Viewing Angle on ISCCP Problems
Posted by Ari Jokimäki on January 20, 2010
International Satellite Cloud Climatology Project (ISCCP) provides measurements of global cloud cover. See the project website for the introduction (see also Rossow & Schiffer, 1991, for basic description of ISCCP cloud data products). They offer the longest satellite dataseries for global cloud cover. ISCCP data has been widely used to study trends in global cloud cover. For example, Pallé et al. (2004) have used ISCCP data to determine a decrease in Earth’s reflectance. Figure 1 shows the global cloud cover trends in ISCCP data, and there we can see that the global cloud cover has apparently decreased rather strongly between about 1987 and 2000.
Figure 1. The global cloud cover from ISCCP data. (Image is loaded from ISCCP website.)
Couple of other examples of papers using ISCCP data in cloud cover research:
Long-term variations in solar radiation at Earth’s surface (S) can affect our climate, the hydrological cycle, plant photosynthesis, and solar power. … Here we present an estimate of global temporal variations in S by using the longest available satellite record. We observed an overall increase in S from 1983 to 2001 at a rate of 0.16 watts per square meter (0.10%) per year; this change is a combination of a decrease until about 1990, followed by a sustained increase. The global-scale findings are consistent with recent independent satellite observations but differ in sign and magnitude from previously reported ground observations. Unlike ground stations, satellites can uniformly sample the entire globe.
The monthly mean shortwave (SW) radiation budget at the Earth’s surface (SRB) was computed on 2.5-degree longitude-latitude resolution for the 17-year period from 1984 to 2000, using a radiative transfer model accounting for the key physical parameters that determine the surface SRB, and long-term climatological data from the International Satellite Cloud Climatology Project (ISCCP-D2). … Significant increasing trends in DSR and net DSR fluxes were found, equal to 4.1 and 3.7 Wm−2, respectively, over the 1984–2000 period (equivalent to 2.4 and 2.2 Wm−2 per decade), indicating an increasing surface solar radiative heating. … The surface solar heating occurs mainly in the period starting from the early 1990s, in contrast to the commonly reported decreasing trend in DSR through the late 1980s, found also in our study.
The apparent decrease in global cloud cover has also been used as an argument against anthropogenic warming (but mainly outside of scientific literature). But when the sources of these arguments are consulted, almost without exception we run into ISCCP cloud cover data being used as the source. In the following, I will show you why it is false to claim global cloud cover trends based on ISCCP data.
Problematic satellite viewing angle
Norris (2000) searched for suspicious spatial patterns from ISCCP cloud data, and found some:
The most striking feature of Figure 2 is the circular patch of positive correlation centered on 0°N, 0°E. It seems unlikely that a natural process would produce such a regular pattern, but it is interesting to note that the pattern almost exactly resembles the footprint of Meteosat data incorporated into ISCCP (Meteosat is the European geostationary satellite).
Accordingly, the ISCCP time series presented in Figure 1 is probably spurious, and any resemblance to time series of other parameters (e.g., Marsh and Svensmark, 2000) is merely coincidental.
Campbell (2004) showed images of the same problem. Campbell showed simply the cloudiness on the map of the world in certain time period and pointed out that there were clear “seams” showing in the maps. Campbell noted:
Qualitatively one suspects that different view angles are affecting the products.
Campbell then showed a graph that had cloudiness plotted for one latitude band, and the satellite positions were marked to the graph. The cloudiness clearly increased further from the satellite positions, so the cloudiness seemed to be increasing with satellite viewing angle. Campbell also made an effort of quantifying the apparent effect; the cloudiness data was divided to different bins by viewing angle, and average cloudiness was calculated for the bins. The resulting graph shows almost linear increase in cloudiness with the increase in viewing angle. Campbell presented the finding also in the form of an equation that describes the viewing angle dependence of cloudiness in ISCCP data.
