The influence of the coast on the dynamics of upwelling fronts: Part II. Numerical simulations

We investigated the dynamics of upwelling fronts near a coast. This work was first motivated by laboratory experiments [Bouruet-Aubertot, Linden, Dyn. Atmos. Oceans, 2002] in which the front is produced by the adjustment of a buoyant fluid initially confined within a bottomless cylinder. It was shown that cyclonic eddies consisting of coastal waters are enhanced when the front is unstable near the coast (the outer vertical boundary). The purpose of this paper is to provide further insights into this process. We reproduced the experimental configuration using a three-dimensional model of the primitive equations. We first show that for coastal fronts more potential energy, in terms of the maximum available potential energy, is released than for open-ocean fronts. Therefore, waves of larger amplitude are generated during the adjustment and the mean flow that establishes has a higher kinetic energy in the former case. Then as baroclinic instability starts and wave crests reach the boundary, cyclonic eddies are enhanced as in the laboratory experiments and in a similar way. However, in contrast to the laboratory experiments, offshore advection of cyclonic eddies can occur in two stages, depending on the spatial organization of the baroclinic wave. When the baroclinic wave consists of the sum of different modes and is thus highly asymmetric, the offshore advection of cyclonic eddies occurs just after their enhancement at the boundary, as in the laboratory experiments. By contrast, when a single-mode baroclinic wave develops, neighboring cyclonic eddies first merge before being advected offshore. Very different behavior is observed for open-ocean fronts. First a mixed baroclinic–barotropic instability grows. Then the eddies transfer their energy to the mean flow and the barotropic and baroclinic instabilities start again. An excellent agreement is obtained with the main result obtained in the laboratory experiments: the ratio between growth rates of surface cyclonic and anticyclonic vorticity increases as the instability develops nearer to the coast.     

Phillips模式的斜压不稳定的非线性饱和问题:能量情形

A conservation law for the Phillips model is derived. Using this law, the nonlinear saturation of purely baroclinic instability caused by the vertical velocity shear of the basic flow in the Phillips model-the case of energy-is studied within the context of Arnold's second stability theorem. Analytic upper bounds on the energy of wavy disturbances are obtained. For one unstable region in the parameter plane, the result here is a second-order correction in ε to Shepherd's; For another unstable region, the analytic upper bound on the energy of wavy disturbances offers an effective constraint on wavy (nonzonal) disturbances φ'i at any time.     

大气中对称不稳定机制的动力学分析及暴雨的分析与预报

用气块法从力学角度对对称不稳定机制及在对称不稳定条件下产生的环流进行分析，更全面地定义了对称不稳定及它的产生机制和中尺度环流形成的过程。对暴雨个例诊断分析表明，有些强降水区在降水前和降水过程中存在着不对称不稳定，说明对称不稳定条件下形成的环流对暴雨有引发和加强作用。在以往研究原基础上，总结出了利用常规观测资料分析对称不是否存在，及其能量大小的方法，用它来预报是否降水及降水的强度，从而提高降水预报准确率。     

简单的热带海气耦合波——Rossby和Kelvin波的相互作用

本文分析了当大气和海洋中未经耦合前的自由波分别为Kelvin波和Rossby波,经相互作用后所产生的耦合波的性质。结果表明,不管大气的自由波为Kelvin波或Rossby波,而海洋的自由波为Rossby波或Kelvin波,经相互作用后的耦合波可以分成两类。一类耦合波的色散关系接近自由的Kelvin波;另一类则由不同经圈模的Rossby波经相互作用后的耦合波。这两类波都具有不稳定性。文中讨论了耦合波的传播和不稳定的物理机制,并指出这类不稳定的热带耦合波,对研究ENSO事件中的某些现象有一定的参考意义。     

弱耗散作用下非线性Rossby波的调制不稳定

本文使用WKB方法对弱耗散作用下基本气流具有弱切变的行星大气中非线性Rossby波进行了研究,得到了弱耗散的非线性Rossby波振幅所满足的复系数非线性Schrdinger方程,并分别就无耗散和弱耗散的Rossby波的边带稳定性问题进行了讨论,得到了一些新结果.     

Observation of gravity waves in a boreal forest

In this paper we report the results of the analysis of two 60-min wave events that occurred in a boreal aspen forest during the 1994 BOREAS (Boreal Ecosystems-Atmosphere Study) field experiment. High frequency wind and temperature data were provided by three 3-D sonic anemometer/thermometers and fourteen fine-wire thermocouples positioned within and above the forest. Wave phase speeds, estimated from information revealed by spectral analysis and linear plane wave equations, are 2.2 and 1.3 m s^-1 for the two events. The wavelengths are 130 m and 65 m respectively and are much larger than the vertical wave displacements. There is strong evidence from the present analysis and from the literature supporting our postulate that these waves are generated by shear instability. We propose that wind shear near the top of the stand is often large enough to reduce the gradient Richardson number below the critical value of 0.25 and thus is able to trigger the instability. When external conditions are favorable, the instability will grow into waves.     

