Parameterization of Shortwave Clear-sky Global Radiation Observations at a Tropical Rural Site

Vidya Saraswathi A

St. Philomena College, Centre for PG Studies and Research, Philonagar, Darbe, Puttut, DK 574202, Karnataka

Dr. M. Venkat Ratnam

Scientist, National Atmospheric Research Laboratory (NARL), Department of Space (DoS), Govt. of India, Gadanki, Post Box: 123, Tirupati 517 502, India

Abstract

The sun is the main source of energy on the Earth. The solar radiation reaching the Earth’s surface is highly variable, due to the prevailing state of the atmosphere at a specific location. Both the scattering and absorption significantly affect the incoming solar radiation. Due to these processes, a small fraction of radiation reaches the Earth’s surface out of the whole spectrum of solar radiation. From the total amount of solar energy emitted by the Sun, about 29% arriving at the top of the atmosphere is reflected back to space by clouds, aerosols, or bright ground surfaces like sea, ice and snow. This energy plays no role in the Earth’s climate system. About 23% of incoming solar energy is absorbed in the atmosphere by water vapor, dust and ozone, and 48% passes through the atmosphere and is absorbed by the surface. The Shortwave Global Radiation (0.25 to 4 micron) comes directly from the Sun and strongly controls the energy budget of the Earth. It even drives processes such as photosynthesis and evapotranspiration. Monitoring of shortwave global radiation at the surface is significant in a variety of areas such as hydrological, agricultural and photo-voltaic energy-technological studies. Apart from these, long-term measurements provide vital information about climate change. Under clear-sky conditions, shortwave global radiation can be modelled based on various sensitive parameters. Accurate shortwave clear-sky models are useful for photovoltaic modelling, and also to estimate the cloud's radiative effects. There exist many clear-sky models from simple to complex for shortwave global radiation at different locations.

The present study is aimed at exploring the clear-sky shortwave global-horizontal radiation measurements from a pyranometer at Gadanki. The clear-sky global radiation (G) as a function of solar zenith angle (Ө) will be parameterized as: G = a.(cosӨ)^b, where a and b are coefficients. The significance and variability of these coefficients will be studied. Further, we will evaluate the online McClear model simulations of clear-sky global radiation at Gadanki with the clear-sky measurements on several clear days.

Keywords: GHI, Pyranometer, Adnot Model, McClear Model