Flow over and around Hofsjökull
Final Report Abstract
The data obtained during the field campaign were collected and assembled by R. Kühnel within his work on a diploma thesis. The data are available. The paper "Transience in mountain induced gravity waves: Hofsjokull" by J. Egger and R. Kühnel (see Appendix II) describes first results from the permanent stations. Main findings are that there are relatively slow oscillations with periods of - 3 hours at individual stations which form, however, rarely a 'global1 pattern covering the mountain domain. Principal oscillation pattern (POP) analysis shows that such global patterns are almost stationary. The pressure at individual stations reacts to changes of the mean wind but these responses are more complicated than one would expect on the basis of simple linear arguments, namely, a fairly fast adaptation towards an upslope high and downslope low. There are indications that the mountain drag affects the zonal near surface flow. Surprisingly, a conspicuous high pressure anomaly is found in the valley leading from the glacier towards Ingolfsskali both for southerly and northerly mean flows. Katabatic flow events along this valley have been investigated and a progression of katabatic regimes has been found although the events tend to be of only moderate strength. The katabatic winds are reasonably strong and frequent at the edge of the glacier nearly all the time. Late in the night (06 UTC) they are almost absent at a distance of less than 6 km (site N3b) downstream. In the afternoon (15 UTC), the katabatic winds often reach the site N3b, but not N3c, indicating that they typically reach only a distance of 6-10 km from the edge of the glacier. Indications for the katabatic flow at site N3b, about 5 km from the edge of the glacier, can also be found in form of "Microfronts" recorded by the energy balance station at site IM3b. With a distinct turn in wind direction, colder and drier air with higher C02 content is reaching the station. During clear nights, such events have been observed periodically with a repeat time of 30-60 min. For the SUMO system the FLOHOF campaign was the first and environmental test. Atmospheric profiles of temperature, humidity and wind could be obtained up to more than 3000 m above ground. First high resolution WRF simulations have been conducted for days with SUMO ascents during the campaign. Together with the wind profiles from pilot balloons and the tethersonde, these model simulations will be used to test a newly developed method for the derivation of wind profiles from GPS data of the SUMO autopilot These tests yielded quite satisfactory and promising results. First results are displayed at a poster (see appendix II) shown at this year's EGS meeting in Vienna and in the paper submitted to Earth and Environmental Science, Three more papers are contained in Appendix II. One of them (accepted by Met. Zeitschrift) is concerned with effect of variable surface winds on the temperature perturbation near the ground as far as they are dynamically generated. Another one (also submitted to Met Zeitschrift) deals with the effect of irregular station distribution on the result of the POP analysis. A third one (submitted to Earth and Environmental Science) describes the SUMO system and first results from the FLOHOF campaign. The diploma thesis of R. Kühnel is not yet completed but will also contain some results from linear modeling in addition to data analysis results. The interpretation of the atmospheric profiles measured during FLOHOF will also be an important part of the ongoing PhD project of Stephanie Mayer at GIB.
Publications
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J. Reuder et al., SUMO: A Small Unmanned Meteorological Observer for atmospheric boundary layer research. Invited talk for the ISARS 14th International Symposium for the Advancement of Boundary Layer Remote Sensing, 23-25 June 2008, Rise, Denmark
