Meteorol Atmos Phys 99, 129–153 (2008) DOI 10.1007/s00703-007-0275-2 Printed in The Netherlands 1 Institute of Physics and Meteorology, University of Hohenheim, Stuttgart, Germany 2 Max-Planck-Institut fuur€ Meteorologie, Hamburg, Germany The representation of a synoptic-scale weather system in a thermodynamically adjusted version of the ECHAM4 general circulation model H.-S. Bauer1, V. Wulfmeyer1, L. Bengtsson2 With 16 Figures Received 19 July 2006; Accepted 7 August 2007 Published online 26 November 2007 # Springer-Verlag 2007 Summary to systematic deviations in near-surface temperature and wind fields. The general situation is very similar in both In this work, a strong cyclone event is simulated by the model representations. Errors were detected in the simulation general circulation model (GCM) ECHAM4 for studying of the convective boundary layer behind the cold front. The the representation of weather systems in a climate model. observed strong convective activity is missed both by the The system developed along the East Coast of the U.S.A. adjusted ECHAM4simulationandERA15.Thisismost between the 12th and 14th of March 1993. likely caused by weaknesses in the cloud and convection The GCM simulation was started from climatological con- schemes or by a too strong downdraft compensating the ditions and was continuously forced to the analyzed state by frontal lifting and suppressing the vertical transport of mois- a thermodynamical adjustment based on the Newtonian re- ture from the boundary layer to higher levels. This work laxation technique (nudging). Relaxation terms for vorticity, demonstrates for the investigated case the value of simulating divergence, temperature, and the logarithm of surface pres- single weather events in climate models for validating model sure were added at each model level and time step. The physics. necessary forcing files were calculated from the ECMWF re-analysis (ERA15). No nudging terms were added for the components of the water cycle. Using this forcing, the model was able to reproduce the 1. Introduction synoptic-scale features and its temporal development realis- tically after a spin-up period. This is true even for quantities Usually, validation of GCMs is performed for that are not adjusted to the analysis (e.g., humidity). Detailed long-term averages, for example by comparing comparisons of the model simulations with available obser- monthly mean values of temperatures or 500 hPa vations and the forcing ERA15 were performed for the cy- clone case. Systematic errors were detected in the simulation geopotential heights. These results have been ap- of the thermodynamic state of the atmosphere, which can be plied for ‘‘tuning’’ of parameterization schemes traced back to deficiencies in model parametrizations. in order to obtain a satisfactory large-scale re- Differences in the representation of the surface fluxes lead sponse (Moncrieff 1995). This tuning and the temporal averaging hide model weaknesses oc- curring on shorter temporal and smaller spatial Correspondence: Hans-Stefan Bauer, Institute of Physics and Meteorology, University of Hohenheim, Garbenstraße 30, 70599 scales. Long integration periods are necessary Stuttgart, Germany (E-mail: [email protected]) to create such climatologies and corresponding 130 H.-S. Bauer et al. observational data is only available for some mospheric state, which should be as accurate as of the interesting quantities. It is essential that possible. This analysis is produced by a data as- GCMs are not only able to produce correct similation system, which combines the infor- long-term averages but also the statistics of mation of an earlier model forecast with more atmospheric variables due to weather events, par- recently measured observations of the atmo- ticularly with respect to the simulation of region- spheric variables. al water cycles. Only if the latter performance is In our study we use a slightly different ap- acceptable, GCMs can be used as a guidance for proach. The simulation was started from a cli- decision making, e.g., for the change of water matological state and was continuously forced management due to climate change. to the observed state using a four-dimensional So far, in the majority of studies concerning data assimilation (FDDA) approach based on the validation of ECHAM4, as for many other the ‘‘Newtonian relaxation technique’’ (nudging) climate models, the climatological validation of (Krishnamurti et al. 1991; Jeuken et al. 1996). long-term averages was preferred. Only a few It forces the model thermodynamics to the ob- studies dealt with the simulation of single weath- served state by adding a non-physical relaxation er events. One of the first who discussed the evo- term to the model equations. Such a dynamical lution of an Atlantic cyclone in a GCM was adjustment reduces the model spin-up charac- Gilchrist (1971). He used a five-layer model with teristics compared to other data assimilation a horizontal resolution of 5 longitude and 3 schemes, since it takes place at every