Evidence for freeze–thaw events and their implications for rock weathering in northern Canada

Discussions regarding weathering in cold environments generally centre on mechanical processes and on the freeze–thaw mechanism in particular. Despite the almost ubiquitous assumption of freeze–thaw weathering, unequivocal proof of interstitial rock water actually freezing and thawing is singularly lacking. Equally, many studies have used the crossing of 0 °C, or values close to that, as the basis for determining the number of ‘freeze–thaw events’. In order to assess the weathering regime at a site in northern Canada, temperatures were collected at the surface, 1 cm and 3 cm depth for sets of paving bricks, with exposures both vertical and at 45°, orientated to the four cardinal directions. Temperature data were collected at 1 min intervals for 1 year. These data provide unequivocal proof for the occurrence of the freezing and thawing of water on and within the rock (freeze–thaw events). The freeze event is evidenced by the exotherm associated with the release of latent heat as the water actually freezes. This is thought to be the ?rst record of such events from a ?eld situation. More signi?cantly, it was found that the temperature at which freezing occurred varied signi?cantly through the year and that on occasion the 1 cm depth froze prior to the rock surface. The change in freeze temperature is thought to be due to the chemical weathering of the material (coupled with on‐going salt inputs via the melting of snowfall), which, it is shown, could occur throughout the winter despite air temperatures down to ?30 °C. This ?nding regarding chemical weathering is also considered to be highly signi?cant. A number of thermal stress events were also recorded, suggesting that rock weathering in cold regions is a synergistic combination of various chemical and mechanical weathering mechanisms. Copyright © 2003 John Wiley & Sons, Ltd.     

Evidence for freeze–thaw events and their implications for rock weathering in northern Canada: II. The temperature at which water freezes in rock

Many undertakings have used either a single value or a narrow window of temperatures as a threshold for the freezing of water within rock. These temperatures vary from 0 to ?5 C, with most windows being in the range ?1 to ?4 C. Based on thermal data, these thresholds are commonly used to ‘count’ the number of freeze–thaw events as a basis for determining freeze–thaw weathering. Data collected from northern Canada indicate that the temperature at which freezing occurs can vary substantially, even for the same site. Using exotherm and zero curtain observations from bricks, at angles of 90 and 45, aligned to the four cardinal aspects, the various temperatures at which water froze are shown. Bricks on the north and east commonly did exhibit freezing, based on exotherms, within the window ?1 to ?5 C, while data for the south and west aspects showed substantial variation, with freezes sometimes between ?6·4 and ?8·9 C. The data were evaluated for evidence of zero curtain effects (indicative of water freezing), but no unequivocal events could be found, and it is suggested that, at the scale of observation used here, they are unlikely. It would therefore appear that the use of thermal thresholds may not be meaningful for evaluation of freeze–thaw events. The available data also indicate many instances when temperatures went substantially sub‐zero (e.g. ?20 C) and yet no indication of water freezing occurred – most likely because there was no water available to freeze. This indicates that any form of freeze–thaw event counting, in the absence of some indicator of the presence of water and that it actually froze, is flawed, as thermal conditions alone are not adequate to indicate the occurrence or not of actual freeze–thaw weathering events. These data suggest that evaluations of freeze–thaw occurrence based simply on thermal thresholds may be substantially in error. Copyright © 2006 John Wiley & Sons, Ltd.     

Shore platform lowering due to frost shattering during the 2009 winter at mesnil Val,English channel coast,NW France

In 2008–2009, a severe cold snap affected the otherwise temperate mid‐latitude coasts of the English Channel of France. In March 2009, we gathered rock spalling observations at Mesnil Val, NW France, to document the effect of frost on platform lowering in macro‐tidal environments. Six epochs of frost were recorded in 2 months, the two longest lasted 16 and 8 days, with minimum air temperature dropping to –9.5°C. Semi‐diurnal tides flood the entire platform, imposing up to 25 freeze–thaw cycles below –2.5°C, the freezing temperature of seawater. 19 cycles occurred at neap tidal elevation lasting at most 3.5 h. Our integrated observations indicate that these frost cycles were responsible for a platform lowering of about 0.8±0.5 mm during a single winter. No clear spatial trend appears, nor do macroscopic chalk types clearly stand out as being more susceptible to erosion. Assuming that the long‐term platform retreat model preserves a constant slope, frost shattering would then account for 10 to 20% of the expected annual platform erosion rate. Under more contrasted climates, frost is thus likely to be a prominent shaping process for rocky coastal platforms. Copyright © 2015 John Wiley & Sons, Ltd.     

