| Under the background of global warming,significant climate change has taken place in the polar regions.The polar regions is covered by the elements of Cryosphere,the potential benefits and disadvantages of precipitation change depend on the form of precipitation.The changes of precipitation form in spring-summer period have important impacts on mass balance and energy flux of the underlying Cryosphere that closely related to hydrological cycles and the health of the ecosystem over the polar regions.This paper focuses on the characteristics,causes and effects of precipitation form changes during spring-summer period in Arctic and Antarctic.Firstly,we developed a special standard of optimum temperature threshold to separate rainfall from the total precipitation by use of the observational data of 19station in Alaska and northern Canada,where the solid and liquid precipitation are recorded respectively.This optimum temperature was used to analyze the changes of precipitation form at 253 stations in Arctic over the last 50 years.Secondly,we validate the performance of ERA-Interim reanalysis data on the temperature and precipitation in the polar regions.And the causes of precipitation form change in Arctic were analyzed from the perspective of surface temperature,vertical temperature,vertical humidity and atmospheric circulation by using this data.Thirdly,the characteristics and causes of precipitation form in Antarctic over the past 30years were analyzed by using the observational data of the Great Wall station,Zhongshan station and Antarctic Peninsula stations,and the ERA-Interim reanalysis data.Finally,the effects of rainfall on snow cover during spring-summer period were discussed by using field observation data and the Polar-WRF model.Results shows that:1.The first-rain occured earlier and the proportation of precipitation occurring as rain was increased in Alaska and northern Canada during 1961-2015 spring-summer period.We derived the seasonal optimum temperature threshold of 2.01??spring?,0.54??summer?,1.14??fall?,1.17??mean annual?and 0.74??spring-summer?,respectively.By using these temperature thresholds,we distinguished the solid and liquid phase from total precipitation archived at 253stations and investigated their variation trend during 1961-2010.The proportion of rainfall in total precipitation decreased with latitudes in the region north of 60°N.The precipitation forms showed obvious seasonal and interannual variability.Specially,the spring RPR increased over most of the terrestrial Arctic?82.46%stations?,22.37%of which passed 95%statistical significance test,indicating that there was a general transition trend from solid precipitation to liquid precipitation in the spring Arctic continents over the past decades.In summer and autumn,there was obvious regional variability in the south of the research area,but in the arctic coastal stations the increasing trend appeared uniform.Additionally,more precipitation had fallen as rainfall instead of snowfall in Alaska,Siberia and Northern Europe during spring-summer period.Temperature and atmospheric circulation influence the change of RPR.Our results indicate that both the spring-summer mean temperature?0.4-1?/decade?and the RPR?2-6%/decade?increased significantly in the Canadian Arctic Archipelago from 1979–2015.Further analysis found that,the near-surface temperature?below 700hPa?,averaged over Arctic,was increased significant?1?/10a?in recent decades.This is beneficial for the transition of precipitation form from solid to liquid.The near-surface specific humidity?below 700hPa?was also increased significantly(0.1g·kg-1/10a).However,due to the low content of water vapor in Arctic,most of the water vapor is transported from low latitudes.The change of atmospheric circulation has a more profound effect on the RPR.Except for the contribution of climate warming,the North Atlantic Oscillation?NAO?is another key factor influencing temporal and spatial differences in the RPR.In the positive phase of NAO,more rainfall will occur in the areas near to the North Atlantic Ocean.While in the negative phase of NAO,more rainfall will occur in northern Canada.Different precipitation form data?rainfall days and snowfall days?were analyzed based on the observations of automatic weather stations at the Great Wall station?GW?and Zhongshan station?ZS?in Antarctica during 1985-2015 and 1989-2015,respectively.The results showed that the precipitation days at GW was more than at ZS.In the GW,the number of rainfall days is more than that of snowfall day during January-March.However,almost all the precipitation occurring as snow in the ZS.The precipitation form is affected by the temperature change.The increasing?decreasing?number of the total summer rain days?snow days?reversed and began significantly decreasing?increasing?in approximately 2001 in the GW,agreeing well with the changes of the surface air temperature over the Antarctic Peninsula.Further,we analyzed the movements of the Amundsen Sea Low?ASL?and found that the location of the ASL correlated well with the changes of precipitation form.The westward movement of the ASL may have led to a cold southern wind anomaly and fewer?more?rain?snow?days over the Antarctic Peninsula during the summer seasons of recent years.The rainfall only occurs in the coastal areas in Antarctica.The Antarctic Peninsula is one of the largest RPR areas which can reach 20-70%in summer.In the last 30 years,most areas of Antarctica had no remarkable trends in RPR.The RPR have a significant decrease trend in the Antarctic Peninsula,the Ronne Ice Shelf and Queen Mary coast,and a significant increase trend in the Auth coast.In Austral summer,RPR has a positive correlation with SAM in the coastal areas.When SAM in a positive phase,the pressure is abnormal high in mid-latitudes and unusual low in the polar regions,the strengthened westerlies are bad for the water vapor transmission.When SAM in a negative phase,the changes of pressure are opposite.It is conducive to the transportation of water vapor to the Antarctica,which increases the RPR in the coastal areas.There is a significant positive correlation between the RPR and ENSO in coastal areas of Marie Byrd Land and Rathman hills.The RPR will increase in these regions when the sea surface temperature in the area of Nino 3.4 is abnormal high.Based on the data obtained from the field observation experiment in Barrow area,this paper analyzes the influence of rainfall or snowfall on the melting of snow in the polar region.It is found that when the rainfall occurred,the dry snow on the surface changed to wet snow,the snow particle size increased,the surface albedo reduce to about 0.7,which were beneficial to the surface absorbing more heat.While the occurrence of snow can increase the surface albedo to about 0.85 and hinder the beginning of ablation.Simulation through the Polar-WRF model found that the snow melts mainly through the exchange of sensible heat between the atmosphere and the surface during spring-summer period.The heat from rainfall and the latent heat of the phase change contribute to the melting of the snow.Even after the surface covers by the new snow,the melting and refreezing continue in the snow cover.A large amount of latent heat has a great effect on the melting of the snow.In general,under the background of global warming,the temperature of Arctic and Antarctic Peninsula increased significantly.Combined with the changes of atmospheric circulation,the precipitation form will change form solid to liquid,the global cryosphere region will become wetter.How to deepen the understanding of the cascad impact of this change is one of the important issues in climate change. |
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