ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-8-13479-2008 Total and partial cloud amount detection during summermonths 2005 at Westerland (Sylt, Germany) Schade N. H. 1 Macke A. 2 Sandmann H. 1 Stick C. 1 Institut für Medizinische Klimatologie, Hermann-Rodewald-Str. 5, 24098 Kiel, Germany Leibniz-Institut für Meereswissenschaften, Düsternbrooker Weg 20, 24105 Kiel, Germany 15 07 2008 8 4 13479 13505 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-discuss.net/8/13479/2008/acpd-8-13479-2008.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/8/13479/2008/acpd-8-13479-2008.pdf

The detection of cloudiness is investigated by means of partial and total cloud amount estimations from pyrgeometer radiation measurements and all-sky imager observations. The measurements have been performed in Westerland, a seaside resort on the North Sea island of Sylt, Germany, during summer 2005. <br><br> An improvement to previous studies on this subject results from the fact that for the first time partial cloud amount (PCA), defined as total cloud amounts without high clouds, calculations from longwave downward radiation (LDR) according to the APCADA-Algorithm (Dürr and Philipona, 2004) are validated against both human observations from the German Weather Service DWD at the nearby airport of Sylt and digital all-sky imaging. <br><br> Differences between the resulting total cloud amounts (TCA's), defined as total cloud amount for all-cloud situations, derived from the camera images and from human observations are within &plusmn;1 octa in 72% and within &plusmn;2 octa in 85% of the cases. Compared to human observations PCA measurements according to APCADA underestimate the observed cloud cover in 47% of all cases and the differences are within &plusmn;1 octa in 60% and &plusmn;2 octa in 74% of all cases. Since high cirrus clouds can not be derived from LDR, separate comparisons for all cases without high clouds have been performed showing an agreement within &plusmn;1(2) octa in 73(90)% for PCA and also for camera derived TCA. For this coastal mid-latitude site under investigation we find similar though slightly smaller agreements to human observations as reported in Dürr and Philipona (2004). Though limited to day-time the cloud cover retrievals from the sky imager are not much affected by cirrus clouds and provide a more reliable cloud climatology for all-cloud conditions than APCADA.

 References Beaubien, M. and Bisberg, A.: A new CCD-based instrument for the automatic determination of cloud cover, Paper presented at the 10th Atmospheric Radiation Conference, Am. Meteorol. Soc., Madison, Wisconsin, USA. Berk, A., Anderson, G. P., Acharya, P. K., Chetwynd, J. H., Bernstein, L. S., Shettle, E. P., and Matthew, M. W.: MODTRAN4 users manual, Air Force Res. Lab., Hanscom Air Force Base, Mass, 2000. Brutsaert, W.: On a derivable formula for longwave radiation from clear sky, Water Resour. Res., 11(3), 742–744, 1975. Buck, A.L.: New equations for computing water vapour pressure and enhancement factor, J. Appl. Meteorol., 20, 1527–1532, 1981. Dürr, B. and Philipona, R.: Automatic cloud amount detection by surface longwave downward radiation measurements, J. Geophys. Res., 109, D05201, doi:10.1029/2003JD004182, 2004. Feister, U., Shields, J., Karr, M., Johnson, R., Dehne, K., and Woldt, M.: Ground based cloud images and sky radiance in the visible and near infrared region from whole sky imager measurements, Paper presented at Climate Monitoring: Satellite Application Facility Training Workshop, DWD, Dresden, 2000. Hollmann, R., Mueller, R. W., and Gratzki, A.: CM-SAF surface radiation budget: First results with AVHRR data, Adv. Space Res., 37(12), 2166–2171, 2006. Long, C. N. and Ackermann, T. P.: Identification of clear skies from broadband pyranometer measurements and calculation of downwelling shortwave cloud effects, J. Geophys. Res., 105, 15 609–15 626, 2000. Long, C. N., Slater, D. W., and Tooman, T.: Total sky imager model 880 status and testing results, ARM TR-006, 1–17, 2001. Marty, C. and Philipona, R.: The clear-sky index to separate clear-sky from cloudy-sky situations in climate research, Geophys. Res. Lett., 27, 2649–2652, 2000. Morris, V.: Total Sky Imager (TSI), Handbook. US Department of Energy, DOE/SCARM/ TR017, 2004. Ohmura, A., Gilgen, H., Hegner, H., Müller, G., Wild, M., et al.: Baseline surface radiation network (BSRN/WCRP): New precission radiometry for climate research, B. Am. Meteorol. Soc., 79, 2115–2136, 1998. Oznovitch, I., Yee, R., Schiffler, A., McEwen, D. J., and Sofko, G. J.: The all-sky camera revitalized, Appl. Optics, 33, 7141–7150, 1994. Pfister, G., McKenzie, R. L., Liley, J. B., and Thomas, A.: Cloud coverage based on all-sky imaging and its impact on surface solar irradiance, J. Appl. Meteorol., 42, 1421–1434, 2003. Ruffieux, D., Nash, J., Jeannet, P., and Agnew, J. L.: The COST 720 temperature, humidity, and cloud profiling campaign: TUC, Meteorol. Z., 15(1), 510, 2006. Schade, N. H.: Experimentelle Erfassung und Interpretation der solaren Einstrahlung bei durchbrochener Bewölkung, Leibniz Institut fuer Meereswissenschaften an der Christian-Albrechts-Universität Kiel, Diplomarbeit, http://www.ifm-geomar.de/index.php?id=1645#5651, 2005. Schade, N. H., Macke, A., Sandmann, H., and Stick, C.: Enhanced solar global irradiance during cloudy sky conditions, Meteorol. Z., 16(3), 295–303, 2007. Shields, J. E., Karr, M. E., Tooman, T. P., Sowle, D. H., and Moore, S. T.: The Whole-Sky Imager – A year of Progress, www.arm.gov/publications/proceedings/conf08/extended_abs/shields_je.pdf, 12 February 2004. Sutter, M., Dürr, B., and Philipona, R.: Comparison of two radiation algorithms for surfacebased cloudfree sky detection, J. Geophys. Res., 109, D17202, doi:10.1029/2004JD004582, 2004.