Recent improvements to the Hadley Centre climate model include the introduction of a new land surface scheme called “MOSES” (Met Office Surface Exchange Scheme). MOSES is built on the previous scheme, but incorporates in addition an interactive plant photosynthesis and conductance module, and a new soil thermodynamics scheme which simulates the freezing and melting of soil water, and takes account of the dependence of soil thermal characteristics on the frozen and unfrozen components. The impact of these new features is demonstrated by comparing 1×CO2 and 2×CO2 climate simulations carried out using the old (UKMO) and new (MOSES) land surface schemes. MOSES is found to improve the simulation of current climate. Soil water freezing tends to warm the high-latitude land in the northern Hemisphere during autumn and winter, whilst the increased soil water availability in MOSES alleviates a spurious summer drying in the mid-latitudes. The interactive canopy conductance responds directly to CO2, supressing transpiration as the concentration increases and producing a significant enhancement of the warming due to the radiative effects of CO2 alone.

The impact of new land surface physics on the GCM simulation of climate and climate sensitivity

In the Arctic, where wind transport of snow is common, the depth and insulative properties of the snow cover can be determined as much by the wind as by spatial variations in precipitation. Where shrubs are more abundant and larger, greater amounts of drifting snow are trapped and suffer less loss due to sublimation. The snow in shrub patches is both thicker and a better thermal insulator per unit thickness than the snow outside of shrub patches. As a consequence, winter soil surface temperatures are substantially higher, a condition that can promote greater winter decomposition and nutrient release, thereby providing a positive feedback that could enhance shrub growth. If the abundance, size, and coverage of arctic shrubs increases in response to climate warming, as is expected, snow‐shrub interactions could cause a widespread increase (estimated 10%‐25%) in the winter snow depth. This would increase spring runoff, winter soil temperatures, and probably winter CO 2 emissions. The balance between these winter effects and changes in the summer energy balance associated with the increase in shrubs probably depends on shrub density, with the threshold for winter snow trapping occurring at lower densities than the threshold for summer effects such as shading. It is suggested that snow‐shrub interactions warrant further investigation as a possible factor contributing to the transition of the arctic land surface from moist graminoid tundra to shrub tundra in response to climatic warming.

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Snow-Shrub Interactions in Arctic Tundra: A Hypothesis with Climatic Implications

Several mechanisms can extend the equilibrium domain of a liquid phase into the solid region of the normal phase diagram. The causes of premelting, which include surface melting, interface curvature and substrate disorder, occur in all types of substances, including H2O. In the case of H2O, premelting can have important environmental consequences, among which are the heaving of frozen ground, breakdown of rock and concrete, sintering of snow, flow of glaciers, scavenging of atmospheric trace gases by snow and ice, and the electrification of thunderclouds. The article reviews the basic mechanisms of premelting and discusses their roles in the environmental phenomena. The principal results of numerous studies are reviewed, and trends in current research are outlined.

http://iopscience.iop.org/article/10.1088/0034-4885/58/1/003/pdf

The premelting of ice and its environmental consequences

It is well known that temperature has a pervasive effect on insects. Nearly every aspect of an insect’s life is influenced by temperature, from direct effects on the kinetics of enzymatic reactions, to defining the limits of physiological function and behavior, and ultimately to shaping of evolutionary pathways. As a group, insects, more than any other eukaryotic taxon, have evolved not only to survive but to flourish in a wide variety of thermal environments.

Principles of Insect Low Temperature Tolerance

Precise temperature data from four Alaskan permafrost sites (Prudhoe Bay, Barrow and two sites near Fairbanks) combined with computer modelling provide quantitative measures of the existence and dynamics of unfrozen water in the active layer and permafrost. Unfrozen water contents are negligible for living and dead moss layers, small in the peat layers and larger in the silts, and show significant site-to-site variation. The effect of unfrozen water on the ground thermal regime is largest immediately after freeze-up and during cooling of the active layer. It is less important during warming and thawing of the active layer and during freezing and thawing of seasonally frozen ground. The effects last less than a month in cold permafrost and throughout most of the freeze-up period in warm permafrost. Physically, unfrozen water introduces a spatially distributed latent heat and changes thermal properties which retards the thermal response of an active layer or permafrost. Unfrozen water in the freezing and frozen active layer and near-surface permafrost also protects the ground from rapid cooling and creates a strong thermal gradient at the ground surface that increases the heat flux out of the ground. This enlarged heat flux also enhances the insulating effect of the snow cover. There do not appear to be any inherent difficulties in using conductive heat modelling for the active layer during the period when the zero curtain exists. Copyright © 2000 John Wiley & Sons, Ltd.



