Structures of dynamically unstable shear flows and their implications for shallow internal gravity waves

Summary In this study, the response of a dynamically unstable shear flow with a critical level to periodic forcing is presented. An energy argument is proposed to explain the upshear tilt of updrafts associated with disturbances in two-dimensional stably stratified flows. In a dynamically unstable flow, the energy equation requires an upshear tilt of the perturbation streamfunction and vertical velocity whereU _z is positive. A stability model is constructed using an iteration method. An upshear tilt of the vertical velocity and the streamfunction fields is evident in a dynamically unstable flow, which is required by energy conversion from the basic shear to the growing perturbation wave energy according to the energy argument. The momentum flux profile indicates that the basic flow is decreased (increased) above (below) the critical level. Thus, the shear instability tends to smooth the shear layer. Following the energy argument, a downshear tilt of the updraft is produced in an unstably stratified flow since the perturbation wave energy is negative. The wave energy budget indicates that the disturbance is caused by a thermal instability modified by a shear flow since the potential energy grows faster than the kinetic energy.With 4 Figures     

The impact of nonlinear stability and instability on the validity of the tangent linear model

l.Intr0ducti0nTheequationsgoverningthemotionsoftheatmosphereandoceanaren0nlinear0nes.Itisdiffcultandcomplicatedtodealwiththenonlinearproblem.Sothetangentlinearmodel(TLM),whichisobtainedbylinearizingthenonlinearmodelinthevicinityofthebasicflow.iswidelyutilizedinthetheoreticalresearchandpracticalimplementationintheatmosphericandoceanicsciences.First-TLMcanbeusedtoestimatetheev0luti0nofsmallPerturbati0nsinaf0recastmodel.Second,itisadoptedinextendedKalmanfilterinthedataassimilation(Evensen,l99…     

Enhanced response of an atmospheric flow to a line-type heat sink in the presence of a critical level

Summary Response of the atmosphere to a line-type heat sink, which represents a cold pool induced by evaporative cooling in falling precipitation, is investigated. Two-dimensional, steady-state, linear perturbations forced by the diabatic cooling in the presence of a critical level are solved analytically. The ambient flow is assumed to have a three-layer structure which is the same as that in Lindzen and Tung (1976, denoted as LT hereafter) except that an isolated diabatic cooling is specified in the lower layer. The shear layer with a critical level is assumed to be either dynamically stable or unstable. A steady-state solution is possible even for a dynamically unstable flow in the middle layer because the wave energy is allowed to propagate to infinity in the upper layer.Thermally induced wave response near the critical level depending on the stability and wind shear in terms of the Richardson number exhibits the same behavior as the eigenvalue problem solved by Jones (1967) and LT because the source mechanism does not change the critical level behavior. That is, when the shear layer is dynamically stable, almost all of the wave energy is absorbed near the critical level, while for dynamically unstable case waves can be partially-or over-reflected from the critical level depending on the Richardson number. Waves can be over-reflected and over-transmitted simultaneously as they travel through the critical level in the process of the energy extraction from the unstable mean shear flow.Using the duct condition proposed by LT, it is found that the magnitude of the perturbation vertical velocity in the lower layer is 6 times larger than that for a nonducting case with the same cooling rate. This implies that under a proper choice of the critical level height and stability profile, the response of the atmosphere to a line-type heat sink can be significantly enhanced through the over-reflection process.With 7 Figures     

Kinematic models of a dry convective boundary layer compared with dual-Doppler radar observations of wind fields

The kinematic structure of the convective boundary layer, observed by a dual-Doppler radar system, is compared with the structure predicted by simple shear models. We first consider the models to be inviscid, then add viscous effects. Model 1 assumes a linear ambient wind profile from the surface through the boundary layer, and a constant wind above. The shear layer is assumed to be statically neutral, but static stability is permitted in the region above the shear. Model 2 has a hyperbolic tangent ambient wind profile.After considering the inviscid models, some of the effects of viscosity are incorporated into the models in a crude way, and the results are compared.We conclude that although the presence of shear is important, the kinematic structure is relatively independent of the details of the wind and temperature profiles. Viscosity has important effects, especially near the critical level where the disturbance velocity is equal to the wind speed. The patterns predicted by both models agree very well with the dual-Doppler radar observations when viscosity is included.     