time step latitude, which covered the northern hemisphere. rather than every few hours. This ensures a more Despite the coarse resolution, many typical syn- gradual forcing of the model. optic-scale features during the life-cycle were In our study, only the model thermodynamics reproduced realistically. For the ECHAM model, is relaxed so that all components of the water first studies concentrating on the development cycle can develop freely. We evaluate how this of synoptic-scale systems were carried out by relaxation process forces the model to the ‘‘real’’ Bengtsson et al. (1995) and Chen and Roeckner thermodynamic state as prescribed by the (1997). The former investigated the representa- ECMWF re-analysis (hereafter ERA15). To be tion of hurricane-type vortices in the ECHAM3 able to separate errors caused by the initialization model and found a good representation of such procedure from errors in the model representa- systems in the evolving model climate. Chen tion of the dynamical and physical processes, and Roeckner (1997) validated the represen- we use ERA15 as a surrogate for the ‘‘true’’ tation of clouds during the development of an atmospheric state. Differences between the re- average North Atlantic cyclone in ECHAM4. analysis and the ECHAM4 simulation are then Their average cyclone was calculated from most likely caused by model differences. In ad- an ensemble of systems developed during one dition, comparisons with observations are used to winter season. study whether remaining deviations in the ther- In our work, the ability of ECHAM4 to repro- modynamic fields are due to the nudging tech- duce a synoptic-scale weather system is investi- nique itself, or due to model initialization. It will gated in more detail, using a low computational be shown that our approach can indeed be used effort and permitting the validation of more quan- to detect deficiencies in boundary layer and con- tities without the need of long-term averages. We vection parameterizations. Deviations in the rep- study the temporal evolution of a distinct syn- resentation of the components of the water cycle optic system using a high temporal and spatial are discussed in more detail in a companion resolution (from the point of view of climatolog- paper. ical applications). As synoptic system, the development of an In order to simulate a single weather event that extratropical cyclone along the East Coast of can be compared with synoptic data, it is essen- the U.S.A. between the 12th and 14th of March tial that the model simulation is forced to the 1993 was chosen. This system was selected since observed state. In numerical weather prediction it is one of the best documented cases of that (NWP), this is usually achieved by starting the kind in literature. Its development was predicted simulation with a representation of the initial at- accurately by the models of the different fore- The representation of a synoptic-scale weather system 131 casting centers. The latter point was very impor- al days, we had to apply the continuous nudging tant for the selection of the case since we use approach. ERA15 as a surrogate for the ‘‘true’’ atmospheric In Sect. 2, a brief description of the main set-up state. of the model is given. Furthermore, the adjust- In addition, we mention results from a second ment technique, its advantages and disadvantages, synoptic situation that was investigated using this and the influence on the model thermodynamics approach. Between the 15th and 23rd of February are described. In Sect. 3, we investigate the 1993 a blocking anticyclone developed over the representation of the selected system in the central and eastern North Atlantic. For this sit- ECHAM4 model. First, a general introduction uation we focus on relating the results to the into East Coast Storms is given, before the syn- findings for the cyclone development along the optic development of the selected case is more East Coast of the USA. closely investigated. The validation is divided The following scientific questions are addressed: into two parts. In this paper the representation of the thermodynamic state during the develop- – Can ECHAM4 be forced to the analyzed ther- ment of the system is investigated to assess the modynamical evolution of such systems? general usability of the proposed approach for – What systematic differences in thermodynamic validation purposes. The representation of the fields occur during the evolution of the system? hydrological cycle in general, and clouds in – How far are these deviations due to the nudg- particular, as well as sensitivity studies are in- ing technique, due to errors in initial fields, vestigated in a companion paper. In Sect. 4, we and due to model physics? finish with a summary and some concluding – What are the most important problems in the remarks. representation of model physics related to thermodynamical processes? Recently, work to validate climate models in 2. Technical introduction ‘‘numerical weather prediction mode’’ was per- formed by Phillips et al.