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Model description

The Kühlungsborn Mechanistic general Circulation Model (KMCM) (Becker and Schmitz, 2001) is a dry idealized GCM. The model is run at moderate resolution with triangular truncation at wavenumber 29 in the horizontal and 24 hybrid levels, up to 0.3 hPa. Temperature relaxation parametrizes the radiation. The relaxation time is 16 days and drops down to 4 days at the upper rim. All simulations are ``perpetual January''. In the troposphere, relaxation temperature 1$T_e$ corresponds to observational temperature with increased meridional gradients. In the stratosphere $T_e$ is related to the radiatively determined state.

In order to examine the influence of different stationary wave forcing mechanisms, KMCM yields the possibility to turn them on or off independently. These mechanisms are world orography ($\Phi_s$) as well as additional diabatic heating in the deep tropics ($Q_c$) and middle latitudes ($Q_m$):

\begin{displaymath} Q = Q_c + \frac{\vert\omega \vert \mbox{h}(-\omega )}{40 \mbox{hPa d}^{-1}} Q_m \mbox{ .} \end{displaymath} (1)

The first term on the l.h.s. is used to mimic convective heating in the tropics. The second term describes self-induced condensational heating in middle latitudes in order to mimic land-sea heating contrasts. It depends linearly on the pressure velocity $\omega$ and is only active for rising motions due to the Heavyside function $\mbox{h}(-\omega )$. $Q_c$ and $Q_m$ are prescribed functions of longitude, latitude and pressure (Becker and Schmitz, 2001, their Fig. 2). For a more detailed model description, including turbulent boundary layer mixing and definition of the surface temperature, the reader is referred to Becker and Schmitz (2001).

The nomenclature of the model runs consists of the three abbrevations for the forcing mechanisms. By writing them in brackets or as function of longitude $\lambda$ it is indicated whether the zonal-mean or the longitude-dependent field is included. Starting from the rotationally invariant reference run $[\Phi_s]/[Q_m]/[Q_c]$, orographic (run $\Phi_s(\lambda)/[Q_m]/[Q_c]$), midlatitudinal (run $[\Phi_s]/Q_m(\lambda)/[Q_c]$) and tropical thermal forcing of stationary waves(run $[\Phi_s]/[Q_m]/Q_c(\lambda)$) are added and can be combined with each other (e.g. run $\Phi_s(\lambda)/Q_m(\lambda)/[Q_c]$). All mechanisms are present in run $\Phi_s(\lambda)/Q_m(\lambda)/Q_c(\lambda)$, that generates a realistic mean state of the atmosphere (Fig. 1). The length of each model run is 1801 days.

Prior to the EOF analysis the model data was smoothed with a binomial 30-day low-pass filter and weighted with the square root of cosine latitude. The low-pass filter reduced each data set by 128 days. All EOFs are calculated for the northern hemisphere ($20^o$N to $90^o$N). One-point correlation maps are used for comparison.

Figure 1: a) Zonal-mean zonal wind and b) zonal wind at 200hPa for run $\Phi_s(\lambda)/Q_m(\lambda)/Q_c(\lambda)$(contour interval 10 m/s, negative values shaded).

 
\begin{figure}\begin{center} \epsfig{figure=c:/eigene/physik/sparc/bilder/ufull.eps,width=13cm}\end{center} \end{figure}

Previous: Introduction Next: Annular Modes in the surface pressure Up: Ext. Abst.