Laboratory for
Climatology and Remote Sensing

Abstract:

The distinction made by satellite data between ground fog and low stratus is still an open problem. A proper detection scheme would need to make a determination between low stratus thickness and top height. Based on this information, stratus base height can be computed and compared with terrain height at a specific picture element. In the current paper, a procedure for making the distinction between ground fog and low-level stratus is proposed based on Moderate Resolution Imaging Spectroradiometer (MODIS, flying on board the NASA Terra and Aqua satellites) daytime data for Germany. Stratus thickness is alternatively derived from either empirical relationships or a newly developed retrieval scheme (lookup table approach), which relies on multiband albedo and radiative transfer calculations. A trispectral visible–near-infrared (VIS–NIR) approach has been proven to give the best results for the calculation of geometrical thickness. The comparison of horizontal visibility data from synoptic observing (SYNOP) stations of the German Weather Service and the results of the ground fog detection schemes reveals that the lookup table approach shows the best performance for both a valley fog situation and an extended layer of low stratus with complex local visibility structures. Even if the results are very encouraging [probability of detection (POD) = 0.76], relatively high percentage errors and false alarm ratios still occur. Uncertainties in the retrieval scheme are mostly due to possible collocation errors and known problems caused by comparing point and pixel data (time lag between satellite overpass and ground observation, etc.). A careful inspection of the pixels that mainly contribute to the false alarm ratio reveals problems with thin cirrus layers and the fog-edge position of the SYNOP stations. Validation results can be improved by removing these suspicious pixels (e.g., percentage error decreases from 28% to 22%).

Abstract:

Die Arbeit stellt die Entwicklung eines Verfahrens zur Erkennung der Niederschlagsfläche in optischen Satellitendaten dar. Die Abgrenzung regnender Wolken beruht dabei auf dem Modellkonzept, dass diese über eine ausreichend große Kombination aus optischer Dicke und effektivem Tropfenradius verfügen müssen.

Abstract:

Gradients of fog and rain along an altitudinal transect (1800 to 3185 m asl) are presented for a tropical montane forest of southern Ecuador (ECSF area). The investigation is performed with a combination of satellite and ground based remote sensing, in-situ measurements and numerical models. The ECSF area is characterised by above-average cloudiness and an increase of rainfall and fog water intake with height. Even if concentrations of soluted matters are generally low, an increase of e.g. sulfate concentrations can be observed during special events as e.g. volcanic eruptions.

Abstract:

The scope of the EU Concerted Action on Science and Technology (COST) 722 “Short range forecasting methods of fog, visibility and low clouds” is presented. Results of the inventory phase are discussed and some examples of the current work within the single working groups are given.

Abstract:

Two new approaches for the detection of fog/low stratus during daylight are presented. The methods are based on second generation space technology from the polar (TERRA/AQUA MODIS) and geostationary (MSG-SEVIRI) orbits. The MODIS approach makes use of a threshold technique which relies on eight spectral bands. Minimum and maximum fog thresholds are determined by spectral threshold functions which are derived from radiative transfer calculations. Validation with SYNOP data points to a good performance of the algorithm. The SEVIRI approach profits from spectral and spatial cloud characteristics. Fog detection is performed by using dynamic thresholding and a test due to cloud homogeneity.

Abstract:

The detection of clouds and the analysis of cloud frequency play an important role for operational weather prediction as well as for climate– ecological studies. A threshold technique for cloud detection in the tropical mountainous area of Ecuador is presented which is based on National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (NOAA-AVHRR) data. Cloud classification is performed for both day and night overpasses by applying several threshold tests which also include ancillary terrain information. From a set of 155 images, maps of relative cloud frequency are calculated for Ecuador and adjacent areas as well as the central study area of an ecological project in southern Ecuador (Loja). Results show a clear relation between topography, main airflow and cloudiness due to barrage and lee-effects. The spatial extension of high mountain ecosystems such as the Ecuadorian Pa´ramo has been proven to be significantly influenced by the spatial pattern of cloud frequency.

Abstract:

Konzentrierende Solarsysteme können einen sehr großen Teil der zukünftigen Strom- und Wärmeerzeugung leisten. Die Technik dieser Systeme steht kurz vor der Markteinführung, da die Stromgestehungskosten beinahe konkurrenzfähig geworden sind. Eine zentrale Größe ist dabei die Direkt-Normal-Strahlung, sozusagen der "Brennstoff" solcher Kraftwerke, die einen sehr hohen Einfluss auf deren Leistung und Stromerträge hat. Konzentrierende Solarkraftwerke sind eine Großinvestition, bei der schon in der Planungsphase eine Risiko- und Kostenreduktion angestrebt wird. Daher müssen heute Aussagen über die vorhandenen Strahlungsressourcen gemacht werden, um in Zukunft Solarenergie wirtschaftlich nutzen zu können. Die vorliegende Arbeit erarbeitet ein operationelles Verfahren, das mit Hilfe von sichtabaren und infraroten Daten des geostationären Satelliten Meteosat und Atmosphärendaten für Ozon, Wasserdampf und Aerosol mittels eines Parametrisierungsmodells die stündliche Direkt-Normal-Strahlung berechnet. Diese kann für große Regionen, Länder oder einzelne Standorte innerhalb des Sichtbereiches des Satelliten Meteosat in einer räumlichen Auflösung von bis zu 5km x 5km für Zeitreihen bis zu 20 Jahren bestimmt werden. Die so gewonnenen Informationen der verfügbaren Strahlungsressource sind ein wichtiger Bestandteil bei der Planung und Auslegung von konzentrierenden Solarkraftwerken und bei der Abschätzung des Strahlungspotenzials einer Region.