Validation and analysis of regional present-day climate and climate change simulations over Europe

In the European Commission (EC) project "Regionalization of Anthropogenic Climate Change Simulations, RACCS, recently terminated, 11 European institutions have carried out tests of dynamical and statistical regionalization techniques. The outcome of the "dynamical part" of the project, utilizing a series of high resolution LAMs and a variable resolution global model (all of which we shall refer to as RCMs, Regional Climate Models), is presented here. The per- formance of the dqterent LAMs had first, in a preceding EC project, been tested with "perfect" boundary forcing fields (ECMWF analyses) and also multi-year present-day climate simula- tions with AMIP "perfect ocean " or mixed layer ocean GCM boundary conditions had been validated against available climatological data. The present report involves results of vali- dation and analysis of RCM present-day climate simulations and anthropogenic climate change experiments. Multi-year (5 - 30 years) present-day climate simulations have been per- formed with resolutions between 19 and 70 km (grid lengths) and with boundary conditions from the newest CGCM simulations. The climate change experiments involve various 2xCO2 - ]xCO2 transient greenhouse gas experiments and in one case also changing sulphur aerosols. A common validation and inter-comparison was made at the coordinating institution, MPIfor Meteorology. The validation of the present-day climate simulations shows the importance of systematic errors in the low level general circulation. Such errors seem to induce large errors in precipitation and surface air temperature in the RCMs as well as in the CGCMs providing boundary conditions. Over Europe the field of systematic errors in the mean sea level pressure (MSLP) usually involve an area of too low pressure, often in the form of an east-west trough across Europe with too high pressure to the north and south. New storm-track analyses confirm that the areas of too low pressure are caused by enhanced cyclonic activity and similarly that the areas of too high pressure are caused by reduced such activity. The precise location and strength of the extremes in the MSLP error field seems to be dependent on the physical param- eterization package used. In model pairs sharing the same package the area of too low pressure is deepened further in the RCM compared to the corresponding CGCM, indicating an increase of the excessive cyclonic activity with increasing resolution. From the experiments performed it seems not possible to decide to what extent the systematic errors in the general circulation are the result of local errors in the physical parameterization schemes or remote errors trans- mitted to the European region via the boundary conditions. Additional errors in precipitation and temperature seems to be due to direct local effects of errors in certain parameterization schemes and errors in the SSTs taken from the CGCMs. For all seasons many biases are fOund to be statistically significant compared to estimates of the internal model variability of the time- slice mean values. In the climate change experiments statistically significant European mean temperature changes which are large compared to the corresponding biases are found. How- ever, the changes in the deviations from the European mean temperature as well as the changes in precipitation are only partly sign wcan ce and are of the same order of magnitude or smaller than the corresponding biases found in the present-day climate simulations. Cases of an inter- action between the systematic model errors and the radiative forcing show that generally the errors are not canceling out when the changes are computed. Therefore, reliable regional cli- mate changes can only be achieved after model improvements which reduce their systematic errors sufficiently. Also in future RCM experiments sujiciently long time-slices must be used in order to obtain statistically sign ijicant climate changes on the sub-continental scale aimed at with the present regionalization technique

Validation of present-day regional climate simulations over Europe: nested LAM and variable resolution global model simulations with observed or mixed layer ocean boundary conditions

Multi-year high resolution present-day climate simulations were made with two limited area models (LAMs) at UKMO and MPI and with a global variable resolution spectral model at Meteo-France. We shall refer to these models as the regional climate models (RCMs). Together with the RCM simulations we verify the similar multi-year simulations made with the corresponding coarse resolution global models. We refer to these models as the GCMs. They are the two coarse resolution GCMs whose output were used for boundary conditions to the LAM simulations and a homogeneous coarse resolution version (T42) of the Meteo-France GCM. In the Meteo-France and the MPI simulations observed (AMIP) SST and sea-ice distributions were used whereas in the UKMO simulations we used SST and sea-ice distributions determined from a mixed layer ocean model coupled to the GCM. In the present assessment the main emphasis is put on the validation of precipitation and surface air temperature simulations. The relatively large biases or systematic errors in these parameters in both the GCM and RCM simulations seem in most cases to be explained as the result of systematic errors in the surface pressure (or the low level flow) and the cyclone activity. In most remaining cases they seem to be due to defects in specific physical parameterization schemes. The UKMO and Meteo-France simulations are 10-year integrations whereas the MPI simulations are integrations of 46-months only

MPI Workshop on Semi-Lagrangian Methods

Conclusions of discussions: Following the presentations on the first day of the workshop a round-table discussion were held on the second day with the participants (explecitely) mentioned in the list above. It was concluded that the operational semi—Lagrangian models developed at CMC and ECMWF seemed to work satisfactory in weather forecast mode. Control Eulerian forecasts with a short time-step and semi—Lagrangian forecasts with relatively long time—steps had been found to give almost equivalent accuracy. So far they had not been tested in climate mode, i.e. with lower resolution in very long term integrations. The semi—Lagrangian version of the CCM2 had been developed at NCAR for applications in climate research, but so far it had been tested only in relatively short term integrations. It was decided to initiate coordinated experiments with the semi-Lagrangian schemes currently available at the three centres in order to test the suitability for the use in climate simulations. Each center should perform two five year integrations, an Eulerian control integration and a semi-Lagrangian integration, with similar resolution and sea—surface temperature (SST). CMC would run its model at resolution T63/L20, NCAR would run the CCM2 at T42/L18 and ECMWF would run at T63/Ll9. Annual—cycle climatological SST's and sea-ice distributions (AMIP) should be used and mass conservation would be (approximately) enforced by fixing the horizontal mean of In ps (pg beeing the surface pressure). The results obtained from the ECMWF experiments should be analysed at MP1. The cell-inte grated semi—Lagrangian scheme proposed by Machenhauer were found intriguing and it was recommended that this work should be continued as intended with tests of the scheme at first in a shallow water model. A coorporation'might be established with Rene Laprice and his student Andre Plante at the University of Quebec in Montreal who independently had developed and tested cell—integrated semi—Lagrangian schemes for horizontal advection of a passive scalar

The implementation of the semi-implicit scheme in cell-integrated semi-lagrangian models

The cell-integrated semi-Lagrangian method, in which trajectories from the corner points of a grid cell define its extent at a previous time, may be applied to a set of equations in Lagrangian form derived from the complete set of primitive equations to construct a numerical model which conserves exactly the discrete forms of the global integral constraints of mass, momentum, entrophy and total energy. Due to the fulfilment of these integral constraints one should expect the numerical model to be absolutely stable. Experiments with a simple one-dimensional shallow water model show, however, that a time step dependent instability develops when a CFL criterion for gravity waves is exceeded. Analysis of experiments with one-dimensional shallow water models unveils the mechanism of the instability. Using again for simplicity one-dimensional models a main achievement is a successful implementation of the semi-implicit time stepping scheme in the cell-integrated models. © 1997 Taylor Francis Group, LLC