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1.
A seasonal snowcover blankets much of Canada during wintertime. In such an environment, the frequency of blowing snow events is relatively high and can have important meteorological and hydrological impacts. Apart from the transport of snow, the thermodynamic impact of sublimating blowing snow in air near the surface can be investigated. Using a time or fetch-dependent blowing snow model named 'PIEKTUK' that incorporates prognostic equations for a spectrum of sublimating snow particles, plus temperature and humidity distributions, it is found that the sublimation of blowing snow can lead to temperature decreases of the order of 0.5 °C and significant water vapour increases in the near-surface air. Typical predicted snow removal rates due to sublimation of blowing snow are several millimetres snow water equivalent per day over open Arctic tundra conditions. The model forecast sublimation rates are most sensitive to humidity, as well as wind speed, temperature and particle distributions, with a maximum value in sublimation typically found approximately 1 km downstream from blowing snow initiation. This suggests that the sublimation process is self-limiting despite ongoing transport of snow by wind, yielding significantly lower values of blowing snow sublimation rates (nearly two-thirds less) compared to situations where the thermodynamic feedbacks are neglected. The PIEKTUK model may provide the necessary thermodynamic inputs or blowing snow parameterizations for mesoscale models, allowing the assessment of the contribution of blowing snow fluxes, in more complex situations, to the moisture budgets of high-latitude regions.  相似文献   

2.
An Intercomparison Among Four Models Of Blowing Snow   总被引：4，自引：3，他引：1  
Four one-dimensional, time-dependent blowing snow models areintercompared. These include three spectral models, PIEKTUK-T,WINDBLAST, SNOWSTORM, and the bulk version of PIEKTUK-T,PIEKTUK-B. Although the four models are based on common physicalconcepts, they have been developed by different research groups. Thestructure of the models, numerical methods, meteorological field treatmentand the parameterization schemes may be different. Under an agreed standardcondition, the four models generally give similar results for the thermodynamic effects of blowing snow sublimation on the atmospheric boundary layer, including an increase of relative humidity and a decrease of the ambient temperature due to blowing snow sublimation. Relative humidity predicted by SNOWSTORM is lower than the predictions of the other models, which leads to a larger sublimation rate in SNOWSTORM. All four models demonstrate that sublimation rates in a column of blowing snow have a single maximum in time, illustrating self-limitation of the sublimation process of blowing snow. However, estimation of the eddy diffusioncoefficient for momentum (Km), and thereby the diffusion coefficients for moisture (Kw) and for heat (Kh), has a significant influence on the process. Sensitivitytests with PIEKTUK-T show that the sublimation rate can be approximately constant with time after an initial phase, if Km is a linear function with height. In order to match the model results with blowing snow observations, some parameters in the standard run, such as settling velocity of blowing snow particles in these models, may need to be changed to more practical values.  相似文献   

3.
Variations in wind speed and air temperature during blowing snow are considered in detail using data from the Canadian weather observation network with high spatiotemporal resolution. It is revealed that blowing snow considerably affects the lower atmospheric layer regime. The analysis of observational data illustrates the fact of wind speed increase during the snowstorm. The local minima of air temperature during the period of blowing snow are identified. The method is determined for calculating the threshold wind speed that provokes the onset of blowing snow. The highest skill scores were obtained for the method which takes into account air temperature and humidity.  相似文献   

4.
A Bulk Blowing Snow Model   总被引：1，自引：0，他引：1  
We present in this paper a simple and computationally efficient numerical model that depicts a column of sublimating, blowing snow. This bulk model predicts the mixing ratio of suspended snow by solving an equation that considers the diffusion, settling and sublimation of blowing snow in a time-dependent mode. The bulk model results compare very well with those of a previous spectral version of the model, while increasing its computational efficiency by a factor of about one hundred. This will allow the use of the model to estimate the effects of blowing snow upon the atmospheric boundary layer and to the mass balance of such regions as the Mackenzie River Basin of Canada.  相似文献   

5.
A density current model was extended for use in katabatic flow over the steep slopes of Antarctica through the inclusion of the Coriolis effect and weight flux terms corresponding to blowing snow and cooling caused by sublimation. The model was calibrated and tested against data obtained during two flights in Adelie Land, Antarctica, along a trajectory starting about 170 km inland and extending to Dumont d'Urville. The predicted trend in water vapor flux agrees with measurements of this flux, lending support to empirical formulae for both snow flux and sublimation rate. Model predictions of velocity were in good agreement with measured quantities when reasonable estimates of radiation divergence and surface heat exchange were provided as input to the model. The potential temperature gradient above the katabatic layer was found to play a major role in flow stability for high velocity and deep katabatic flows. Velocity predictions were in better agreement with the data when a locally determined value was used for the coefficient in the empirical snow flux expression.  相似文献   

