青海三江源区高寒植被地表反照率变化及其辐射温度效应

Albedo changes of alpine vegetation and its radiative temperature effect in the Three-River Headwaters Region of Qinghai

地表反照率是影响地表辐射收支与能量平衡的重要地表物理参数,是区域和全球气候变化研究中的一个关键要素。以三江源为案例区,基于2001—2018年生长季(6—8月)中分辨率成像光谱仪(MODIS)地表反照率(MCD43C3)产品,以及同期气候和植被指数数据,应用随机森林回归算法量化植被和气候对地表反照率时空变化的影响分析了土地利用/覆被变化导致的地表辐射特性参数(地表反照率)的改变引起的辐射温度效应。结果表明:三江源区域生长季地表反照率在空间上呈自东南向西北递增的特征,其均值为(0.163±0.027),集中分布在0.12—0.18。长江源园区、黄河源园区以及澜沧江源园区地表反照率差异较大,分别是(0.177±0.036)、(0.153±0.037)和(0.156±0.002)。从年际变化来看,研究区地表反照率以每10年(0.152±0.763)%速率不显著下降(P=0.47),但其变化趋势存在明显空间分异,其中显著减少(增加)的区域占8.4%(1.9%),长江源园区、黄河源园区以及澜沧江源园区以不同速率下降,分别为每10年下降(0.078±0.900)%、(0.215±0.740)%、(0.152±0.450)%。三江源地表反照率时空变化受气候和植被因子双重影响,而植被因子是主导因子。在气候因子中,最高气温对地表反照率年际变化的影响最大,其次是降水,影响最小的是最低气温。生长季地表反照率变化的辐射温度效应总体上表现为辐射增温效应,值为(0.11±0.42)℃,空间上既有增温效应,也有降温效应。植被覆盖度、地表反照率与辐射温度效应间为正反馈,生长季草地覆盖度增加(降低),地表反照率减少(增加)引起辐射温增(降温)效应。统计分析表明高寒退化植被恢复对区域地表反照率影响引起了辐射增温效应,需对该问题予以更多关注,建议开展进一步的观测和模拟研究,以更深入地揭示高寒草地生物地球物理过程和机制。

Albedo is an important variable affecting radiation budget and energy balance of the Earth surface, and is a hot topic in regional and global climate change studies. The Three-River Headwaters Region(TRH) was studied based on the albedo product(MCD43 C3) from Medium Resolution Imaging Spectrometer(MODIS), and climate and vegetation data in the growing season(June to August) from 2001 to 2018, the random forest algorithm was applied to analysis the impact of vegetation and climate on the spatial and temporal changes in albedo timeseries. And the radiative temperature effect of the albedo was quantified due to the changing fraction of vegetation coverage. The results showed that the regional averaged albedo during the growing season was(0.163±0.027) and mostly in the range from 0.12 to 0.18 with a spatial gradient increasing from the southeast to northwest region. Among the three parts of the National Park, the albedos were(0.177±0.036),(0.153±0.037) and(0.156±0.002) for the Yangtze River headwater, the Yellow River headwater and the Lancang River headwater, respectively. From the perspective of inter-annual changes, the regional mean albedo insignificantly decreased by the rate of(0.152±0.763)% per ten years(P=0.47) with an obviously spatial pattern that significantly decreased(increased) over the area of 8.4%(1.9%), and the decreasing trends of(0.078±0.900)%,(0.215±0.740)% and(0.152±0.450)%, per ten years, respectively for the three parts of the National Park. The spatial and temporal variability of surface albedo in the TRH was influenced by both climatic and vegetation factors, with vegetation being the dominant factor;among the climatic factors, the maximum temperature had the greatest influence on interannual variability of surface albedo, followed by precipitation, and the minimum temperature the least. The radiative temperature effect of surface albedo variation during the growing season was generally a radiative warming effect, with a value of(0.11±0.42) ℃. Spatially, there was both a warming and a cooling effect. There was a positive feedback between vegetation cover, surface albedo and radiative temperature effect, the increasing(decreasing) fraction of grass coverage resulted a decreasing(increasing) surface albedo in the growing season, which caused a warming(cooling) effect of the radiative temperature. The statistical analysis of this paper implyes that the effect of alpine degraded vegetation restoration on regional surface albedo caused radiative warming effect. However, more attention should be paid to this issue and further observation and modelling are recommended to reveal the biogeophysical processes and mechanisms of alpine grasslands in future researches.