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The Arctic can be seen as part of a European ''climate engine''. Understanding arctic climate processes is therefore a necessity for the understanding of climate variability in Europe. While the first German ACSYS project 1997-1999 was strongly concentrating on observational studies, the purpose of the present project is the effort to join the german capacities in arctic climate modelling because natural variability as well as model errors in the arctic tend to be large. Based on this general view there are three major aims of the joint program.

1. The measurement, the phenomenology and the modelling of atmospheric boundary layer (abl) processes with special emphasis upon the arctic temperature inversion and the modulation of abl processes through inhomogeneities of sea ice like leads and polynas. Simulations using micro- and mesoscale atmospheric models will be performed to derive an upscaling parametrization for use in large scale models.
2. The systematic model study using a set of regional (atmospheric amd coupled) climate models for the Arctic during the period of joint experiment between Greenland and Spitsbergen in late winter 2002/03. The experiment will provide a high resolution data set for the validation of models especially for the abl processes in the inner arctic and around Greenland.
3. The analysis of large scale water and energy cycle variations over the arctic cap based on reanalysis data, ECHAM4-T42 forced with observed sea surface temperatures and sea ice distributions during the 20th century and the extension and analysis of the ACSYS Arctic Precipitation Data Archive. This part of the project will also provide the large scale information necessary for the more regional climate studies during the selected periods.

In polar regions the exchange of energy and momentum between atmosphere and ocean is influenced by inhomogeneities of the sea ice cover such as leads and polynyas and by the variability of the sea ice roughness. This project aims at a better understanding of the meso- and small scale atmospheric processes caused by these inhomogeneities. The effect of sea ice inhomogeneities on the atmospheric boundary layer will be quantified and parameterizations will be improved/developed suitable for regional climate models such as REMO and HIRHAM. Parameterizations will be based on results obtained with the nonhydrostatic mesoscale model METRAS which will be run with different grid resolutions and for different sea ice scenarios.

Data obtained from different subprojects of the German ACSYS will be used for a validation of the model. Additionally, measurements will be carried out in the boundary-layer over leads in the pack ice region around Svalbard from helicopter and ship during Polarstern cruise ARK19 (spring 2003).

http://www.awi-bremerhaven.de/ATM/ACSYS/

Background:
The atmosphere over the Arctic sea ice is characterized by a nearly permanent temperature inversion. It regulates the exchange between the Arctic boundary layer and the free atmosphere and influences the turbulent, radiation and cloud processes within the boundary layer and thus the interaction between atmosphere and sea ice. Weather and climate models face large difficulties in forecasting reliably the atmospheric conditions over Arctic sea ice. On one hand climate models predict the largest rise in temperature in the Arctic, but on the other hand the Arctic is the region where models show the largest differences between each other. The insufficient simulation of Arctic inversion and boundary layer processes as well as their interaction may be a possible reason.

Objective:
During a field experiment in the region north of Spitsbergen in April 2003 a comprehensive data set will be collected in order to analyse and quantify the complex interaction between Arctic inversion and boundary layer processes. Within an area of about 200 km x 200 km the mean atmospheric conditions and the turbulent fluxes, radiation fluxes and cloud properties will be sampled applying the following platforms:

• research aircraft Falcon and Polar 2 (horizontal and vertical structure)
• research vessels Polarstern and Aranda (temporal variability)
• automatic Argos ice buoys (synoptic scale and sea ice drift)
• satellites NOAA-AVHRR, Radarsat and others (cloud and ice conditions).

The data set will be used to validate routine model analyses (ECMWF, German Met Office, HIRLAM) as well as the simulations by the models used in this ACSYS research compound.

http://wetter.dkrz.de/acsys

The understanding and quantification of interaction processes between atmosphere, ice and ocean on different spatial and temporal scales are major goals of the international ACSYS programme. The current project aims at contributing to these goals by investigating the interactions between the katabatic wind and the oceanic circulation and the sea ice in the coastal areas of Greenland, and by investigating the energy and momentum transfer over sea ice.