6.
A model is proposed to determine the electric field strength in blowing snow. To test this model, the electric field strength was measured over an 80-day period during the Canadian Arctic Shelf Exchange Study (CASES) in 2004. The electric field strength at 0.5 m correlates well with the difference between 10-m wind speed and a threshold wind speed, although there is a large amount of variation between the electric fields generated during different blowing snow events. Although the model predicts that the electric field should be proportional to particle number density, the correlation is weak. The correlation of wind speed and electric field strength suggests that particles become charged primarily due to friction-induced temperature difference as they impact upon the surface. The strength of the electric field is likely influenced by a large number of other factors that are difficult to measure. However, the model predicts electric field strengths in excess of 25 kV m−1 near the surface, which would have a significant effect on particle motion.  相似文献   

7.
The object under study is the blowing snow, i.e., the transport of snow lifted from the snow surface. The method is described for predicting the blowing snow initiation using the output data of the WRF-ARW numerical atmospheric model. The skill scores are presented for the forecasts for January 2013 calculated from data of 10 stations of the Canadian weather observation network.  相似文献   

8.
Sublimation and melt disturb the environmental information obtained from ice core records in the Andes. In two case studies we demonstrate to what extent these post-depositional processes may remove major parts of the accumulated snow cover. Dark ash layers from the Tungurahua eruption changed the albedo of surface snow on Chimborazo glacier (6268 m, 1°30 S,78°36 W, Ecuador) between two ice core drilling campaigns and forced substantial melt. Re-distribution and washout of the chemical constituents shifted the concentration profiles obtained in December 1999 as compared to an equivalent core drilled in December 2000. The stable isotope records showed that approximately the water equivalent (weq) of an annual layer had melted, and that the percolating melt water penetrated within the firn layer to a depth of at least 16.5 m without refreezing. In the second example, from a site on the dry axis between the tropical and extra-tropical precipitation belts, significant loss of accumulated snow layers occurred by sublimation. A surface experiment at Cerro Tapado glacier (5536 m, 30°08 S,69°55 W, Chile) revealed that losses of 2 mm weq (5 mm snow) per day occurred during the dry period following the 1997/98 El Niño. This loss generally included the entire surface layer enriched in stable isotopes, and thus caused minimal disturbance of the isotopic signature (and hence climatic information) of the net accumulation, yet chemical constituents again experienced considerable changes in concentration. From annual layer counting and direct dating it is obvious that the major part of the accumulated ice on both glaciers is younger than 100 years; however, isotopic and chemical variations at least in the basal ice from Cerro Tapado clearly point to climate conditions different from the recent centuries. This evidence is supported by mass balance considerations derived from a glacier-climate model. The possibility of a third type of disturbance aside from sublimation and melting – in this case a significant hiatus in the environmental chronology – also deserves consideration for other icecore records from this region. Potential disruptions or discontinuities need to be carefully evaluated given the profound changes in climatic and glaciological conditions since the Last Glacial Maximum throughout Holocene times.  相似文献   

9.
For the first time a simulation of blowing snow events was validated in detail using one-month long observations (January 2010) made in Adélie Land, Antarctica. A regional climate model featuring a coupled atmosphere/blowing snow/snowpack model is forced laterally by meteorological re-analyses. The vertical grid spacing was 2 m from 2 to 20 m above the surface and the horizontal grid spacing was 5?km. The simulation was validated by comparing the occurrence of blowing snow events and other meteorological parameters at two automatic weather stations. The Nash test allowed us to compute efficiencies of the simulation. The regional climate model simulated the observed wind speed with a positive efficiency (0.69). Wind speeds higher than 12 m s ?1 were underestimated. Positive efficiency of the simulated wind speed was a prerequisite for validating the blowing snow model. Temperatures were simulated with a slightly negative efficiency (?0.16) due to overestimation of the amplitude of the diurnal cycle during one week, probably because the cloud cover was underestimated at that location during the period concerned. Snowfall events were correctly simulated by our model, as confirmed by field reports. Because observations suggested that our instrument (an acoustic sounder) tends to overestimate the blowing snow flux, data were not sufficiently accurate to allow the complete validation of snow drift values. However, the simulation of blowing snow occurrence was in good agreement with the observations made during the first 20 days of January 2010, despite the fact that the blowing snow flux may be underestimated by the regional climate model during pure blowing snow events. We found that blowing snow occurs in Adélie Land only when the 30-min wind speed value at 2 m a.g.l. is >10 m s ?1. The validation for the last 10 days of January 2010 was less satisfactory because of complications introduced by surface melting and refreezing.  相似文献   