特殊地形对鄂东北一次局地强降水过程的作用机制分析

基于FNL1°×1°再分析资料和来自国家气象信息中心的区域自动站与CMORPH小时降水融合产品,通过高分辨率的WRF数值模拟,本文重点分析了2015年7月22-24日期间在西南涡东移过程中,受长江中下游特殊地形影响,在鄂东北江汉平原河谷地区诱发生成的一次短时局地强降水天气过程,围绕特殊地形对局地降水增幅的作用机制展开一系列的深入研究,研究结果表明:此次局地强降水过程是在长江中下游特殊的中尺度地形影响下,配合东移西南涡前部偏南暖湿气流的输送,两者共同作用而诱发产生,此次局地降水过程持续时间短,降水增幅显著。之后,通过研究单一地形对局地降水的影响发现,大别山脉作为单一地形的作用效果为提升局地降水增幅,扩大强降水范围;幕阜山脉作为单一地形的作用效果为削弱降水增幅,缩小强降水范围。进一步深入分析上述单一地形影响降水变化的作用机制得出,大别山脉的地形作用有利于局地强降水区附近对流层低层的层结对流不稳定性增强,以及降水区近地面层冷池的维持和增强,有利于提升局地降水的增幅。而与大别山脉对局地降水作用效果不同,就幕阜山脉单一地形而言,地形对偏南暖湿气流的阻挡作用,削弱了局地强降水期间进入降水区的水汽通量,继而对局地降水的增幅有抑制作用。此外通过研究组合地形对局地降水的影响时发现,大别山脉、幕阜山脉、皖南山地,以及幕阜山脉和皖南山地之间的狭窄河谷地形共同构成的类似"喇叭口"地形,其产生的狭管效应,使进入地形区内的偏南气流辐合加强,而幕阜山脉和大别山脉之间的河谷地区,作为偏南气流从幕阜山脉东侧绕流进入江汉平原的重要通道,有效保证了强降水区域内充足的正涡度平流输送,上述有利的地形组合配置对于局地降水发展增强起到了至关重要的作用。     

黄土高原一次β中尺度大暴雨特征及成因分析

利用NCEP／NCAR逐日6h分析资料和常规观测资料,分析了2011年2月25日一28日山西连续降雪天气过程（以下简称2．25降雪）。①2．25降雪过程经历了三个阶段：2月25日为回流降雪阶段,26日一27日为倒槽冷锋与回流降雪共同影响降雪阶段,28日为低空切变线影响降雪阶段。②2．25降雪过程涵盖了华北地区大到暴雪的三个类型：回流类降雪、倒槽冷锋类降雪、低空切变线类降雪。③通过温湿场分析得出,对于低空切变线类降雪,对流层中层的湿核对降雪的开始有一定的指示意义。降雪未开始之前对流层中层有湿核,随着时间的推移,湿核向低层扩展,整个对流层中低层变为高湿区,降雪开始。当对流层中层变为干区,并向低层扩展,降雪过程结束。对于回流类降雪,低层回流对回流降雪起到冷垫的作用。④通过涡度场分析得出,对流层低层的负绝对涡度中心对其东侧的降水有指示意义,如果其东侧对流层低层配合有正涡度核,降雪强度较大,维持时间较长。⑤地面层出现的负绝对涡度中心说明近地面层有小高压系统的存在,这是因为低层回流冷垫作用形成的孤立小高压体。⑥28日降雪维持机制是条件性对称不稳定。     

"2003.1"黔东南暴雪天气过程的对称不稳定分析

采用对称不稳定判据,对发生在2003年1月5～6日一次罕见的黔东南暴雪天气过程进行了分析,结果表明：暴雪产生在对称不稳定大气中,低空急流促使对流层低层暖湿气流辐合上升,触发对称不稳定能量释放,产生暴雪天气。西南涡、横切变和暴雪区有向对称不稳定区移动的趋势。     

Instabilities of the tidally induced bottom boundary layer in the rotating frame and their mixing effect

To investigate the stability of the bottom boundary layer induced by tidal flow (oscillating flow) in a rotating frame, numerical experiments have been carried out with a two-dimensional non-hydrostatic model. Under homogeneous conditions three types of instability are found depending on the temporal Rossby number Ro_t, the ratio of the inertial and tidal periods. When Ro_t < 0.9 (subinertial range), the Ekman type I instability occurs because the effect of rotation is dominant though the flow becomes more stable than the steady Ekman flow with increasing Ro_t. When Ro_t > 1.1 (superinertial range), the Stokes layer instability is excited as in the absence of rotation. When 0.9 < Ro_t < 1.1 (near-inertial range), the Ekman type I or type II instability appears as in the steady Ekman layer. Being much thickened (100 m), the boundary layer becomes unstable even if tidal flow is weak (5 cm/s). The large vertical scale enhances the contribution of the Coriolis effect to destabilization, so that the type II instability tends to appear when Ro_t > 1.0. However, when Ro_t < 1.0, the type I instability rather than the type II instability appears because the downward phase change of tidal flow acts to suppress the latter. To evaluate the mixing effect of these instabilities, some experiments have been executed under a weak stratification peculiar to polar oceans (the buoyancy frequency N²  10⁻⁶ s⁻²). Strong mixing occurs in the subinertial and near-inertial ranges such that tracer is well mixed in the boundary layer and an apparent diffusivity there is evaluated at 150–300 cm²/s. This suggests that effective mixing due to these instabilities may play an important role in determining the properties of dense shelf water in the polar regions.