ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-10705-2010 Some implications of sampling choices on comparisons between satellite and model aerosol optical depth fields Sayer A. M. 1 2 Thomas G. E. 1 Palmer P. I. 3 Grainger R. G. 1 Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, OX11 0QX, UK School of GeoSciences, University of Edinburgh, Edinburgh, EH9 3JW, UK 16 11 2010 10 22 10705 10716 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/10/10705/2010/acp-10-10705-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/10705/2010/acp-10-10705-2010.pdf

The comparison of satellite and model aerosol optical depth (AOD) fields provides useful information on the strengths and weaknesses of both. However, the sampling of satellite and models is very different and some subjective decisions about data selection and aggregation must be made in order to perform such comparisons. This work examines some implications of these decisions, using GlobAerosol AOD retrievals at 550 nm from Advanced Along-Track Scanning Radiometer (AATSR) measurements, and aerosol fields from the GEOS-Chem chemistry transport model. It is recommended to sample the model only where the satellite flies over on a particular day; neglecting this can cause regional differences in model AOD of up to 0.1 on monthly and annual timescales. The comparison is observed to depend strongly upon thresholds for sparsity of satellite retrievals in the model grid cells. Requiring at least 25% coverage of the model grid cell by satellite data decreases the observed difference between the two by approximately half over land. The impact over ocean is smaller. In both model and satellite datasets, there is an anticorrelation between the proportion <i>p</i> of a model grid cell covered by satellite retrievals and the AOD. This is attributed to small <i>p</i> typically occuring due to high cloud cover and lower AODs being found in large clear-sky regions. Daily median AATSR AODs were found to be closer to GEOS-Chem AODs than daily means (with the root mean squared difference being approximately 0.05 smaller). This is due to the decreased sensitivity of medians to outliers such as cloud-contaminated retrievals, or aerosol point sources not included in the model.

 References Birks,~A.: Improvements to the AATSR IPF relating to land surface temperature, ESA Technical note, 2004. Brennan,~J I., Kaufman,~Y J., Koren,~I., and Li,~R R.: Aerosol-cloud interaction-misclassification of MODIS clouds in heavy aerosol, IEEE Trans. Geosci. Remote Sens., 43, 911–915, doi:10.1109/TGRS.2005.844662, 2005. Forster,~P., Ramaswamy,~V., Artaxo,~P., Berntsen,~T., Betts,~R., Fahey,~D W., Haywood,~J., Lean,~J., Lowe,~D C., Myhre,~G., Nganga,~J., Prinn,~R., Raga,~G., Schulz,~M., and Van Dorland,~R.: Changes in atmospheric constituents and in radiative forcing, Cambridge University Press, in: Climate Change 2007, the physical science basis. Contribution of Working Group 1 to the fourth assessment report of the Intergovernmental Panel on Climate Change, 2007. Grey,~W M F., North,~P R J., Los,~S O., and Mitchell,~R M.: Aerosol optical depth and land surface reflectance from multiangle AATSR measurements: global validation and intersensor comparisons, IEEE Trans. Geosci. Remote Sens., 44, 2184–2197, doi:10.1109/TGRS.2006.872079, 2006. Kinne,~S., Lohmann,~U., Feichter,~J., Schulz,~M., Timmreck,~C., Ghan,~S., Easter,~R., Chin,~M., Ginoux,~P., Takemura,~T., Tegen,~I., Koch,~D., Herzog,~M., Penner,~J., Pitari,~G., Holben,~B., Eck,~T., Smirnov,~A., Dubovik,~O., Slutsker,~I., Tanré,~D., Torres,~O., Mishchenko,~M., Geogdzhayev,~I., Chu,~D A., and Kaufman,~Y.: Monthly averages of aerosol properties: A~global comparison among models, satellite data, and AERONET ground data, J. Geophys. Res., 108, 4634, doi:10.1029/2001JD001253, 2003. Kinne,~S., Schulz,~M., Textor,~C., Guibert,~S., Balkanski,~Y., Bauer,~S E., Berntsen,~T., Berglen,~T