NUMERICAL STUDIES OF PLANETARY BOUNDARY LAYER PARAMETERIZATION SCHEMES ON SUPER TYPHOON SANBA (2012) DURING ITS INITIAL STAGE

The formation and development of typhoons are closely related to the disturbed low vortexes at the planetary boundary layer (PBL). The effects of five PBL parameterization schemes (PBL schemes hereinafter) on the trajectory, intensity, and distribution of physical quantities are studied using the mesoscale WRF model on Super Typhoon Sanba (2012) during its initial stage. Results show that the five PBL schemes exhibit significant different effects on the simulated intensity and path. The results simulated by QNSE and ACM2 without the Bogus method are close to the best track data in the numerical experiments. When the Bogus method is adopted, the simulated trajectories improve significantly because the initial field is close to the true data. Among the five PBL schemes, QNSE and ACM2 with the Bogus method present improved simulated path and intensity compared with the three other schemes. This finding indicates that the two schemes deal with the initial PBL process satisfactorily, especially in the formation and development of disturbed low vortexes. The differences in the treatment methods of the five PBL schemes affect the surface layer physical quantities and the middle and upper atmospheres during the middle to late periods of the typhoon. Although QNSE and ACM2 present better simulation results than other schemes, they exhibit a few differences in the internal structure of the typhoon. The results simulated by MYJ are worse, and this method may be unsuitable for studying the formation and development of typhoons.     

Numerical Simulation of the Rapid Intensification of Hurricane Katrina(2005):Sensitivity to Boundary Layer Parameterization Schemes

Accurate forecasting of the intensity changes of hurricanes is an important yet challenging problem in numerical weather prediction. The rapid intensification of Hurricane Katrina(2005) before its landfall in the southern US is studied with the Advanced Research version of the WRF(Weather Research and Forecasting) model. The sensitivity of numerical simulations to two popular planetary boundary layer(PBL) schemes, the Mellor–Yamada–Janjic(MYJ) and the Yonsei University(YSU) schemes, is investigated. It is found that, compared with the YSU simulation, the simulation with the MYJ scheme produces better track and intensity evolution, better vortex structure, and more accurate landfall time and location. Large discrepancies(e.g.,over 10 hPa in simulated minimum sea level pressure) are found between the two simulations during the rapid intensification period. Further diagnosis indicates that stronger surface fluxes and vertical mixing in the PBL from the simulation with the MYJ scheme lead to enhanced air–sea interaction, which helps generate more realistic simulations of the rapid intensification process. Overall, the results from this study suggest that improved representation of surface fluxes and vertical mixing in the PBL is essential for accurate prediction of hurricane intensity changes.     

A NUMERICAL STUDY OF THE PBL CONDITIONS OVER RECLAIMED GRASSLAND

PBL conditions over reclaimed grassland in North China(49°19′N, 119°55′E) are examined using the improved ver-sion of the model developed by Nickerson et al. (1986). Basic results are presented as follows: i) the reclaimed section, orthe "heat island", has noon intensity of 4—6℃, the surface disturbance can be felt at 700 m AGL and convectiveperturbance at the PBL top, with a convective speed of 1—2 cm / s; ii) once grassland is reclaimed on a large scale, thePBL climate will develop into further aridity; iii) irrigation can alleviate the intensity and dryness of the "island" but failsto prevent the climate from aridization; iv) the local circulation in the PBL keeps its direction unchanged in 24 hours, i.e.,wind blows toward the "island" at the height below 150—200 m at roughly 2 m/s and returns at higher levels with aslightly smaller speed. Further, moist flow can extend 30—50 km horizontally inside the reclamation.     

对流层物质垂直交换模式中的参数化处理

为了研究对流层(TL)中垂直涡旋扩散系数(K_z)随高度变化的参数化形式对物质垂直交换的贡献,用“高分辨TL物质交换模式”(EM3)对5种不同K_z型进行了数值对比模拟,结果表明:KK_z型较好地描述了行星边界层(PBL)内受下垫面影响的高频湍流和PBL以上TL由各种动力学和热力学过程产生的切变湍流;垂直运动和K_z对PBL下层的物质输送到TL上层和从源高度向下输送到PBL低层两者均有重要作用;垂直运动的作用具有间隙性,而K_z是连续的.     

