ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-7-2937-2007 Statistical variability of top of atmosphere cloud-free shortwave aerosol radiative effect Jones T. A. 1 Christopher S. A. 1 Department of Atmospheric Sciences, The University of Alabama in Huntsville, Huntsville, AL, USA 12 06 2007 7 11 2937 2948 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/7/2937/2007/acp-7-2937-2007.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/7/2937/2007/acp-7-2937-2007.pdf

The statistical variability of globally averaged MODIS aerosol optical thickness at 0.55 μm (AOT) and top of atmosphere CERES cloud-free shortwave radiative effect (SWRE) is presented. Statistical variability is defined as the robustness of globally averaged statistics relative to data distribution. At the CERES footprint level, which we label "raw data", both the AOT and SWRE data derived from clear-sky CERES-SSF products show significant deviations from a normal distribution as evidenced by high skewness values. The spatial and temporal distribution of the data is also not uniform, with a greater concentration of data occurring in aerosol heavy-regions. As a result, globally averaged AOT and SWRE are overestimated when derived from raw data alone. To compensate, raw data are gridded into 2×2 degree grid-cells (called "gridded" data) to reduce the effect of spatial non-uniformity. However, the underlying non-normal distribution remains and manifests itself by increasing the uncertainty of grid-cell values. Globally averaged AOT and SWRE derived from a gridded dataset are substantially lower than those derived from raw data alone. The range of globally averaged AOT and SWRE values suggests that up to a 50% statistical variability exists, much of which is directly tied to how the data are manipulated prior to averaging. This variability increases when analyzing aerosol components (e.g. anthropogenic) since component AOT (and SWRE) may not exist at all locations were AOT is present. As a result, regions where a particular component AOT does not exist must either not be included in the global average or have data within these regions set to null values. However, each method produces significantly different results. The results of this work indicate simple mean and standard deviation statistics do not adequately describe global aerosol climate forcing data sets like the one used here. We demonstrate that placing raw observations on to a uniform grid is a necessary step before calculating global statistics. However, this by no means eliminates uncertainty in globally averaged AOT and SWRE values, while adding its own set of assumptions. When reporting any globally averaged statistic, it is important to report corresponding distribution and coverage information, in the form of skewness values, probability density functions, and spatial distribution plots, to help quantify its usefulness and robustness.

 References Anderson, T. L., Charlson, R. J., Bellouin, N., Boucher, O., Chin, M., Christopher, S. A., Haywood, J., Kaufman, Y. J., Kinne, S., Ogren, J. A., Remer, L. A., Takemura, T., Tanré, D., Torres, O., Trepte, C. R., Wielicki, B. A., Winker, D. M., and Yu, H.: An "A-Train" Strategy for Quantifying Direct Climate Forcing by Anthropogenic Aerosols, Bull. Amer. Meteorol. Soc., 86, 1795–1809, 2005. Bellouin, N., Boucher, O., Haywood, J., and Reddy, M. S.: Global estimate of aerosol direct radiative forcing from satellite measurements, Nature, 438, 1138–1141, doi:10.1038/nature04348, 2005. Christopher, S. A. and Zhang, J.: Shortwave aerosol radiative forcing from MODIS and CERES observations over the oceans, Geophys. Res. Lett., 29, 1859, doi:10.1029/2002GL014803, 2002. Christopher, S. A., Zhang, J., Kaufman, Y. J., and Remer, L. A.: Satellite-based assessment of top of atmosphere anthropogenic radiative forcing over cloud-free oceans, Geophys. Res. Lett., 33, L15816, doi:10.1029/2005GL025535, 2006. Christopher, S. A. and Jones, T. A.: Satellite-based Assessment of Cloud-free Net Radiative Effect of Dust Aerosols over the Atlantic Ocean, Geophys. Res. Lett., 34, L02810, doi:10.1029/2006GL027783, 2007. Kaufman, Y. J., Koren, I., Remer, L. A., Tanré, D., Ginoux, P., and Fan, S.: Dust transport and deposition observed from the Terra-Moderate Resolution Imaging Spectroradiometer (MODIS) spacecraft over the Atlantic Ocean, J. Geophys. Res., 110, D10S12, doi:10.1029/2003JD004436, 2005a. Kaufman, Y. J., Boucher, O., Tanré, D., Chin, M., Remer, L. A., and Takemura, T.: Aerosol anthropogenic component estimated from satellite data, Geophys. Res. Lett., 32, L17804, doi:10.1029/2005GL023125, 2005b. Li, F., Vogelmann, A. A., and Ramanathan, V.: Saharan Dust Radiative Forcing Measured from Space, J. Climate, 17, 2558–2571, 2004. Loeb, N. G. and Manalo-Smith, N.: Top-of-atmosphere direct radiative effect of aerosols over global oceans from merged CERES and MODIS observations, J. Climate, 18, 3506–3526, 2005. Ramana, M. V. and Ramanathan, V.: Abrupt transition from natural to anthropogenic aerosol radiative forcing: Observations at the ABC-Maldives Climate Observatory, J. Geophys. Res., 111, D20207, doi:10.1029/2006JD007063, 2006. Wilks, D. S.: Statistical Methods in the Atmospheric Sciences, Academic Press, 627 pp., 2006. Yu, H., Dickinson, R. E., Chin, M., Kaufman, Y. J., Zhou, M., Zhou, L., Tian, Y., Dubovik, O., and Holben, B. N.: The direct radiative effect of aerosols as determined from a combination of MODIS and GOCART simulations, J. Geophys. Res., 109, D03206, doi:10.1029/2003JD003914, 2004. Yu, H., Kaufman, Y. J., Chin, M., Feingold, G., Remer, L. A., Anderson, T. L., Balkanski, Y., Bellouin, N., Boucher, O., Christopher, S., DeCola, P., Kahn, R., Loeb, N., Reddy, M. S., Schulz, M., Takemura, T., and Zhou, M.: A review of measurement-based assessment of aerosol direct radiative effect and forcing, Atmos. Chem. Phys. 6, 613–666, 2006. Zhang J., Christopher, S. A., Remer, L. A., and Kaufman, Y. J.: Shortwave aerosol radiative forcing over cloud-free oceans from Terra: 2: Seasonal and global distributions, J. Geophys. Res., 110, D10S24, doi:10.1029/2004JD005009, 2005a. Zhang, J., Christopher, S. A., Remer, L. A., and Kaufman, Y. J.: Shortwave aerosol radiative forcing over cloud-free oceans from Terra: Angular models for aerosols, J. Geophys. Res., 110, D10S23, doi:10.1029/2004JD005008, 2005b. Zhang, J. and Reid, J. S.: MODIS aerosol product analysis for data assimilation: Assessment of over-ocean level 2 aerosol optical thickness retrievals, J. Geophys. Res., 111, D22207, doi:10.1029/2005JD006898, 2006.