Satellite Remote Sensing Retrieval And Application Of Radiative Effects Of Light-absorbing Particles In Snow

Snow is the most reflective natural material on the Earth’s surface.Light-absorbing particles（LAPs）that deposit on the surface of the snow can significantly reduce its albedo and enhance its absorption of solar radiation,resulting in a positive radiative forcing（RF）and accelerating snowpack melting.Continuous changes in snow have important impacts on ecosystems and socio-economic systems.Therefore,it is of great importance to analyze the radiation effects of LAPs in snow under the influence of global climate change in order to gain a better understanding of climate system radiation balance.We developed an algorithm to retrieve the radiation effects of LAPs in snow using MODIS surface albedo data and radiative transfer models in this dissertation.This algorithm was used to analyze the spatiotemporal characteristics of RF by LAPs in snow in the northern hemisphere and the darkening effect of LAPs released by Australian bushfires in snow in New Zealand in 2019-2020.To begin with,a method is presented for calculating the surface blue-sky albedo under clear and cloudy skies by using MODIS data as well as Snow,Ice,and Aerosol Radiative model（SNICAR）and Santa Barbara DISORT Atmospheric Radiative Transfer（SBDART）.The blue-sky albedo is under topographic correction and subsequently the blue-sky albedo of the surface is calculated under all-sky condition.The snow albedo,snow coverage,and snow particle size are retrieved based on the surface albedo data,as well as the content of LAPs in snow as well as the decrease in snow albedo reduction and radiative forcing caused by LAPs.In-situ measurement data are used to validate the retrieval results,and the correction factors are negatively correlated with snow pollution levels.Moreover,we discuss the uncertainty in the retrieval caused by ignoring snow depth and assuming a semi-infinite plane.The results show that the total uncertainty is highest in the Greenland,Northern Russia,and Northeastern Canada regions,approximately±80%,while the uncertainty in the mid-latitude regions is approximately±40%.The developed satellite-based remote-sensing retrieval algorithm is employed to calculate the RF of LAPs in snow across the Northern Hemisphere from 2003 to 2018.Additionally,the spatial distribution characteristics of RF vary throughout the seasons according to an attribution analysis.The results reveal significant spatial variations in RF within the snow of the Northern Hemisphere.Specifically,RF can only be retrieved in mid-latitude regions from December to January and in high-latitude regions starting from February after the polar night.In mid-latitude regions,RF reaches peak value in March,coinciding with the onset of snow melting,and completely dissipates in April.Conversely,in high-latitude regions,RF steadily increases from March and reaches its maximum in May.The results indicate that the factors influencing the spatial distribution patterns of RF vary across seasons when performing a quantitative attribution analysis of the monthly spatial distribution changes of RF.During winter,the primary factors determining the spatial distribution of RF are the snow water equivalent and solar irradiation,with their combined contribution exceeding 50%.In spring,the dominant factor influencing the spatial distribution of RF is the LAPs content,accounting for approximately 73%of the relative contribution.Furthermore,variations in solar irradiation throughout the season are responsible for most of the seasonal fluctuations in RF at both regional and hemispheric scales,contributing approximately 50-95%.LAPs represent the second most influential factor affecting the seasonal variation of RF,contributing approximately 5-30%.The inter-annual variation of RF across various regions over a span of multiple years are also examined.The results indicate that there were no significant changes in RF within the global snow during the period from 2003 to 2018.However,notable decreases in RF were observed in the high-latitude Eurasian（HEUA）and the northeastern China（NEC）over the same time frame.Specifically,the average RF decrease rate in HEUA amounted to-0.04 W m^-2 a^-1,while NEC experienced a more pronounced decrease at-0.14 W m^-2 a^-1.To investigate the factors contributing to the decline in RF within these two regions,sensitivity experiments were conducted to isolate the RF generated by different factors.The results reveal that the decrease trend in RF in both HEUA and NEC can be attributed to a reduction in the LAP content.On average,the RF decrease rate associated with LAPs in HEUA and NEC over the course of multiple years equated to-0.024 W m^-2 a^-1 and-0.068 W m^-2 a^-1,respectively.Furthermore,a series of ground-based in situ observations corroborate the decline of atmospheric LAPs in both HEUA and NEC during the observation period.Finally,a quantitative assessment of the darkening effect induced by the 2019-2020 Australian wildfires on the snow and glaciers of New Zealand is presented,employing the constructed algorithm.The results reveal that during the catastrophic wildfire event in Australia,a substantial quantity of LAPs was released into the atmosphere.These particles were subsequently transported over long distances and deposited on the snow and glacier surfaces of New Zealand.Consequently,there was a reduction of 0.08±0.03 in broadband snow reflectance,persisting for a period of three months and impacting over 90%of the snow-covered areas on New Zealand’s South Island.Simultaneously,the wildfire event accelerated the rate of snow melting at an average of 0.41±0.2 cm day^-1,equivalent to an effective temperature increase of 1.8°C.This indicates that the impact of the wildfire disaster on New Zealand’s hydrological cycle surpassed the overall temperature rise of approximately 1.5°C since the pre-industrial era.Moreover,considering the background of climate change,the frequency and magnitude of future wildfire events are projected to rapidly escalate across various temperature increase scenarios.These natural sources of LAPs will exert significant darkening effects on the broader cryosphere,altering the local radiation balance and hydrological cycle.