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https://doi.org/10.5194/esd-2019-51
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/esd-2019-51
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 18 Dec 2019

Submitted as: research article | 18 Dec 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Earth System Dynamics (ESD).

Mesoscale atmospheric circulation controls of local meteorological elevation gradients on Kersten Glacier near Kilimanjaro summit

Thomas Mölg1, Douglas R. Hardy2, Emily Collier1, Elena Kropač1, Christina Schmid1, Nicolas J. Cullen3, Georg Kaser4, Rainer Prinz4, and Michael Winkler5 Thomas Mölg et al.
  • 1Climate System Research Group, Institute of Geography, Friedrich-Alexander-University Erlangen-Nürnberg (FAU), Germany
  • 2Department of Geosciences, University of Massachusetts, Amherst, USA
  • 3Schoolof Geography, University of Otago, Dunedin, New Zealand
  • 4Department of Atmospheric and Cryospheric Sciences (ACINN), University of Innsbruck, Austria
  • 5Zentralanstalt für Meteorologie und Geodynamik (ZAMG), Austria

Abstract. Elevation gradients of meteorological variables in mountains are of interest to a number of scientific disciplines and often required as parameters in modeling frameworks. Measurements of such gradients on glaciers, however, are particularly scarce and strongly skewed towards the mid latitudes and valley glaciers. This article adds a tropical perspective and presents four years of overlapping measurements at 5603 m and 5873 m on Kersten Glacier, Kilimanjaro (East Africa), between 2009 and 2013. Mean gradients in near-surface air temperature (T), water vapor pressure (VP) and snow accumulation (ACC) per 100 m elevation are −0.75 °C, −0.16 hPa and −114 ± 16 mm w.e. per year, respectively. An intriguing feature is a strong diurnal cycle of the T and VP gradients, which are (depending on season) 2–4 times larger between early and late morning than in the hours of weak gradients. The ACC decrease with elevation, furthermore, is mostly the result of a lower frequency of ACC events at the upper measurement site and not due to contrasting amounts at the two altitudes during events. A novel facet of our study is to link the measured on-glacier gradients to a high-resolution atmospheric modeling data set, which reveals the importance of the mesoscale atmospheric circulation. A thermally direct circulation is established over the mountain in response to diabatic surface heating/cooling with upslope flow during the day and downslope flow in the night. This persistent circulation communicates heat and moisture changes in the lowlands to the higher elevations during morning and early afternoon, which is evident in the advection patterns of potential temperature and VP, and shapes the time-variability of gradients as recorded by our weather stations on the glacier. A few local processes seem to matter as well (glacier sublimation, turbulent heat fluxes), yet they show a secondary influence only during limited time windows. Atmospheric model data also demonstrate that declining moist entropy and water vapor fluxes in the summit zone favor formation of the negative ACC gradient. The results extend the empirical basis of elevation gradients in high mountains, and in particular over glacier surfaces, by the unusual case of a slope glacier on an equatorial, free-standing massif. Our measurement/model link, moreover, demonstrates an approach for future studies to put observations of elevation gradients more systematically in a multiscale process context.

Thomas Mölg et al.
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Short summary
The glaciers on Kilimanjaro summit are unique sample spots of the climate in the tropical mid-troposphere. Measurements of air temperature, air humidity, and precipitation with high-altitude weather stations show that the differences in these meteorological elements between two elevations (~ 5600 and ~ 5900 m) vary significantly over the day and the seasons, in concert with the air flow dynamics around the mountain. Knowledge of these variations will improve atmosphere/cryosphere models.
The glaciers on Kilimanjaro summit are unique sample spots of the climate in the tropical...
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