积雪研究进展

2017年5月，清华大学HydroSky团队博士后陈晓娜等在Journal of Geophysical Research: Atmospheres上发表论文《Observed radiative cooling over theTibetan Plateau for the past three decades driven by snow cover-induced surface albedo anomaly》。

  　积雪变化是气候变化背景下研究者普遍关注的一个课题。从大尺度来说，积雪与地球表层能量平衡、大气循环、湿度、降雨和流域水文状况等有着重要的联系。从小尺度来说，积雪对局部气温、干旱状况、土壤湿度、融雪径流量等起着决定性作用。但青藏高原地形复杂，青藏高原积雪变化及其对气候系统的反馈方面的研究依然很少。本文综合利用高精度积雪遥感数据、地表反照率数据、辐射核数据、站点径流数据以及再分析数据，首次对青藏高原积雪变化驱动下地表反照率异常所产生的短波辐射胁迫作用（S_nRF）进行了估算。研究发现，青藏高原积雪覆盖丰度的变化与地表反照率的异常高度相关；地表反照率的变化与积雪覆盖地表状态时的反照率高度相关，而与无雪地表的反照率变化关联较小。基于该关系，本研究量化了1982-2014年间青藏高原地表反照率变化，并利用辐射核的方法估算了1982-2014年间反照率变化对大气顶端短波辐射胁迫作用。结果显示，1982-2014年间，青藏高原的S_nRF呈微弱的增强的趋势，但并不显著；与年际变化相关，1982-2014年间，青藏高原S_nRF呈现出年内变化加剧的趋势，表现为积雪累积期S_nRF增强，消融期S_nRF减弱。本研究为气候变化背景下青藏高原积雪的反馈与响应提供了理论与数据支撑。

Figure 1. 14-year (a) annual-mean SCF, (b) changes, (c) inter-annual variability in annual-mean SCF and linear trend in SCF, and (d) intra-annual variability in monthly-mean SCF and 14-year changes of SCF in each month during 2001–2014 over the TP. Black dots in (b) indicate that changes are statistically significant at the 95 % level. The orange (green) color indicates an increase (decrease) in SCF in a given month from 2001 to 2014. The error bar in (d) isexpressed as the standard deviation of SCF in a given month during 2001–2014.

Figure 2. 13-year annual-mean (a) land surface albedo (a_s) , (b) a_sf, changes in (c) a_s , (d) a_sf and (e) albedo contrast (∆a_s) between a_s and a_sf, and (f) coefficient of determination (R²) between snow cover fraction (SCF) and ∆a_s over the TP during 2002–2014. Black dots in (f) indicate that the linear correlationbetween SCF and ∆a_s are statistically significant at the 95 % level.

Figure 3. 33-year (a) annual-mean S_nRF, (b) changes, (c) interannual variability ofannual-averaged S_nRF and linear trend in S_nRF and (d) intra-annual variability of monthly averaged S_nRF and 33-year changes of S_nRF in each month over the TP during 1982–2014.The annual-mean values in (a) and (c) were averaged from two S_nRF results using CAM3 and AM2 kernels. The error bars in (c) and (d) are expressed as the range derived from the two S_nRF results in clear-sky condition. Black dots in (b) indicate that changes are statistically significant at the 95% level. The solid green bars in (d) indicate that the S_nRF was enhanced, and the changes are significant at the 95 % levelin a given month over the TP during1982–2014.

   论文作者包括：陈晓娜（清华大学）、龙笛（清华大学）、洪阳（清华大学）等。该研究得到国家自然基金委“西南河流重大计划”等项目支持。论文链接：http://onlinelibrary.wiley.com/doi/10.1002/2017JD026652/full。