Understanding the surface temperature cold bias in CMIP5 AGCMs over the Tibetan Plateau
The temperature biases of 28 CMIP5 AGCMs are evaluated over the Tibetan Plateau (TP) for the period 1979–2005. The results demonstrate that the majority of CMIP5 models underestimate annual and seasonal mean surface 2-m air temperatures (Tas) over the TP. In addition, the ensemble of the 28 AGCMs and half of the individual models underestimate annual mean skin temperatures (Ts) over the TP. The cold biases are larger in Tas than in Ts, and are larger over the western TP. By decomposing the Ts bias using the surface energy budget equation, we investigate the contributions to the cold surface temperature bias on the TP from various factors, including the surface albedo-induced bias, surface cloud radiative forcing, clear-sky shortwave radiation, clear-sky downward longwave radiation, surface sensible heat flux, latent heat flux, and heat storage. The results show a suite of physically interlinked processes contributing to the cold surface temperature bias. Strong negative surface albedo-induced bias associated with excessive snow cover and the surface heat fluxes are highly anticorrelated, and the cancelling out of these two terms leads to a relatively weak contribution to the cold bias. Smaller surface turbulent fluxes lead to colder lower-tropospheric temperature and lower water vapor content, which in turn cause negative clear-sky downward longwave radiation and cold bias. The results suggest that improvements in the parameterization of the area of snow cover, as well as the boundary layer, and hence surface turbulent fluxes, may help to reduce the cold bias over the TP in the models.
Key wordssurface temperature cold bias CMIP5 AMIP Tibetan Plateau surface energy budget
在28个CMIP5-AMIP试验的模式中, 大多数模式对高原气温的模拟存在着冷偏差, 多模式集合和半数以上的模式模拟的地表温度偏低, 主要特征为气温的冷偏差强于地表温度, 高原西部强于高原东部. 通过地表能量平衡分解法, 将地表温度的冷偏差定量分解为反照率反馈项、云辐射强迫项、晴空短波辐射项、晴空向下长波辐射项、感潜热项和地表热通量项. 结果表明, 这些分解项对冷偏差的贡献存在物理上相联系的过程, 积雪覆盖面积偏大引发的反照率反馈作用和晴空向下长波辐射强迫造成了地表温度模拟的冷偏差, 其物理过程是: 低温模式模拟的积雪覆盖面积偏大, 使得地表反照率增大, 地表吸收的短波辐射减少、地表感、潜热通量减少, 使得地表向大气输送的热量和水汽(尽管很小)偏少, 大气温度偏低、水汽含量减少, 导致晴空向下长波辐射减小, 地表温度偏冷. 鉴于积雪覆盖面积在模式模拟中的重要性, 有必要对积雪覆盖面积的参数化方案做出改进, 并提高地表湍流通量, 这可能会有助于减少模式模拟的地表温度冷偏差.
关键词地表温度 冷偏差 CMIP5 AMIP 青藏高原 地表能量平衡
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We are indebted to Prof. Jianhua LU for his countless suggestions in conducting the study. This research is supported by the National Natural Science Foundation of China (Grant Nos. 91437219 and 91637312); the Third Tibetan Plateau Scientific Experiment (Grant No. GYHY201406001); the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (Grant No. QYZDY-SSW-DQC018); and the Special Program for Applied Research on Super Computation of the NSFC–Guangdong Joint Fund (second phase).
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