This contribution was motivated by a certain discrepancy between some of the goals of MAP concerning the boundary layer over the Alpine region on one hand, and the rather sparse efforts that have been made so far to develop ideas and concepts to meet these goals on the other hand. Particularly, it is stated in the MAP design proposal (Binder and Schär, 1995) that turbulent fluxes of mass, momentum, heat and moisture should be investigated with respect to their temporal and spatial evolution over the (entire) Alpine topography. However, the present knowledge on the structure, the temporal evolution and the exchange processes within the atmospheric boundary layer is restricted to either horizontally homogeneous and stationary conditions (It was pointed out in the talk of William Blumen that not even the concepts for the homogeneous and equilibrium boundary layer are completely settled and accepted at present.), or to certain ideal settings of inhomogeneity, such as a small scale topography of `cosine type' (e.g., Xu and Taylor, 1995) or a structured surface patchiness (Schmid and Bünzli, 1995). Over the Alpine range, effects of such surface irregularities (which have a vertical extension that is generally much smaller than the height of the boundary layer itself) will not only have to be studied at the same time and taking the non-idealized, `real' inhomogeneity into account: also, the meso-scale modification of the turbulence structure and evolution of the Alpine boundary layer as a whole due to non-equilibrium conditions (a horizontal mean pressure gradient, the mean surface temperature varying due to elevation changes - among others) will have to be considered.
In view of these difficulties, one might argue that it is too early to attack the problem of developing concepts for and perform observations on the problem of turbulent exchange (and the boundary layer structure as a whole) over a topography as complex as the Alpine. However, the position is taken here that the current knowledge on turbulent exchange processes over complex terrain is sufficient that meaningful experiments and modeling studies can be planned and accomplished in order to make a step forward towards the boundary layer related goals of MAP. Even if it is not likely that these goals can be met in completeness as formulated in the design proposal, efforts should nevertheless be made to increase the knowledge and understanding of turbulent exchange processes over the Alpine region. Particular emphasis should thereby be put on the problem of identifying potential parameterizations for turbulent exchange (and boundary layer processes) in meso-scale numerical models and the treatment of the subgrid scale variability of the topography and the surface properties.
For the design of potential observational studies in order to provide information on the surface turbulent fluxes (i.e. the spatial distribution of turbulent fluxes within a `grid box' and the resulting effective fluxes) it will have to be kept in mind that the lowest atmospheric layer over a rough surface such as over most of the Alpine range is not characterised by surface layer similarity. Rather, a so-called roughness sublayer will cover the lower part of the surface layer, wherein the flow and turbulence structure is fully three-dimensional and affected by single roughness elements. This means that `surface fluxes' cannot simply be measured close to the surface and assumed to be representative for a larger area. Instead, profiles of turbulent fluxes will have to be measured within the roughness sublayer with a certain, relatively dense horizontal resolution, that takes the `source area' (area of influence for a measurement at a certain height, also called footprint) for the observations into account. Since it will not be possible to cover the whole domain of interest with such a network of observations, it is suggested to select a number of characteristic topographic features of the Alpine range and to investigate the turbulence characteristics and boundary layer structure for these entities in some detail.
Discussions among a few of the participants
of the meeting in Bad Toelz concerning the above mentioned issues
led to the formation of an
Informal MAP-Working Group on Boundary Layer ProcessesThis yet informal group is interested to initiate, promote and support boundarylayer studies in the framework of MAP. It is open to everybody interested in the boundary layer over the Alps. The work of this group is based on the Supporting Objective 2.4.2 of the MAP Design Proposal (p.26f) and will start its work via communication on the INTERNET. Tentative sequence of work:
Every participant in this informal group is invited to work at point 1 of the above list by making contributions to a draft document that can be obtained from Stefan Emeis (emeis@ifu.fhg.de). He will integrate the submissions into the document and redistribute it. Ideally there would be an abstract to each topic with a few key references describing the present state of the art. If this document has reached some (yet to be defined) degree of completeness we could proceed to point 2 of the list. |
Binder, P. and Ch. Schär (Eds.), 1995: Mesoscale Alpine Programme, Design Proposal 1/95
Xu, D. and P.A. Taylor, 1995: Boundary-Layer Parameterizations of drag over small-scale topography, Quarterly J. Roy Meteorol. Soc., 121, 433-444.
Schmid, H.P. and D. Buenzli, 1995: The influence of surface texture
on the effective roughness length, Quarterly J. Roy Meteorol.
Soc., 121, 1-22.
MAP Data Centre - April '05 - MAP WebMaster