Downscaling of the global climate model data for the mass balance calculation of mountain glaciers

In this paper, we consider a hybrid method of downscaling of the GCM‑generated meteorological fields to the characteristic spatial resolution which is usually used for modeling of a single mountain glacier mass balance. The main purpose of the study is to develop a reliable forecasting method to evaluate future state of moun‑ tain glaciation under changing climatic conditions. The method consists of two stages. In the first or dynamical stage, we use results of calculations of the regional numerical model HadRM3P for the Black Sea‑Caspian region with a spatial resolution of 25 km [22]. Initial conditions for the HadRM3P were provided by the GCM devel‑ oped in the Institute of Numerical Mathematics of RAS (INMCM4) [18]. Calculations were carried out for two time periods: the present climate (1971–2000) and climate in the late 21st century (2071–2100) according to the scenario of greenhouse gas emissions RCP 8.5. On the second stage of downscaling, further regionalization is achieved by projecting of RCM‑generated data to the high‑resolution (25 m) digital altitude model in a domain enclosing a target glacier. Altitude gradients of the surface air temperature and precipitation were derived from the model data. Further on, both were corrected using data of observations. Incoming shortwave radiation was calculated in the mass balance model separately, taking into account characteristics of the slope, i.e. exposition and shading of each cell. Then, the method was tested for glaciers Marukh (Western Caucasus) and Jankuat (Central Caucasus), both for the present‑day and for future climates. At the end of the 21st century, the air tem‑ perature rise predicted for the summer months was calculated to be about 5–6 °C, and the result for the winter to be minus 2–3 °C. Change in annual precipitation is not significant, less than 10%. Increase in the total short‑ wave radiation will be about 5%. These changes will result in the fact that the snow line will be higher than the body of the glacier itself. This will inevitably cause degradation of the glacier and its gradual disappearance. The main contribution to the glacier shrinking and disappearance will be made by air temperature rise, because it will affect the change in the ratio of the areas of ablation and accumulation. Besides, a rise of temperature will increase the average melting season duration. These are, of cause, preliminary results just to illustrate how the downscaling method works. We did not take into account dynamic effects and gradual reducing of the glaciated area. In future, we plan to couple mass balance and dynamical models [17] and to adjoin them with downscaled climate change in order to account for transient glacier changes.