Habilitation
Identification of micrometeorological parameters controlling the energy- and matter exchange of polar, high-arctic ecosystems
Johannes Lüers (06/2007-01/2014)
To observe and to assess the current and near-future climate change in the Arctic and higharctic tundra ecosystems it requires improvements in weather and permafrost monitoring and water and carbon cycle research, accomplished by setting up long-term observation sites with high-quality in-situ measurements of meteorological, soil/snow/ice physical and biological or plant physiological parameters, and of process orientated flux-parameters at any scale, to find the relevant driving forces of the exchange and cycles of momentum, heat, water and trace gases or elements in Arctic landscapes.
This energy and matter exchange in polar environments and the crucial transfer functions between the surface and atmospheric boundary layer and the energy balance of sea-ice and tundra ecosystems – is still only partly understood.
The work summarized in this Habilitation will take part in that survey to identify meteorological parameters controlling the energy and matter exchange of high-arctic ecosystems.
This work based on two own measurement campaigns performed in the Kongsfjord region at the west coast of the Svalbard Island (78°55'N, 11°55'E). The first was the Arctic-Turbulence- Experiment during the winter/spring transition 2006 at Ny-Ålesund (ARCTEX-2006), and second, the Arctic-Turbulence-Experiment in the summer of 2009 located in the Bayelva River catchment (ARCTEX-2009), both applying in-situ Ultrasonic-Anemometry and Laser- Scintillometry. In addition, the long-term database of the permanent, high-arctic observation sites of the Alfred Wegener Institute for Polar- and Marine Research (AWI) was used for this research.
Mainly forced by the human activity the ecosphere of the earth’s polar ice caps will undergo a severe change. Especially the observed and expected changes of the atmosphere will force the other ecospheres, most of all the arctic cryosphere and biosphere, to react.
Therefore, the major objective of this thesis is to identify some of the relevant meteorological parameters controlling the energy and matter exchange of high-arctic ecosystems. To fulfill that survey long-term monitoring and observation of the atmosphere and the permafrost soil, and of the carbon-cycle, which is representing the biological activity in the broad sense, is requested.
To be more precise it is necessary
- to measure the energy and matter fluxes if possible directly or otherwise indirectly,
- to describe the transfer functions between soil, water, snow/ice, vegetation and atmosphere,
- to estimate seasonal or annual balances or budgets,
- to improve or to find appropriate parameterizations of the key processes,
- to contribute to (ecosphere) models, forecasts, scenarios or climate projections.
The work summarized in this thesis tries to follow this roadmap of goals.
Considering the process understanding the ARCTEX and Bayelva datasets help to identify and to understand some of the relevant meteorological parameters controlling the energy and matter exchange of high-arctic ecosystems. The frequent effect of the air mass change and the shift of the prevailing wind direction down (SE) or up (NW) the Kongsfjord valley, forced by the change of the macro- and meso-scale atmospheric circulation pattern is interfering with the near surface atmospheric exchange conditions. This frequent wind field change in the arctic spring and snow melt season specifically could causes short but developed unstable situations or even free-convection events mostly around noon or early afternoon.
These short periods can have a positive feedback and a growing contribution to the snow and ice melting processes during spring if the change of the tundra surface condition from a snow covered landscape and from still water saturated soils to snow free periods and dry soils is supported by such convection events which allow greater sensible and latent heat fluxes than usual. Another example linked to changes in weather and air mass transitions is that during the snow covered winter/spring season a considerable CO2-exchange through the snow-pack can occur. Remarkable CO2-fluxes in both directions are observable especially during strong, rapid air pressures changes, with high wind speed and advection of, relative to the snow-pack, CO2- enriched or CO2-depleted air masses.
But not only is the annual carbon budget of interest, which is close to zero. More important are the strong seasonal signals of the CO2-exchange, which have a great influence to the whole Arctic and therefore to the global ecosphere matter cycles and their appropriate parameterization in ecosphere models. Furthermore, based on the results of the analysis of the annual surface energy budget of a high-arctic permafrost tundra ecosystem the energy allocation, for example during the snow melting season, could be quantified.
In addition, the existence of an untypical temperature profile close to the surface in the Arctic spring could among other effects be proven to be one of the major issues hindering estimation of the appropriate ground surface temperature. Results of a comparison of different sensible heat-flux parameterizations with direct measurements indicate that the use of the semi-analytical Launiainen & Cheng (1995) approach together with an iterative hydrodynamic bulk profile model achieves enough accuracy for heat flux calculations as it reliably reproduces the temporal variability of the surface temperature. This allows also an applicable gap-filling of the heat fluxes QH and QE.