The Atmospheric Radiation Measurement Program

The scientific motivation for the ARM program arose from a decade of comparisons of different climate models. Those comparisons sought to elucidate the underlying reasons for the substantial and disconcerting differences, from one model to another, in their predictions of climatic responses to such perturbations as the doubling of atmospheric carbon dioxide. The comparisons produced two important results in the late 1980s. One such undertaking, called the Inter-comparison of Radiation Codes in Climate Models, showed that when radiative transfer through the atmosphere was understood at high spectral resolution, it was possible to create a fast, lower-resolution model appropriate for climate modeling. The low-resolution models were needed because of limitations on computing power and on the availability of sufficient measurements to validate the high-resolution models, especially in the context of complex three-dimensional cloud fields. Other comparisons of climate models suggested that the predicted sensitivity of global temperature to greenhouse gases was directly related to how the models treated the interaction of clouds with radiation.