摘要：
随着数值预报模式分辨率的提高,当模式网格距与含能湍涡的长度尺度相当时,模式动力过程可解析一部分湍流运动,而剩余的湍流运动仍需参数化,此时便产生了湍流参数化的“灰色区域”问题.对传统的PBL(Planetary Boundary Layer)方案在“灰色区域”下的适用性评估,是改进PBL方案以使其能够适应分辨率变化的前提和基础.本研究基于干对流边界层的大涡模拟试验,比较了WRF (Weather Research and Forecast Model)模式中四种常用的边界层参数化方案[YSU (Yonsei University)、MYJ (Mellor-Yamada-Janjic)、MYNN2.5 (Mellor-Yamada-Nakanishi-Niino Level 2.5)、MYNN3)]在“灰色区域”尺度下的表现.研究表明,混合层内总热通量对所使用的参数化方案和水平分辨率均不敏感.不同参数化方案中次网格与网格通量的比例表现出对水平网格距不同的依赖性.局地PBL方案(MYJ、MYNN2.5)在混合层内的平均位温随网格距减小而增大,次网格通量随网格距减小而减小,较参考湍流场对次网格通量有所低估.YSU方案的非局地项几乎不随水平格距改变而变化,对次网格通量的表征并未表现出较强的分辨率依赖性,且过强的非局地次网格输送使混合层内温度层结呈弱稳定,抑制了可分辨湍流输送,不易于激发次级环流.MYNN3方案的非局地次网格通量(负梯度输送项)随网格距减小而减小,使其对次网格通量的表征具有较好的分辨率依赖性.PBL方案在“灰色区域”尺度下的适用性与具体分辨率有关.以分辨率500m为例,四种PBL方案中不存在一种最佳方案,能对边界层的热力结构和湍流统计特征均有准确的描述.%As the resolution in numerical weather prediction models increases,turbulent motion can be partially resolved by grid scale dynamics.However,a substantial part of turbulent motion still needs to be parameterized,which results in the so-called "grey zone" problem.Assessment of the suitability of traditional PBL (Planetary Boundary Layer) schemes at "grey zone" resolutions would provide foundation for future improvement of PBL schemes,making them more adaptive to the variation of resolutions.In this study,the authors assess the performance of four PBL schemes that are commonly used in WRF,i.e.YSU (Yonsei University),MYJ (Mellor-Yamada-Janjic),MYNN (Mellor-Yamada-Nakanishi-Niino Level 2.5) and MYNN3 in the simulation of convective PBL at "grey-zone" resolutions using LES (Large-Eddy Simulation).Results indicate that at higher resolutions,domain-averaged potential temperature increases in the mixing layer with local PBL schemes such as MYJ and MYNN2.5,while subgrid scale flux decreases and falls below the reference value constructed from LES.The nonlocal terms in YSU hardly change with grid size and do not show distinct scale-sensitivity.In contrast,the counter-gradient term in MYNN3 decreases with reduced grid-size and exhibits a certain degree of scale-sensitivity.The strong nonlocal subgdd mixing in YSU results in a weak,stable stratification in the mixing layer and suppresses resolved-scale turbulence mixing,thus impeding the onset of convectively induced secondary circulations in the model.The suitability of PBL schemes at "grey zone" resolutions varies with certain resolutions.For example,at the resolution of 500 m,none of the four PBL schemes performs well to accurately simulate both the thermal structure and turbulent statistics in the PBL.
