季节内时间尺度上青藏高原雪盖变化对东亚冬季风的影响

Snow cover is a crucial lower boundary condition of the atmosphere. Tibetan Plateau (TP) snow cover (TPSC), as an important predictor of East Asian monsoon, influences East Asian large-scale atmospheric circulation by modulating the TP thermal forcing. However, systematic understanding of the impact of fast TPSC variation on East Asian winter monsoon (EAWM) at the intraseasonal time-scale is stall lacking. Based on the above background, we propose to investigate the influence of intraseasonal variation of TPSC on EAWM from a new perspective at intraseasonal time-scale. This work aims to investigate the lag relationship between TPSC and EAWM by using statistical analysis method based on multiple sets of remote sensing snow cover data, meteorological station data and atmospheric circulation reanalysis data. Major components of EAWM, such as near surface temperature and wind, precipitation, sea level pressure, East Asian trough, together with various EAWM indices will be discussed. To further reveal causality of relationship and physical mechanism, statistical analysis and dynamical diagnosis for both reanalysis of atmospheric circulation and numerical experiments will be performed. We also expect to improve the skill of an empirical model for extended-range forecast of winter surface air temperature, precipitation and EAWM indices by using the intraseasonal variation of TPSC as an additional new predictor with physical mechanism.

雪盖是重要的大气下边界条件。青藏高原雪盖通过改变青藏高原热力强迫作用影响东亚大尺度大气环流活动，对东亚季风有重要预测意义。然而，目前我们对季节内时间尺度上青藏高原雪盖快速变化对东亚冬季风的影响尚缺乏系统性认识。基于上述背景，本项目将从季节内时间尺度新视角，系统研究青藏高原雪盖季节内变化对东亚冬季风的影响。本项目拟利用多套遥感雪盖资料、气象台站资料和大气环流再分析资料，通过统计分析方法，揭示东亚近地层气温和风场、降水、海平面气压场、东亚大槽等东亚冬季风关键大气要素及多种东亚冬季风指数相对于青藏高原雪盖季节内变化的滞后变化关系。并对大气环流再分析资料和数值试验结果同时进行统计分析和动力诊断，明晰统计关系的因果性和物理机制。进而将青藏高原雪盖季节内变化作为新的有物理意义的预报因子加入延伸期统计预报模型，以期提高我们对冬季气温、降水和冬季风指数等的延伸期预报技巧。

青藏高原雪盖在季节内时间尺度上有较强变化，且对大气活动有重要影响。本项目围绕季节内时间尺度上青藏高原雪盖变化对东亚冬季风的影响研究主题，完成了以下研究内容：（1）研究了季节内时间尺度上青藏高原雪盖快速变化的时空特征，并讨论了青藏高原雪盖季节内变化与大气环流的联系；（2）研究了青藏高原雪盖季节内变化对东亚季风区冬季对流层中层和低层大气环流的影响，并揭示了影响机理；（3）研究了次季节预报模式中青藏高原雪盖的系统性偏差及其影响。本项目得到的主要结论有：（1）青藏高原雪盖的季节内变率对其总变化贡献明显，青藏高原雪盖与同期气温负相关、与前期降雪正相关，垂直上升运动引起的异常绝热加热和水汽平流异常都影响青藏高原雪盖的季节内变化；（2）青藏高原雪盖季节内变化通过雪–反照率效应，迅速造成高原季节内热力异常，进而造成的温度平流过程和绝热过程影响东亚冬季风（包括对流层中层和低层大气风场、温度场、东亚大槽等）；（3）次季节预报模式中的青藏高原雪盖存在系统性高估，并随着模式的积分时间而累积，这种系统性正偏差会影响模式预报的青藏高原地面气温，随着预报时效增加模式中的青藏高原地表气温变得更冷。