冻融作用下带构造柱砖墙抗震性能试验研究

以4榀相同特性的带构造柱砖墙为研究对象,分别进行0、40、80及120次冻融循环试验和低周反复荷载试验,来研究冻融环境对砌体结构抗震性能的影响。试验结果表明:随着冻融循环次数的增加,裂缝出现更早,发展更为迅速,破坏程度更加严重,承载力、刚度、耗能、延性均出现明显的衰减。本文从宏观和微观两个方面对遭受冻融作用砖墙进行了损伤机理的分析。     

Dynamic shear modulus and damping ratio of frozen compacted sand subjected to freeze–thaw cycle under multi-stage cyclic loading

Frozen soil plays an important role on the stability of railway and highway subgrade in cold regions. However, the dynamic properties of frozen soil subjected to the freeze–thaw cycles have rarely been investigated. In this study, cryogenic cyclic triaxial tests were conducted on frozen compacted sand from Nehe, Heilongjiang Province in China which was subjected to the closed-system freeze–thaw cycles. A modified Hardin hyperbolic model was suggested to describe the backbone curves. Then, dynamic shear modulus and damping ratio versus cyclic shear strain were analyzed under the different freeze–thaw cycles, temperatures, initial water contents, loading frequencies and confining pressures. The results indicate that the freeze–thaw process plays a significant effect on the dynamic shear modulus and damping ratio, which slightly change after one freeze–thaw cycle. Dynamic shear modulus increases with increasing initial water content, temperature, loading frequency and confining pressure. Damping ratio increases with increasing initial water content, while decreases with increasing temperature and loading frequency. The effect of confining pressure on the damping ratio was found not significant. Furthermore, the empirical expressions were formulated to estimate dynamic shear modulus and damping ratio of the frozen compacted sand. The results provide guidelines for evaluating the infrastructures in cold regions.     

Effect of seasonal freeze–thaw process on spatial and temporal distribution of soil water and its infiltration to recharge groundwater

Clarifying the distribution and dynamics of soil moisture during the freeze–thaw process is crucial for surface ecology and is an objective requirement to investigate the mechanism of changes during the groundwater recharge process in a freeze–thaw zone. Based on the monitoring data of soil moisture and temperature in the Changbai Mountain area, the freeze–thaw process is classified into four periods. This study investigates the hydrothermal migration processes during different periods. The simultaneous heat and water model is used to simulate and analyse the infiltration of soil moisture into groundwater under five precipitation insurance rates. The results are as follows: (1) The smaller the soil depth, the stronger is the correlation between soil temperature and air temperature during the freeze–thaw process. (2) The redistribution of soil moisture before and after freeze–thaw is significantly affected by the soil texture, and soil permeability affects the recharge of soil moisture from the upper region to the lower region during the thawing period. (3) Groundwater receives vertical infiltration recharge mainly during non-freezing and is supplied by freezing and snowmelt recharge during the stable thawing period. The percentage of soil water infiltration during the stable thawing period in the total annual infiltration increases gradually with the precipitation insurance rate.     

Temporal and spatial variations in periglacial soil movements on alpine crest slopes

This paper describes up to ten years of continuous monitoring of frost heave, creep and associated parameters on high mountain crest slopes in the Japanese and Swiss Alps, aiming to evaluate spatial and interannual variations in the rates and controls of soil movement. Shallow frost creep re?ecting diurnal frost heave activity dominates the crest slopes that lack a vegetation mat and have a thin debris mantle with good drainage. Seasonal frost heave activity can induce slightly deeper movement where ?ne soil exists below the depth reached by diurnal freeze–thaw penetration, although the shallow bedrock impedes movements below 20 cm depth. As a result, downslope velocity pro?les display strong concavity with surface velocities of 2–50 cm a^?1. The frost creep rates vary spatially, depending on the soil texture, slope gradient, frequency of temperature cycling across 0 °C and moisture availability during freeze–thaw periods. Soil movements recur in every freeze–thaw period, although with some interannual variations affected by the length of seasonal snow cover and the occurrence of precipitation during freeze–thaw periods. The Swiss Alps encounter more signi?cant interannual variations than the Japanese Alps, re?ecting the large variability of the annual snow regime. Copyright © 2005 John Wiley & Sons, Ltd.     

