青藏高原土壤冻融过程的气候效应:进展和展望

Climate Effects of Soil Freeze-Thaw Process over Qinghai-Xizang Plateau:Progress and Perspectives

青藏高原(简称高原)热、动力作用对东亚乃至全球大气环流及气候有着影响。高原的热力作用主要来自于高原地表的非绝热加热异常变化,高原陆面过程决定着地表非绝热加热。本文回顾总结了高原陆面过程中的土壤冻融过程对土壤水热传输、地表非绝热加热影响及其气候效应的研究。主要体现在如下几个方面的进展:(1)土壤冻融对土壤水分具有"水分存储"效应,冻融过程可将土壤中90%以上的水分从前一年秋季保存到春季释放出来。(2)高原地表非绝热加热估算仍是一个挑战性的问题,再分析资料中的地表感、潜热通量存在较大偏差,且在春季最为显著,数值模式对土壤冻融过程模拟的偏差较大,数值模式和再分析资料对高原地表非绝热加热估算的偏差,影响了对高原热力作用的深入认识和理解。(3)水热完全耦合的参数化方案和冻融参数化方案改进可有效减小模式对土壤温、湿度的模拟偏差。(4)冻融过程将前秋的土壤湿度异常保持到次年春季,进而引起春季地表非绝热加热异常,这可作为跨季节气候预测的"信号"。通过对高原冻融区土壤信息的同化,可显著提高模式对后期东亚天气气候的模拟效果。(5)春季高原融冻异常通过引起土壤湿度异常产生的地表非绝热加热异常,通过改变高原南、北两侧大气的斜压性,激发出西风带内的Rossby波列传播,影响中国东部地区夏季降水的异常。对土壤冻融及融雪物理过程的认识和参数化,是高原和寒区陆气相互作用研究中具有挑战性的问题和未来研究的重要方向。

The thermal effects of the Qinghai-Xizang Plateau(QXP)on the East Asian and global atmospheric circulation and climate are due to the surface diabatic heating,which is closely related to the land surface processes. This paper reviewed the research progress of soil frozen-thawing process on soil hydrothermal transport,surface diabatic heating over QXP and its climate effects,the main points are as follows:(1)soil frozen-thawing process has "water storage" effect,and the water storage index can reach 0. 99 in the whole soil layer.(2)The estimation of surface diabatic heating on QXP is still a challenging problem,and different reanalysis data have large biases,especially in spring,one of the main reasons is the bias of the atmospheric model in simulating the soil frozen-thawing process.(3)The fully coupled water-heat transport scheme and the modified frozenthawing parameterization scheme can effectively reduce the model biases in the simulation of soil temperature and moisture.(4)Soil moisture anomalies in the previous autumn and winter can persist to spring through the soil frozen-thawing process,causing the surface diabatic heating anomalies in spring,can be a signal of climate prediction.(5)The anomalous thawing of frozen ground in spring over QXP affects the surface diabatic heating by causing soil moisture anomalies,and changes the baroclinicity of the atmosphere on the north and south sides of the QXP,which excites the Rossby wave train,leads the anomalies of atmospheric circulation over the downstream regions,causes summer precipitation anomalies in eastern China. The large biases of the numerical model and reanalysis data in estimation of the surface diabatic heating of QXP limit the deep understanding of the thermal effects of the QXP. How to improve the model parameterization by deepening the understanding of the physical processes of freeze-thaw and snow is a challenging problem and an important part of future research.