High-resolution Simulation of an Extreme Heavy Rainfall Event in Shanghai Using the Weather Research and Forecasting Model: Sensitivity to Planetary Boundary Layer Parameterization

In this study,an extreme rainfall event that occurred on 25 May 2018 over Shanghai and its nearby area was simulated using the Weather Research and Forecasting model,with a focus on the effects of planetary boundary layer(PBL)physics using double nesting with large grid ratios(15:1 and 9:1).The sensitivity of the precipitation forecast was examined through three PBL schemes:the Yonsei University Scheme,the Mellor?Yamada?Nakanishi Niino Level 2.5(MYNN)scheme,and the Mellor?Yamada?Janjic scheme.The PBL effects on boundary layer structures,convective thermodynamic and large-scale forcings were investigated to explain the model differences in extreme rainfall distributions and hourly variations.The results indicated that in single coarser grids(15 km and 9 km),the extreme rainfall amount was largely underestimated with all three PBL schemes.In the inner 1-km grid,the underestimated intensity was improved;however,using the MYNN scheme for the 1-km grid domain with explicitly resolved convection and nested within the 9-km grid using the Kain?Fritsch cumulus scheme,significant advantages over the other PBL schemes are revealed in predicting the extreme rainfall distribution and the time of primary peak rainfall.MYNN,with the weakest vertical mixing,produced the shallowest and most humid inversion layer with the lowest lifting condensation level,but stronger wind fields and upward motions from the top of the boundary layer to upper levels.These factors all facilitate the development of deep convection and moisture transport for intense precipitation,and result in its most realistic prediction of the primary rainfall peak.     

近中性条件下青藏高原行星边界层厚度及其特征分析

本文利用1979年5—8月常规气象资料,采用三种不同定义的确定边界层厚度的方法,得到近中性弱稳定条件下夏季青藏高原平均边界层厚度为600m左右。     

长江流域一次暴雨过程的模拟对行星边界层参数化的敏感性研究

用MM5对长江流域的一次暴雨进行模拟考察其对行星边界层参数化的敏感性。不同的边界层参数化表现在不同的地表通量和垂直混合设计上，本文分析了MM5中4个主要方案地表通量和垂直混合参数化方案的不同以及它们对降水强度、落区和时间的影响。研究还发现，地表通量对暴雨模拟结果的影响比垂直混合方案要大。     

小尺度深凹地形边界层影响因子的数值试验研究

建立了一个针对小尺度深凹地形的、高分辨的、三维非静力的边界层试验模式,通过求解非定常的方程组获得边界层的定常解。有选择地对影响深凹地形边界层结构的不同因子进行了数值试验研究。结果表明,深凹地形的深度Ｈ、坡度α以及初始地面风速ｕ０的大小和初始风速的垂直切变（Ｐ）等因子对边界层的结构有较大的影响,且模式具有较好的稳定性。     

成都边界层风场演变天气意义的检验

应用１９９３年４月２５日￣５白４日四川盆地加密观测探空资料，分析了四川盆地边界层风场变化规律及其与４月３０日￣５月１日盆地初夏少见的大范围大风、冰雹、暴雨天气过程的关系。通过分析进一步证实：西邻青藏高原的成都是四川盆地天气变化的关键信息点；在高原盆地特定地形的影响下，成都边界层风场呈规律性变化，其全息图像是：当成都边界层为东北风时，四川盆地边界层维持气旋式偏东流场，处于辐合、正涡度、上升运动区中，     

Effect of Boundary Layer Latent Heating on MJO Simulations

A latent heating peak in the PBL was detected in a simulation by a global GCM that failed to reproduce Madden-Julian Oscillation(MJO).The latent heating peak in the PBL was generated by very shallow convection,which prevented moisture from being transported to the free troposphere.Large amount of moisture was therefore confined to the PBL,leading to a dry bias in the free atmosphere.Suffering from this dry bias,deep convection became lethargic,and MJO signals failed to occur.When the latent heating peak in the PBL was removed in another simulation,reasonable MJO signals,including the eastward propagation and the structure of its large-scale circulation,appeared.We therefore propose that the excessive latent heating peak in the PBL due to hyperactive shallow convection may be a reason for a lack of MJO signals in some simulations by other GCMs as well.