Ninth International Conference on Permafrost ... - IARC Research

Phase Changes of Water as a Basis of the Water and Energy Exchange Function ofthe CryosphereV.V. ShepelevMelnikov Permafrost Institute, SB RAS, Yakutsk, RussiaWater exists on our planet in all three of its phases withinthe temperature and pressure ranges found on Earth, resultingin very high activity and extent of phase transitions of water.This involves intensive phase interaction of water, that is,water transfer from the liquid state to the gaseous state andback, from the solid state to the gaseous or liquid state, etc.Many authors emphasized the importance of the interphaseform of water movement. Priklonsky (1958), in his generalcharacterization of groundwater formation, consideredsuch phase changes as evaporation and freezing to be themain types of water movement in the ground. Khodkov &Valukonis (1968), in their discussion of the main types ofmovement of natural waters, ranked the phase transition ofwater above other forms of water movement or migration.It is this form of water movement that Vernadsky (1960)had in mind, suggesting the existence of the phase field ofthe Earth, which encompasses the upper lithosphere andthe near-surface atmosphere. The necessity of applying thetheoretical knowledge on water physics to investigations ofglobal water cycles was underlined by Sokolov (1966). Henoted, in particular, that the role of the cryosphere as a waterexchangesystem is very high, though remains little studied.Research on interphase interactions of water is complicatedby the fact that all the basic states of water are not phasehomogeneous,but phase-heterogeneous. In other words, thephase mixing effect is inherent in water. In the atmosphere,water is present in liquid, solid, and gaseous states. Similarly,surface ice and ice-rich permafrost contain some amounts ofthe liquid and gas forms of water. The phase heterogeneityof the basic states of water can be explained by the fact thatin nature, there exists no absolutely pure water in either ofthe states—liquid, solid, or gas. Water in any macroscopicvolume is, above all, a dispersion medium in whichvarious microscopic impurities, such as mineral, organicand other solid particles and compounds, are dispersed.The high surface energy of these microscopic particlescauses the formation of water microphases on their surface,which intensively exchange water with the surroundingmacroscopic medium, determining to a large measure itswater- and energy-exchange function.In the ground, the microscopic phase dispersion of wateris boosted, so to say, by the mechanical dispersion of thesoil. For example, pellicular water in the zone of aerationis a liquid microphase of water, which actively exchangeswater and interacts with the surrounding pore air medium.Similarly, unfrozen interfacial water in ice-rich permafrostcan be considered a liquid microphase of water which is indynamic equilibrium with the pore ice medium. This explains,among other factors, the well-known concept of equilibriumunfrozen water content in frozen ground (Tsytovich 1945,1959). This concept implies that the unfrozen water contentin frozen ground varies with changes in temperature andexternal pressure of soil or rock. Phase changes of unfrozenwater to ice and back occur even under very slight changesin ground temperature or external pressure.Hence the basic physical states of water reflect onlyits macroscopic phase homogeneity, characterizing oneor another phase of bulk water as a continuum. From themicroscopic point of view, the main physical states ofwater are phase heterogeneous, and this drives the waterand energy exchange and other important processes in theEarth’s atmosphere, hydrosphere, lithosphere, and, certainly,cryosphere.Considering the crucial role the phase changes andphase interactions of water play in the cryosphere, and inorder to quantify the water and energy exchange functionof the cryosphere, it is proposed that the global cyclesin climatic circulation of water are distinguished asfollows: cryoatmogenic, cryohydrogenic, atmolithogenic,glaciogenic, and cryolithogenic (Table 1).The cryoatmogenic cycle is related to sublimation of watervapour in the atmosphere, that is, the phase changes of waterfrom gas to solid and back, and subsequent falling on theearth surface as large snow and ice formations.The cryohydrogenic cycle is driven by the formation ofseasonal river ice, lake ice, icings, and ground ice in theactive layer of permafrost and the subsequent melting ofthese seasonal ice forms, as well as snow cover.The atmolithogenic cycle involves evaporation,condensation and sublimation of water in the zone ofaeration, which can be viewed as subsurface atmosphereand where intensive moisture transfer occurs, linking theatmosphere and the lithosphere.The glaciogenic and cryolithogenic cycles are caused bylong-term, rather than seasonal, climatic fluctuations. In coldperiods, the solid phase of water in glaciers and permafrostincreases in volume and mass. Conversely, in warm periods,liquid water resources increase due to melting of glaciers andground ice. Hence the glaciogenic and cryolithogenic cyclesTable 1. Characteristics of the cryosphere as a water and energyexchange system.Main water andenergy cyclesMass of waterinvolved in annualwater cycle, kgEnergy released (+)or taken up (-), WtCryoatmogenic 1.0 · 10 13 ± 0.9 · 10 12Cryohydrogenic 2.6 · 10 16 ± 2.75 · 10 14Atmolithogenic 0.2 · 10 11 ± 0.18 · 10 10Glaciogenic 0.25 · 10 16 - 2.6 · 10 14Cryolithogenic 2.5 · 10 13 - 0.26 · 10 12283