A new soil freeze/thaw discriminant algorithm using AMSR‐E passive microwave imagery

The soil freeze–thaw controls the hydrological and carbon cycling and thus affects water and energy exchanges at land surface. This article reported a newly developed algorithm for distinguishing the freeze/thaw status of surface soil. The algorithm was based on information from Advanced Microwave Scanning Radiometer Enhanced (AMSR‐E) which records brightness temperature (Tb) in the afternoon and after midnight. The criteria and discriminant functions were obtained from both radiometer observations and model simulations. First of all, the microwave radiation from freeze–thaw soil was examined by carrying out experimental measurements at 18·7 and 36·5 GHz using a Truck‐mounted Multi‐frequency Microwave Radiometer (TMMR) in the Heihe River of China. The experimental results showed that the soil moisture is a key component that differentiates the microwave radiation behaviours during the freeze–thaw process, and the differences in soil temperature and emissivity between frozen and thawed soils were found to be the most important criteria. Secondly, a combined model was developed to consider the impacts of complex ground surface conditions on the discrimination. The model simulations quite followed the trend of in situ observations with an overall relation coefficient (R) of approximately 0·88. Finally, the ratio of Tb18·7H (horizontally polarized Tb at 18·7 GHz) to Tb36·5V was considered primarily as the quasi‐emissivity, which is more reasonable and explicit in measuring the microwave radiation changes in soil freezing and thawing than the spectral gradient. By combining Tb36·5V to indicate the soil temperature variety, a Fisher linear discrimination analysis was used to establish the discriminant functions. After being corrected by TMMR measurements, the new discriminant algorithm had an overall accuracy of 86% when validated by 4‐cm soil temperature. The multi‐year discriminant results also provided a good agreement with the classification map of frozen ground in China. Copyright © 2011 John Wiley & Sons, Ltd.     

Implications of measurement metrics on soil freezing curves: A simulation of freeze–thaw hysteresis

Soil freeze–thaw events have important implications for water resources, flood risk, land productivity, and climate change. A property of these phenomena is the relationship between unfrozen water content and sub-freezing temperature, known as the soil freezing characteristic curve (SFC). It is documented that this relationship exhibits hysteretic behaviour when frozen soil thaws, leading to the definition of the soil thawing characteristic curve (STC). Although explanations have been given for SFC/STC hysteresis, the effect that ‘scale’ – particularly ‘measurement scale’ – may have on these curves has received little attention. The most commonly used measurement scale metric is the ‘support’, which is the spatial (or temporal) unit within which the measured variable is integrated or soil volume sampled. We show (a) measurement support can influence the range and shape of the SFC and (b) hysteresis can be attributed, in part, to the support and location of the measurements comprising the SFC/STC. We simulated lab measured temperature, volumetric water content (VWC), and permittivity from soil samples undergoing freeze–thaw transitions using Hydrus-1D and a modified Dobson permittivity model. To assess the effect of measurement support and location on SFC/STC, we masked the simulated temperature and VWC/permittivity extent to match the instrument's support and location. By creating a detailed simulation of the intra- and inter-support variability associated with the penetration of a freezing front, we demonstrate how measurement support and location can influence the temperature range over which water freezing events are captured. We show it is possible to simulate hysteresis in homogenous media with purely geometric considerations, suggesting that SFC/STC hysteresis may be more of an apparent phenomenon than mechanistically real. Lastly, we develop an understanding of how the location and support of soil temperature and VWC/permittivity measurements influence the temperature range over which water freezing events are captured.     

