3.2. Solar and Thermal Atmospheric Radiation
B. Bodhaine (Editor),
E. Dutton, B. Halter, G. Koenig, D. Longenecker,
D. Nelson, R. Stone, R. Tatusko, and J. Wendell
3.2.1. Radiation Measurements
The CMDL radiation group, known as the Solar and Thermal Atmospheric Radiation (STAR) group, has undertaken several new projects in addition to the traditional surface radiation monitoring that was the primary emphasis of the group in the past. Current projects include baseline surface radiation monitoring, solar and infrared (IR) calibration facilities, spectral and wideband ultraviolet (UV) measurements, various interactions with the World Meteorological Organization (WMO) related radiation programs, validation of surface radiation budget measurements from space, instrument modification and development, polar surface and climate studies, and radiative transfer model testing and verification. This expansion of the CMDL radiation program is in response to, and partially responsible for, a general increase in interest in atmospheric radiation measurements over the past decade. Also as a result of this increased scientific interest, two relatively large programs were established by the National Aeronautics and Space Administration (NASA) and the Department of Energy (DOE) to address pressing issues in climate and radiation. The impetus for this increase in interest and the expanded role of the CMDL project has been the existence of ever improved general circulation models and satellite-derived surface radiation quantities, and the need for high quality in situ surface measurements for validation. The STAR groups movement into UV measurements resulted from many overlapping interests between the radiation/climate community and those of atmospheric chemistry.
Fundamental to STARs efforts are the long-term monitoring of surface radiation components at the four CMDL baseline observatories. Upgrades to those sites continued in 1996 and 1997 as we incorporated diffuse solar irradiance into our core measurements and updated solar tracking capabilities at some sites. For the past 8 years we maintained surface radiation monitoring sites at Bermuda and Kwajalein and for the last 12 years at the Boulder Atmospheric Observatory (BAO) near Erie, Colorado. The measurement programs at these three sites and at Barrow, Alaska (BRW) and at South Pole, Antarctica (SPO) make up the CMDL contribution to the World Climate Research Program (WCRP) Baseline Surface Radiation Network (BSRN), for which CMDL also provides the international project manager.
Additional Measurement Programs
In addition to the basic monitoring activities, other major projects currently being undertaken include satellite validation and algorithm testing, operational radiometer improvements, new instrument technology testing and evaluation, spectral UV measurements, polar radiation research, aerosol optical depth studies, and cloud optical properties investigations. This report will cover the status and progress of each of the topics.
We are participating in the validation of the surface radiation quantities that will be observed and derived as part of the NASA Earth Observing System (EOS) Clouds and Earths Radiant Energy System. The CMDL participation will be to enhance current field site observing capabilities and to provide data and analysis relative to the understanding and interpretation of the magnitude and variability of the Earths surface radiation budget. The effort will include further refinements of field operations and continued pursuit of full compliance with the BSRN measurement recommendations. Initial activities involve the comparison of surface-based irradiance observations with the computational procedures using radiative transfer models intended to eventually be used with the satellite data to project top-of-the-atmosphere observations to estimated surface quantities. The project is expected to last a minimum of 3 years and should help provide the information necessary to not only diagnose current climate energy but further to provide climate modeling efforts with supporting quantities against which intermediate climate predictions can be tested.
The BRW site was chosen as the third in a series of Clouds And Radiation Testbed sites of the DOE/Atmospheric Radiation Measurements (ARM) program, which has an extensive surface irradiance measurement mission. We are cooperating with the ARM program in providing consultation and other assistance, and we are providing fast-turn-around data from our program to the ARM research team for the purpose of crosscheck and quality control. The extensive observational capability of ARM will enhance the CMDL efforts to determine the source and extent of variations in the radiation budget of the Barrow region.
In August of 1997 the STAR group hosted an intercomparison between traditional solar radiometry and developing technology. This new technology developed by the Scripps Institution of Oceanography (SIO) has begun to be used extensively by the radiation community. The purpose of the comparison was to determine how the newer instrumentation performed relative to the CMDL standards that are traceable to the international standards. Analysis of the comparison data is being performed independently so that personal specific biases would not be a factor in the conclusions. Preliminary results suggest a close agreement between the systems much of the time, even during some extreme events when midday readings on both instrument systems decreased to near <2 W m-2. Differences between the instruments were generally less than 10 W m-2 with a few cases showing differences of about 25 W m-2.
MLO Apparent Transmission
One of the longest of the radiation-related data records maintained by STAR is the Mauna Loa Atmospheric Transmission record. This record is explained by Dutton et al.  and was updated through 1997 as shown in Figure 3.15. The principal features now evident in the record are the major impact of stratospheric aerosols from certain volcanic eruptions, an annual cycle caused by the seasonal enhanced transport of Asian dust aerosol over the central Pacific, and the lack of any pronounced or significant trend in the data over the 40-year record. This lack of a significant trend helps bound the maximum possible change that could have occurred in upper tropospheric or stratospheric aerosol. Less obvious in the record is the year-to-year variability in the annual cycle that has been shown to have a coherence with the Quasi-Biennial Oscillation [Dutton, 1992]. Also, not at all obvious in the record, but shown in Figure 3.15, is the fact that over the past 15 years or so, the intensity of the mean amplitude of the annual cycle has increased. This is manifested in the fact that the spring minimum in transmission (maximum in aerosol content) is more consistently present in recent years. Figure 3.15 shows this in the ever increasing baseline of the amplitude of the MLO transmission annual cycle caused by the lack of the minimal amplitudes seen earlier in the record.
Fig. 3.15. Monthly mean (dots) of the apparent solar transmission at Mauna Loa. Apparent transmission is defined in references given in the text and is an inherently stable measure of the long-term composition of the atmosphere for those constituents that can attenuate solar wavelengths of electromagnetic radiation. A lowess smoother (solid line) was fit to the data and emphasizes the variable annual cycle in the data.
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