Abstract
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 midlatitudes and valley glaciers. This article adds a tropical perspective and presents 4 years of overlapping measurements at 5603 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. yr−1, 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 recorded 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 the linking of 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 or 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 in 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, 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.
Highlights
As the world’s mountain glaciers are shrinking increasingly fast, employment and development of glacier-wide mass balance models have intensified over the past decade
From here on we report all elevation gradients per 100 m, which seems practical with regard to the altitude extent of most present mountain glaciers and distinguishes them from micrometeorological gradients that are usually given per meter (e.g., Greuell and Böhm, 1998)
Four years of overlapping automatic weather stations (AWSs) measurements at 5603 and 5873 m on Kersten Glacier (2009–2013) provide a great opportunity to extend our empirical knowledge of elevation gradients in temperature, humidity, and precipitation to a special environment, characterized by its equatorial location and a massive free-standing mountain, Kilimanjaro
Summary
As the world’s mountain glaciers are shrinking increasingly fast, employment and development of glacier-wide mass balance models have intensified over the past decade. These models were realized for the local (e.g., Klok and Oerlemans, 2002; Mölg et al, 2009a), regional (e.g., Machguth et al, 2009; Farinotti et al, 2015), and global scales (e.g., Radicand Hock, 2011; Marzeion et al, 2012). Since that requirement involves challenges (e.g., model setups, retrieval algorithms, and record length), there is no question that ground measurements are the centerpiece for advancing our understanding of the given problem
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