Hysteretic freezing characteristics of riparian peatlands in the Western Boreal Forest of Canada

Freezing characteristics were investigated for a sedge covered floating fen and spruce covered swamp located beside a shallow lake in the Western Boreal Forest of Canada. Thermal properties were measured in situ for one freeze‐thaw cycle, and for two freeze‐thaw cycles in laboratory columns. Thermal conductivity and liquid water content were related to a range of subsurface temperatures above and below the freezing thresholds, and clearly illustrate hysteresis between the freezing and thawing process. Thermal hysteresis occurs because of the large change in thermal conductivity between water and ice, high water content of the peat, and wide variation in pore sizes that govern ice formation. Field and laboratory results were combined to develop linear freezing functions, which were tested in a heat transfer model. For surface temperature boundary conditions, subsurface temperatures were simulated for the over‐winter period and compared with field measurements. Replication of the transient subsurface thermal regime required that freezing functions transition gradually from thawed to frozen state (spanning the ?0·25 to ?2 °C range) as opposed to a more abrupt step function. Subsurface temperatures indicate that the floating fen underwent complete phase change (from water to ice) and froze to approximately the same depth as lake ice thickness. Therefore, the floating fen peatland froze as a ‘shelf’ adjacent to the lake, whereas the spruce covered swamp had a higher capacity for thermal buffering, and subsurface freezing was both more gradual and limited in depth. These thermal properties, and the timing and duration of frozen state, are expected to control the interaction of water and nutrients between surface water and groundwater, which will be affected by changes in air temperature associated with global climate change. Copyright © 2009 John Wiley & Sons, Ltd.     

Snow cornice dynamics as a control on plateau edge erosion in central Svalbard

Snow cornices grow extensively on leeward edges of plateau mountains in central Svalbard. A dominant wind direction, a snowdrift source area and a sharp slope transition largely control the formation of snow cornices in a barren peri‐glacial landscape. Seasonal snow cornice dynamics control bedrock weathering and erosion in sedimentary bedrock on the Gruvefjellet plateau edge in the valley Longyeardalen. Air, snow and ground temperature sensors, as well as automatic time‐lapse cameras on a leeward facing plateau edge were used to study seasonal cornice dynamics. These techniques allowed for monitoring of cornice accretion, deformation and collapse/melting in great detail. The active layer of the top plateau edge is characterized by high moisture content due to rain before freeze‐up in autumn and cornice meltdown during spring thaw. Thus frost weathering there can be very efficient in this otherwise cold and dry environment. Within the first autumn snowstorms, a vertical fully developed cornice was in place (190 cm thick). The backwall surface beneath the thickest part of the cornice remained in the ice segregation ‘frost cracking window’ for almost nine months. Highly weathered rock material from the plateau edge is thus incorporated into the cornice during cornice accretion. Brittle snow deformation leads to the opening of cornice tension cracks between the cornice mass and the snowpack on the plateau. These cracks are a prerequisite for cornice collapses, and often trigger cornice fall avalanches on the slope beneath. In these open cornice tension cracks, weathered rock debris, plucked from the plateau edge, can be visible, demonstrating the erosional property of the cornices. The cornice will either collapse or melt, resulting in suspended sediment transport downslope by cornice fall avalanche or release as rock fall respectively. Therefore, cornices both promote and trigger high weathering rates on Gruvefjellet, and thus control presently the development of the rockwall free faces and the talus cones. Copyright © 2012 John Wiley & Sons, Ltd.     

A portable experimental hillslope for frozen ground studies

Frozen ground hydrological effects on runoff, storage, and release have been observed in the field and tested in numerical models, but few physical models of frozen slopes (at scales from 1 to 15 m) exist partly because the design of such an experiment requires new engineering design for realistic whole‐slope freezing and physical model innovation. Here, we present a new freezable tilting hillslope physical model for hydrological system testing under a variety of climate conditions with the ability to perform multiple (up to 20 per year) freeze–thaw cycles. The 4 × 2 m hillslope is mobile and tiltable on the basis of a modified tri‐axle 4.88‐m (16′) dump trailer to facilitate testing multiple configurations. The system includes controllable boundary conditions on all surfaces; examples of side and baseflow boundary conditions include permeable membranes, impermeable barriers, semipermeable configurations, and constant head conditions. To simulate cold regions and to freeze the hillslope in a realistic and controlled manner, insulation and a removable freezer system are incorporated onto the top boundary of the hillslope. The freezing system is designed to expedite the freezing process by the addition of a 10,130‐KJ (9,600‐BTU) refrigeration coil to the top‐centre of the insulated ceiling. Centre placement provides radial freezing of the hillslope in a top‐down fashion, similar to what natural systems encounter in the environment. The perimeter walls are insulated with 100 mm of spray foam insulation, whereas the base of the hillslope is not insulated to simulate natural heat fluxes beneath the frozen layer of soil. Our preliminary testing shows that covers can be frozen down to ?10 °C in approximately 7 days, with subsequent thaw on a similar time frame.     

