Inter-Comparison of Snow Penetrometers (Ramsonde, Avatech SP2 and SnowMicroPen) in the Framework of Avalanche Forecasting

Snow hardness, defined as the resistance against penetration of an object into snow, is a good mechanical indicator of the snowpack stratigraphy and is therefore a fair predictor of stability. Moreover, its measurement by a penetrometer does not require the time-consuming digging of a snow pit. For these reasons, hardness is a measure of choice in snowpack observations. The ramsonde can reliably quantify hardness but its resolution in depth and hardness is too poor to resolve thin features such as weak layers or soft features such as non-permanent slab structures. However, this instrument remains the worldwide standard to measure hardness. There is thus a need for highly resolved penetrometers. The SnowMicroPen fits these requirements but its high price and fragility limit its usage to research purposes. Recently, the company Avatech has released a new penetrometer, the SP2, intended to fill the gap between the robust and lowly resolved ramsonde and the costly and highly resolved SnowMicroPen. We conducted an objective comparison of co-located vertical profiles measured by these three instruments in combination with manual stratigraphy and stability tests, in the French Alps during winter 20152016. The dataset comprises more than one hundred co-located SP2 and SnowMicroPen profiles measured for ten different snowpack configurations. The profiles are evaluated in terms of hardness and depth but we also focus on the features relevant to assess the snowpack stability. The SP2 is shown to successfully recover the main stratigraphic sequences but its depth measurement suffers from variable accuracy and its force sensor cannot resolve the vertical hardness variations in soft layers, which are measured by the SnowMicroPen or indirectly revealed by stability tests.