MODIS, empirical/indirect comparison between TM and MODIS was also implemented. Both types of validation showed that the absolute error of bidirectional reflectance from atmospheric correction (MOD09) is less than 9.4%, and for nadir bidirectional reflectance distribution function (BRDF)-adjusted reflectance (MOD43B4) it is less than 3.1%, in which the error of visible bands of two data sets is less than 1.35% and 0.95%, respectively. This validation will help improve the accuracy of MODIS products used in this area.

The influence of clouds on earth radiation budget — a regional study: The North Sea

Abstract The influence of clouds over the North Sea on climate is investigated by analyzing NOAA AVHRR data. The main interest is on high clouds due to their ambivalent behaviour in the radiation field. After a cloud classification, especially for high clouds, and the determination of cloud optical properties, the obtained information was applied to calculate the cloud-climate efficiency. This index is similar to the cloud forcing, but is valid for an individual classified satellite image pixel. The cloud forcing is the sum of the cloud-climate efficiencies over an area. Using NOAA-AVHRR data the annual cycle (October 1989, October 1990, April 1991 – July 1992) of cloud forcing at the top of atmosphere were calculated. Due to the strong dependence on solar insolation, high clouds with the same optical properties lead to an heating or a cooling of the earth/atmosphere system. For thin cirrus clouds the heating effect is well correlated with an increase of the surface temperature. A further approach to compare the increasing/decreasing cloud forcing with an analysis of relative topography 300/1000 hPa shows that an increase is positively correlated with an increase of the temperature in this layer.

Validation of optical cloud parameters inferred from satellite measurements by ground observations

Abstract Cloud reflectances and emittances are inferred from satellite measurements in NOAA-AVHRR channels 1, 2, 4 and 5. For a few cases during field experiments in France (La Crau), Italy and during the International Cirrus Experiment (ICE) 1987 observations from the ground are available to validate the inferred data.

Radiation budget components at surface and at top of atmosphere for convective cloud cases in Central Europe

Abstract The paper is focused on the determination of radiation budget components at surface as well as at top of atmosphere using remotely sensed data for a few convective cloud cases between 1990 and 1993. The target area for this investigation is Central Europe and covers an area of approx. 1800 × 1800 km2. Preliminary results from heating rate computations will further be given. To infer the individual radiation budget components at surface from NOAA-AVHRR and Meteosat data, an inverse remote sensing technique was applied. This techniques uses partly look-up tables, which are generated for different atmospheric conditions. For the atmospheric radiative transfer a delta two-stream approximation scheme was used. The tables consider different variables, like solar zenith angle, cloud optical depth, cloud base height, and for the longwave standard temperature and humidity profiles. The cloud optical properties were computed with the same delta two-stream approximation scheme, where a cloud classification was carried out in advance to distinguish between cloud types. This information was used to define the microphysical cloud properties. For the atmospheric conditions monthly means of horizontal visibility and relative humidity were taken into account. The radiation budget components at top of atmosphere were computed using a narrow-to-broadband conversion considering all geometrical conditions. Finally, the heating rates for four atmospheric layers were also calculated based on the inverse remote sensing technique.

How accurate is a classification of thin clouds using NOAA AVHRR data

Abstract To classify different cloud types June 1991 NOAA AVHRR data were analysed. Applying the maximum likelihood classification method, it was difficult to detect thin clouds over land due to the variability of surface reflectance and to the unknown amount and type of aerosols. This study is about the accuracy by which clouds can be detected considering the two aforementioned difficulties. Thus AVIRIS data with high spatial and spectral resolution were used to determine the surface reflectance characteristics of a chosen test site. A pixel by pixel comparison between AVHRR and spatially and spectrally integrated AVIRIS data showed a systematic error less than 2 %. Starting with the test sites surface reflectances inferred from AVIRIS data the signal (corresponding to a NOAA pixel) at top of atmosphere (TOA) has been simulated using a radiative transfer scheme (5S). The aerosol amounts were varied in these simulations. The maximal error in the reflectance at TOA due to these variations of aerosol amount was found in an order of 3 % in channel 1 and 4 % in channel 2. This corresponds to an error in the optical depth of a cloud of about 0.5 or about 30 W/m 2 in the downward net shortware flux at surface.

Detection of clouds and their influence on radiation budget determined by multisensor satellite data

In several instances during the International Cirrus Experiment (ICE) 1989, cloud types were detected by multisensor satellite data over the North Sea. The first cloud classification scheme is based on the maximum likelihood method using NOAA AVHRR and Meteosat data. The second is an algorithm using a combination of Meteosat and SSM/I data. By comparing these results together and with synoptical observations, good agreement can be achieved. The discrepancies can be explained either by time delay or different spatial resolution. Comparing the monthly mean cloudiness inferred from NOAA AVHRR-data with ISCCP C2 data, it seems that the ISCCP C2 results underestimate the real cloudiness for the North Sea area (approx. 55 degree(s) N latitude, 5 degree(s) E longitude). To determine the influence of clouds on the earth radiation budget and on the climate the cloud-climate efficiency was used. This index is similar to the cloud forcing, but is valid for an individual classified satellite image pixel. The cloud forcing is the sum of the cloud-climate efficiencies over an area, i.e., the heating or cooling of the earth/atmosphere system can be estimated. Using NOAA-AVHRR data the annual cycle of cloud forcing at top of atmosphere was calculated for the North Sea.