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Cloud Analysis - Part 1

Climatology of Global Cloud and Surface Properties

Figure 1: Monthly Global Mean Deviations from Long-Term Global Mean

Cloud Amount (%) Cloud Top Pressure (mb) Cloud Top Temp. (K) Cloud Optical Depth Surface Temperature (K) Surface Reflectance

Expressed as deviations of monthly averages from the average over the whole time record, the figures above (solid lines) show that month-to-month variations in globally-averaged cloud and surface properties are very small (SD = standard deviation): cloud amounts vary by about 1-3% compared to a mean value of 66.7%, cloud top pressures vary by about 10-40 mb compared to a mean value of 578.1 mb, cloud top temperatures vary by about 1-4 K compared to a mean value of 261.7 K, cloud optical thicknesses vary by about 0.1-0.4 compared to a mean value of 3.4, surface temperatures vary by about 2-3 K compared to a mean value of 288.4 K, and surface visible reflectances vary by 0.02-0.04 compared to a mean value of 0.13.

The short ISCCP time record, covering only about 22 years, shows some signs of a slower variation. Cloud amount increased by about 2% during the first three years of ISCCP and then decreased by about 4% over the next decade. ISCCP began right after one of the largest El Ninos this century (in 1982-83) and the eruption of the El Chichon volcano, both of which may have caused some changes in clouds. There were other, weaker El Ninos in 1986-87, 1990-91 and 1992-94 and another volcanic eruption (Mt. Pinatubo) in 1991. Note however, that there appear to be no related changes in cloud top pressure/temperature or optical thickness (the small change in 1991 is caused by including the extra sunlight reflected by volcanic aerosol with clouds which affects the cloud top pressure/temperature determined for very thin cirrus clouds). The surface temperature shows a decrease of about 1 K from 1983 to about 1991. Since there a number of events occurring during this time period, the cause of these cloud variations is not yet understood.

Such variations are referred to as "natural" variability, that is the climate varies naturally for reasons that are not fully understood. The problem for understanding climate changes that might be produced by human activities is that the predicted changes are similar in magnitude to those shown here. The difference between natural and human-induced climate change will only appear clearly in much longer ( ≥ 50 years) data records.

Figure 2: Zonal Mean Anomaly Maps

Cloud Amount (%) Cloud Top Pressure (mb) Cloud Top Temp. (K) Cloud Optical Thickness Surface Temperature (K) Surface Reflectance

This figure shows the variations of zonal monthly mean values as a function of latitude, where the long-term average for each month of the year is subtracted to remove the average seasonal cycle. The figure illustrates the fact that cloud variations exhibit regional dependence that is hidden in the global averages shown in the first figures. This figure shows that the slow changes in cloud amount are concentrated mostly in the tropics and subtropics, whereas the changes in cloud optical thickness are concentrated in the polar regions. Surface temperature shows changes in both the tropics/subtropics and polar regions; the pattern suggests that these changes are associated with the El Ninos.

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8

Data Analysis