Validation of components of the water cycle in the ECHAM4 general circulation model based on the Newtonian relaxation technique: a case study of an intense winter cyclone

The representation of a simulated synoptic-scale weather system is compared with observations. To force the model to the observed state, the so-called Newtonian relaxation technique (nudging) is applied to relax vorticity, divergence, temperature, and the logarithm of surface pressure to the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis fields. The development of an extraordinary strong cyclone along the East Coast of the USA during 12–14 March 1993 was chosen as the case study. The synoptic-scale features were well represented in the model simulation. However, systematic differences to observations of the International Satellite Cloud Climatology Project (ISCCP) occurred. The model underestimated clouds in lower and middle levels of the troposphere. Low-level clouds were mainly underestimated behind the cold front of the developing cyclone, while the underestimation of mid-level clouds seems to be a more general feature. The reason for the latter is the fact that the relative humidity has to exceed a critical threshold before clouds can develop. In contrast, thin upper-level cirrus clouds in pre-frontal regions were systematically overestimated by the model. Therefore, we investigated the effects of changed physical parameterizations with two sensitivity studies. In the PCI experiment, the standard cloud scheme operated in ECHAM4 was replaced by a more sophisticated one which defines separate prognostic equations for cloud liquid water and cloud ice. The second experiment, RHCRIT, changed the profile of the critical relative humidity threshold for the development of clouds in the standard scheme. Both experiments showed positive changes in the representation of clouds during the development of the cyclone as compared to the ISCCP. PCI clearly reduced the upper-level cloud amounts by intensifying the precipitation flux in the middle troposphere. The changed condensation threshold in the RHCRIT experiment led to a sharper represented cold front and a better represented cloudiness on its rear side as compared to the PCI and the CONTROL simulations.