New Climate Records Released Are Based on SORCE Measurements
by: Odele Coddington
Laboratory for Atmospheric and Space Physics, University of Colorado – Boulder.
In November 2015, a new Solar Irradiance Climate Data Record (CDR) consisting of total and spectral solar irradiance and their associated time- and wavelength-dependent uncertainties was presented to the public. The 400+ year climate data record extends from 1610 to present day and is operationally updated from LASP on a quarterly basis. The advances in the Naval Research Laboratory’s (NRL) solar variability models used to create this long-term record are directly attributable to improved knowledge in the variability in the Sun’s energy output obtained from satellite observations by the NASA SOlar Radiation and Climate Experiment (SORCE) mission. The overall approach of the Solar Irradiance CDR algorithm builds on, and advances, the original NRL models developed by Dr. Judith Lean over a decade ago and used in a variety of model simulations of climate and atmospheric change.
The Sun’s power per unit area summed over all wavelengths – the total solar irradiance, or TSI – and the dispersion of this power across the electromagnetic spectrum – the solar spectral irradiance, or SSI – varies with changes in the flux of magnetic activity emerging from the Sun’s interior into its atmosphere. These magnetic features manifest as dark spots on the visible surface of the sun called “sunspots” and as brighter features called faculae that accompany the sunspots. The presence of sunspots reduces the Sun’s irradiance for most visible wavelengths while the faculae increase the Sun’s irradiance. The net effects from these opposing features changes the Sun’s irradiance over the 11-yr cycle of magnetic activity when the enhanced facular emission at maximum solar activity exceeds that of the irradiance reduction from sunspots. On shorter time scales when the Sun’s approximate 27-day rotation period alters the distribution of sunspots and facular features on the projection of the solar disk viewed at Earth, sunspots may reduce irradiance more than the facular features increase it.
Approximately 11-years of data from three instruments on the SORCE satellite were used to improve the NRL models of TSI and SSI, NRLTSI2 and NRLSSI2 respectively, which form the Solar Irradiance Climate Data Record. Both of these models have empirical relationships of the irradiance variability as related to sunspot and faculae indicators (proxies) that extend back to 1610, and these relationships are based on SORCE and other solar irradiance measurements. The Total Irradiance Monitor (TIM) on SORCE detects changes in the TSI at unprecedented accuracy and stability. The TIM observations on SORCE and on the TSI Calibration Transfer Experiment (TCTE) contribute to what is now a 38+ year record of TSI observations from space. The SORCE SOLar STellar Irradiance Comparison Experiment (SOLSTICE) instrument measures SSI between 115 and 309 nm and the SORCE Spectral Irradiance Monitor (SIM) instrument measures SSI between 310 nm to 2400 nm. SORCE SSI observations at wavelengths longer than 400 nm are the first ever spectrally resolved measurements of the Sun’s irradiance from space and the observations at wavelengths shorter than 400 nm contribute to an existing record of satellite ultraviolet (UV) observations that is not as lengthy as the TSI record nor consistently sampled in time (or spectral resolution).
Figure 1a is an example of a comparison of the modeled TSI with SORCE TIM observations (v17 processing) between 2003 and 2014. NRLTSI2 results are highly correlated with SORCE TIM observations and explain much (92%) of the variability over this time period. Additional comparisons of modeled TSI and SSI with SORCE observations over solar rotational time scales, as well as comparisons of modeled TSI and SSI with the original model formulation can be found in Coddington et al., Bull. Amer. Meteor. Soc., 97(7), 1265-1282, doi:10.1175/BAMS-D-14-00265.1.
Deliverables: The National Oceanographic and Atmospheric Administration (NOAA) National Center for Environmental Information (NCEI) is the definitive source of the Solar Irradiance CDR (https://www.ncdc.noaa.gov/cdr/atmospheric). Users may also directly download the data from LASP’s LISIRD server (http://lasp.colorado.edu/lisird3/data/nrl2_files. Due to file size considerations, the NRLSSI2 data delivered to NOAA NCEI does not contain uncertainties; however, these uncertainties are available from LASP LISIRD. A summary of the data products delivered as the Solar Irradiance CDR and their operational update cadence are given in Table 1.
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Figure 1: Comparison of NRLTSI2 modeled output with SORCE TIM observations (v17) between 2003-2014. The NRLTSI2 uncertainties (grey shading) do not include the uncertainty due to the TSI absolute scale, which is approximately 350 ppm.