Abstract. In countries like Japan, particular solid precipitation particles
(PPs), such as unrimed PPs and graupel, often form a weak layer in snow,
which triggers slab avalanches. An understanding of weak PP layers is
therefore essential for avalanche prevention authorities to design a
predictive model for slab avalanches triggered by those layers. Specific
surface area (SSA) is a parameter that could characterize the physical
properties of PPs. The SSAs of solid PPs in Nagaoka – a city in Japan
experiencing the heaviest snowfall in the country – were measured for four
winters (from 2013/2014 to 2016/2017). More than 100 SSAs of PP
were measured during the study period using the gas absorption method. The
measured SSA values range from 42 to 153 m2 kg−1. Under melting
conditions, PPs showed comparatively smaller values. Unrimed and slightly
rimed PPs exhibited low SSA, whereas heavily rimed PPs and graupel exhibited
high SSA. The degree of PP riming depends on the synoptic meteorological
conditions. Based on the potential of weak PP layer formation with respect
to the degree of riming of PPs, the results indicate that SSA is a useful
parameter for describing the characteristics of PP, and consequently
predicting avalanches triggered by weak PP layers. The study found that the
values of SSA strongly depend on wind speed (WS) and wet-bulb temperature
(Tw) on the ground. SSA increases with increase in WS and decreases with
increase in Tw. An equation to empirically estimate the SSA of fresh PPs in
Nagaoka using WS and Tw was established. The equation successfully reproduced
the fluctuation of SSA. The SSA equation, along with the meteorological
data, is an efficient first step toward describing the development of weak
PP layers in the snow cover models.
Measurement of specific surface area of fresh solid precipitation particles in heavy snowfall regions of Japan
