3.2.6. Baseline Surface Radiation Network (BSRN)
CMDL continues to play an active role in the management of the WCRP BSRN. One of the recommendations of the BSRN is the routine use of cavity radiometers at the BSRN field sites. The requirement has proven to be particularly problematic. Our approach has been to use the cavity sparingly under supervision at the field sites and depend primarily on a calibrated pyrheliometer. This has resulted in too-infrequent field cavity operations to meet the spirit of the BSRN recommendations. The STAR group has embarked on a course to develop an alternate approach that significantly improves upon pyrheliometer performance and only falls slightly short of the standard cavity performance. The approach is the use of an extremely wide spectral window, Supersill 300, on the front of the cavity that not only seals the interior of the cavity from the elements but protects it from adverse convective heat transfer and intrusions by particulate matter. This mode of operation was possible on earlier cavities available from Eppley Instruments, but Eppley has been working on modifications of the cavity heating cycles to account for continuous windowed operations. These modifications account for excess heat in the unwindowed version that readily escapes out the aperture. Temperature stabilization of the actual cavity area is desired. We will soon be receiving and testing one of the modified units. Radiative transfer calculations show a 5 to 7 W m-2 effect on the window, depending on column water vapor at SZA less than about 60° and decreasing to near 0.0 at a SZA of 90°. The full range of the variability, because of different amounts of vertical water column is about ±1 W m-2, thus permitting a rather straight forward correction to the windowed measurement as a function of the SZA with a simple refinement for the water vapor amount to bring the adjusted value to within about 1 W m-2 of the true value.
A new BSRN activity by CMDL involved the purchase of equipment for a BSRN monitoring site at Balbina in the Amazon region of Brazil. A site visit was made during September 1997 and assistance was provided during the installation and setup of the instrumentation and data acquisition system.