Some aspects of internal gravity waves in the multicell-type convective system

Summary Some aspects of internal gravity waves in the multicell-type convective system are examined using a linear theory and a nonlinear numerical model. The basic-state wind is assumed to increase linearly with height and then remain constant.In the theoretical part, the two-dimensional, linear, steady-state response of a stably stratified atmosphere to specified cooling representing the evaporative cooling of falling precipitation in the subcloud layer is analytically considered. It is shown that there exist an updraft on the upstream side of the cooling and a downdraft on the downstream side. As the wind shear increases enough, the magnitude of the updraft decreases. This is because a large portion of the specified cooling is used to compensate for the positive vorticity associated with the positive wind shear and accordingly the effective cooling necessary to produce perturbations is reduced.In the numerical part, a two-dimensional version of the ARPS (Advanced Regional Prediction System) that is a nonhydrostatic, compressible model with detailed physical processes is employed. Results from the dry simulation, in which the steady cooling is specified in the model, show that the simulated quasi-steady field resembles the linear, steady-state solution field because the nonlinearity factor of thermally-induced waves in this case is small. For the moist simulation, the quasi-steady perturbations obtained from the dry simulation are used as initial conditions. It is shown that gravity waces can effectively initiate convection even with small amplitude and that updraft at the head of the density current somewhat resembles the linear, steady-state response of a stably straified flow to the specified cooling. The updraft, that is, forced internal gravity waves, at the head of the density current is responsible for the initiation of consecutive convective cells that move downstream and develop as a main convective cell. This study suggests that internal gravity waves play a major role in the initiation of consecutive convective cells in the multicell-type convective system and hence in its maintenance.     

Effects of variable wind shear on the mesoscale circulation forced by slab‐symmetric diabatic heating

Abstract

We examine the response of stably stratified airflow to a slab‐symmetric diabatic forcing associated with condensation in long‐lasting precipitation bands. The steady‐state linearized Boussinesq equations are used to model the diagnostic relationship between the vertical motion field, the heating source and the ambient flow. The basic‐state flow is assumed to be horizontally uniform and non‐rotating, but the static stability and wind vary in the vertical. Linear theory shows that the speed of the along‐band wind component is unimportant for slab‐symmetric heating since it cannot contribute towards the advection of buoyancy or vertical motion.

For typical atmospheric stratification and a moving heating source associated with a cloud band, the Taylor‐Goldstein equation is solved numerically. The numerical results show that the cross‐band wind shear tilts the updraft core and broadens it. While the magnitude of the shear is increased, the circulation becomes stronger. The details of the wind profile are also important in determining the intensity and structure of the circulation. When the wind profile indicates a convex bulge (i.e. the low‐level shear is weaker than the upper‐level shear), the circulation becomes slightly weaker in comparison with the linear wind profile. Conversely, the circulation becomes stronger when the wind profile has a concave shape. Increasing the concave bulge tends to enhance the circulation but not in a monotonic fashion. This non‐monotonic relation between the vertical motion and the parabolic wind profile is interpreted in terms of kinetic energy changes of parcels that interchange their altitudes.     

启动对流的初始扰动对热带飑线模拟的影响

在中尺度数值模拟及数值预报中,正确的对流启动是模拟或预报是否取得成功的关键。作者针对一次热带海洋飑线过程,利用一个风暴尺度云分辨数值模式进行数值模拟,采用不同的初始扰动触发与启动对流发展,重点讨论这些不同启动对流的初始扰动对飑线演变、生命史及其成熟结构等模拟的影响。结果表明,初始扰动的结构、形态分布及其与环境场的不同配置对具有深对流的飑线生命史存在重要影响,而对飑线的成熟结构影响较小。     

强风暴个例电荷结构及云闪放电差异的数值模拟

利用耦合电过程的冰粒子分档模式对长春地区两个降雹型和非降雹型强风暴个例的闪电特征进行了模拟和对比分析。结果表明,在雷暴云的初始发展期,由于上升气流较弱,两者电荷分布相似,均表现为弱的正偶极结构。随着云体不断发展,两者电荷分布开始表现出差异:降雹型个例中的上升气流较强,风切变较大,过冷水能被携带到较高的高度,冰相粒子也能被带到较高处或在较高处继续增长,使得不同区域均存在冰相粒子含量中心。因此,冰相粒子的发生范围不同、环境参数不同及荷电符号不同的非感应起电过程,形成多个电荷中心,电荷结构易出现多层分布。在不同的发展时期电荷结构均呈现出不同的形态,放电既可能在上升气流区触发,也可能在气流辐散区触发。相对而言,非降雹型个例中的上升气流较弱,风切变较小,冰相粒子的分布较规则,非感应起电过程较均匀,从而导致电荷分布始终较均匀。不同发展时期的电荷结构都相对有规则,满足放电条件的位置具有一定的相似性。     

A New Mechanism of Convective Cell Regeneration and Development Within a Two-Dimensional Multicell Storm