Des donnees precises de temperature du sol provenant de quatre sites ou existe un pergelisol en Alaska (Prudhoe Bay, Barow et deux sites pres de Fairbanks) sont comparees avec des modeles obtenus par ordinateur, et de cette comparaison, sont tirees des mesures quantitatives de l'existence et de la dynamique de l'eau non gelee du pergelisol et de la couche active. Les contenus en eau non gelee sont negligeables dans les couches de mousses vivantes et mortes, petites dans les couches de tourbe, et plus importantes dans les limons ou lse variations sont significatives d'un endroit a l'autre. L'effet de l'eau non gelee sur le regime thermique du sol est le plus marque immediatement apres le gel et pendant le refroidissement de la couche active. Il est moins important pendant le rechauffement et le degel de la couche active et pendant le gel et le degel de la couche saisonnierement gelee. Les effets de cette eau non gelee persistent moins d'un mois dans les reegions de pergelisol ‘froid’ mais se manifestent pendant la majorite de la periode de gel dans le pergelisol dit ‘chaud’. Physiquement, l'eau non gelee introduit une chaleur latente qui modifie les proprietes thermiques et retarde la variation de temperature de la couche active on du pergelisol. L'eau non gelee dans la couche gelant et gelee, et dans le pergelisol proche de la surface, protege le sol d'un refroidissement rapide et cree un fort gradient thermique dans le sol proche de la surface du sol, gradient qui augmente le flux de chaleur. Celui-ci accroit aussi l'effet d'isolation de la couverture neigeuse. Aucune difficulte particuliere n'apparait dans l'emploi de la conduction thermique pour modeliser le tarnsfert de chaleur dans la couche active pendant la periode zero. Copyright © 2000 John Wiley & Sons, Ltd.

Effects of unfrozen water on heat and mass transport processes in the active layer and permafrost.

Unfrozen water content as a function of temperature was measured in the laboratory using pulsed nuclear magnetic resonance (PNMR) for a Windsor sandy loam soil. The PNMR data were related to previously measured soil moisture retention data through the modified Clausius-Clapeyron equation, with suitable adjustment for surface tension. The transformed measured unfrozen water content data and the previously measured soil moisture retention data were expressed by a Brooks and Corey type of equation with the required set of regression parameters determined. It was found that a single set of parameters were sufficient to correctly express the behavior of these data when suitable constraints were imposed on the unfrozen water content data. Additional insight into the traditional form of expressing unfrozen water content data is presented in terms of air or ice entry pressure.

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Comparison of soil freezing curve and soil water curve data for Windsor sandy loam

Unfrozen Water Contents of Submarine Permafrost Determined by Nuclear Magnetic Resonance

: An experiment is described that demonstrates the balance between the ice and the unfrozen water in a frozen soil as water is removed. Nuclear magnetic resonance (NMR) is used to monitor the unfrozen water content as the soil is dehydrated by a molecular sieve material. Our results show that the unfrozen water content of a Morin clay soil remains constant until the total water content has been reduced to the point where no ice remains in the system. Once the ice is depleted, the unfrozen water content determined by NMR corresponds to the total water content of the soil determined by the weight of water removed by the molecular sieve material. Thus the validity of utilizing NMR in determining unfrozen water contents vs temperature is established. (Author)

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Allen R. Tice

Papers

Comparison of soil freezing curve and soil water curve data for Windsor sandy loam

Comparison of soil freezing curve and soil water curve data for Windsor sandy loam

Unfrozen Water Contents of Submarine Permafrost Determined by Nuclear Magnetic Resonance

Relationship between the ice and unfrozen water phases in frozen soil as determined by pulsed nuclear magnetic resonance and physical desorption data

Partial glass formation: A novel mechanism of insect cryoprotection