The main scientific goals are:
(1) Investigation of the forcing of the katabatic wind on the system sea ice/ocean in the coastal areas of Greenland
(2) Investigation of arctic inversions over sea ice and of transfer processes at the atmosphere-ice-ocean interface
(3) Validation of mesoscale model simulations and parameterizations

An aircraft-based field experiment is planned for spring 2003, mesoscale model simulations will be carried out on a scale of about 1km using a non-hydrostatic weather prediction model.

The first aim of this subproject is to improve our understanding of the interactions between atmosphere, ocean, and sea ice in the northern polar region on seasonal to decadal time scales. For this purpose, the high-resolution regional atmospheric climate model HIRHAM is to be coupled with the ocean-sea ice model MOM for an integration domain covering the whole Arctic. The application of a complex atmosphere-ocean-ice model with high resolution is required for a realistic representation of the interactions between its components. The model development is one focal point of the subproject.

A validation of the coupled model is to be done by comparing the model simulations with different observational data. On the one hand detailed measurements are available for the SHEBA (Surface Heat Budget of the Arctic Ocean) period (October, 1997 to October, 1998) from the SHEBA ice camp over the Beaufort/Chukchi Sea, from the station Barrow, as well as from extensive airplane and satellite data collected during this time. On the other hand a new observational data set for the inner Arctic will be provided by the ACSYS field experiment scheduled for March/April 2003.

The impact of a regionally varying ocean- and sea ice forcing on the atmospheric variability is to be analyzed by comparing several long-standing simulations of the coupled atmosphere-ocean-ice model with those of the pure atmosphere model HIRHAM. The results can be used to improve the model parameterizations for the exchange of energy, momentum, and water between atmosphere and ocean as well as to improve our understanding of regional feedbacks in the Arctic climate system and their relevance for climate changes. A second aim of this subproject is the investigation of the impact of a changed land surface/soil model on the simulation of the atmospheric circulation, temperature and precipitation, etc. For this, an alternative land surface parameterization (NCAR LSM) will be installed in HIRHAM. Long-standing HIRHAM runs with the old ECHAM3 LSM and the new NCAR LSM will be carried out and validated with available observational data. Particularly, the impact of soil freezing on the climate and possible feedbacks will be investigated.

Numerical simulations with regional climate models (REMO/LM) are planned to determine the mass balance of the Greenland ice sheet, including the contributions to the fresh water flux into the neighbouring oceans. For this purpose the parameterisation of the thermal and hydrological processes on land and on ice sheets will be improved or developed and implemented in the model. The reference run and the improved simulations will be compared and validated using field campaign data.

The main objective of APDA is to provide an observational basis for research on climatic changes related to the hydrological regime of the Arctic zone. The archive is operated by the Deutscher Wetterdienst on request of WCRP (ACSYS). The specific aims of this project comprise:

• Updating of the archive by additional precipitation and snow depth data from surface networks, satellite data and model results (ERA-40).
• Quality assessment based on statistical studies for the data being hold in the archive.
• Development of a method for using snow depth data for quality-control of precipitation data and complementation of precipitation analysis.
• Further development of the method for correction of precipitation data with regard to the systematic gauge measurement error.
• Verification of precipitation predicted by NWP re-analyses of ERA-40.
• Development of an improved large-scale precipitation climatology for the Arctic catchment area based on both precipitation and snow depth data.

APDA Homepage: http://www.dwd.de/en/FundE/Klima/KLIS/int/GPCC/Projects/APDA/index.htm

The understanding of the spatial-temporal variability of the hydrological cycle of the Arctic is one of the mean key issues of the ACSYS program. Aim of the subproject is to analyse and quantify the influence of large scale atmospheric phenomena like the AO or NAO upon the hydrological cycle in view of the observed long period NAO/AO changes during the last 20 years. The analysis will be done jointly using reanalysis data and model simulations of the ECHAM4-T42 model forced with observed sea surface temperatures and sea ice distribution of the 20th century. An ensemble of six runs differing solely in their initial conditions is available and can be used to distinguish random variability from forced one.

This project is funded by the German Federal Ministry of Education and Research under fund no. 03PL034.

The project is also described on the "Planet Earth" web pages under http:/www.planeterde.de/Members/huenken/BMBF%20Projekte/Projektinformationen/PI_UW_Acsys.