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Snow cover is an important part of cryosphere and the most seasonally changing land surface cover, which is sensitive to climate change. Previous studies showed that climate warming has already altered the extent and phenology of snow cover, which influences the plant phenology of the forest ecosystem. This research investigates the spatial distribution and temporal trend of snow cover in the forest area of mid and high latitude in the Northern Hemisphere (50°N-90°N,180°W-180°E) based on a satellite-derived snow dataset. Results showed that the spatial distribution of snow cover exhibits a latitudinal gradientin the mid and high latitudes of the Northern Hemisphere. The snow cover onset week (SCOW) and snow cover end week (SCEW) shortened significantly at a rate of 0.23 weeks/10 yr. and 0.48 weeks/10 yr., respectively (PPPPP

The immersed boundary method is applied to simulate the flow around a guide vane of hydro-turbine. The complex dynamic boundary effect caused by the moving guide vanes is modelled as the forcing term in the Navier-Stokes equation. Numerical simulation of threedimensional transient turbulence is conducted to flow around one moving guide vane of a benchmark hydro-turbine unit in a channel. The dynamical Smagorinsky-Lilly model of large eddy simulation is also used. The evolution process of the dynamic vortex structure is obtained by the interaction between the guide vane and the flow around vane to reveal the physical mechanism of wake oscillation flow around vane. This method serves as a reference for the fine simulation and study of the flow around the multi-stage cascade flow in hydro-turbine. The research on the transient effects of the coupling upstream pipeline system and hydroturbine has theoretical guiding significance.