Annual Forcing of the Surface Radiation Balance Diurnal Cycle Measured from a High Tower near Boulder, Colorado

Abstract

The radiation balance consisting of upward and downward components of solar and thermal infrared broadband irradiances is continuously measured from the top of a 300-m tower situated on the Colorado high plains. The data are representative of a weighted areal average over a variety of surface and vegetation types within about a 1.5-km radius of the tower. Data from a three-year period, 1986?88, appears to be sufficient to define smooth annual cycles in monthly averages and 1-h resolution diurnal cycles in seasonal averages. It is found that even though infrared cycles are out of phase with cycles of corresponding solar components, the overall net radiation balance is in phase with surface solar forcing. The latter follows closely the extraterrestrial forcing but with some phase modifications by clouds and surface reflectance variations. The value of the correlation coefficient squared between the extraterrestrial radiation and the measured surface radiation balance quickly increases from 0.89?0.99 as averaging time increases from 1?90 days, respectively.