Characteristics of Stratiform Cloud over the East of Tibetan Plateau Simulated by Two Versions of the FGOALS2 Climate System Model
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摘要: 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。
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关键词:
- 耦合气候系统模式 /
- 大气环流模式试验(AMIP) /
- 层云 /
- 云辐射强迫 /
- 青藏高原东侧
Abstract: 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. -
图 1 1984年1月至2007年12月冬季青藏高原东侧大气层顶的短波云辐射强迫（SWCRF）的地区分布（阴影，单位：W m−2）：（a）ISCCP的观测结果；（b）FGOALS-s2的模拟结果；（c）FGOALS-g2的模拟结果；（d）FGOALS-s2的模拟结果减去ISCCP结果的差值；（e）FGOALS-g2的模拟结果减去ISCCP结果的差值。图中黑色方框表示研究区域（22°N～32°N，102°E～118°E），下同
Figure 1. Geographical distribution of shortwave Cloud radiative forcing (shading, units: W m−2) at the top of the atmosphere during winter time from Jan 1984 to Dec 2007 on the east of Tibetan Plateau: (a) Observation results of ISCCP (International Satellite Cloud Climatology Project) data; (b) simulation results of FGOALS-s2 (Flexible Global Ocean–Atmosphere–Land System) model; (c) simulation results of FGOALS-g2 (Flexible Global Ocean–Atmosphere–Land System) model; (d) differences between FGOALS-s2 and ISCCP, (e) differences between FGOALS-g2 and ISCCP. Black boxes are the east of Tibetan Plateau (22°N–32°N, 102°E–118°E)，the same below
图 4 1984年1月至2007年12月青藏高原东侧（22°N～32°N，102°E～118°E）层云区内冬季N（−SWCRF/LWCRF）对净云辐射强迫（NETCRF，单位：W m−2）的散点图：（a）ISCCP的观测结果；（b）FGOALS-s2的模拟结果；（c）FGOALS-g2的模拟结果
Figure 4. N vs. NETCRF (N = −SWCRF/LWCRF, NETCRF = SWCRF + LWCRF) from (a) the observation results of ISCCP, (b) the simulation results of FGOALS-s2, (c) the simulation results of FGOALS-g2 within the stratiform cloud region to the east of Tibetan Plateau (22°N–32°N, 102°E–118°E)
图 5 1984年1月至2007年12月冬季青藏高原东侧整层积分的水汽收支诊断（阴影，单位：mm d−1）：（a–c）
$ - \left\langle \overline {\nabla \left({vq} \right)} \right\rangle$ 垂直积分水汽输送通量的总贡献；（d–e）$ - \left\langle \overline {u \cdot {\partial _x}q} \right\rangle$ 纬向水汽输送通量的贡献；（g–i）−$\left\langle {\overline {v \cdot {\partial _y}q} } \right\rangle $ 经向水汽输送通量的贡献；（j–l）−$\left\langle {\overline {{\partial _p}\omega q} } \right\rangle $ 垂直水汽输送通量的贡献；（m–o）蒸发项的贡献。左列为ERA-Interim的结果，中列和右列分别为FGOALS-s2和FGOALS-g2的模拟结果Figure 5. Climatology of December–February vertically integrated moisture budget from Jan 1984 to Dec 2007 on the east of Tibetan Plateau (shading，units: mm d−1): (a–c) The contribution of vertically integrated moisture convergence flux
$ - \left\langle {\overline {\nabla \left({vq} \right)} } \right\rangle $ ; (d–e) The contribution of zonal moisture convergence flux-$ \left\langle {\overline {{{u}} \cdot {\partial _{{x}}}q} } \right\rangle $ ; (g–i) The contribution of meridional moisture convergence flux −$\left\langle {\overline {{{v}} \cdot {\partial _{{y}}}q} } \right\rangle $ ; (j–l) The contribution of vertical moisture convergence flux −$\left\langle {\overline {{\partial _p}\omega q} } \right\rangle $ ; (m–o) The contribution of evaporation. Left: results of ERA-Interim reanalysis data, middle: simulation results of FGOALS-s2, right: simulation results of FGOALS-g2
图 6 1984年1月到至2007年12月冬季青藏高原东侧平均大气低层稳定度（阴影；700 hPa位温与地面位温的差值，单位：K）与850 hPa的风场（矢量，单位：m s−1）。（a）ERA-Interim的结果与（b）FGOALS-s2、（c）FGOALS-g2模式的模拟结果；（d）FGOALS-s2模式、（e）FGOALS-g2模式与再分析资料的差值。地面高度超过700 hPa区域的结果并未在图中给出
