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Evaluation of surface solar irradiance in coupled climate models

PI: Beate Liepert and Anastasia Romanou
Institution: Columbia University
Up-to-date observations from satellites and in situ measurements of surface downwelling irradiance will be analyzed and compared with global general circulation coupled model simulations of the late 19th century and 20th century runs to evaluate the model climatology, variability and cloud and aerosol effects. The incoming shortwave irradiance at the surface is one driving mechanism of the ocean/ice and land submodels and while it is directly affected by the physics in the atmosphere such as clouds, aerosols, water vapor, ozone and other gases it is only indirectly affected by the ocean/ice/land. Hence, aerosol and cloud variations have a strong direct signal on the downwelling shortwave surface irradiance which can be easily interpreted. At the same time, surface solar irradiance and its link to clouds and aerosols represents one crucial part of the cloud feedback loop. Furthermore, there has been increased interest in the scientific community regarding the long-term variability of the surface incoming solar irradiance. Observations show that solar irradiance has recently been decreasing in stations worldwide by about 4% from 1961-1990, possibly due to changed aerosol concentrations and cloud variations. This has been described as a global dimming effect. Current research (from satellite observations) shows a reversal of this effect from the late 1980s and during the 1990s. We will use two data sets, which represent two different classes of data retrieval methods, from satellites and in situ measurements: ISCCP-FD surface radiation fluxes (2.50 2.50, 3 hourly, 1984-2000) and broadband surface solar irradiance records from pyranometer measurements at sites worldwide from 1950s onwards (GEBA, USNSRDB, BSRN). We will compare the observed climatology and variability of the surface downwelling shortwave flux to four coupled climate models, the GISS model with two ocean submodels, the GFDL and NCAR models, on all resolved temporal and spatial scales. At the same time, and since the models use different aerosol datasets and cloud parameterizations, some use indirect cloud-aerosol effects, the inter-model comparisons of the solar irradiance at the surface will be instructive of the quality of each models representation of cloud and aerosol impacts. In the proposed study we will a) assess the long-term variability and trend of the model downwelling shortwave flux at the surface and compare it to observations b)assess the aerosol and cloud effects in the models and, c) examine the models capability in simulating specific events (volcanic eruptions and El Nino conditions).
  • Romanou, A., B.Liepert, G. A. Schmidt, W. B. Rossow, R. A. Ruedy, and Y. Zhang, 2007: 20th Century changes in surface solar irradiance in simulations and observations. Geophys. Res. Lett., 34, doi:10.1029/2006GL028356.. Abstract. Full Article. Edit.

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