Volcanic Effects on Turbidity and Irradiances and Their Dependence on Surface Wind Direction

Abstract

Solar irradiances, atmospheric turbidity and meteorological variables measured at the University of Michigan are analyzed to determine effects of the 30 m wind direction on irradiance and turbidity changes caused by the El Chichón volcanic cloud. Results for the period with the largest volcanic effects, from 26 October 1982 through mid-June 1983, are compared with results for the same eight-month period in 1979?80, 1980?81, 1981?82 and 1983?84. Effects of boundary-layer turbidity for winds from southerly directions occasionally masked effects of the stratospheric volcanic cloud on solar irradiances and sunphotometer estimates of Angström turbidity parameters. As a result, volcanic effects could be quantified most reliably for winds only from northerly directions. The largest changes occurred with 30 m winds from the north-northwest octant, for which the volcanic cloud caused the average 500 nm aerosol optical depth to increase from 0.15 to 0.28, the turbidity coefficient to increase from 0.08 to 0.17, and the wavelength exponent to decrease from near unity to 0.6. The ratio of diffuse-to-direct solar irradiance increased from 0.10 to 0.21. For winds from southerly directions, average turbidity was larger and more variable, and irradiance and turbidity changes due to the volcanic cloud were smaller. The smallest changes occurred with 30 in winds from the east-southeast octant, for which the cloud caused the optical depth to increase from 0.29 to 0.31, the turbidity coefficient to increase from 0.09 to 0.16, and the wavelength exponent to decrease from 1.3 to 0.9. The ratio of diffuse-to-direct irradiance increased from 0.15 to 0.22. Daily totals of solar irradiances for 22 cloudless days from 26 October 1982 to 8 June 1983 showed the following average percentage changes from average conditions caused by the cloud: direct normal, ?24; diffuse, +87; global, ?5; global spectral (630?2800 nm), ?1; and south-facing 42.3°-inclined, ?5.