10.
Extraordinary blowing snow transport events in East Antarctica   总被引：1，自引：1，他引：0  
In the convergence slope/coastal areas of Antarctica, a large fraction of snow is continuously eroded and exported by wind to the atmosphere and into the ocean. Snow transport observations from instruments and satellite images were acquired at the wind convergence zone of Terra Nova Bay (East Antarctica) throughout 2006 and 2007. Snow transport features are well-distinguished in satellite images and can extend vertically up to 200 m as first-order quantitatively estimated by driftometer sensor FlowCapt?. Maximum snow transportation occurs in the fall and winter seasons. Snow transportation (drift/blowing) was recorded for ~80% of the time, and 20% of time recorded, the flux is >10?2 kg m?2 s?1 with particle density increasing with height. Cumulative snow transportation is ~4 orders of magnitude higher than snow precipitation at the site. An increase in wind speed and transportation (~30%) was observed in 2007, which is in agreement with a reduction in observed snow accumulation. Extensive presence of ablation surface (blue ice and wind crust) upwind and downwind of the measurement site suggest that the combine processes of blowing snow sublimation and snow transport remove up to 50% of the precipitation in the coastal and slope convergence area. These phenomena represent a major negative effect on the snow accumulation, and they are not sufficiently taken into account in studies of surface mass balance. The observed wind-driven ablation explains the inconsistency between atmospheric model precipitation and measured snow accumulation value.  相似文献   

11.
This paper presents a new triple-moment blowing snow model PIEKTUK-T by including predictive equations for three moments of the gamma size distribution. Specifically, predictive equations for the total number concentration, total mass mixing ratio, and total radar reflectivity for blowing snow are included. Tests in the context of idealized experiments and observed case studies demonstrate that the triple-moment model performs better than the double-moment model PIEKTUK-D in predicting the evolution of the number concentration, mixing ratio, shape parameter, and visibility in blowing snow, provided that the fall velocities for the total number concentration, mass mixing ratio, and radar reflectivity are weighted by the same order of the respective moments in both models. The power law relationship between the radar reflectivity factor and particle extinction coefficient found in PIEKTUK-T is consistent with one observed in snow storms. Coupling of the triple-moment blowing snow model to an atmospheric model would allow realistic studies of the effect of blowing snow on weather and climate.  相似文献   

12.
To estimate the sublimation rate of snow during relocation by wind, sizes and concentration of ice crystal fragments were measured at 6 levels in the lowest 1 m, during ten 10-min runs, in a nocturnal blizzard. Power-law functions of height described the decrease in mean particle diameter and concentration. The vertical gradient of water vapor, measured with a thermocouple psychrometer, was approximately linear from 0.2 to 1.0m above the surface. Evaporation of blowing snow over 3 km of transport distance was estimated to be 39% of transport rate, under conditions of the experiment.  相似文献   

13.
An atmospheric surface-layer model is used to investigate the interactionbetween suspended snow particles and the near-surface flow. Themodel incorporates the effects of upward diffusion, gravitational settling and sublimation of snow particles in 48 size classes, the effects of snowdrift sublimation on the heat and moisture budget of the surface layer, and the buoyancy destruction of turbulent kinetic energy (TKE) caused by the presence of suspended particles. A new term in the E- closure model representing the buoyancy destruction due to suspended particles is included in the prognostic equation for TKE. Generally, model results indicate that the presence of suspended particles causes significant decreases in TKE, the dissipation rate, turbulent length scales and eddy exchange coefficients (up to 40%). It is found that the reduction in the eddy exchangecoefficients is due mainly to reductions in turbulent length scales. Theassociated particle Richardson number peaks near the saltation-suspensioninterface, but at higher levels in the surface layer the particle-induced buoyancy can also significantly affect the flow. A detailed analysis of the various snowdrift quantities, the TKE budget and the particle buoyancy effects on the flow is presented.  相似文献   