The freeze-thaw weathering regime at a section of the Niagara escarpment on the Bruce Peninsula,Southern Ontario,Canada

The bedrock freeze-thaw and moisture regimes at an actively eroding site on the Niagara Escarpment, Bruce Peninsula, southern Ontario, were monitored between December 1983 and April 1984, and the results compared with amounts of debris collected in an adjacent rock trap. Frost wedging in pre-existing rock fissures is the primary mechanism responsible for the observed rockfall events; hydration effects are negligible. Debris production was more closely related to the duration of the freezing leg of the freeze-thaw cycle than to intensity or to cycle frequency. Release also coincided with periods of high pore saturation (> 60 per cent) and the seepage of water from cracks and fissures. Fewer freeze-thaw cycles were recorded in the air than at 1 and 3.5 cm in the bedrock. The number of cycles that could be declared geomorphologically effective according to established temperature criteria was normally less than half the total number of freeze-thaw cycles recorded in both air and bedrock. Under the current temperature regime at the field site, few effective cycles are capable of penetrating more than 5 cm into the free face.     

Effects of riparian vegetation on stream bank subaerial processes in southwestern Virginia,USA

Riparian vegetation is frequently used for stream bank stabilization, but the effects of vegetation on subaerial processes have not been quantified. Subaerial processes, such as soil desiccation and freeze–thaw cycling, are climate‐related phenomena that deliver soil directly to the stream and make the banks more vulnerable to fluvial erosion by reducing soil strength. This study compares the impact of woody and herbaceous vegetation on subaerial processes by examining soil temperature and moisture regimes in vegetated stream banks. Soil temperature and water tension were measured at six paired field sites in southwestern Virginia, USA, for one year. Results showed that stream banks with herbaceous vegetation had higher soil temperatures and a greater diurnal temperature range during the summer compared to forested stream banks. Daily average summer soil water tension was 13 to 57 per cent higher under herbaceous vegetation than under woody vegetation, probably due to evapotranspiration from the shallow herbaceous root system on the bank. In contrast to summer conditions, the deciduous forest buffers provided little protection for stream banks during the winter: the forested stream banks experienced diurnal temperature ranges two to three times greater than stream banks under dense herbaceous cover and underwent as many as eight times the number of freeze–thaw cycles. During the winter, the stream banks under the deciduous forests were exposed to solar heating and night time cooling, which increased the diurnal soil temperature range and the occurrence of freeze–thaw cycling. Study results also indicated that freeze–thaw cycling and soil desiccation were greater on the upper stream bank due to thermal and moisture regulation of the lower bank by the stream. Therefore, subaerial erosion and soil weakening may be greater on the upper stream banks. Additional research is needed on the influence of subaerial processes on both subaerial and fluvial erosion. Copyright © 2006 John Wiley & Sons, Ltd.     

