| The climate of the Qinghai-Tibet Plateau is warming and humidifying against the background of global warming.The water and heat exchange between ground and air caused by subgrade construction continues to change with climate change,which has a significant impact on the freezing and thawing characteristics of the active layer and the hydrothermal state of permafrost subgrade.Therefore,it is of great engineering significance to deeply explore the single effect of rised air temperature,increased rainfall,and the composite effects of warming and humidifying on the surface energy balance process of the permafrost active layer and the hydrothermal stability of the permafrost subgrade.Based on this,this paper firstly conducts a permafrost subgrade model test by considering the effect of rainfall energy through multiple working conditions,exploring the impact of rainfall under the effect of rainwater sensible heat on the surface energy balance and hydrothermal dynamics of permafrost subgrade,and clarifying the important role of rainwater sensible heat.Furthermore,a permafrost water-heat coupling model with surface energy balance that considers rainwater sensible heat is constructed through theoretical improvement,and the effect mechanism of rainfall on the surface energy balance and water and heat transfer in the active layer of permafrost is simulated,verified,and analyzed.Finally,a comparative study was conducted by simulating the setting of working conditions on the differences between the single and composite effects of rised air temperature and increased rainfall on the surface energy balance and the hydrothermal state in the active layer,and the model was expanded to predict the long-term stability of a permafrost subgrade under the future climate model SSP2-4.5.The main research progress is as follows:(1)The indoor model test of permafrost subgrade under the condition of rainwater sensible heat rainfall shows that the seasonal difference between potential rainfall temperature and air temperature in a permafrost environment is significant,and the maximum temperature difference in summer is nearly 3°C.The summer rainfall causes a significant decrease in the pavement temperature,surface upward radiation flux,and shallow soil heat flux under the impact of rainwater sensible heat.The autumn rainfall has a weak cooling effect on the subgrade soil.The spring rainfall promotes the reduction of soil heat flux and temperature under the pavement and shoulders,which makes it easy to cause the temperature to rise on the slope and the natural ground soil.The change in moisture content in the active layer of the subgrade exhibits similar seasonal characteristics.Ignoring the rainwater sensible heat effect underestimates the effect of rainfall on the reduction of pavement long wave radiation and temperature,overestimates the water accumulated amount in the subgrade,and weakens the soil cooling effect under the pavement,leading to temperature rises in the shoulder,slope,and natural ground.Overall,it affects the evaluation of the rainfall effect in different seasons.(2)A frozen soil water-heat coupling model with surface energy balance considering rainwater sensible heat is constructed.The average deviation errors of soil volume moisture content,temperature,and heat flux are within±1.198%,±0.704℃,and±1.66W/m~2 through COMSOL Multiphysics simulation,and the consistency index is greater than 0.877,0.929,and 0.937,respectively.The optimized model is used to explore the changes in surface energy and temperature during rainfall.The results show that the summer rainfall increases the surface evaporation latent heat and rainwater sensible heat,decreases the surface net radiation,sensible heat,and surface soil heat flux to cool the ground.The cooling effect is positively correlated with rainfall intensity and influenced by rainfall periods.The cooling effect of daytime rainfall events in the summer is significant.The rainwater sensible heat promotes the ground surface cooling.The rainwater briefly heats the ground surface at night,but the significant effects of evaporation latent heat cause the ground surface to continue to cool.Overall,the high frequency and small rainfall amounts in the Beiluhe region,Qinghai-Tibet Plateau have a relatively small rainwater sensible heat effect,but the impact of extreme rainfall needs to be taken seriously.(3)The simulated studies on different climate scenarios indicate that compared to the single effect of rised air temperature and increased rainfall,the composite effect of warming and humidifying resulted in a significant increase in surface net radiation and evaporation latent heat,a more significant decrease in surface sensible heat,and a smaller impact of rainwater sensible heat,leading to an increase in surface soil heat flux.Under the composite climatic effects,the increase in temperature gradient liquid water flux is significant,and the increase in water potential gradient liquid water flux in the shallow layer is also greater than the single effect of rised air temperature but less than the single effect of increased rainfall.The comprehensive influences lead to a smaller increase in soil water content in the warm season than the single effect of increased rainfall.In addition,the heat conduction flux of the active layer increases significantly in the cold season but is smaller than the single effect of rised air temperature.The liquid water convection increases significantly in the warm season but is less than the single effect of increased rainfall.As a result,the active layer heats up,but the increased amplitude of the soil temperature is smaller than the single effect of the rised air temperature.When the air temperature rises by 1.0°C,the permafrost table declines by 0.1m,and the average temperature of the active layer increases by 0.43°C.The rainfall increases by100mm,the permafrost table upraises by 0.08m,and the average temperature decreases by0.11°C.Under the composite effect,the influence of increased rainfall to the temperature reduction of the permafrost is 16.3%.Under the trend of warming and humidifying,the thermal effect of environmental warming on permafrost is still dominant.(4)The prediction of permafrost subgrade stability in the warming and humidifying climate in the next 30a shows that the surface sensible heat flux decreases with the increase of ground surface latent heat,net radiation,and rainwater sensible heat,the increase of latent heat and rainwater sensible heat is significantly smaller than the sum of the changes in net radiation and sensible heat,resulting in an increase in the surface soil heat flux.The increase in surface heat flux at the subgrade center is greater than that of the shoulder and natural ground.The soil temperature of the subgrade is significantly increased,and the permafrost table declines rapidly,which is very likely to cause thermal disturbance of the permafrost at the depth of the subgrade.The increase in soil evaporation effect on the natural ground and the upper part of the subgrade leads to a decrease in the soil water content,with the decrease on the shoulder being greater than that of the natural ground and the subgrade center.The process accelerates the water accumulation at the junction between the lower part of the subgrade and the original ground.Based on this,it is necessary to strengthen the control of water migration and accumulation at the interface between the lower part of the subgrade and the original ground,and reduce the rainfall infiltration at the shoulder.The comprehensive subgrade cooling measures combining active and passive methods are taken to improve the whole temperature control effect of the subgrade,especially the effective control of the temperature rise of the subgrade center,and reduce the impact of climate change on the frozen subgrade to maintain the stability of the lower permafrost. |
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