Changes of South-Central Pacific Large-scale Environment Associated with Hydrometeors-Radiation-Circulation Interactions in a Coupled GCM

A common deficiency in coupled atmosphere-ocean models is the lack of stratocumulus 27 clouds near the west coasts of continents and shallow cumulus clouds over the trade wind regions. 28 In this study, we examine the changes of large-scale trade wind environment associated with 29 hydrometeors-radiation-circulation interactions, focusing over the south-central Pacific, by 30 carrying out experiments with falling ice (snow) radiative effects (FIREs) on or off using the 31 CESM1-CAM5 coupled model. We found that experiment with FIREs off (NOS) tends to have 32 too-warm (0.3—0.9 K) sea surface temperature (SST) over the east half of the Pacific, weaker 33 surface wind stress over the northeast side but stronger stress over the southwest side of the study 34 domain, lower-tropospheric wind fields with a cyclonic-like wind pattern that is similar to surface 35 wind stress, and weaker subsidence over the northeast side against observations/reanalysis data 36 and experiment with FIREs on (SON). The cyclonic-like wind pattern for changes between NOS 37 and SON causes stronger moisture convergence, accompanied with horizontal warm advection, 38 horizontal moist advection, and stronger effective ascending motion over the northeast side of the 39 study domain, building large-scale environmental conditions to facilitate middle- and high-clouds 40 instead of shallow cumulus clouds in nature. This large-scale environment in conjunction with the 41 warmer SSTs may be also responsible for the lacking of the stratocumulus clouds near the coast 42 of America. Remaining biases in SON such as the double intertropical convergence zone and 43 parameterization deficiencies may also prevent a more realistic simulation of trade wind clouds 44 and their associated large-scale environments.

当前大气-海洋耦合模式普遍存在的一项不足，是在大陆西海岸附近缺乏层积云（stratocumulus clouds），且在信风区（trade wind regions）难以模拟出浅积云（shallow cumulus clouds）。本研究以太平洋中南部海域为重点研究区域，借助CESM1-CAM5耦合模式开展开启/关闭下落冰（雪）辐射效应（FIREs，falling ice (snow) radiative effects）的敏感性试验，探讨与水成物-辐射-环流相互作用（hydrometeors-radiation-circulation interactions）相关的大尺度信风环境变化。研究结果表明，关闭FIREs的试验（下称NOS试验）在太平洋东半部海域会出现偏高0.3—0.9 K的海表温度（SST，sea surface temperature）；研究区域东北侧的表面风应力（surface wind stress）偏弱，西南侧则偏强；对流层低层风场（lower-tropospheric wind fields）呈现类气旋式分布，与表面风应力的分布特征一致；且相对于观测/再分析资料（observations/reanalysis data）与开启FIREs的试验（下称SON试验），研究区域东北侧的下沉运动（subsidence）偏弱。NOS试验与SON试验的差异所对应的类气旋式风场型（cyclonic-like wind pattern），会使得研究区域东北侧的水汽辐合（moisture convergence）增强，同时伴随水平暖平流（horizontal warm advection）、水平湿平流（horizontal moist advection）以及更强的有效上升运动（effective ascending motion），进而构建出利于形成中高云（middle- and high-clouds）而非自然状态下浅积云的大尺度环境条件。这种大尺度环境与偏高的海表温度相结合，或许也是美洲海岸附近层积云缺失的成因之一。此外，SON试验仍存在双热带辐合带（double intertropical convergence zone）等偏差，以及参数化缺陷（parameterization deficiencies），这些问题同样可能阻碍信风云及其相关大尺度环境的更真实模拟。