Past and projected future changes in snowpack and soil frost at the Hubbard Brook Experimental Forest,New Hampshire,USA

Long‐term data from the Hubbard Brook Experimental Forest in New Hampshire show that air temperature has increased by about 1 °C over the last half century. The warmer climate has caused significant declines in snow depth, snow water equivalent and snow cover duration. Paradoxically, it has been suggested that warmer air temperatures may result in colder soils and more soil frost, as warming leads to a reduction in snow cover insulating soils during winter. Hubbard Brook has one of the longest records of direct field measurements of soil frost in the United States. Historical records show no long‐term trends in maximum annual frost depth, which is possibly confounded by high interannual variability and infrequency of major soil frost events. As a complement to field measurements, soil frost can be modelled reliably using knowledge of the physics of energy and water transfer. We simulated soil freezing and thawing to the year 2100 using a soil energy and water balance model driven by statistically downscaled climate change projections from three atmosphere‐ocean general circulation models under two emission scenarios. Results indicated no major changes in maximum annual frost depth and only a slight increase in number of freeze–thaw events. The most important change suggested by the model is a decline in the number of days with soil frost, stemming from a concurrent decline in the number of snow‐covered days. This shortening of the frost‐covered period has important implications for forest ecosystem processes such as tree phenology and growth, hydrological flowpaths during winter, and biogeochemical processes in soil. Published in 2010 by John Wiley & Sons, Ltd.     

Using stable isotopes to estimate travel times in a data‐sparse Arctic catchment: Challenges and possible solutions

Use of isotopes to quantify the temporal dynamics of the transformation of precipitation into run‐off has revealed fundamental new insights into catchment flow paths and mixing processes that influence biogeochemical transport. However, catchments underlain by permafrost have received little attention in isotope‐based studies, despite their global importance in terms of rapid environmental change. These high‐latitude regions offer limited access for data collection during critical periods (e.g., early phases of snowmelt). Additionally, spatio‐temporal variable freeze–thaw cycles, together with the development of an active layer, have a time variant influence on catchment hydrology. All of these characteristics make the application of traditional transit time estimation approaches challenging. We describe an isotope‐based study undertaken to provide a preliminary assessment of travel times at Siksik Creek in the western Canadian Arctic. We adopted a model–data fusion approach to estimate the volumes and isotopic characteristics of snowpack and meltwater. Using samples collected in the spring/summer, we characterize the isotopic composition of summer rainfall, melt from snow, soil water, and stream water. In addition, soil moisture dynamics and the temporal evolution of the active layer profile were monitored. First approximations of transit times were estimated for soil and streamwater compositions using lumped convolution integral models and temporally variable inputs including snowmelt, ice thaw, and summer rainfall. Comparing transit time estimates using a variety of inputs revealed that transit time was best estimated using all available inflows (i.e., snowmelt, soil ice thaw, and rainfall). Early spring transit times were short, dominated by snowmelt and soil ice thaw and limited catchment storage when soils are predominantly frozen. However, significant and increasing mixing with water in the active layer during the summer resulted in more damped steam water variation and longer mean travel times (~1.5 years). The study has also highlighted key data needs to better constrain travel time estimates in permafrost catchments.     

Suspended sediment and discharge relationships to identify bank degradation as a main sediment source on small agricultural catchments

Variability of suspended sediment concentration (SSC) versus discharge relationships in streams is often high and illustrates variable particle origins or availability. Particle availability depends on both new sediment supply and deposited sediment stock. The aim of this study is to improve SSC–discharge relationship interpretation, in order to determine the origins of particles and to understand the temporal dynamics of particles for two small streams in agricultural catchments from northwestern France. SSC and discharge were continuously recorded at the outlets and data were examined at different time‐scales: yearly, monthly, with distinction between flood periods and non‐flooding periods, and individual flood events. Floods are classified in relation to SSC–discharge hysteresis, and this typology is completed by the analysis of SSC–discharge ranges during rising and falling flow. We show that particles are mainly coming from channel, banks, either by hydraulic erosion or by cattle trampling. Particle availability presents a seasonal dynamics with a maximum at the beginning of autumn when discharge is low, decreasing progressively during autumn to become a minimum in winter when discharge is the highest, and increasing again in spring. Bank degradation by cattle is the determining factor in the suspended sediment dynamics. Cattle bank‐trampling produces sediment, mostly from spring to autumn, that supplies the deposited sediment stock even outside floods. This hydrologically independent process hides SSC–discharge correlation classically linked to hydraulic erosion and transport. Differences in SSC–discharge relationships and suspended sediment budgets between streams are related to differences in transport capacity and bank degradation by cattle trampling and channelization. Copyright © 2007 John Wiley & Sons, Ltd.     

