Optical Thickness of Winter Clouds from Ground-based Visible Images
Kalinka Bakalova(1) and Dimitar Bakalov(2)
(1) Solar-Terrestrial Influences Laboratory, BAS, Bl.3, G. Bonchev str. , 1113 Sofia, Bulgaria
(2) Institute for Nuclear Research and Nuclear Energy, 72, Tsarigradsko ch., 1784 Sofia, Bulgaria
An interdisciplinary project, aimed at developing tools for the determination of the physical characteristics of the atmosphere by means of ground-based digital imaging in the visible range is now in progress. As a part of it, here we study the possibility of evaluating the optical thickness of winter clouds from ground-based images. Optical thickness is the main parameter related to the interaction of clouds with radiation. In many studies of the properties of clouds the optical thickness is retrieved from remote sensed data. Estimations of the optical thickness from satellite data on the brightness of the solar radiation reflected by clouds depend on the assumptions about the size distribution and the shape of water particles. The advantage of ground-based observations of clouds as compared to satellite ones is in the possibility of measuring the attenuation of solar radiation through the clouds, which in turn depends exponentially on the optical thickness. The main problem in such an approach is the need to take into account the effects of scattering and extinction of radiation through the atmosphere.
The method presented here is based on relative photometric measurements in the visible spectral band of the clouds and of the brightness of an isotropic reflecting white surface. The estimates of the solar radiation scattered by the atmosphere are obtained using a model that also involves data on the horizontal visibility and other meteorological parameters. The brightness is measured from digital images taken at the earth surface.
The proposed method for ground-based measurements of the optical thickness of clouds has been applied to winter clouds above the city of Sofia. The fresh snow cover, which practically satisfies the conditions for an etalon diffuse reflector is used. The possibility of comparing our results to the estimates of the optical thickness of clouds, retrieved from satellite data, is of great interest.
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,