Atmospheric Controls of Snow Accumulation on Glaciers and Ice Caps in High Asia

Snowfall is the major contributor to snow accumulation on glaciers and ice caps. Unfortunately, its quantification is rather difficult, both by observations and by numerical modelling. Field measurements of snowfall are generally problematic, and particularly inaccurate in mountainous regions. This holds true also for data from remote sensing systems like the TRMM. Numerical modelling of precipitation in general, and of snowfall in particular, is depending on parameterization of sub-grid processes occurring at a wide range of spatial scales. The scarcity of reliable observational data on snowfall required to test and validate the relevant parameterization schemes is one of the major obstacles for deepening our understanding of atmospheric controls of snow accumulation on glaciers and ice caps. In addition, the often made assumption that easy-to-measure snow accumulation equals snowfall is not valid in areas where other processes like snowdrift or avalanches cause snow deposition or erosion. Besides a general discussion of the above-mentioned problems, the presentation will focus on results obtained from a gridded atmospheric data set, i.e., the so-called High Asia Refined analysis (HAR), covering the study region by two nested domains of 30 km and 10 km grid spacing. Starting from autumn 2000, three-hourly (30 km) and hourly (10 km) data are available for a comprehensive set of atmospheric variables (see www.klima.tu-berlin.de/HAR). HAR data was used to analyse annual and seasonal patterns of precipitation and atmospheric water transport, as well as to drive numerical models for surface mass balance of glaciers and ice sheets. A new study, which is the main subject of this presentation, reveals specific regimes of dynamic controls of precipitation in different regions of High Asia. One of the striking results is that the analysis identified a specific regime that is able to explain some of the atmospheric controls behind the so-called Karakoram anomaly (glaciers in this region show less negative or even positive mass balance than those in other regions), which has been highly debated in the scientific literature over the last years.