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. The mass balance of very small glaciers is often governed byanomalous snow accumulation, winter precipitation being multiplied by snowredistribution processes (gravitationally or wind driven), or suppressed snowablation driven by micrometeorological effects lowering net radiation and/orturbulent heat exchange. In this case study, we analysed the relativecontribution of snow accumulation and ablation processes governing the long-and short-term mass balance of the lowest perennial ice field of the Alps,the Ice Chapel, located at 870 m a.s.l. in the Berchtesgaden NationalPark (Germany). This study emphasizes the importance of the localtopographic setting for the survival of a perennial ice field located farbelow the climatic snow line. Although long-term mass balance measurements ofthe ice field surface showed a dramatic mass loss between 1973 and 2014, theice field mass balance was rather stable between 2014 and 2017 and evenshowed a strong mass gain in 2017/2018 with an increase in surface height by50 %–100 % relative to the ice field thickness. Measurements suggestthat the winter mass balance clearly dominated the annual mass balance. Atthe Ice Chapel surface, 92 % of snow accumulation was gained by snowavalanching, thus clearly governing the 2017/2018 winter mass balance of theice field with mean snow depths of 32 m at the end of the accumulationperiod. Avalanche deposition was amplified by preferential deposition ofsnowfall in the wind-sheltered rock face surrounding the ice field. Detailed micrometeorological measurements combined with a numerical analysisof the small-scale near-surface atmospheric flow field identified themicrometeorological processes driving the energy balance of the ice field.Measurements revealed a katabatic flow system draining down the ice fieldthroughout the day, showing strong temporal and spatial dynamics. Thespatial origin of the thermal flow system was shown to be of particularimportance for the ice field surface energy balance. Numerical simulationindicates that deep katabatic flows, which developed at higher-elevation shadedareas of the rock face and drained down the ice field, enhance sensible heatexchange towards the ice field surface by enhancing turbulence close to theice surface. Conversely, the shallow katabatic flow developing at the icefield surface appeared to laterally decouple the local near-surfaceatmosphere from the warmer adjacent air suppressing heat exchange. Numericalresults thus suggest that shallow katabatic flows driven by the coolingeffect of the ice field surface are especially efficient in lowering theclimatic sensitivity of the ice field to the surrounding rising airtemperatures. Such micrometeorological phenomena must be taken into accountwhen calculating mass and energy balances of very small glaciers orperennial ice fields at elevations far below the climatic snow line.

Avalanches and micrometeorology driving mass and energy balance of the lowest perennial ice field of the Alps: a case study