So Campbell had established that the amount of measured cloudiness in ISCCP data is larger when the satellite viewing angle is larger. Next Campbell entered into a discussion of trends. Campbell noted an important thing:
As the ISCCP project progressed, different numbers of geo satellites were available. This had the unforeseen effect of changing the average view angle as the data set has been accumulated.
And it turns out that the number of satellites increased, and that then decreased the average viewing angle. As higher viewing angle was associated with higher cloudiness, decreasing average viewing angle decreases the amount of measured clouds. Campbell showed that it has indeed happened in the ISCCP data and the result is that the long decreasing trend in ISCCP cloud data seems to be an artifact of satellite viewing angle. Campbell then applied a correction based on the equation for the viewing angle dependency. The result still shows a decreasing trend but clearly smaller than without the correction. However, the corrected map still showed some “seams” so the correction was only rough one, and there were some calibration issues Campbell suspected might still be at play. Campbell concluded:
A substantial part of the ISCCP trend is due to systematic changes in the view angle over the 18 years of the data analysis. From an analysis of the AVHRR data alone, increasing cloudiness at steeper view angles is obvious.
Pallé (2005) also noted that there possibly is viewing angle problems with ISCCP data. Campbell (2006) continued to define a correction to the ISCCP viewing angle problem among some other problems (some diurnal corrections). After corrections, not much cloud trends were left. However, Campbell noted that there still were a discontinuity in the data:
But there is a very distinct discontinuity in all the cloud amount time series in October 2001 when NOAA 16 AVHRR data replaced NOAA 14 data.
Evan et al. (2007) also studied the viewing angle issue. They first noted how the ISCCP data had been used, including most of the above mentioned studies, and they then said:
However, these trends in total cloudiness have not been observed in surface [Norris, 2005] and other satellite [Jacobowitz et al., 2003; Wylie et al., 2005] cloud records.
In their figure 2 they showed similar map as Norris and Campbell did previously of the cloud variability, showing the circular features suggestive of the viewing angle problem. They said:
These circles correspond to so-called geostationary ‘‘footprints’’ that describe the area observed by each satellite. At the center of these footprints a satellite sensor’s viewing angle is perpendicular to the surface, corresponding to a satellite zenith angle of 0 degrees. At the footprint edges the satellite zenith angle is much higher, corresponding to a longer path length through the atmosphere that light must travel before it is detected by the sensor.
They then performed a test where they created two dataseries from ISCCP data; one from large satellite viewing angles and one from small viewing angles. It turned out that the dataseries with large viewing angles (black solid line in their figure 3) showed far stronger decrease in clouds than the one with small viewing angle (black dashed line in their figure 3). They also showed how changes in viewing angle over time from large viewing angle areas (grey solid line in figure 3) matched very well the step changes in the change in cloudiness (compare grey and black solid lines in figure 3). They also listed some events in the satellite network (launches of new satellites, repositioning of satellites, etc.) that might have affected the data, and the listed events matched the step changes in tha data rather well. One such event was a shift that occurred in 2001. That shift seems to be related to a change in reference satellite (Knapp, 2008).
Evan et al. then showed an example of one location which had large satellite viewing angle where there was a known satellite network event, and showed how the cloud data clearly jumped at the event (see their figure 4), while a nearby region that had small viewing angle showed trendless line at the same time. To finish the paper, Evan et al. suggested some actions that could be done to correct the problem.
Berthier et al. (2008) also found some potential problems with ISCCP data:
Comparisons of CTH developed from LITE, for 2 weeks of data in 1994, with ISCCP (International Satellite Cloud Climatology Project) cloud products show that the cloud fraction observed from spaceborne lidar is much higher than that from ISCCP. Another key result is that ISCCP products tend to underestimate the CTH of optically thin cirrus clouds.
They considered the viewing angle problem to be one possible source for the differences they found. The viewing angle problem was also briefly touched in Norris (2008).
