Release of latent heat associated with the freezing of pore water results in the maintenance ofisothermal temperatures at or just below zero-degrees centigrade within the freezing active-layer. Thisperiod of isothermal conditions within the active layer, commonly termed the zero-curtain, is both aphysical boundary preventing cooling in the underlying permafrost and a length of time (Outcalt et al.1990). The zero curtain establishes in the active layer about the phase-equilibrium temperature anddecouples the permafrost from the atmosphere for its duration (Osterkamp and Romanovsky 1997).Following the closure of the zero curtain temperature at the top of permafrost is permitted to declineat a rate governed by the thermal conductivity of the snow pack and the frozen active layer and theprevailing thermal gradient between the atmosphere and permafrost (Burn and Zhang 2009). Theclosure of the zero curtain, expressed by the decline in temperature at the top of permafrost, denotesthe completion of active-layer freezeback and the onset of the cooling period (Burn and Zhang 2009;Karunaratne 2011). Given that the zero curtain is associated with latent heat released from waterduring soil freezing its development in dry material is not possible. This effect was described for thefirst time by Sumgin in Russian Arctic in 1940.• Zero-curtain effect was analyzed at 4 different sites in Tiksi during fall seasons in 2012-2014 with focuson September-October 2013. Soil water contents at these sites ranges from wet to dry depending onthe site location.• The duration of zero-curtain effect occurring in Tiksi at the 4 different locations varies from 25 to 50days for different years. Its onset depends on the air temperature falling below 0ºC. Its durationdepends on the soil water content of the specific area.• Our study shows that Tiksi Observatory Site is horizontally inhomogeneous containing 10% dry soil and90% wet soil regions. Depending on the soil water content different regions create different impact tothe energy budget.• Our measurements show that surface (“skin”) temperature follows the air temperature profile rather thansoil temperature, that is, its dependence on soil type and water content is weak. In other words, thezero-curtain effect has a weak impact on the skin temperature. This result is important for atmosphericmodels and bulk flux parameterization. However, the zero-curtain effect creates substantial vertical andhorizontal temperature gradients in upper soil layers during transition season
Release of latent heat associated with the freezing of pore water results in the maintenance of<br>isothermal temperatures at or just below zero-degrees centigrade within the freezing active-layer. This<br>period of isothermal conditions within the active layer, commonly termed the zero-curtain, is both a<br>physical boundary preventing cooling in the underlying permafrost and a length of time (Outcalt et al.<br>1990). The zero curtain establishes in the active layer about the phase-equilibrium temperature and<br>decouples the permafrost from the atmosphere for its duration (Osterkamp and Romanovsky 1997).<br>Following the closure of the zero curtain temperature at the top of permafrost is permitted to decline<br>at a rate governed by the thermal conductivity of the snow pack and the frozen active layer and the<br>prevailing thermal gradient between the atmosphere and permafrost (Burn and Zhang 2009). The<br>closure of the zero curtain, expressed by the decline in temperature at the top of permafrost, denotes<br>the completion of active-layer freezeback and the onset of the cooling period (Burn and Zhang 2009;<br>Karunaratne 2011). Given that the zero curtain is associated with latent heat released from water<br>during soil freezing its development in dry material is not possible. This effect was described for the<br>first time by Sumgin in Russian Arctic in 1940.<br>• Zero-curtain effect was analyzed at 4 different sites in Tiksi during fall seasons in 2012-2014 with focus<br>on September-October 2013. Soil water contents at these sites ranges from wet to dry depending on<br>the site location.<br>• The duration of zero-curtain effect occurring in Tiksi at the 4 different locations varies from 25 to 50<br>days for different years. Its onset depends on the air temperature falling below 0ºC. Its duration<br>depends on the soil water content of the specific area.<br>• Our study shows that Tiksi Observatory Site is horizontally inhomogeneous containing 10% dry soil and<br>90% wet soil regions. Depending on the soil water content different regions create different impact to<br>the energy budget.<br>• Our measurements show that surface (“skin”) temperature follows the air temperature profile rather than<br>soil temperature, that is, its dependence on soil type and water content is weak. In other words, the<br>zero-curtain effect has a weak impact on the skin temperature. This result is important for atmospheric<br>models and bulk flux parameterization. However, the zero-curtain effect creates substantial vertical and<br>horizontal temperature gradients in upper soil layers during transition season
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