Measurement of specific surface area of fresh solid precipitation particles in heavy snowfall regions of Japan

Yamaguchi, Satoru ; Ishizaka, Masaaki ; Motoyoshi, Hiroki ; Nakai, Sent ; Vionnet, Vincent ; Aoki, Teruo ; Yamashita, Katsuya ; Hashimoto, Akihiro ; Hachikubo, Akihiro

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<strong>Abstract.</strong> 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 m<span class="inline-formula"><sup>2</sup></span> kg<span class="inline-formula"><sup>-1</sup></span>. 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 (<span class="inline-formula"><i>T</i><sub>w</sub></span>) on the ground. SSA increases with increase in WS and decreases with increase in <span class="inline-formula"><i>T</i><sub>w</sub></span>. An equation to empirically estimate the SSA of fresh PPs in Nagaoka using WS and <span class="inline-formula"><i>T</i><sub>w</sub></span> 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.</p>

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