In this study, based on simulations of a two-dimensional multicell storm under a ground-layer upshear （Uz〈0） by a mesoscale numerical model, a new mechanism of cell regeneration and development within the multicell storm at the ＂less than optimal shear＂ state.is proposed. In the presence of a ground-layer upshear, the circulation associated with the surface cold pool is not counteracted by that associated with the ambient wind sl~ear, and the density current extends out faster, making the multicell storm stay at the ＂less than optimal shear＂ state. As a result, a new cell is triggered by the strong vertical perturbation ahead of the mature convection, rather than by the split-up from the updraft at the leading edge of the surface cold pool as well as the gust front. The latter is the mechanism at the ＂optimal＂ state proposed by Lin et al. in 1998. In the new mechanism, the regenerated cell grows fast with the incident warm moist air from the upstream of the multicell storm, and tends to cut off the moist airflow into the mature convection at its western sector. Consequently, the mature convection would weaken, be replaced, and eventually decay. Actually, these two different mechanisms come into play in a way depending on the relationship between the circulation of the low-level shear and that of the cold pool. When the circulation of the cold pool is stronger than that of the wind shear, the multicell storm is at the ＂less than optimal shear＂ state, and the new convective cell is produced by the disturbance ahead of the mature cell. When the circulation of the cold pool is weaker, the cell regeneration is dominated by the mechanism at the ＂optimal＂ state, and the new cell is split from the gust front updraft. Therefore, these two mechanisms are not contradictive. With a moderate ground-layer upshear, they can alternately operate within a multicell storm.     

Nonlinear meridional scale interaction in baroclinic instability

Baroclinic instability of a zonal flow with latitudinal structure is examined using a nonlinear quasi-geostrophic, two-level β-plane model. An initially small perturbation with the structure of the linearly most unstable mode is allowed to grow to finite amplitude through nonlinear interaction. Because of latitudinal asymmetries of the basic zonal flow, a spectrum of meridional modes is generated in the perturbation. The time evolution of zonal wind and perturbation meridional structures, and their Fourier meridional mode spectra are examined. The radius of deformation is an important meridional scale in both the zonal flow and perturbation. This is especially true during the barotropic decay phase of the baroclinic wave. Time series of energy conversion terms show there is no energy accumulation.     

Low-level wind response to mesoscale pressure systems

Observations are presented which show a strong correlation between low-level wind behaviour (e.g., rotation near the surface) and the passage of mesoscale pressure systems. The latter are associated with frontal transition zones, are dominated by a pressure-jump line and a mesoscale high pressure area, and produce locally large horizontal pressure gradients. The wind observations are simulated by specifying a time sequence of perturbation pressure gradient and subsequently solving the vertically-integrated momentum equations with appropriate initial conditions. Very good agreement is found between observed and calculated winds; in particular, (i) a 360 ° rotation in wind on passage of the mesoscale high; (ii) wind-shift lines produced dynamically by the pressure-jump line; (iii) rapid linear increase in wind speed on passage of the pressure jump.     

Calculations of hailstone growth in a sloping steady updraft

Abstract

Analytical solutions to the equations of growth and motion of hailstones in updrafts with cloud water contents that vary linearly with height are used to investigate hail growth in a model cloud. The model storm is steady and two‐dimensional and is constructed by compositing sections with linear variations of vertical air velocity and cloud water content so as to approximate the Fleming storm. Hail embryos are introduced at various positions in the updraft and their subsequent history calculated. A strong correlation is found between the embryo starting position and its trajectory and final size. Embryos starting in the central portions of the updraft are carried up rapidly and displaced far from the updraft. Embryos starting near the upshear edge of the updrafr have a simple arcing up and down trajectory and produce small hailstones that can be expected to melt before reaching the ground. Embryos starting near the downshear edge of the updraft give rise to the largest hailstones and have recirculating trajectories. Effects of changing some of the model parameters are investigated.     

弱垂直风切变下台前飑线不同发展阶段的热、动力特征分析

以2014年7月18日09号超强台风"威马逊"影响期间,发生在湘赣地区的一次台前飑线过程为例,讨论了在垂直风切变明显弱于中纬度飑线情况下台前飑线的生成与发展机制。研究表明:1)在飑线初生阶段,弱垂直风切变与较弱冷池相平衡使得飑线垂直发展,其前部上干冷下暖湿的不稳定环境条件是其发展加强的热力因素;台风倒槽右侧风向水平切变、飑线前侧的阵风锋是其发展的动力条件。2)在飑线成熟阶段,飑线后侧的地面冷池范围变大、强度变强,导致飑线前方的水汽及能量补给减弱;同时飑线后部中层干冷空气入侵加强,飑线上升气流向冷池方向倾斜,垂直抬升条件减弱,不利于台前飑线的发展维持。成熟阶段的这两个特点表明台前飑线由盛转衰。3)在飑线消散阶段,由于飑线远离台风,台风的影响减弱,导致台前飑线水汽和动力条件不足,从而消亡。     

Further analysis of singular vector and ENSO predictability in the Lamont model—Part I: singular vector and the control factors

