Solar Radiation Transfer In Lake Ice/Snow Cover And Its Impacts On Lake Heat Balance During Ice-covered Period

In autumn and winter,the temperature drops,and the water body of the lake freezes and seals,forming an ice cap covering the water surface.Compared with the ice free period,the existence of the closed ice cap has changed the original physical,chemical and ecological environment of the lake.(1)The existence of ice layer causes most of the solar radiation to return to the atmosphere,changing the light field environment that enters the water body;(2)The existence of the ice layer weakens or even isolates the direct exchange of heat,momentum and matter between the atmosphere and the lake water body.Atmospheric reoxygenation is stopped,which reduces the dissolved oxygen concentration of the lake water body or even lacks oxygen.These will have a significant impact on the lake environment and the operation of the ecosystem.As the global climate continues to warm,the lake environment and ecological problems caused by the shortening of the freezing period and the thinning of ice thickness have attracted more and more attention.Light(radiation)and temperature are the most basic physical elements of the lake ecological environment.It is very important to accurately determine the influence of the ice/snow process on the solar radiation transmission and temperature of frozen lakes.Based on this,this paper takes the Baotou Nanhai Lake,a typical lake in the cold and arid regions of China,and the BLH-A lake,a typical thermal karst shallow lake in the hinterland of the Qinghai-Tibet Plateau,as the research objects.Shallow thermal karst lakes are produced in permafrost regions of the plateau.The depressions continue to accumulate water and the underlying frozen soil layer melts,making the depressions deeper and deeper,and the lake surface is getting larger and more water.When the accumulation of water reaches a certain level,the water in the low-lying land will no longer dry up in winter,and the permafrost below it will melt away,and the low-lying land will completely become a plateau lake.Generally speaking,the lakes formed in this way are small in scale,but in large numbers,they are very common along the Qinghai-Tibet Road.This paper uses on-site high-frequency monitoring data to determine the optical characteristics and radiation transmission process of the air-(snow)-ice-water system during the freezing period.The effects of the radiation transfer of ice and snow layer on the thermodynamic structure and energy balance of the lake were analyzed.The results are helpful to understand the evolution of lake light field and temperature structure and its environmental and ecological effects under freezing conditions.The research based on this article has mainly obtained the following results:1,Carry out on-site observations of the radiation transmission process of the"air-snow-ice-water"system under different snow conditions,including the incident,reflection and transmission spectrum radiation under the conditions of bare ice and 0.5,1.0,3.5,and 7.0 cm snow thickness.Illumination,calculate the albedo and extinction coefficient of snow and bare ice with different thicknesses.The results show that:(1)The albedo of the bare ice surface increases first and then decreases with the increase of the wavelength,and the average value of the total albedo of the bare ice surface is 0.28;When snow appears,the total albedo increases rapidly to 0.96 as the thickness of the snow layer increases from 0 to 3.5-4.0 cm.The albedo of different snow thicknesses all show a trend of first increasing and then decreasing with the increase of wavelength,and then snow layer continues to thicken and the albedo remains basically unchanged.(2)When there is no snow layer,the change trend of the extinction coefficient of the lake ice layer with wavelength is opposite to the albedo,showing a trend of first decreasing and then increasing.The average spectrum of the overall extinction coefficient of the bare ice layer is 3.09 m^-1;The extinction coefficient of the new snow layer on the ice surface is 9.72 m^-1,and the extinction coefficient of the frozen ice is 2.26 m^-1.(3)The change of transmission spectrum with wavelength is the same as that of solar radiation spectrum,both showing a trend of first increase and then decrease.However,compared with reflection spectrum and solar radiation spectrum,the peak of transmission spectrum moves to the long-wave region,and the peak of radiation flux appears Between 530-600 nm.2,Using the above-mentioned ice and snow optical properties to evaluate the influence of solar radiation and ice and snow layers on the ice-water thermodynamic structure and processes of lakes and ponds on the Qinghai-Tibet Plateau,the results show that:(1)The water temperature of plateau lakes and ponds is kept above 3.0°C during the icing period,and gradually rises to 7-9°C during the melting period.The vertical water temperature structure is composed of an upper stable inversion layer and a middle and lower mixed isothermal layer.(2)Develop an ice-water heat flux(F_w)calculation model,and find that F_w is on the order of 10¹Wm^-2 with obvious seasonal changes,which is a key factor in controlling the growth and melting of the bottom of the ice layer.(3)The analysis of the energy budget of ice and water area shows that the solar radiation absorbed by the ice layer accounts for 21%of the energy income of the ice layer,which is a key factor affecting ice temperature;transmitted radiation is the most important energy source that affects water temperature changes and F_w.F_w accounts for about 95%of the transmitted radiation,and the intense solar radiation over the Qinghai-Tibet Plateau is the key indirect reason for the thin lake ice.Based on the field observation and analysis of Baotou Nanhai Lake and the thermal karst shallow lake BLH-A lake on the Qinghai-Tibet Plateau,this paper summarizes the solar radiation transmission process and energy balance process of the shallow lake during the freezing period,which is helpful to reveal the dynamic change mechanism of light and temperature in frozen lakes.To provide a basis for accurately evaluating the impact of the ice and snow process on the lake ecological environment.