Night-time cloud cover estimation

Abstract

In this paper a method for cloud cover assessment at night-time (when only thermal infrared data are available) is presented. It is based on the analysis of long wave radiation transfer processes in partially cloudy areas, which led to the formulation of a simplified model of the surface–cloud–atmosphere system. The model was implemented in an operational and iterative algorithm to solve the radiative equations. The algorithm was validated using ground data collected at four meteorological stations in Argentina during November 1997 and May–June 1998, which were compared to cloudiness derived from National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer thermal data. Differences between observed and calculated cloudiness were within ±1 okta in 78% of the cases studied, giving a bias of +0.5 oktas and a standard deviation of ±1.0 oktas in cloudiness estimation.

Acknowledgments

This research was financially supported by the European Community through the Environment and Climate Research Program (contract ENV4-CT95-0166) and the FEDER funds, by the Spanish Ministerio de Ciencia y Tecnología (projects AMB96-1928-CE and REN2001-3116/CLI), and “Ramóu y Cajal” research contracts of Dr. E. Valor and Dr. E. Rubio and by the Generalitat Valenciana (program of financial support for contracting doctors). Dr F. Sospedra wishes to express his gratitude to the Conselleria d'Educació i Ciència of the Valencian Government for the grant received. The authors thank the Instituto de Clima y Agua of Castelar (Argentina) and in particular its director Mr Cesar Rebella, for allowing the use of their facilities to perform the validation experiments. The collaboration of Mr Roberto de Ruyver, Mr Patricio Oricchio and Mr Alejandro Bisigato in the cloud observation was also essential.