F., Boucher,~O., Chin,~M., Collins,~W., Dentener,~F., Diehl,~T., Easter,~R., Feichter,~J., Fillmore,~D., Ghan,~S., Ginoux,~P., Gong,~S., Grini,~A., Hendricks,~J., Herzog,~M., Horowitz,~L., Isaksen,~I., Iversen,~T., Kirkevåg,~A., Kloster,~S., Koch,~D., Kristjansson,~J E., Krol,~M., Lauer,~A., Lamarque,~J F., Lesins,~G., Liu,~X., Lohmann,~U., Montanaro,~V., Myhre,~G., Penner,~J., Pitari,~G., Reddy,~S., Seland,~O., Stier,~P., Takemura,~T., and Tie,~X.: An AeroCom initial assessment – optical properties in aerosol component modules of global models, Atmos. Chem. Phys., 6, 1815–1834, doi:10.5194/acp-6-1815-2006, 2006. Koren,~I., Remer,~L A., Kaufman,~Y J., Rudich,~Y., and Martins,~J V.: On the twilight zone between clouds and aerosols, Geophys. Res. Lett., 34, L08805, doi:10.1029/2007GL029253, 2007. Koren,~I., Oreopoulous, ~L., Feingold, ~G., Remer, ~L A., and Altaraz, ~O.: How small is a small cloud?, Atmos. Chem. Phys., 8, 3855–3864, doi:10.5194/acp-8-3855-2008, 2008. Lee,~Y H. and Adams,~P J.: Evaluation of aerosol distributions in the GISS-TOMAS global aerosol microphysics model with remote sensing observations, Atmos. Chem. Phys., 10, 2129–2144, doi:10.5194/acp-10-2129-2010, 2010. Levy,~R C., Leptoukh,~G G., Kahn,~R., Zubko,~V., Gopalan,~A., and Remer,~L A.: A~critical look at deriving monthly aerosol optical depth from satellite data, IEEE Trans. Geosci. Remote Sens., 47, 2942–2956, doi:10.1109/TGRS.2009.2013842, 2009. Liu,~L., Lacis,~A A., Carlson,~B E., and Mishchenko,~M I.: Assessing Goddard Institute for Space Studies ModelE aerosol climatology using satellite and ground-based measurements: A~comparison study, J. Geophys. Res., 111, D20212, doi:10.1029/2006JD007334, 2006. Martonchik,~J V., Diner,~D J., Kahn,~R A., Ackerman,~T P., Verstraete,~M M., Pinty,~B., and Gordon,~H R.: Techniques for the retrieval of aerosol properties over land and ocean using multiangle imaging, IEEE Trans. Geosci. Remote Sens., 36(4), 1212–1227, doi:10.1109/36.701027, 1998. Martonchik,~J V., Kahn,~R A., and Diner,~D J.: Retrieval of Aerosol Properties over Land Using MISR Observations, Springer (Berlin), in: Aerosol Remote Sensing Over Land, edited by: Kokhanvosky,~A A. and de Leeuw,~G., 267–293, 2009. Marshak,~A., Web,~G., Coakley Jr.,~J A., Remer,~L A., Loeb,~N G., and Cahalan, ~R F.: A simple model for the cloud adjacency effect and the apparent bluing of aerosols near clouds, J. Geophys. Res., 113, D14S17, doi:10.1029/2007JD009196, 2008. Mishchenko,~M I. and Geogdzhayev,~I V.: Satellite remote sensing reveals regional tropospheric aerosol trends, Opt. Express, 15, 7423–7438, 2007. Park,~R A., Jacob,~D J., Kumar,~N., and Yantosca,~R M.: Regional visibility statistics in the United States: Natural and transboundary pollution influences, and implications for the Regional Haze Rule, Atmos. Env., 40, 5405–5423, doi:10.1016/j.atmosenv.2006.04.059, 2006. Penner,~J E., Quaas,~J., Storevlmo,~T., Takemura,~T., Boucher,~O., Guo,~H., Kirkevåg,~A., Kristjánsson,~J E., and Seland,~Ø.: Model intercomparison of indirect aerosol effects, Atmos. Chem. Phys., 6, 3391–3405, doi:10.5194/acp-6-3391-2006, 2006. Poulsen,~C A., Siddans,~R., Kerridge,~B J., Thomas,~G E., Grainger,~R G., Perez-Navarro,~O., Portela-Arjona,~O., and Deschamps,~P.-Y.: GlobAEROSOL Data User Element: Preliminary Validation and Assessment Report, version 1.2, available from http://www.globaerosol.info/, last access: March 2010, 2007. Quaas,~J., Ming,~Y., Menon,~S., Takemura,~T., Wang,~M., Penner,~J E., Gettelman,~A., Lohmann,~U., Bellouin,~N., Boucher,~O., Sayer,~A M., Thomas,~G E., McComiskey,~A., Feingold,~G., Hoose,~C., Kristjánsson,~J E., Liu,~X., Balkanski,~Y., Donner,~L J., Ginoux,~P A., Stier,~P., Grandey,~B., Feichter,~J., Sednev,~I., Bauer,~S E., Koch,~D., Grainger,~R G., Kirkevåg,~A., Iversen,~T., Seland,~Ø., Easter,~R., Ghan,~S J., Rasch,~P J., Morrison,~H., Lamarque,~J.