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Radiation measurement at Lewis Glacier, Mount Kenya, Kenya

  • Stefan Hastenrath (a1) and J. K. Patnaik (a2)

Abstract

Short- and long-wave radiation on variously oriented vertical surfaces, direct solar radiation, global radiation, and long–wave radiation on a horizontal surface were measured on Lewis Glacier, Mount Kenya, at 4800 m. For the orientation of vertical surfaces, the following azimuths were selected: 45°, facing the steep slope of the upper glacier; 135°, facing a rock ridge and some glacier surface in the foreground; 225°, facing down–glacier towards the Teleki valley with open sky occupying much of the view; and 315°, directed towards the steep south-east face of the Nelion peak.

The horizontal components of diffuse short-wave radiation reach a magnitude comparable to those of direct radiation. As a result of contrastingly different albedos of natural surfaces, the horizontal component of diffuse short–wave radiation is particularly large from the direction of the upper glacier, with values around 330–500 W m−2, and smallest from the direction of the rock face of Nelion peak, where values are around 150–330 W m−2. Long–wave radiation seems enhanced from the direction of the Nelion face, and reduced from the azimuth of the upper glacier, thus apparently reflecting differences in emissivity and temperature.

Résumé

On a mesuré sur le Lewis Glacier au Mount Kenya à 4800 m les rayonnements de courte et de grande longueur d'onde sur des surfaces verticales d'orientation variée, ainsi que le rayonnement solaire direct, le rayonnement global et le rayonnement de grande longueur d'onde sur une surface horizontale. Pour l'orientation des surfaces verticales, on a chois les azimuths suivants: 45° face à la pente forte du glacier supérieur, 135° face à une paroi rocheuse et un peu de surface en glace au premier plan, 225° face à l'aval du glacier vers la vallée de Teleki, avec le ciel libre occupant la plus grande part du champ de vision, et 315° en direction de la face sud–est trés raide du Pic Nelion.

Les composantes horizontales de la radiation diffuse de courte longueur d'onde atteignent un ordre de grandeur comparable à ceux de la radiation directe. En raison des différences trés marquées d'albédos des surfaces naturelles, la composante horizontale de la radiation diffuse de courte longueur d'onde est particuliérement forte en provenance de la direction du glacier supérieur avec des valeurs d'environ 330 à 500 W m−2 et est minimum en provenance de la paroi rocheuse face au Pic Nelion où les valeurs sont de l'ordre de 150 à 330 W m−2. Les radiations de grande longueur d'onde semblent presque nulles en provenance de la direction de la face du Nelion et réduites en provenance de l'azimuth du glacier supérieur, ces phénoménes etant apparemment dus à des différences d'émissivité et de température.

Zusammenfassung

Auf dem Lewis–Glacier am Mount Kenya wurden in 4800 m Höhe kurz- und langwellige Strahlung auf vertikalen Flächen unter-schiedlicher Orientierung, direkte Sonnenstrahlung, Globalstrahlung und langwellige Strahlung auf eine horizontale Fläche gemessen. Für die Orientierung der Vertikalflächen wurden die folgenden Azimute ausgewählt: 45°, auf den Steilhang des oberen Gletschers zu; 135°, mit Blick auf einen Felsrücken und etwas Eisfläche im Vordergrund; 225°, gletscherabwärts auf des Teleki- Tal zu, wo das Blickfeld vornehmlich freien Himmel einschliesst; 315°, mit Blick auf die steile Felswand des Nelion-Gipfels.

Die horizontalen Komponenten der diffusen kurzwelligen Strahlung erreichen eine ähnliche Grösse wie die der direkten Strahlung. Infolge der unterschiedlichen Albedo natürlicher Flächen ist die Horizontal-komponente der diffusen kurzwelligen Strahlung aus der Richtung des oberen Gletschers, mit Werten um 330–500 W m−2 besonders gross; dagegen ergeben sich die geringsten Werte aus der Richtung der Nelion-Felswand, wo sie nur 150–330 Wm−2 betragen. Die langwellige Strahlung ist vergleichsweise gross aus Richtung der Nelion–Felswand, und wesentlich geringer im Azimut des oberen Gletschers, offenbar infolge unterschiedlicher Emissivität und Temperatur.

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References

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Radiation measurement at Lewis Glacier, Mount Kenya, Kenya

  • Stefan Hastenrath (a1) and J. K. Patnaik (a2)

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