Evaluation of meteorological controls of reconstructed rockfall activity in the Czech Flysch Carpathians

Rockfall is an important process in the final sculpturing of escarpments and scree slopes that originate in bedrock landslides in the Flysch Carpathians. The spatio‐temporal characteristics of rockfall activity were studied at four localities representative of old landslides in the highest part of the Czech Flysch Carpathians (Moravskoslezské Beskydy Mountains). Historical activity, chronology, and spatial context of rockfall activity were reconstructed using dendrogeomorphic techniques and rockfall rate index (RR). A total of 1132 increment cores from 283 trees growing in the rockfall transport and accumulation zones enabled the dating of 989 rockfall events. Reconstruction of a 78‐year‐long RR chronology suggests similar rockfall histories and trends at all study sites, indicating the existence of major common factors driving rockfall dynamics in the region. Temporal analysis and correlation of the RR series obtained with monthly mean temperatures, numbers of days with temperature transitions through 0 °C and monthly precipitation totals show that meteorological characteristics have evident but variable influence on rockfall activity. The most important factor is the effect of freeze–thaw cycles throughout the year, supplemented by low temperatures, especially during autumn. The influence of precipitation totals is of lesser importance. Copyright © 2011 John Wiley & Sons, Ltd.     

Characteristics of land surface heat and water exchange under different soil freeze/thaw conditions over the central Tibetan Plateau

Freezing and thawing processes at the soil surface play an important role in determining the nature of Tibetan land and atmosphere interactions. In this study, land surface water and heat exchanges under different freezing and thawing conditions over the central Tibetan Plateau were investigated using observations from the Coordinated Enhanced Observing Period/Asia‐Australia Monsoon Project on the Tibetan Plateau, and the Simultaneous Heat and Water Model. During the freezing and thawing stages, significant diurnal variation of soil temperature resulted in a diurnal cycle of unfrozen water content at the surface. Radiation and energy components and evapotranspiration averaged over four freeze/thaw stages also changed diurnally. On average, the surface albedo (0·68) during the completely frozen stage was sharply higher than those during the freezing, thawing, and completely thawed stages due to the snow cover. The Bowen ratios were 3·1 and 2·5 in the freezing and thawing stages, respectively, but the ratio was only 0·5 in the completely thawed stage. Latent heat flux displayed distinctly better correlation with unfrozen soil water content during the freezing and thawing stages than during the completely frozen and thawed stages. This implies that the diurnal cycle of unfrozen soil water, resulting from diurnal freeze/thaw cycles at the surface, has a significant impact on latent heat flux. A surface energy imbalance problem was encountered, and the possible sources of error were analysed. Copyright © 2011 John Wiley & Sons, Ltd.     

A land surface model incorporated with soil freeze/thaw and its application in GAME/Tibet

Land surface process is of great importance in global climate change, moisture and heat exchange in the interface of the earth and atmosphere, human impacts on the environment and eco- system, etc. Soil freeze/thaw plays an important role in cold land surface processes. In this work the diurnal freeze/thaw effects on energy partition in the context of GAME/Tibet are studied. A sophisti- cated land surface model is developed, the particular aspect of which is its physical consideration of soil freeze/thaw and vapor flux. The simultaneous water and heat transfer soil sub-model not only reflects the water flow from unfrozen zone to frozen fringe in freezing/thawing soil, but also demon- strates the change of moisture and temperature field induced by vapor flux from high temperature zone to low temperature zone, which makes the model applicable for various circumstances. The modified Picard numerical method is employed to help with the water balance and convergence of the numerical scheme. Finally, the model is applied to analyze the diurnal energy and water cycle char- acteristics over the Tibetan Plateau using the Game/Tibet datasets observed in May and July of 1998. Heat and energy transfer simulation shows that: (i) There exists a negative feedback mechanism between soil freeze/thaw and soil temperature/ground heat flux; (ii) during freezing period all three heat fluxes do not vary apparently, in spite of the fact that the negative soil temperature is higher than that not considering soil freeze; (iii) during thawing period, ground heat flux increases, and sensible heat flux decreases, but latent heat flux does not change much; and (iv) during freezing period, soil temperature decreases, though ground heat flux increases.