公里级分辨模式中次网格地形动力效应参数化研究

Due to the heterogeneity of the complex terrain, high-density surface observations are required to resolve the fine-scale gradients in surface weather produced by topographic forcing. However, the observational stations are even sparser over orographic regions compared to those over plains. Therefore, it is hard to exactly understand various terrain-induced phenomenons. Besides, these phenomenons are too complex to be given full consideration in numerical weather prediction (NWP) models. As a result, the performance of the NWP model is poor due to the unresolved orographic processes. Unfortunately, hazardous weathers are of frequent occurrences over complex terrain, such as snowstorms, ice storms or cold-air damming, orographic rainfall induced floods or landslides, wildfire spread driven by fuels, downslope windstorms and air quality hazards induced by cold-air pools. To improve the skills of NWP model over orographic regions, (1) the multiple terrain-induced processes are necessary to be investigated extremely and (2) the unresolved processes should be correctly parameterized in NWP models. As for the resolutions of operational NWP will be increased to kilometer scale (1-5km) in few years, some middle scale orography-induced processes will be resolve explicitly and orographies of 1-10km horizontal scales will not be resolved and needed to be studied eagerly. Therefore, the main purpose of this program is to develop the subgrid orographic parameterization schemes appropriate to kilometer-scale model..Benefit from “Dali Mountain Meteorological Field Observational Base” and “2022 Winter Olympic/winter Paralympic meteorological service and action project”, multiple-source observations over Cang Mountain and Haituo Mountain are available. The two mountains are of 5-10 km horizontal scales which are concerned in this program. More recently, real-case WRF based large-eddy simulations (WRF-LESs) were carried out for many micro-scale meteorological processes, such as rolls in planetary boundary layer (PBL), cold pools and slope flow induced by orography, and stable boundary layer turbulence. These simulations can explicitly resolve the large eddies in PBL and are robust to unveil the details of the flows over complex terrain. Ideal LES is easy to manipulate the inflow properties and ideal hills in the model so that various mountain-induced drags could be calculated due to the simulation results. Therefore, to accomplish this program, several steps will be carried out. Fisrtly, observations over Cang Mountain and Haituo Mountain are used to investigate the orographic flows in real atmosphere. Secondly, real-case WRF-LESs will be carried out to enrich the observed data and display the details of various orographic flows. Thirdly, ideal LESs, evaluated by the data of WRF-LESs and observations, will be employed to diagnose orographic drags due to different orographic parameters and inflow properties. Finally, based on the above analysis and preexist subgrid orographic parameterization schemes, new schemes appropriate for kilometer-scale NWP models will be developed.

山地区域极易发生灾害性气象事件，为了提高模式在复杂地形区域的预报准确率，需要对模式次网格尺度地形效应进行深入研究。当分辨率提高到公里级，小尺度和介于小到中尺度之间地形（尺度≤10km）的动力效应将会在模式中突显。因此，本项目将致力于研究适用于公里级分辨率（3km左右）数值模式的次网格尺度地形动力效应参数化方案。首先，利用大理山地气象野外科学试验基地和2022年冬奥会北京延庆海坨山的野外气象观测资料，研究真实大气中的地形动力效应；其次，利用理想大涡模拟和WRF实际大涡模拟，精细化描述不同大气条件下的地形流场特征和地形拖曳力特征；最后，基于现有次网格尺度地形参数化方案和大涡模拟的诊断结果，研究公里级分辨率模式中次网格地形动力效应参数化方案。本项目的实施，将会揭示出小尺度和介于小到中尺度之间地形的动力流场特征，改进公里级分辨率模式在复杂地形区域的预报准确率。

本研究针对复杂地形区域天气预报困难问题，研究了公里尺度地形的动力效应及其在对流可分辨模式中的参数化。（1）利用大理山地气象野外科学试验基地的观测资料和冬奥会北京延庆海坨山的野外气象观测资料，揭示了小到中尺度地形的热力和动力效应；（2）利用仿真大涡模拟，揭示了大理苍山东侧午后大风与背风波之间的关系；（3）利用理想大涡模拟，揭示了背风波特征，包括振幅、波长和传播距离，与边界层稳定度和高空急流之间的关系；（4）利用理想大涡模拟，给出不同地形参数（高度，长宽比，坡度，坡向）条件下地形湍流拖曳力的大小与方向；（5）在对流可分辨模式中进行了小尺度地形湍流拖曳力参数化研究，发现该参数化方案能够显著降低复杂地形区域地面和低空风模拟误差，改进模式降水预报；（6）针对理想大涡模拟结果，设计了适用于公里-次公里尺度模式并考虑了地形走向与低空风向夹角的地形湍流拖曳力参数化方案。项目的实施首次揭示了较高山脉（~边界层高度）激发背风波日变化机理及其与边界层稳定度的关系，为复杂地形区域灾害性大风和降水天气预报提供理论依据，并为改进大尺度模式重力波参数化方案提供理论依据；首次提出适用于次公里-公里分辨率模式的次网格地形参数化方案，将为提高该尺度分辨率模式在复杂地形区域模拟提供理论和技术支持。