Cloud Regime Radiative Forcing from ISCCP

May Highlights

Maps of the relative frequency of occurence (RFO) for three tropical cloud regimes (CR) defined by the joint histograms of cloud top pressure and optical thickness to the left.

May 1, 2011Click on image to enlarge.

Global Climate Models (GCMs), our most advanced computer tools for predicting climate change are quite capable in producing, on average, a budget of solar radiation reflected and thermal radiation emitted to space that is close to observations. "On average" is, however, a key qualifier: the agreement with observations is achieved only when both the observations and model output are averaged over long temporal (months, years) and large spatial (global, hemispherical, zonal) scales. The apparent agreement gives us undeserved confidence in the models because often it is a result of compensating errors. One example where it is critical not to bypass the details, is when we deal with the role of clouds in the planet's radiation budget. If the effect of clouds appears in a GCM to be good "on average" one may be tempted to conclude that clouds are well-simulated, which may not be true upon closer scrutiny. In a recent paper (Oreopoulos and Rossow 2011) we propose a method that will provide the tools to examine in more depth the verisimilitude of GCM clouds. Specifically, we break down the radiative effect of clouds by cloud regimes (cloud type mixtures). The regime specification is based on International Satellite Cloud Climatology Project (ISCCP) satellite observations and was combined with another data set from the same project, to conditionally average the radiative effect of each regime. The work reveals the relative contribution to the the overall cloud radiative effect of each cloud regime. We claim that the basic features of the radiative behavior seen in the data should be also reproduced by GCMs.