The annual cycle of Earth radiation budget from Clouds and the Earth's Radiant Energy System (CERES) Data

The annual cycle of Earth radiation budget is the response of the climate system to the annual cycle of solar input. Examining the response of a system to a cyclical forcing is an important method for understanding the dynamics of the system. The absorbed insolation depends on the albedo, which is a dynamic quantity depending largely on the movement of major cloud systems as they interact with the solar heating and other parts of the climate system. Snow and ice cover also affect the albedo to provide a forcing of the absorbed flux of energy. The temperatures of the land, atmosphere, and ocean vary in response to the cyclic heating and together with the clouds cause the longwave flux at the top of the atmosphere to vary.

The annual cycle of Earth radiation budget is investigated by use of data from the Clouds and the Earth's Radiant Energy System (CERES). Monthly-mean maps of reflected solar flux and Earth-emitted flux on a 1° grid are used for the study. The absorbed solar radiation is the heating by the Sun and is computed from the CERES data by subtracting the reflected flux from the insolation. The annual cycles of albedo, absorbed solar radiation, and Earth-emitted radiation fluxes are described by use of principal components for the time variations, for which the corresponding geographic variations are the empirical orthogonal functions. The principal components provide the most concise quantitative description of the data. The immense heat capacity of the ocean causes it to have a slow response to the annual cycle of heating relative to land, and the atmosphere is closely coupled with the surface, so the Earth's surface is partitioned into land and ocean for the analysis.

The first principal component describes a large majority of the variance of the albedo, the absorbed solar radiation, and the Earth-emitted radiation fluxes. This conciseness of the description makes principal component analysis useful as a method for comparing observed radiation budget with that computed by a model. The magnitude of the Earth-emitted radiation and the albedo and their phase angles relative to the absorbed solar radiation are important descriptors of the climate system and are computed for the first principal components.