14.
The sublimation of falling snow may be an important component of the atmospheric water budget of the Mackenzie River Basin and many parts of the Arctic. To investigate this issue, a simple sublimation model is used along with surface precipitation observations and sonde data obtained during the autumn 1994 Beaufort and Arctic Storms Experiment (BASE). Model results are then compared with actual precipitation measurements at Inuvik and Tuktoyaktuk, sites in Northern Canada, to approximate mass loss due to sublimation. The sublimation results are found to vary in concert with cloud base height, precipitation intensity aloft and the nature of the precipitation. Atmospheric conditions are furthermore examined over a wide range of the Arctic, especially the Mackenzie River Basin, to assess to what degree the results can be generalized. The presence of a relatively dry near-surface layer, a favourable environment for sublimation, is a key feature of most sites during the early autumn storm period. Estimates of sublimational mass losses are found over Inuvik and Tuktoyaktuk using sonde derived cloud base heights and temperature and humidity profiles. Sublimation losses for such sites are found to be of the order of 40–60%, which shows that sublimation is indeed a significant process over the Mackenzie Basin and needs to be well handled in climate models. However, increasing the vertical resolution of the sublimation model to that of climate scales can dramatically affect predicted sublimation amounts; how to properly account for sublimation then remains a difficult task.  相似文献   

15.
Atmospheric response to soil-frost and snow in Alaska in March   总被引：2，自引：0，他引：2  
Summary A hydro-thermodynamic soil-vegetation model including soil freezing/thawing (soil-frost) and snow-metamorphism has been integrated into the PennState/NCAR Mesoscale Meteorological Model MM5 in a two-way coupled mode. A hierarchy of simulations with and without the soil-frost module, each combined with and without the snow module, shows the influence of snow-cover and soil-frost on weather in Alaska. Herein the landscape is featured as it is typically by mesoscale models.Theoretical considerations suggest that organic soil types should be considered in mesoscale modeling because of their different thermal and hydrological behavior as compared to mineral soils. The Ludwig-Soret and Dufour effects are small, but increase appreciably during freezing/thawing and snow-melt.The snow and soil-frost processes have a demonstrable impact on the surface thermal and hydrological regimes and on the near-surface atmospheric conditions even on the short (synoptic) timescales. The presence of snow-cover results in a highly stable stratification. In cloud-free areas, the enhanced loss of radiant energy and cooling of the air over snow-cover lead to a positive feedback to relatively colder, drier conditions. In cloudy areas, a positive feedback to warmer, moister conditions develops over snow-cover. As the changes in atmospheric humidity and temperature caused by snow-cover propagate into the pressure field, sea level pressure is lower by more than 1hPa in the simulations with snow-cover. Although the effect of soil-frost alone is an order of magnitude smaller, the soil-frost snow system leads to an increase of the pressure difference to 1.2hPa. The changes in the pressure field alter wind speed and direction slightly.Soil-frost results in soil temperature differences of 2–5K in the upper soil layers, while snow results in differences of 3–10K. Soil-frost has a notably greater impact in cloud-free than cloudy areas. When a snow-cover is present, frozen soil enhances the insulating effect of a snow-cover in cloudy areas, but reduces it in cloud-free areas. In cloudy areas, soil-frost without snow-cover leads to cooler, drier atmospheric conditions relative to no frost. In cloudy areas, soil-frost under a snow-cover reduces the water supply to the atmosphere as compared to snow-covered conditions without soil-frost. The combined effects of soil-frost and snow increase precipitation locally by as much as 12.2mm/ 48h. If mesoscale modeling does not consider the soil-frost snow system, predicted water vapor fluxes will be too high in cloud-free areas, and too low in cloudy areas.  相似文献   

16.
A large nuclear war could produce massive quantities of smoke from burning cities and industries. A portion of this smoke would fall out on Arctic sea ice, thus lowering its albedo and potentially increasing the solar energy absorbed by the ice and the snow that covers it. We use a one-dimensional thermodynamic sea ice model to examine the effect of smokefall on the seasonal variation of sea ice. In particular, we test the sensitivity of the model results to the time of year, duration, and latitude of smokefall.Sea ice thickness variations and the period of summer ice-free conditions are sensitive to the season of smokefall. The largest sea ice perturbations are generated by smokefall in spring. In this case the period of ice-free conditions during the summer can increase by 2 – 3.5 months between 67.5° N and 82.5° N. In any given season, the annual cycle of sea ice is not very sensitive to the duration of smokefall. The equilibrium annual cycle of sea ice variation is restored within a few years of smokefall when the smoke is flushed out of the ice/snow system.Since the sea ice model used here is not a comprehensive global climate model, it is difficult to predict the mid-latitude climate effects of the massive, but temporary, Arctic sea ice changes. However, our results suggest that future global climate model simulations of the effects of nuclear war smoke include interactive sea ice calculations.The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

