“灰色区域”尺度次网格云过程参数化的研究

Currently, operational global NWP models are running at grid size close to 10 km, while regional NWP models are in 2-5 km grid-size range. Updraft in convective could not be resolved explicitly by the dynamics at these resolutions yet; on the other hand, some assumptions in conventional convective parameterization are not valid. Cloud parameterization in models meets “gray zone” problem. At “gray zone” scales, the assumption of equilibrium between subgrid cloud and large scale environment in grid box is invalid in classic mass flux based convective parameterization; some treatments of microphysics process in subgrid cloud are no longer reasonable , and the absence of the interaction between the hydrometeor in subgrid cloud and the microphysics process in models may cause “double counting” problem. Using the simulation results from a high resolution cloud resolved model, firstly this project will confirm the grid size threshold which could resolve convection system explicitly, and then based on a classic convective parameterization develop a subgrid cloud parameterization scheme which are applicable to “gray zone” scales. This study will focus on threes aspects including the assumption of equilibrium, the treatments of microphysics in subgrid cloud and the interaction of subgrid cloud scheme and microphysics process. Hopefully, this project will help to increase the performance of high resolution NWP models.

当前，全球业务天气预报模式水平分辨率接近10 km，区域模式达到2-5 公里。在此分辨率下，对流云引起的上升气流仍不能被模式完全显式分辨，同时传统对流参数化方案的某些假定不再成立，模式中云参数化过程面临“灰色区域”问题。在“灰色区域”尺度，经典的基于质量通量的对流参数化方案中次网格云与网格内大尺度环境的准平衡假定不再成立、次网格云内的微物理过程的考虑不尽合理、次网格云内的水凝物处理也因不考虑与模式中微物理过程的相互作用，极易引起双重处理。本申请项目拟通过高分辨率云分辨模式的结果给出可显式模拟对流系统的最低分辨率，并基于一个经典的对流参数化方案，针对准平衡闭合假定、次网格云中微物理过程的处理和次网格云方案与微物理过程相互作用三个方面开展工作，发展一个适用于“灰色区域”尺度的次网格云参数化方案。这将有助于提高高分辨率天气数值预报模式的预报性能。

当前，全球业务天气预报模式水平分辨率接近10 km，区域模式达到2-5 公里。在此分辨率下，对流云引起的上升气流仍不能被模式完全显式分辨，同时传统对流参数化方案的某些假定不再成立，模式中云参数化过程面临“灰色区域”问题。本项目通过大涡模拟结果给出可显式模拟对流系统的最低分辨率，证实了次网格通量的效应与次网格云量之间的关系，发现随着模式水平分辨率的提高，次网格云内的不均一对次网格通量的贡献将显著增加，从而在对次网格云参数化过程无法仅考虑质量通量项。同时基于一个经典的对流参数化方案，针对准平衡闭合假定、对流时间尺度和次网格云的卷入率效应三个方面开展工作，初步发展了一个适用于“灰色区域”尺度的次网格云参数化方案。试验结果表明该方案可适用于高分辨率，对降水的总体预报性能优于原对流参数化方案和无对流参数化方案试验。