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Cloud Analysis - Part 3

Seasonal Variations of Cloud and Surface Properties

On time scales longer than one month, the largest time variation is the systematic cycle of the seasons. The next figures illustrate the seasonal variations of cloud and surface properties in various ways.

Figure 4: Seasonal Mean Distributions

Winter Cloud Amount (%) - WINTER Cloud Top Pressure (mb) - WINTER Cloud Top Temperature (K) - WINTER Cloud Optical Thickness - WINTER Surface Temperature (K) - WINTER Surface Reflectance - WINTER
Spring Cloud Amount (%) - SPRING Cloud Top Pressure (mb) - SPRING Cloud Top Temperature (K) - SPRING Cloud Optical Thickness - SPRING Surface Temperature (K) - SPRING Surface Reflectance - SPRING
Summer Cloud Amount (%) - SUMMER Cloud Top Pressure (mb) - SUMMER Cloud Top Temperature (K) - SUMMER Cloud Optical Thickness - SUMMER Surface Temperature (K) - SUMMER Surface Reflectance - SUMMER
Autumn Cloud Amount (%) - AUTUMN Cloud Top Pressure (mb) - AUTUMN Cloud Top Temperature (K) - AUTUMN Cloud Optical Thickness - AUTUMN Surface Temperature (K) - AUTUMN Surface Reflectance - AUTUMN

These maps show the average regional distribution of cloud and surface properties for each of the four seasons, where the names apply to the northern hemisphere seasons. Superficially, they all resemble the annual mean plots shown in Part 2. The seasonal variations are better shown by comparing the latitude distributions for the different seasons as done in the plots below.

Figure 5: Seasonal and Zonal Mean Distributions

All Seasons Cloud Amount (%) - All Seasons Cloud Top Pressure (mb) - All Seasons Cloud Top Temperature (K) - All Seasons Cloud Optical Thickness - All Seasons Surface Temperature (K) - All Seasons Surface Reflectance - All Seasons

These plots show that the seasonal variation of clouds in the tropics appears as a shifting of the position of the convective zone: local summer has higher cloud amount, much lower cloud top pressure/temperature, and much larger optical thicknesses associated with much more rainfall. Note that the seasonal variations of clouds at lower latitudes are not associated with any significant seasonal variation in surface temperature. In northern midlatitudes, the storm track shifts to higher latitudes in local summertime and the clouds become somewhat thinner; whereas in southern midlatitudes the storm track position remains the same year-round, but the cloud amount increases, the cloud top pressure decreases and the optical thickness decreases in local summertime. This difference in behavior is associated with a difference in the seasonal cycle of surface temperature in the two hemispheres: the seasonal variations of surface temperature in the land-dominated northern hemisphere are much larger in magnitude than those in the ocean-dominated southern hemisphere. Both polar regions show a decrease in cloud amount in local summertime; however, since the satellites have difficulty detecting very low-level clouds that are ubiquitous in summertime, this change in cloud amount is not corret. If anything, the cloud amount is actually slightly larger in summertime than in wintertime. Although there is little change in cloud amount, the summertime cloud top pressures and optical thicknesses are much larger than the wintertime values (for ISCCP, since there is no sunlight available in polar winter, the optical thickness values can only be estimated by interpolating over time but these values are not shown in the plot). The large seasonal variations of surface temperature in the polar regions are associated with dramatic changes of cloud vertical extent and optical thickness: colder temperatures lead to larger vertical extent but smaller optical thicknesses.

Figure 6: Zonal, Monthly Mean Distributions

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

This figure shows the whole time record of monthly and zonal mean cloud and surface properties illustrating the dominance of the seasonal cycle when weather variations are removed. A notable feature is that there are significant seasonal variations of cloud properties at low latitudes even though there are almost no variations of surface temperature, whereas at midlatitudes where there is a very large seasonal variation of surface temperature (particularly in the northern hemisphere), there is almost no seasonal variation of cloud properties. The polar regions exhibit large variations of both clouds and surface temperature.

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

Data Analysis