17.
We present results from numerical experiments made with a GCM, the NCAR CCM1, that were designed to estimate the annual balance between snow-fall accumulation and ablation for geographically important land regions for a variety of conditions. We also attempt to assess the reliability of these results by investigating model sensitivity to changes in prescribed physical parameters. Experiments were run with an initial imposition of 1 m of (midwinter) snowcover over all northern hemisphere land points. Over Alaska, western Canada, Siberia, and the Tibetan Plateau the model tended to retain this snow cover through the summer and in some cases increase its depth as well. We define these regions as glaciation sensitive and note some correspondence between them and source regions for the Pleistocene ice sheets. An experiment with greatly reduced CO2 (100 ppm) showed a tendency towards spontaneous glaciation, i.e., the model remained snow-covered throughout the summer over the same geographic regions noted above. With 200 ppm CO2 (roughly equal to values at the last glacial maximum), snow cover over these regions did not quite survive the summer on a consistent basis. Combining 200 ppm CO2 and 1 m of initial northern hemisphere snow cover yielded glaciation-sensitive conditions, agreeing remarkably well with locations undergoing glaciation during the Pleistocene. To assess the reliability of these results, we have determined minimal model uncertainty by varying two of the empirical coefficients in the model within physically plausible ranges. In one case surface roughness of all ocean gridpoints was reduced by an order of magnitude, leading to local 10% reductions in precipitation (snowfall), a change hard to distinguish from inherent model variability. In the other case, the fraction of a land grid square assumed to be occupied by snow cover for albedo purposes was varied from one-half to unity. Large changes occurred in the degree of summer melting, and in some cases the sign of the net balance changed as fractional snow cover was changed. We conclude that the model may be able to reveal regions sensitive to glaciation, but that it cannot yield a reliable quantitative computation of the magnitude of the net snow accumulation that can be implicitly or explicitly integrated through time.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dilmenil  相似文献   

18.
Canopy-level humidity is often less at night during fine weather in a mid-latitude city, compared to its rural surroundings. This feature has been attributed, in part, to reduced urban dew, but links are largely unproven, because urban dew data are rare. In this study, surface moisture (i.e., dew + guttation by blotting) and dewfall (by mini-lysimeter) were measured at rural and urban residential sites in Vancouver, Canada, during the summer of 1996. Air temperature and humidity were measured at both sites, and on rural-to-urban vehicle traverses. Weather and location effects were evident. Humidity data suggested the small (< 1 g m–3) urban moisture excess observed on fine nights was linked to reduced urban dew. For grass, the frequency of moisture events, and surface moisture amounts, were similar for both sites. However, on grass, rural dewfall (mean=0.10 mm per night) was more than urban dewfall (mean=0.07 mm per night). On the other hand, data for a roof lysimeter (mean dewfall=0.12 mm per night) showed that an urban roof could rival rural grass as a favoured location for dewfall in Vancouver.  相似文献   

19.
Snowdrift is one of the manymanifestations of two-phase flow, in which theinteraction between suspended particles and theambient fluid brings about some interesting features.Specifically, the drag required to keep particles insuspension against the downward gravitational pullrequires expenditure of turbulent kinetic energy(TKE). Other effects include the increased density of theair-snow mixture and the stable thermal stratificationcaused by the snowdrift sublimation-induced cooling.An atmospheric surface-layer model that includes snowdriftsuspension is described that includes the effects ofupward diffusion, gravitational settling andsublimation of snow particles in 48 size classes, theeffects of snowdrift sublimation on the heat andmoisture budget of the surface layer and the dampingof turbulence in the presence of suspended particles. Thewell-known E- closure model is applied toevaluate the eddy exchange coefficient, with a newterm representing buoyancy reduction induced by thestably stratified suspended particle profile includedin the prognostic equation for TKE.  相似文献   

20.
While turbulent bursts are considered critical for blowing-snow transport and initiation, the interaction of the airflow with the snow surface is not fully understood. To better characterize the coupling of turbulent structures and blowing-snow transport, observations collected in natural environments at the necessary high-resolution time scales are needed. To address this, high-frequency measurements of turbulence, blowing-snow density and particle velocity were made in the Canadian Rockies. During blowing-snow storms, modified variable-interval time averaging enabled identification of periods of near-surface blowing-snow coupling with shear-stress-producing motions in the lowest 2 m of the atmospheric surface layer. The identification of those turbulent motions responsible for blowing snow yields a better understanding of the event-driven mechanics of initiation and sustained transport. The type of coherent structures generating the Reynolds stress are just as important as the magnitude of the Reynolds stress in initiating and sustaining near-surface blowing snow. Our results suggest that blowing-snow models driven by merely the time-averaged shear stress lack physical realism in the near-surface region. The next phase of the development of blowing-snow models should incorporate parametrizations of coherent turbulent structures.  相似文献   

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