[lug] Diffuse shortwave measurements

[Gary Hodges]

Hi Martin,

Here is the diffuse shortwave email I promised a couple of weeks ago. 
You are probably aware of efforts in the past few years to improve the
diffuse shortwave measurement.  Without going into a lot of details,
problems with this measurement have been attributed to the PSP's
thermopile "cooling" resulting in a signal "loss."  A generally accepted
method to go back and "correct" diffuse measurements using the
thermopile signal from collocated PIR's is being done by ARM.  This
affects data prior to the use of the Eppley 8-48 for the diffuse
measurement.  In general this means diffuse data prior to year 2001.  We
have also switched to the 8-48 instrument at SURFRAD sites for the
diffuse measurement, but at this time we (SURFRAD) are not planning on
altering the pre 8-48 diffuse data.  This is because we use a radiometer
(Spectrosun SR-75) that doesn't have the same characteristics as the
Eppley PSP.

ARM believes the adjusted historical diffuse data to be more accurate
than the original data, and because of this, they have asked me about
going back and submitting the adjusted data in place of the original
data.  I can of course do this, but I'd like to know if BSRN wants the
adjusted data before proceeding.  I'm sure this will be a topic at the
upcoming BSRN meeting.  BTW, I'll be attending the meeting with my
colleague John Augustine.  The details of the diffuse correcting
procedure are available in the following paper.

Dutton, E. G., Michalsky, J. J., Stoffel, T., Forgan, B. W., Hickey, J.,
Nelson, D. W., Alberta, T. L., Reda, I., 2001. "Measurement of broadband
diffuse solar irradiance using current commercial instrumentation with a
correction for thermal offset errors", J. Atmos. and Ocean. Tech., Vol.
18, pp. 297-314,

Cheers,
Gary