It seems that the finding of Norris, Campbell, and Evan et al. about ISCCP satellite viewing angle (by changes in satellite network that affect the viewing angles) creating a spurious trend is a real problem. I think they have shown it beyond reasonable doubt. ISCCP website lists known and fixed errors in ISCCP data but there doesn’t seem to be any mention of this problem.
Many studies have made conclusions relating to clouds based on the apparent long term decrease in ISCCP data, but all that research is in doubt because the trend at least partly is not real. Furthermore, ISCCP cloud data is the longest satellite record we have on the subject, so it would be important to issue an official correction to this problem. When the trend is corrected, interesting things might be found, as shown by Clement et al. (2009) who corrected for the problem and found that cloud feedback is positive. As a long record, ISCCP data would be suitable to that kind of studies.
Berthier, S., Chazette, P., Pelon, J., Baum, B., 2008, “Comparison of cloud statistics from spaceborne lidar systems”, Atmos. Chem. Phys., 8, 6965-6977, 2008, [Full text]
Campbell, G. Garrett, 2004, “View angle dependence of cloudiness and the trend in ISCCP cloudiness”, 13th Conference on Satellite Meteorology and Oceanography, [Full text]
Campbell, G. Garrett, 2006, “Diurnal and Angular Variability of Cloud Detection: Consistency Between Polar and Geosynchronous ISCCP Products”, 14th Conference on Satellite Meteorology and Oceanography, [Full text]
Clement, Amy C., Burgman, Robert, Norris, Joel R., 2009, “Observational and Model Evidence for Positive Low-Level Cloud Feedback”, Science 24 July 2009, Vol. 325. no. 5939, pp. 460 – 464, DOI: 10.1126/science.1171255, [Full text]
Evan, Amato T., Heidinger, Andrew K., Vimont, Daniel J., 2007, “Arguments against a physical long-term trend in global ISCCP cloud amounts”, Geophys. Res. Lett., 34, L04701, doi:10.1029/2006GL028083, [Full text]
Hatzianastassiou, N., Matsoukas, C., Fotiadi, A. Pavlakis, K. G., Drakakis, E., Hatzidimitriou, D., Vardavas, I., 2005, “Global distribution of Earth’s surface shortwave radiation budget”, Atmos. Chem. Phys. Discuss., 5, 4545-4597, [Full text]
Knapp, Kenneth R., 2008, “Calibration Assessment of ISCCP Geostationary Infrared Observations Using HIRS”, Journal of Atmospheric and Oceanic Technology, Volume 25, Issue 2 (February 2008), DOI: 10.1175/2007JTECHA910.1, [Full text]
Norris, Joel R., 2000, “What Can Cloud Observations Tell Us About Climate Variability?”, Space Science Reviews, Volume 94, Numbers 1-2 / November, 2000, DOI 10.1023/A:1026704314326, [Full text]
Norris, J. R., 2008, “Observed Interdecadal Changes in Cloudiness: Real or Spurious?”, Advances in Global Change Research, 33, DOI 10.1007/978-1-4020-6766-2, [Full text]
Pallé, E., Goode, P. R., Montañés-Rodríguez, P., Koonin, S. E., 2004, “Changes in Earth’s Reflectance over the Past Two Decades”, Science 28 May 2004, Vol. 304. no. 5675, pp. 1299 – 1301, DOI: 10.1126/science.1094070, [Full text]
Pallé, E., 2005, “Possible satellite perspective effects on the reported correlations between solar activity and clouds”, Geophys. Res. Lett., 32, L03802, doi:10.1029/2004GL021167, [Full text]
Pinker, R. T., Zhang, B., Dutton, E. G., 2005, “Do Satellites Detect Trends in Surface Solar Radiation?”, Science 6 May 2005, Vol. 308. no. 5723, pp. 850 – 854, DOI: 10.1126/science.1103159
Rossow, William B., Schiffer, Robert A., 1991, “ISCCP Cloud Data Products”, Bulletin of the American Meteorological Society, Volume 72, Issue 1 (January 1991), DOI: 10.1175/1520-0477(1991)0722.0.CO;2, [Full text]