Lagrangian transport of water vapor and cloud water in the ECHAM4 GCM and its impact on the cold bias |
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Authors: | A Stenke V Grewe M Ponater |
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Institution: | 1.Institut für Physik der Atmosph?re,Wessling,Germany |
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Abstract: | The Lagrangian advection scheme ATTILA has been applied for the transport of water vapor and cloud water in the general circulation
model (GCM) ECHAM4.L39(DLR) (E39) instead of the operational semi-Lagrangian transport scheme (SLT). ATTILA is a purely Lagrangian
scheme that is numerically non-diffusive, while the operational semi-Lagrangian scheme exhibits a considerable numerical diffusion
in the presence of sharp gradients. The model version E39/SLT significantly overestimates the water vapor mixing ratio in
the extratropical lowermost stratosphere (wet bias) by a factor of 3–5 compared to HALOE observations. Compared to E39/SLT,
E39/ATTILA shows substantially reduced water vapor mixing ratios in the extratropical lowermost stratosphere up to 70%, and
a steeper meridional water vapor gradient in the subtropics which is in better agreement with observations. Furthermore, the
temperature distribution as simulated with E39/SLT is characterized by a pronounced cold temperature bias in the extratropical
lowermost stratosphere (cold bias) and in the polar stratosphere above 50 hPa in winter (cold pole). The improvements concerning
the water vapor distribution in E39/ATTILA lead to a substantial reduction of the simulated cold bias by approximately 5–7 K
which also results in a better representation of the modeled tropopause, especially in the extratropics. Sensitivity studies
indicate that the warming of the extratropical lowermost stratosphere in E39/ATTILA is directly related to the reduced wet
bias resulting in a less infrared radiative cooling. Additionally, the cold pole problem is also slightly reduced in E39/ATTILA
by approximately 2–5 K. |
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