Linke’s Turbidity Factor Applied to Worldwide Global. Horizontal Irradiance Measurements

Frank Bason

SolData Instruments, Linabakken 13, DK-8600 Silkeborg, Denmark
Corresponding Author, soldata@soldata. dk


The data collection phase of the Danish Galathea III Expedition was conducted from August 2006 until April 2007 [1]. During this period the research vessel Vmdderen undertook a round the world voyage of nearly 100.000 kilometers while acting as a platform for scientific research in a range of disciplines. The researchers and instruments aboard the ship collected data from many locations around the world from 66.90 N to 67.50 S latitude. Among the experiments aboard the ship was an optics table sponsored by SolData Instruments and containing among other instrumentation three pyranometers for continuous measurement of global solar irradiance on a horizontal surface [2]. Knowledge of global solar irradiance is important for studies of the atmosphere and solar radiation and consequently for modeling the evolution of the Earth’s climate. We employ the Linke turbidity factor in our analysis, for this parameter is often referred to in the literature of atmospheric physics [3].

Keywords: Galathea Expedition, Linke turbidity factor, solar irradiance

1. Introduction

The purpose of this paper is to present an analysis of the global irradiance measurements. In particular we will focus on the computation of Linke’s turbidity factor TL at a wide range of locations and the implications of these results for a general description of solar resources around the world. The factor TL is closely related to the transmittance of the atmosphere on clear days [4]. As a general rule the atmosphere is clearer at higher latitudes, and the large amount of data available from the expedition has permitted the development of an algorithm to describe this relationship. Among the useful results obtained is a correlation between the Linke turbidity factor and the latitude. Secondary parameters such as the surface air water vapor content are also examined.

The turbidity factor was obtained from the horizontal radiation data using an incident direct plus diffuse rad­iation model, observed values of the solar irradiance and a numerical algorithm to determine TL. Based on these results it is possible to provide a highly realistic prediction of the global irradiance on a clear day in a maritime environment. The model developed also supplies information about the distribution of diffuse and direct irradiance, information which is important for the design of solar energy systems. Furthermore TL has implications for atmospheric visibility.

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