Low-Level Clouds

Low clouds over the ocean are extremely important for the energy balance of Earth because their albedos are much higher than that of clear sky over ocean and they have only a small effect on escaping longwave radiation. This is clearly illustrated in the following figure, which slows the annual mean abundance of clouds with tops at or below the 680 mb level (heights less than about 3 km) from ISCCP. Low clouds are abundant over the oceans and are especially common in high latitudes and over the eastern margins of the oceans where low sea surface temperatures and atmospheric subsidence encourage their presence. Also shown is the annual mean net cloud radiative forcing, the amount by which the clouds that are present in the current climate change the net energy balance at the top of the atmosphere. There is a remarkable similarity between the distribution of the fractional area coverage by low clouds and the net forcing of the climate by clouds. This is because the low clouds are relatively abundant where they occur. They have a strong negative effect on the energy balance because they reflect a lot of solar radiation and yet their tops are warm enough that they have only a small effect on the amount of emitted thermal radiation. The amount and optical properties of low stratiform clouds are sensitive to both atmospheric and oceanic large-scale properties and to the abundance of cloud condensation nuclei, which may be affected by human activities such as the burning of fossil carbon. These sensitivities combined with their strong effect on the radiation balance give them the potential to play a major role in climate change.

Long-term trends in low clouds and their relation to SST and other variables are being derived from long records of surface observations in order to obtain estimates of how low cloud changes have been related to natural changes in the climate system. The following figure shows the change in low cloud abundance and SST over the North Pacific and North Atlantic oceans during the three decades from 1952 to 1981. During this period rather significant downward trends in SST over the midlatitude oceans have been accompanied by significant upward trends in the abundance of low clouds. Some tantalizing similarities in the structure of the SST and cloudiness trends are apparent. It is believed that these changes are associated with natural variability in the coupled ocean-atmosphere system. Cloud changes are larger in proportion to the SST changes over the Atlantic than they are over the Pacific Ocean. The reasons for this is not yet understood. . More recent data from the 1980's suggest that these downward trends of SST and upward trends of low cloud amount have reversed and shifted toward warming oceans and decreasing low cloud amounts.

More detailed global data on low cloud abundance, structure, and optical properties can be derived from instruments such as MODIS, CERES, and MISR within EOS. Improved data on the abundance of tropospheric aerosols will be provided by EOSP and MISR. These data can be used to test new models of stratocumulus clouds being developed for possible incorporation in global climate models. If a long enough record can be taken with these instruments, additional insight into natural variability and long-term trends can be obtained. The decade from 1998 to 2008 may be one in which anthropogenic climate change becomes measurable (0.1 - 0.2 C/decade), and it will be important to look at these changes with the EOS measurement system.