Heavy snowfalls over mountain regions are often a direct cause of avalanches. Specific synoptic-scale atmospheric situations are responsible for these kinds of extreme snowfall event, and this is indeed the case for Andorra, a small country located in the Pyrenees, between France and Spain. Based on days with an intensity of at least 30 cm of snow in a 24 h period, the present study uses principal component analysis (PCA) and clustering techniques to characterize the synoptic circulation patterns for these days during the winter season. The area of analysis encompasses the region 30-60°N by 30°W-15°E and the period covers the winter seasons from 1986-87 to 2000-01. The methodology proposed involves a preprocessing approach consisting of a spatial standardization of the data used for the PCA, an alternative approximation to decide the centroids and the number of groups for the K-means clustering, and the rejection of the iterations for this algorithm. This approach enables the synoptic classification of every heavy snowfall day, and composite maps were constructed for sea-level pressure, 500 hPa geopotential height, and 1000-500 m thickness (the 5270 m, 5400 m and 5520 m contour lines). The results show seven circulation patterns, most of them with an Atlantic component of the wind, and others with a clear Mediterranean advection that could be combined with cold continental air. The results, as weather charts, could be a useful tool to assist meteorological models in heavy snowfall forecasting, and the day's classification obtained opens up future possibilities for detailed meteorological and climatological analysis of the established types.

In experimental snowdrifting mass-flux measurements many different instruments have been tested (Takeuchi, CEMAGREE Mases, etc.). Very often the results obtained are a function of the gauge used. However, in order to compare data from different instruments, orders of magnitude have to be similar.Since 1992, snowdrifting has been studied at an experimental plot at La Molina ski resort (eastern Spanish Pyrenees). The alpine site, characterized by a plateau topography, is located at 2250 m.In this paper, different gauges used to measure snowdrifting mass flux at this site are presented: one snow-collector column and two types of snow traps. Snow-collector columns (prismatic boxes) are permanent installations and are used to measure the mass-flux episode. Snow traps (Takeuchi, 1980: modified) are lighter and more mobile, and they are used for short experiments during a wind episode during which mass-flux data are obtained.The three different gauges are compared and the rate of trapping efficiency is suggested from a comparison of the field data with estimated mass-flux data deduced from empirical formulae (Mellor and Fellers, 1986; Naaim-Bouvet and others, 1996). The mass-flux values obtained at the experimental site are lower than the estimated values.

ABSTRACT. Snowdrift formation h as been monitored in th e eastern Spa ni sh Pyrenees since 1992 a t a n experimental study plot at the L a M o lina ski reso rt between 2000 a nd 2400 m. Severa l instrum ents were insta lled to cha racteri ze the wind, qua ntify snowmass flux , detec t snowdrifting episodes a nd meas ure the evoluti on o f a natura l snowdrift.This plot is situ ated within a natural snowd rif't a nd is equipped with meas uring in struments such as snow poles, three co lumns of snow co ll ectors (prism atic boxes ) fac ing the domina nt wind s a nd an anemographic sta tion.During th e 4 years of data acquisition, the thickness of the snowdrift has been measured a nd th e snow coll ec tors checked once each week. An active interest has bee n ta ken in the relationship bet ween snowdrift development a nd drifting snow during a wind epi sode.Th e rel a tionship observed betwee n wind, drifting snow a nd th e developm ent of the snowdrift is presented . The res ults show: (1) th e a mount or snow co ll ec ted in th e sn ow collecto rs is rela ted to wind intensity a nd to the ava il a bilit y a nd th e qua lit y of drifting snow, a nd (2) wind intensity principa ll y a ffec ts th e re la ti ve locati o n o f max imum snowdrift . growth rather tha n co ntrolling the thickness of t he snowdrift.By compa ring the evolution of the snowdrift with Ta bl er's (1975) model, we observed that it fits with th e top ographic features of our study a rea for th e three seaso ns shown.

Snowdrift formation has been monitored in the eastern Spanish Pyrenees since 1992 at an experimental study plot at the La Molina ski resort between 2000 and 2400 m. Several instruments were installed to characterize the wind, quantify snow-mass flux, detect snowdnfting episodes and measure the evolution of a natural snowdrift. This plot is situated within a natural snowdrift and is equipped with measuring instruments such as snow poles, three columns of snow collectors (prismatic boxes) facing the dominant winds and an anemographic station. During the 4 years of data acquisition, the thickness of the snowdrift has been measured and the snow collectors checked once each week. An active interest has been taken in the relationship between snowdrift development and drifting snow during a wind episode. The relationship observed between wind, drifting snow and the development of the snowdrift is presented The results show: (1) the amount of snow collected in the snow collectors is related to wind intensity and to the availability and the quality of drifting snow, and (2) wind intensity principally affects the relative location of maximum snowdrift growth rather than controlling the thickness of the snowdrift. By comparing the evolution of the snowdrift with Tabler’s (1975) model, we observed that it fits with the topographic features of our study area for the three seasons shown.

In experimental snowdrifting mass-flux measurements many different instruments have been tested (Takeuchi, CEMAGREE Mases, etc.). Very often the results obtained are a function of the gauge used. However, in order to compare data from different instruments, orders of magnitude have to be similar.Since 1992, snowdrifting has been studied at an experimental plot at La Molina ski resort (eastern Spanish Pyrenees). The alpine site, characterized by a plateau topography, is located at 2250 m.In this paper, different gauges used to measure snowdrifting mass flux at this site are presented: one snow-collector column and two types of snow traps. Snow-collector columns (prismatic boxes) are permanent installations and are used to measure the mass-flux episode. Snow traps (Takeuchi, 1980: modified) are lighter and more mobile, and they are used for short experiments during a wind episode during which mass-flux data are obtained.The three different gauges are compared and the rate of trapping efficiency is suggested from a comparison of the field data with estimated mass-flux data deduced from empirical formulae (Mellor and Fellers, 1986; Naaim-Bouvet and others, 1996). The mass-flux values obtained at the experimental site are lower than the estimated values.