A Proposed Measurement Standard for Diffuse Radiation Flux

Michalsky, J. J., Cooperative Institute for Research in Environmental Science

Radiation Processes

Radiative Processes

Michalsky, J. J., C. Gueymard, P. Kiedron, L. J. B. McArthur, R. Philipona, and T. Stoffel, 2007: A proposed working standard for the measurement of diffuse horizontal shortwave irradiance, J. Geophys. Res., 112, D16112, doi:10.1029/2007JD008651.


The three pyranometers proposed for the international standard for diffuse flux are shown here in action during the 2006 campaign at the ACRF SGP site. Of note are the shadows of the blocking balls on the domes of each pyranometer. The blocking balls are moved by a solar tracker to continuously shade the pyranometers.


The three pyranometers proposed for the international standard for diffuse flux are shown here in action during the 2006 campaign at the ACRF SGP site. Of note are the shadows of the blocking balls on the domes of each pyranometer. The blocking balls are moved by a solar tracker to continuously shade the pyranometers.

Measurements of diffuse solar radiation (the total horizontal flux minus the solar beam flux) are crucial for validating radiative transfer codes used in climate models. Yet, there is no international standard for diffuse solar flux. There is only an international standard for the solar beam, which uses a group of absolute cavity radiometers kept in Davos, Switzerland. Diffuse radiation, or "skylight", consists of sunlight scattered by molecules and cloud and aerosol particles in the sky and is much more difficult to calculate than the solar beam.

As shown in the photo, pyranometers (instruments for measuring solar radiation flux) located at the Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) Southern Great Plains (SGP) site have black balls on long arms that continuously block the solar beam in order to measure only diffuse flux. For ten days in October 2006, the ACRF SGP site hosted four excellent pyranometers that were candidates for an international working standard for diffuse flux, designed to serve both the ARM and the larger baseline Surface Radiation Network communities.

The four candidate pyranometers were selected based on past performance in two previous diffuse flux studies held in September and October of 2001 and in October 2003. These four pyranometers were characterized in the laboratory for angular and spectral response prior to the start of the study. Based on their performance in this experiment, three of the four pyranometers were chosen for the ARM-BSRN diffuse horizontal shortwave flux working standard. A careful analysis of the instruments suggests that the three pyranometers, measuring simultaneously, quantify the flux to within ± 2.2% plus an additional 0.2 W/m2 because of the uncertainty in the dark offset correction. Michalsky et al. 2007 also outlines a procedure for obtaining low-uncertainty diffuse solar flux when a comparison to this trio is not possible.