Extended Abstract (80K)3.4
On the spatial variability of atmospheric radiation in an alpine valley
Nicolas Matzinger, Swiss Federal Institute of Technology, Zurich, Switzerland; and E. van Gorsel, R. Vogt, A. Ohmura, and M. W. Rotach
The MAP-Riviera project seeks to investigate the turbulent transfer and exchange processes in highly complex topography. For this purpose a detailed field program was realized during the SOP (Special Observation Period) of MAP (the Mesoscale Alpine Programme) in fall 1999 in the Riviera Valley in southern Switzerland. Apart from turbulence observations at many sites on a cross-section through the valley and – by means of an instrumented aircraft – in the bulk of the valley atmosphere, the components of atmospheric radiation were also measured at each of the surface sites. Observations are available for a period of approximately three months. In complex topography such as within a valley both the longwave and shortwave components of atmospheric radiation are influenced by the presence of topography. For solar radiation it is mainly the obstruction through the neighboring ridges and multiple reflection from other slopes, which has to be considered. The observed longwave component within a valley is influenced by longwave emission of radiation from different surfaces within the valley. The situation is further complicated as compared to flat terrain in as the slope of the 'receiving' surface plays a crucial role in determining the radiation balance at a given position. In this contribution a method is presented to calculate the radiation components on an inclined surface from observations of a vertically placed radiation sensor. This method uses the observed net radiation from a vertically oriented sensor and theoretical extra-terrestrial solar radiation. On the basis of observed vertical and slope-normal radiation components at one of the sites, it is shown that this procedure yields highly accurate estimates of the slope-normal net radiation. The radiation data set is split up into days with different conditions with respect to insolation: 'clear sky', 'cloudy' and 'overcast'. For each of these categories and each of the nine sites the average diurnal cycle for net radiation is calculated. At the three sites with a more complete observational program also the individual radiation components are treated in the same way. It is shown that the amplitude and the time of the peak of the net radiation cycle vary greatly as a function of slope angle and exposition. Definitely, if only the slope of the station's surface is taken into account and not the surrounding terrain and the exposition, estimates of net radiation can be drastically in error. Since the net radiation is – at least for clear sky conditions – largely driven by the incoming global radiation, the above findings are similarly true for this latter component. The albedo on a slope exhibits a characteristic daily variation due to spectra diffuse reflection and the so-called cavity effect. The longwave incoming radiation has a similar daily variation at all sites but shows an offset, which is dependent on the 'depth within the valley'. The outgoing longwave radiation, on the other hand, exhibits a doubling of the daily cycle's amplitude between the valley floor and a well-exposed site on a steep slope.
Session 3, PBL Structure and Circulations III
Monday, 17 June 2002, 1:30 PM-2:44 PM
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