Geographic Diurnal Variations

3. Geographic, diurnal and seasonal variations

a. Geographic variations The average cloud amount varies seasonally and with time of day. The overall distribution of clouds around the globe can affect the radiation budget and thus tied with the production of precipitation. From ground-based climatology, one can see from Figure 6 that the ocean is more likely to have a greater total cloud cover than land-based stations due to the availability of moisture. This is primarily the case for the midlatitudes, where ocean observations are scarce.

Cumulonimbus over land primarily dominate the equator compared to ocean, while cumulus are dominate over the ocean. Stratocumulus are primarily a feature over land during the spring and summer months and

Diurnal variations

Cloud cover is more pronounced during the day and the total cloud amount is relatively consistent over the oceans. Most clouds tend to follow a diurnal cycle, as can be seen from Figure 7. The total cloud amount over the central North Atlantic tends to remain constant throughout the day. The stratus and stratocumulus clouds found over the southeast Atlantic are boundary layer-type clouds, with peaks occurring at 6 am and a rapid decline at 1600 local time. These clouds are driven by radiative cooling in the boundary layer overnight with dissipation occurring as the sun heats the boundary layer.

Low-level clouds have a maximum in the early morning, while the maximum cloud amount over land occurs in the early afternoon. Over land, cumulus tend to form in the early morning with dissipation in the early afternoon due to mixing. Cumulus clouds are convectively driven, so solar radiation is important for their development. Cumulonimbus tend to take over after the decline in cumulus clouds due to more organized forcing. This forcing takes more time and therefore cumulonimbus make an appearance in the late afternoon and evening hours in the continental

However, seasonal variations in cloud properties seem to be smaller than latitudinal variations, with the exception of the ITCZ shift in summer months.

4. Current issues in cloud climatology

Cloud climatology has been widely studied for years. Despite a large array of instruments to detect cloud properties and various model constraints, there are still problems that remain. One issue is simply identifying from satellite data what constitutes a cloud. Using multiple channels and sensors doesn’t entirely resolve these issues, as there still could be differences in the instrument calibration and data. Discrepancies in the total cloud amount as recorded by satellite could lead to problems in identifying cloudy pixels from clear sky. Most satellite data tends to be in agreement along the lines of general trends, but can contain large differences across latitudes. Some of this disagreement stems from the spectral response of detectors from one satellite to another.

Models help to compare to what is observed directly. If there are uncertainties that exist in the observations, there will be uncertainties that arise in the model from the derived properties of the