Mads Olander Rasmussen

Directional Effects on Land Surface Temperature Estimation From Meteosat Second Generation for Savanna Landscapes

Structured canopies can show pronounced directional effects which influence land surface temperature (LST) estimates from thermal infrared satellite data. The effects depend on illumination and viewing geometries, because changes in these two geometries effectively cause the sensor to “see” different fractions of the canopy and the “background” surface (bare soil or low vegetation). Furthermore, parts of these two components will be in shadow, depending on the specific geometry of the canopy and its structure. This paper investigates these directional effects for a specific savanna site in West Africa and extends the findings to areas with denser tree crown cover. This is achieved by modeling the combined effects of the structured surface with a geometric optics model. The model assumes that the surface consists of four components: shaded and sunlit tree canopies and shaded and sunlit backgrounds. The brightness temperatures of these four surface components are provided by in situ measurements at the validation site, and emissivities are taken from the Land Surface Analysis Satellite Applications Facility (LSA-SAF) project. The LST modeling is performed for the geometry of the geostationary Meteosat Second Generation and for nadir geometry. Analyses of the temperature differences between the LST estimates for the two geometries show that, in many cases, the directional effects exceed 1°C within a day and that the timing and the sign of the effects change with season. Directional errors due to structured canopies are currently not considered in error estimates of operationally available LST products, e.g., the LSA-SAF LST product or the Moderate Resolution Imaging Spectroradiometer (MODIS) LST/emissivity products.

Modeling Angular Dependences in Land Surface Temperatures From the SEVIRI Instrument Onboard the Geostationary Meteosat Second Generation Satellites

Satellite-based estimates of land surface temperature (LST) are widely applied as an input to models. A model output is often very sensitive to error in the input data, and high-quality inputs are therefore essential. One of the main sources of errors in LST estimates is the dependence on vegetation structure and viewing and illumination geometry. Despite this, these effects are not considered in current operational LST products from neither polar-orbiting nor geostationary satellites. In this paper, we simulate the angular dependence that can be expected when estimating LST with the viewing geometry of the geostationary Meteosat Second Generation Spinning Enhanced Visible and Infrared Imager sensor across the African continent and compare it to a normalized view geometry. We use the modified geometric projection model that estimates the scene thermal infrared radiance from a surface covered by different land covers. The results show that the sun-target-sensor geometry plays a significant role in the estimated temperature, with variations strictly due to the angular configuration of more than ±3°C in some cases. On the continental scale, the average error is small except in hot-spot conditions, but large variations occur both geographically and temporally. The sun zenith angle, the amount of vegetation, and the vegetation structure are all shown to affect the magnitude of the errors. The findings highlight the need for taking the angular effects into account when applying LST estimates in models and when comparing LST estimates from different sensors or from different times, both on the daily and seasonal scale.

Regional-scale estimation of evapotranspiration for the North China Plain using MODIS data and the triangle-approach

Abstract A method for the estimation of daily evapotranspiration is tested for the North China Plain. The method is designed to be simple to implement and with very limited requirements for ground data (air temperature and humidity). The method uses MODIS NDVI and Land Surface Temperature (LST) data to derive evaporative fraction, using an adaptation of the “triangle method”. The energy available for evapotranspiration is estimated using a combination of satellite data from MODIS and the (geostationary) Fengyun 2-series of sensors and station-based air temperature data. A gapfilling routine is applied to the time series of evaporative fraction to create complete daily maps for the region, allowing for use of the ET-estimates for applications requiring complete daily coverage (e.g. hydrological models). Results show that ET estimation on a daily scale is feasible with the proposed method, and that seasonal patterns are in accordance with other independent ET-estimates. There are some indications that our ET-estimates are somewhat overestimated when comparing to other RS-based methods and model simulations. It is demonstrated that the proposed method provides a relatively simple way of obtaining spatially distributed daily estimates of ET, making the method suitable for applications in studies where ground data availability is limited.

The Normalization of Surface Anisotropy Effects Present in SEVIRI Reflectances by Using the MODIS BRDF Method

A modified version of the MODerate resolution Imaging Spectroradiometer (MODIS) bidirectional reflectance distribution function (BRDF) algorithm is presented for use in the angular normalization of surface reflectance data gathered by the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) aboard the geostationary Meteosat Second Generation (MSG) satellites. We present early and provisional daily nadir BRDF-adjusted reflectance (NBAR) data in the visible and near-infrared MSG channels. These utilize the high temporal resolution of MSG to produce BRDF retrievals with a greatly reduced acquisition period than the comparable MODIS products while, at the same time, removing many of the angular perturbations present within the original MSG data. The NBAR data are validated against reflectance data from the MODIS instrument and in situ data gathered at a field location in Africa throughout 2008. It is found that the MSG retrievals are stable and are of high-quality across much of the SEVIRI disk while maintaining a higher temporal resolution than the MODIS BRDF products. However, a number of circumstances are discovered whereby the BRDF model is unable to function correctly with the SEVIRI observations-primarily because of an insufficient spread of angular data due to the fixed sensor location or localized cloud contamination.

Nine pheochromocytomas in the same patient: Final mapping with ultrasound and angiography

A 37-year-old man presented with hypertension and elevated urine catecholamine. Ultrasound scanning revealed a solid tumour of the right adrenal gland and two solid tumours in the retroperitoneum. The findings were confirmed with computed tomography and abdominal angiography. At surgery only the tumour of the right adrenal gland was removed. The histopathologic diagnosis was pheochromocytoma. Postoperatively the symptoms and biochemistry were unchanged and the patient was referred for further treatment. At ultrasonography and abdominal aortography 6 remaining tumours were demonstrated. Surgery was performed and 8 pheochromocytomas were extirpated (3 were closely spaced small tumours in a conglomerate corresponding to one of the visualized tumour sites). On histopathologic examination no signs of invasive growth were found. The patient recovered completely. The blood pressure was still normal 2 1/2 years later. Angiography and non-invasive examination of the entire abdomen and pelvis should be routine when pheochromocytomas are searched for.