Last updated: November 2010
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Atmospheric processes such as the development of the boundary layer, low level cloudiness, and the onset and intensity of convection are significantly influenced by the evolution of the lower boundary conditions. Consequently, an accurate representation of exchange processes between the soil and the atmosphere is essential to produce high quality NWP forecasts.
In recent years many activities concerning the lower boundary of the COSMO model have been undertaken, but most of them lack some final efforts to bring them into operations and to make them accessible to the whole COSMO community. Without giving support, COSMO runs the risk of loosing the benefit of recent developments or of repeating the same developments at different centres.
It is the main goal of this project to incorporate all activities related to the lower boundary conditions which have already reached an advanced state, and to consolidate these developments into well tested and documented software packages readily usable by the COSMO community.
Because the relative importance of the proposed tasks may differ for different users or applications, all activities for which resources are available have been included. This will benefit the model versatility and improve its attractiveness.
The soil vegetation atmosphere transfer (SVAT) scheme TERRA and the associated external parameters provide the terrestrial lower boundary conditions for the atmospheric component of the COSMO model. Driven by incoming solar radiation, the evolution of the lower boundary conditions influence the exchange of heat, moisture, and momentum between the surface and the atmosphere and has a large impact on the simulation of near surface weather parameters. It also affects atmospheric processes, such as the development of the boundary layer, low level cloudiness, and the onset and intensity of convection.
The COSMO model uses a multilayer SVAT model with a direct solution of the heat conduction equation and considers moisture transport due to hydraulic processes within the soil and the effects of transpiration by plants. Phase change processes of soil water (freezing/thawing of soil water/ice) are incorporated in the scheme both for their thermodynamic effect and for their impact on the hydraulic properties of the soil. The COSMO model employs a single layer snow module considering melting of a snow pack with prognostic snow density and time dependent snow albedo.
Current uncertainties in the description and simulation of surface characteristics are partly caused by well known problems. A critical issue is the limited knowledge of the physical properties of the surface in the form of the so-called external parameters (e.g. soil type, root depth). These parameters play a vital role, e.g. in the simulation of the hydrological state, but both the quality of the raw databases and the methods used to derive the required model properties from these databases are not optimal. Moreover, the increase in horizontal resolution of current NWP models implies that some surface types previously neglected, such as lake and urban regions, are becoming more important.
The simulation of the evolution of the soil is also hampered by inadequate knowledge of the initial state. Unrealistic drifts in soil moisture are ubiquitous in most NWP models, due to both incorrect representation of soil processes and the accumulation of precipitation errors (Scipal and Drusch, 2007). As already demonstrated for the global model GME, an improvement may be achieved through a change in the root density distribution together with the introduction of seasonal vegetation parameters (Ritter, 2007).
Due to the non-linear relations between surface parameters and fluxes, neglecting subgrid-scale land-surface heterogeneities by using average external parameters, may result in erroneous flux estimates. Studies have shown that, even with a grid spacing of only a few kilometres, considerable aggregation effects may indeed occur and must be corrected for. This can be tackled by introducing one of the so-called tile or mosaic approaches: the surface within a coarse scale atmospheric grid box is subdivided into patches with assumed homogeneous surface characteristics, fluxes are computed for each patch separately and then averaged.
The current representation of the snow does not allow liquid water to stay within the snow pack, which is a severe limitation in spring when the freezing of trapped water during the night slows down the melting of the snow pack. Furthermore, many local effects such as snow transport by wind and influence of slope orientation make a correct representation of the snow cover particularly difficult.
Many of the goals defined at the start of the project have been reached. However, some topics still require some work to reach a production standard (e.g. multi-layers snow model, parameterization of sub-grid scale heterogeneities) and some new promising developments have been started in the frame of COLOBOC which would further benefit from the focus brought by a priority project (e.g. observation data pool, different topics related to external parameters, TERRA consolidation).
After consultation of the interested partners, it has been decided to propose to the COSMO StC a one year extension of this project.
These are the seven proposed actions of the priority project:
Work has been separated into these distinct tasks:
| Task L | Project leadership |
| Task 0 | Document observations sets available for SVAT-model validation |
| Task 1 | Tools - Consolidation of TERRA standalone code |
| Task 2 | Tools - Software for generating external parameters |
| Task 3 | Revision of external parameters (Raw data sources for the generation of external parameter for the numerical weather prediction models COSMO and GME) |
| Task 4 | Revision of TERRA and the associated look-up tables |
| Task 5 | Revision of snow representation |
| Task 6 | Urban model |
| Task 7 | Parameterization of land surface heterogeneity by the tile/mosaic approach |
| CLM community | will@tu-cottbus.de, sonia.seneviratne@env.ethz.ch, edouard.davin@env.ethz.ch, vielib@student.ethz.ch |
| DWD | Juergen.Helmert@dwd.de, Hermann.asensio@dwd.de, Gerd.Vogel@dwd.de Ekaterina.Machulskaya@dwd.de, Claudia.Heret@dwd.de, Frank.Beyrich@dwd.de, Martin.Lange@dwd.de |
| IMGW | Grzegorz.Duniec@imgw.pl , Andrzej.Mazur@imgw.pl |
| MCH | jean-marie.bettems@meteoswiss.ch, guy.demorsier@meteoswiss.ch, matteo.buzzi@meteoswiss.ch, reto.stoeckli@meteoswiss.ch , oliver.fuhrer@meteoswiss.ch |
| RHM | gdaly.rivin@mecom.ru , kaza4ok-87@mail.ru , rozin2004@mail.ru , alla.yurova@gmail.com |
| Heinemann G, Kerschgens M. | 2005 | Comparison of methods for area-averaging surface energy fluxes over heterogeneous land surfaces using high-resolution non-hydrostatic simulations. Int J Clim 25:379–403 |
| Ritter, B., Einführung GME-Version 2.13 | 2007 | Änderungsmitteilung operationelles NWV-System Deutscher Wetterdienst, Offenbach |
| Scipal, K. and Drusch, M. | 2007 | Assimilation of scatterometer derived soil moisture in the ECMWF land surface scheme Geophysical Research Abstracts,Vol. 9, 01822, 2007 |