-F., Iacono,~M J., Kinne,~S., and Schulz,~M.: Aerosol indirect effects – general circulation model intercomparison and evaluation with satellite data, Atmos. Chem. Phys., 9, 8697–8717, doi:10.5194/acp-9-8697-2009, 2009. Quaas,~J., Stevens,~B., Stier,~P., and Lohmann,~U.: Interpreting the cloud cover – aerosol optical depth relationship found in satellite data using a~general circulation model, Atmos. Chem. Phys., 10, 6129–6135, doi:10.5194/acp-10-6129-2010, 2010. Remer, L A., Kleidman,~R G., Levy,~R C., Kaufman,~Y J., Tanr�e,~D., Mattoo,~S., Martins,~J V., Ichoku,~C., Koren,~I., Yu,~H., and Holben,~B N.: Global aerosol climatology from the MODIS satellite sensors, J. Geophys. Res., 113, D14S07, doi:10.1029/2007JD009661, 2008. Schulz,~M., Textor,~C., Kinne,~S., Balkanski,~Y., Bauer,~S., Berntsen,~T., Berglen,~T., Boucher,~O., Dentener,~F., Guibert,~S., Isaksen,~I S A., Iversen,~T., Koch,~D., Kirkevåg,~A., Liu,~X., Montanaro,~V., Myhre,~G., Penner,~J E., Pitari,~G., Reddy,~S., Seland,~Ø., Stier,~P., and Takemura,~T.: Radiative \mboxforcing by aerosols as derived from the AeroCom present-day and pre-industrial simulations, Atmos. Chem. Phys., 6, 5225–5246, doi:10.5194/acp-6-5225-2006, 2006. Stevens,~B. and Feingold,~G.: Untangling aerosol effects on clouds and precipitation in a~buffered system, Nature, 461, 607–613, doi:10.1038/nature08281, 2009. Stier,~P., Feichter,~J., Kinne,~S., Kloster,~S., Vignati,~E., Wilson,~J., Ganzeveld,~L., Tegen,~I., Werner,~M., Balkanski,~Y., Schulz,~M., Boucher,~O., Minikin,~A., and Petzold,~A.: The aerosol-climate model ECHAM5-HAM, Atmos. Chem. Phys., 5, 1125–1156, doi:10.5194/acp-5-1125-2005, 2005. Textor,~C., Schulz,~M., Guibert,~S., Kinne,~S., Balkanski,~Y., Bauer,~S., Berntsen,~T., Berglen,~T., Boucher,~O., Chin,~M., Dentener,~F., Diehl,~T., Easter,~R., Feichter,~H., Fillmore,~D., Ghan,~S., Ginoux,~P., Gong,~S., Grini,~A., Hendricks,~J., Horowitz,~L., Huang,~P., Isaksen,~I., Iversen,~I., Kloster,~S., Koch,~D., Kirkevåg,~A., Kristjansson,~J E., Krol,~M., Lauer,~A., Lamarque,~J F., Liu,~X., Montanaro,~V., Myhre,~G., Penner,~J., Pitari,~G., Reddy,~S., Seland,~Ø., Stier,~P., Takemura,~T., and Tie,~X.: Analysis and quantification of the diversities of aerosol life cycles within AeroCom, Atmos. Chem. Phys., 6, 1777–1813, doi:10.5194/acp-6-1777-2006, 2006. Thomas,~G E., Poulsen,~C A., Sayer,~A M., Marsh,~S H., Dean,~S M., Carboni,~E., Siddans,~R., and Grainger,~R G.: Oxford-RAL Aerosol and Cloud (ORAC): Aerosol retrievals from satellite radiometers, Springer (Berlin), in: Aerosol Remote Sensing Over Land, edited by: Kokhanvosky,~A A. and de Leeuw,~G., 193–225, 2009. Thomas,~G E., Poulsen,~C A., Siddans,~R., Sayer,~A M., Carboni,~E., Marsh,~S H., Dean,~S M., Grainger,~R G., and Lawrence,~B N.: Validation of the GRAPE single view aerosol retrieval for ATSR-2 and insights into the long term global AOD trend over the ocean, Atmos. Chem. Phys., 10, 4849–4866, doi:10.5194/acp-10-4849-2010, 2010. Twohy,~C H., Coakley Jr.,~J A., and Tahnk, ~W R.: Effect of changes in relative humidity on aerosol scattering near clouds, J. Geophys. Res., 114, D05205, doi:10.1029/2008JD010991, 2009. Várnai,~T., and Marshak,~A.: MODIS observations of enhanced clear sky reflectance near clouds, Geophys. Res. Lett., 36, L06807, doi:10.1029/2008GL037089, 2009. Wen,~G., Marshak,~A., Cahalan, ~R F., Remer, ~L A., and Kleidman, ~R G.: 3-D aerosol–cloud radiative interaction observed in collocated MODIS and ASTER images of cumulus cloud fields, J. Geophys. Res., 112, D13204, doi:10.1029/2006JD008267, 2007. Závody,~A M., Mutlow,~C T., and Llewellyn-Jones,~D T.: Cloud clearing over the ocean in the processing of data from the along-track scanning radiometer (ATSR), J Atmos. Ocean. Tech., 17, 595–615, doi:10.1175/1520-0426(2000)017<0595:CCOTOI>2.0.CO;3B2, 2000. 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., 36, D22207, doi:10.1029/2005JD006898, 2006. Zhao,~T X.-P., Laszlo,~I., Guo,~W., Heidinger,~A., Cao,~C., Jelenak,~A., and Sullivan,~J.: Study of long-term trend in aerosol optical thickness observed from operational AVHRR satellite instrument, J. Geophys. Res., 113, D07201, doi:10.1029/2007JD009061, 2008.