Figure 6. Climatology of December-February low-level tropospheric stability from Jan 1984 to Dec 2007 on the east of Tibetan Plateau (shading, differences of potential temperature between 700 hPa and surface, units: K) and 850 hPa winds (arrow, units: m s−1). (a) Results of ERA-Interim, (b) the simulation results of FGOALS-s2, (c) the simulation results of FGOALS-g2; model biases of (d) FGOALS-s2 and (e) FGOALS-g2 from ERA-Interim. Results in the region with surface pressures less than 700hPa is not shown
图 7 1984年1月到至2007年12月冬季青藏高原东侧（22°N～32°N，102°E～118°E）区域平均的垂直廓线：（a）温度（单位：K）；（b）位温（单位：K）；（c）比湿（单位：g kg−1）；（d）相对湿度（单位：%）；（e）垂直速度（单位：hPa d−1）的垂直廓线
Figure 7. December-February mean vertical profiles from Jan 1984 to Dec 2007 on the east of Tibetan Plateau (22°N–32°N, 102°E–118°E): (a) Temperature (units: K); (b) potential temperature (units: K); (c) specific humidity (units: g kg−1); (d) relative humidity (units: %); (e) vertical velocity (units: hPa d−1)
图 8 1984年1月至2007年12月冬季青藏高原东侧平均云—地表气温辐射反馈（阴影）：（a–c）αSWCRF [见公式（2）]；（d–f）公式（2）中第一项
$\left({\partial {\rm{SWCRF}}/\partial {\rm{CLD}}} \right)\left({{\rm d}{\rm{CLD}}/{\rm d}{T_{\rm{s}}}} \right)$ 的贡献；（g–i）公式（2）中第二项$(\partial {\rm{SWCRF}}/\partial {\rm{LWP}})\left({{\rm d}{\rm{LWP}}{\rm d}{T_{\rm{s}}}} \right)$ 的贡献。（a、d、g）观测资料的结果，（b、e、h）FGOALS-s2的模拟结果，（c、f、i）FGOALS-g2的模拟结果。云观测资料来自ISCCP-D2，地表观测资料来自CRU TS3.10Figure 8. SWCRF–Ts (Surface Temperature) feedback from Jan 1984 to Dec 2007 on the east of Tibetan Plateau (shadings) : (a–c) αSWCRF (equation 2); (b–f) item1:
$\left({\partial {\rm{SWCRF}}/\partial {\rm{CLD}}} \right)\left({{\rm d}{\rm{CLD}}/{\rm d}{T_{\rm{s}}}} \right)$ ; (g–l) item2:${\rm{}}\left({\partial {\rm{SWCRF}}/\partial {\rm{LWP}}} \right)\left({{\rm d}{\rm{LWP}}{\rm d}{T_{\rm{s}}}} \right)$ ; (a, d, g) results from observation data; (b, e, h) simulation results from FGOALS-s2; (c, f, i) simulation results from FGOALS-g2. Cloud data are from ISCCP-D2 and surface data are from CRU TS3.10表 1 观测与FGOALS-s2和FGOALS-g2模拟的青藏高原东部层云区（22°N～32°N，102°E～118°E）平均冬季大气层顶短波云辐射迫（SWCRF）、云量（CF）与云水通量（LWP）。
Table 1. Regional means of short wave radiative forcing (SWCRF) at the top of atmosphere, cloud fraction (CF), and grid-box integrated liquid water path (LWP) within the stratiform cloud region on the east of Tibetan Plateau (22°N–32°N, 102°E–118°E) during winter time from observations and the simulations of FGOALS-s2 and FGOALS-g2
SWCRF/W m−2 CF LWP/g m−2 观测 −70 65% 148 FGOALS-s2 −38 47% 114 FGOALS-g2 −29 41% 75 
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表 2 1984年1月至2007年12月冬季青藏高原东侧（22°N～32°N，102°E～118°E）在公式（2）中各项的区域平均值
Table 2. Winter regional mean of budget terms in equation (2), i.e.,
$\partial {\rm SWCRF}/\partial {\rm CLD}$ ,${{\rm d}\rm{CLD}}/{\rm d}{T\rm{s}}$ ,$\partial {\rm{SWCRF}}/\partial {\rm LWP}$ ,${\rm d}{\rm LWP}/{\rm d}{T\rm{s}}$ on the east of Tibetan Plateau (22°N–32°N，102°E–118°E) from Jan 1984 to Dec 2007$\dfrac{{\partial {\rm{SWCRF}}}}{{\partial {\rm{CLD}}}}$ $\dfrac{ {{\rm d}{\rm{CLD} } } }{ {{\rm d}{T_{\rm{s} } } }}$ $\dfrac{{\partial {\rm{SWCRF}}}}{{\partial {\rm{LWP}}}}$ $\dfrac{ {{\rm d}{\rm{LWP} } } }{ {{\rm d}{T_{\rm{s} } } }}$ 观测 −0.73 −0.25 −0.40 0.2 FGOALS-s2 −0.92 −0.32 −0.94 −0.28 FGOALS-g2 0.12 0.06 0.13 0.02
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