青藏高原土壤湿度对低涡降水反馈过程中的边界层垂直混合作用研究

Vertical mixing in the planet boundary layer (PBL) is an important and key factor in PBL parameterization, which has important influences on the vertical transport and structures of momentum, heat and water vapor in PBL, as well as the simulation of Tibetan Plateau Vortex (TPV) and its precipitation. The relationship between soil moisture anomaly and TPV and its precipitation has received widespread attentions. However, the deep investigation and understanding of the impacts of soil moisture in summer over the TP on vertical mixing in the PBL, and its further impacts on thermodynamics of PBL structures, as well as TPV and its precipitation, are still imperative. Based on observations and reanalysis data, the relationship between soil moisture and thermodynamic structures of lower atmosphere, are analyzed before and after the formation of TPV. Furthermore, a series of numerical sensitivity simulations are used to analyze the impacts of surface sensible and latent heat caused by soil moisture anomaly on vertical mixing in the PBL, as well as the relationship between soil moisture and boundary layer vertical mixing. The physical mechanism of boundary layer vertical mixing in the feedback of soil moisture to TPV and its precipitation during its generation, development and evolution over the TP is also analyzed and elaborated. The conduction of the project could provide a scientific basis for the improvement of numerical model performance on the simulation of summer precipitation over the TP.

边界层垂直混合过程是大气边界层参数化的重要和关键环节，通过影响边界层大气动量、热量和水汽的垂直输送及垂直结构对高原低涡降水产生重要的影响。青藏高原夏季土壤湿度与低涡降水的关系已受到广泛关注。关于青藏高原夏季土壤湿度异常如何影响边界层垂直混合过程，进而对边界层大气热动力结构和高原低涡降水产生影响的认识需要进一步深化。本项目利用观测和再分析资料，分析青藏高原低涡生成前和生成后的演变过程中土壤湿度与低层大气热动力结构的关系；利用数值模拟，分析青藏高原夏季土壤湿度异常引起的地表感潜热变化对边界层垂直混合过程的影响，得出土壤湿度影响边界层垂直混合的关系；分析边界层垂直混合过程对土壤湿度的响应在青藏高原夏季低涡降水生成、发展和演变过程中的作用机制。为提高数值模式对青藏高原夏季降水的模拟能力提供理论基础和科学依据。

本项目基于数值模拟试验，结合多源观测资料和再分析资料，揭示出了模式在高原低层大气的干、冷偏差，以及在高层大气的干偏差是造成高原涡及其降水模拟误差的重要原因。基于模式参数化组合试验和敏感性试验，揭示出了大气凝结潜热仅仅是降水的产物，对高原涡强度的贡献较小；白天对流云团在消亡中低层大气的非绝热加热和水平方向上对流云的融合是诱发夜间高原涡的关键。采用数值敏感性试验，得出了陆地中尺度涡旋外部环境场和涡旋内部边界层垂直混合对涡旋强度、移动和非对称结构的影响机制：涡旋内部增强的边界层垂直混合则会显著减弱涡旋强度，涡旋外部环境场增强的边界层垂直混合会通过增强涡旋环境场中的对流不稳定性、大气斜压性，以及风切变逆风方向上的高 θ_e 侧向平流输送，增强垂直涡度和相对螺旋度，从而有助于涡旋强度和对称性结构的维持。通过考虑对流云内的垂直运动，提出了一个新的对流调整时间方案，该方案不再依赖于水平分辨率，可对高原弱而深厚对流云实施快速地对流调整，提高了降水的模拟能力。基于初始土壤湿度扰动集合试验，揭示出初始土壤湿度增加不仅会显著增加边界层的水汽含量，还会增强高原西北部和东南部的热力对比，从而增强高原涡及其降水强度但减弱东移速度。土壤湿度不仅会影响边界层垂直混合的强度，还会影响其最大值所在的高度。将土壤湿度引入到边界层垂直混合参数化方案的敏感性试验结果揭示出，减弱并降低高原边界层垂直混合的作用高度，会通过减弱地表水汽向边界层的传输效率，降低边界层内的水汽含量，抑制边界层内水汽向边界层顶的垂直输送，对降水的模拟产生积极作用。通过本项目的研究，加深了边界层垂直混合对土壤湿度的响应及其在陆地中尺度涡旋强度、移动和非对称结构中作用机制的理解。为提高数值模式对青藏高原夏季降水的模拟能力提供了事实和理论依据。