Robert Sausen

Objective Identification of Cyclones in GCM Simulations

Abstract An objective routine for identifying individual cyclones has been developed. The procedure was designed with the aim to keep the input expenditure low. The method ensures a complete collection of cyclones and an exclusion of short time fluctuations attributed to numerical effects. The cyclones are identified as relative minima of the geopotential height field in 1000 hPa. The initial stages of the cyclones are found by locating relative maxima in the 850-hPa vorticity field. Further on the temporal development of the extrema is taken into consideration. An individual cyclone is regarded only if it exists for at least 24 h and if it attains a mature stage at least once, where a certain margin of the geopotential gradient to the surroundings is exceeded. The identification routine is applied to simulations with the Hamburg general circulation model ECHAM in T21 resolution. Also, cyclone tracks based on ECMWF analyses are evaluated, to which the model results are compared. The effect of different cl...

A model of river runoff for use in coupled atmosphere-ocean models

Abstract A model for simulating continental river runoff is presented. The local runoff is transported by a linear advection scheme. The advection rates are quasi-objectively determined as a function of the slope of the orography. The river runoff is incorporated in the general circulation model ECHAM. The results of a 20 year simulation are presented. Qualitatively, observations are well produced.

Impact of vertical resolution on the transport of passive tracers in the ECHAM4 model

Abstract The transport of the passive tracers 14CO2 and SF6 has been modelled with two versions of the general circulation model ECHAM4 with different vertical resolutions: the standard model with 19 model layers (L19) and a higher-resolution version with 39 layers (L39). We study the impact of vertical resolution on the modelled transport characteristics. Both models are able to capture the observed SF6concentrations in the troposphere, but in the stratosphere the SF6 mixing ratios are overestimated. L39 generally calculates higher stratospheric SF6 mixing ratios than L19. This deviation increases with altitude. The difference between the modelled profiles is partly attributed to the residual mean meridional circulation, which is stronger in L39 than in L19, and partly to the initial globally constant SF6 concentration. The comparison of modelled 14CO2 surface concentrations and vertical profiles with observations has shown that an increased vertical resolution in the climate model ECHAM4 reduces the strength of stratosphere–troposphere exchange. L39 allows a better representation of sharp gradients at the tropopause than L19. This results in a weaker downward transport across the tropopause. However, compared to observations the downward transport is still too strong even in the L39 simulation.

Greenhouse Effect, Radiative Forcing and Climate Sensitivity

Temperature conditions and climate on Earth are controlled by the balance between absorbed solar radiation and outgoing terrestrial radiation. The greenhouse effect is a synonym for the trapping of infrared radiation by radiatively active atmospheric constituents. It generally causes a warming of the planet’s surface, compared to the case without atmosphere. Perturbing the radiation balance of the planet, e.g., by anthropogenic greenhouse gas emissions, induces climate change. Individual contributions to a total climate impact are usually quantified and ranked in terms of their respective radiative forcing. This method involves some limitations, because the effect of the external forcing is modified by radiative feedbacks. Here the current concept of radiative forcing and potential improvements are explained.