# POTSDAM-2 - The Atmospheric Model

The atmospheric component of CLIM*BER*-3α is that of CLIM*BER*-2, called POTSDAM-2 (POTsdam Statistical Dynamical Atmospheric Model 2) (Petoukhov et al., 2000). It is a 2.5-dimensional statistical-dynamical model which includes many of the processes that are also described by more sophisticated GCMs. In contrast to atmospheric GCMs (AGCMs), it does not resolve individual synoptic scale systems, but rather predicts the statistical characteristics connected with ensembles of these systems. However, the large-scale circulation (e.g., the monsoons, jet streams, Hadley circulation) and the main high and low-pressure areas are explicitly resolved.

The model is based on the assumption of a universal vertical structure of temperature and humidity in the atmosphere. This allows reducing the three-dimensional description to a set of two-dimensional, vertically averaged prognostic equations for temperature and humidity. The three-dimensional fields which are needed for the calculation of dynamics and of radiative fluxes can than be obtained by using the vertical profiles.

The atmospheric circulation, as well as the energy and moisture transport, are computed at 10 pressure levels, while long-wave radiation is calculated using 16 levels. An explicit integration scheme is used. In CLIM*BER*-3α the horizontal resolution of POTSDAM-2 was increased to 7.5° × 22.5°, instead of 10° × 51.4° as in CLIM*BER*-2, and the time-step reduced from one to half a day.

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