FGOALS2两个模式版本对青藏高原东侧冬季层云的模拟特征

冬季青藏高原东部(22°N^32°N,102°E^118°E)层云区是唯一存在于副热带陆地的层云密集区,环流特征较为复杂,大多数耦合气候系统模式对该地区层云的模拟存在较大的偏差。对该地区层云模拟能力的系统分析评估是改进模式性能的重要基础。本文基于国际卫星云计划(ISCCP)卫星资料,评估了中国科学院大气物理研究所两个版本的气候系统模式FGOALS-s2和FGOALS-g2的大气环流模式试验(AMIP)对青藏高原东侧层云的模拟能力。通过分析云辐射强迫等相关特征、大气环流、稳定度、以及地表气温和云的关系,探讨了模式偏差的可能原因。结果表明,两个模式都不同程度地低估了青藏高原东侧的低层云量和云水含量。在垂直结构模拟方面,FGOALS-s2模式能较好地模拟出高原东侧低云主导的特征,其模拟的云顶高度与卫星资料更为接近;而FGOALS-g2模式则高估了该地区的平均云顶高度。分析表明,两个模式均低估了高原东侧的低层稳定度,同时不同程度地低估了该地区中低层水平水汽输送,导致层云云量的模拟偏少。此外,FGOALS-g2高估了高原东侧的上升运动和垂直水汽输送,使得模拟的低云偏少而云顶高度偏高。

Characteristics of Stratiform Cloud over the East of Tibetan Plateau Simulated by Two Versions of the FGOALS2 Climate System Model

Eastern China, on the east of Tibetan Plateau (22°N–32°N，102°E–118°E), is covered by stratiform clouds in winter time. This cloud type is unique to like latitudes of the global subtropical continent. A reasonable simulation of these clouds over the leeside of the Tibet Plateau remains a challenge for the current state-of-the art climate models. In this study, we compared the Cloud radiation forcing and other cloud properties and their radiative forcing at the top of the atmosphere from two LASG/IAP climate system models, Atmospheric Model Intercomparison Project (AMIP) of Flexible Global Ocean–Atmosphere–Land System-s2 and g2 (FGOALS-s2 and FGOALS-g2), and observations from the International Satellite Cloud Climatology Project (ISCCP). It was found that both models underestimated the strength of shortwave cloud forcing, cloud water path, and cloud fraction over Eastern China. FGOALS-s2 can reproduce the domination of low stratiform clouds over Eastern China, while FGOALS-g2 overestimated the proportion of high clouds. Such bias in the simulation of stratiform clouds is related to weak lower tropospheric stability and insufficient low-level moisture. In addition, the overestimated average cloud top height in FGOALS-g2 was explained as an unrealistic updraft, which produced stronger vertical moisture transports over the east of Tibetan Plateau.