ABSTRACT. In order to gain more insight into the mountain snow-transport mech anisms wind and drift flux measurements have been executed on a ridge crest (mainly during snow-storms). Horizontal wind-speed profiles, measured between 0.3 and 6 m above snow surface, show a hump-shaped course especially for strong winds. Theoretical approximations substantiate that the Bernoullian pressure decrease on the crest may be the main cause for this type of wind profile. Roughness parameters (zo, U,.) are determined with the aid of the wind profiles and compared with those reported in the literature. Corresponding drift density profiles coincide with steady-state drift theories as long as wind speeds are low (UI < 7 -10 m S-I), at greater wind speeds snow plumes of I to 1.5 m thickness develop immediately above snow surface. Areal measurements on snow mass-ba lance differences between windward and lee slopes are used to approximate the total transport over the ridge crest and to derive a quantitative relationship between crest winds and drift-snow deposition on lee slopes. R ESUME. Transport de neige au-dessus des cretes des montagnes. En vue d'acquerir une meilleure connaissance d es mecanismes de transport de la neige en montagne, on a execute des mesures de vent et de transport par le vent sur une crete montagneuse (principa lement des jours de tempetes de neige). Les profils de vitesse h orizontale du vent mesures entre 0,3 et 6 m au-dessus de la surface de la neige presentent une forme de bosse specialement pour les vents forts. Des approximations th eoriques viennent a l'appui du fait que la chute de pression de Bernoulli sur la crete peut et re la cause principale de ce type de profil de vent. Les parametres de rugosite (zo, u"' ) sont determines a l'aide de ces profils de vent et compares a ceux que rapporte la litterature. Les profils correspondants de densite de chasse-neige coincident avec les theories de l'etat d'equilibre du chasse-neige tant que les vitesses de vent sont faibles (u 1 .;;; 7-10 m S-I), a des vitesses superieures des plumealL'< de neige de I a 1,5 m d'epaisseur se deve\oppent immediatement au-dessus de la surface de la neige. On a utilise des mesures de surface des differences entre les bilans de masse de la neige cote au vent et sous le vent, pour avoir une approximation du transport total au-dessus de la crete et pour en d eduire une relation quantitative entre les vents sur la crete et le depot de neige sur le cote sous le vent.ZUSAMMENFASSUNG. Schneeuerfrachtung iiber Gipfelkuppen. Urn vermehrte Einsicht in die Schneeverfrachtungsvorglinge an Gipfelkuppen zu gewinnen, wurden an einer Gratkante Wi nd-und Schnee-dichte-Profile gemessen. Der griisste Teil der Messungen wurde wlihrend Sturmperioden (Schneefall mit Schneetreiben) ausgefuhrt. Die Wind profile wiesen bei griisseren Windgeschwindigkeiten durchwegs einen bodennahen "Buckel" auf, der mit Hilfe von theoretischen Anslitzen auf den Striimungsunterdruck auf der Gratkante zuruckgefuhrt werden konnte. Die Striimungsparameter (zo, 1/,.) wurd...
Avalanche forecasting for a given region is still a difficult task involving great responsibility. Any tools assisting the expert in the decision-making process are welcome. However, an efficient and successful tool should meet the needs of the forecaster. With this in mind, two models, were developed using a commercially available software: CYBERTEK-COGENSYSTM, a judgment processor for inductive decision-making–a principally data-based expert system. Using weather, snow and snow-cover data as input parameters, the models evaluate for a region the degree of avalanche hazard, the aspect and altitude of the most dangerous slopes. The output result is based on the snow-cover stability. The new models were developed and have been tested in the Davos region (Swiss Alps) for several years. To rate the models, their output is compared to the a posteriori verified hazard. the first model is purely data-based. Compared to other statistical models, the differences are: more input information about the snow cover from snow profiles and Rutschblock tests, the specific method to search for similar situations, the concise output result and the knowledge base that includes the verified degree of avalanche hazard. The performance is about 60%. The second, more-refined model, is both data- and rule-based. It tries to model the decision-making process of a pragmatic expert and has a performance of about 70%, which is comparable to the accuracy of the public warning.
Knowledge about snow mechanics and snow avalanche formation forms the basis of any hazard mitigation measures. The crucial point is the snow stability. The most relevant mechanical properties -the compressive, tensile and shear strength of the individual snow layers within the snow cover -vary substantially in space and time. Among other things the strength of the snow layers depends strongly on the state of stress and the strain rate. The evaluation of the stability of the snow cover is hence a difficult task involving many extrapolations.To gain insight in the release mechanism of slab avalanches triggered by skiers, the skier's impact is measured with a load cell at different depths within the snow cover and for different snow conditions. The study focused on the effects of the dynamic loading and of the damping by snow compaction. In accordance with earlier finite-element (FE) calculations the results show the importance of the depth of the weak layer or interface and the snow conditions, especially the sublayering.In order to directly measure the impact force and to study the snow properties in more detail, a new instrument, called rammrutsch was developed. It combines the properties of the rutschblock with the defined impact properties of the rammsonde. The mechanical properties are determined using (i) the impact energy of the rammrutsch and (ii) the deformations of the snow cover measured with accelerometers and digital image processing of video sequences. The new method is well suited to detect and to measure the mechanicalprocesses and properties of the fracturing layers. The duration of one test is around 10 minutes and the method seems appropriate for determining the spatial variability of the snow cover. A series of experiments in a forest opening showed a clear difference in the snow stability between sites below trees and ones in the free field of the opening.
During two winters, typical meteorological conditions (temperature, humidity, wind, radiation) and snow-surface conditions (snow surface and snow temperatures) were measured to simulate the formation and ablation processes (mainly sublimation) of surface hoar on two level snow plots, situated at 2500 and 1500 m a.s.l. In order to verify the simulated deposition/ablation rates, the surface-hoar mass, thickness and occasionally density were also measured. The evaluation shows that, using the aerodynamic bulk method for the half-hourly simulation periods, >90% of the day/night periods could be rated as either hoar-formation or ablation periods. The simulated sublimation rates (deposition/ablation) deviate in the mean by not more than 10% from the measured amounts! However, some larger deviations are present, mainly during ablation periods with heavy melting. Finally a method is shown for transforming net deposited vapour amounts into surface-hoar layers of a corresponding height in order to produce “layers” which might be integrated into snow-cover simulation models.
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