In this study, singular vector analysis was performed for the period from 1856 to 2003 using the latest Zebiak–Cane model version LDEO5. The singular vector, representing the optimal growth pattern of initial perturbations/errors, was obtained by perturbing the constructed tangent linear model of the Zebiak–Cane model. Variations in the singular vector and singular value, as a function of initial time, season, ENSO states, and optimal period, were investigated. Emphasis was placed on exploring relative roles of linear and nonlinear processes in the optimal perturbation growth of ENSO, and deriving statistically robust conclusions using long-term singular vector analysis. It was found that the first singular vector is dominated by a west–east dipole spanning most of the equatorial Pacific, with one center located in the east and the other in the central Pacific. Singular vectors are less sensitive to initial conditions, i.e., independence of seasons and decades; while singular values exhibit a strong sensitivity to initial conditions. The dynamical diagnosis shows that the total linear and nonlinear heating terms play opposite roles in controlling the optimal perturbation growth, and that the linear optimal perturbation is more than twice as large as the nonlinear one. The total linear heating causes a warming effect and controls two positive perturbation growth regions: one in the central Pacific and the other in the eastern Pacific; whereas the total linearized nonlinear advection brings a cooling effect controlling the negative perturbation growth in the central Pacific.     

改进的切线性模式对一个边界层模式变分资料同化的改善

大气、海洋中边界层物理过程的强非线性会对切线性模式近似（尤其是当积分时间长时）有较大的影响，从而给相应的4维变分资料同化问题的求解造成困难。本构造了Mellor-Yamada湍流封闭模式（level 2.5)的一个改进的切线性模式，相比通常的切线性模式和简化的切线性模式可以提高对非线性扰动的逼近。利用这个改进的切线性模式的伴随模式分别进行了1到7天的变分资料同化试验，得到了满意的结果。而用通常的伴随模式和简化的伴随模式都无法得到令人满意的结果。     

积云团对大气层结影响的模拟研究

文中设计了一个二维完全弹性的积云团数值模式 .模式中考虑了水平均一和时间定常的大尺度抬升速度的影响 .大度抬升速度只对热力学方程和水汽守恒方程有影响 .积云团的启动方式采用冷池和随机扰动相结合的方法 .文中对一个给定温湿层结和大尺度抬升速度情况下 ,就不同扰动结构、水平环境切变、模式区域大小和冷池强度等进行了 1 1个模拟试验 .结果表明 ,在不同扰动结构、是否有水平环境风切变、模式区域不同以及不同云型谱时 ,给定温湿层结和大尺度抬升速度下的积云团活动使得温湿层结向着同一温湿层结调整 .这说明大气由于积云团活动达到准平衡状态 ,这个准平衡状态主要依赖于初始温湿层结和大尺度抬升速度 .这与 Betts积云参数化方案的假设相一致 .     

复杂地形下雷暴增强过程的个例研究

本文基于多普勒雷达变分同化分析系统（VDRAS）反演的对流层低层热力和动力场,并结合多种稠密观测资料,对北京地区2009年7月22日一次弱天气尺度强迫下雷暴在山区和平原增强的机理进行了较深入的分析。研究结果表明：雷暴过程受大尺度天气系统影响不明显,对流前期地面弱冷锋,是此次雷暴新生的触发机制,高层冷平流、低层偏南暖湿气流的稳定维持和对流不稳定能量的聚集是本次雷暴增强的必要条件。雷暴从河北北部移进北京西北山区后,在下山和到达平原地区时,经历了两次明显的发展增强阶段。雷暴第一阶段下山增强,地形强迫起着主要作用,具体表现在三个方面：（1）地形斜坡使得雷暴冷池出流下山加速与稳定维持的偏南气流形成了强的辐合区;（2）地形抬升使得偏南暖湿入流强烈地上升,从而加剧了对流的发展;（3）地形抬高了冷池出流高度,使得出流与近地面偏南气流构成随高度顺转的低层垂直风切变,低层暖空气之上有冷平流叠加,使得雷暴前方的动力和热力不稳定增强。雷暴第二阶段在平原地区再次增强的主要原因是：组织完好的雷暴到达平原地区后,其冷池与低层暖舌在城区（朝阳地区）的对峙,产生了强的扰动温度梯度;强的冷池出流与势力相当的偏南暖湿气流相互作用产生了强的辐合上升气流,并与下沉气流在较长时间内共存;冷池出流形成的负涡度与低层切变产生的正涡度达到近似平衡状态。运用RKW理论,三者导致雷暴前方低层的辐合抬升最强,最有利于雷暴的维持发展。     

一次长寿命风暴的CINRAD/SA雷达反射率及中气旋产品特征与流场结构分析

利用济南观测站的探空、涡度、地面资料和CINRAD/SA雷达产品,对2004年6月24日发生在山东西部地区的强风暴过程进行了分析。结果表明,风暴低层存在中尺度辐合现象,风暴前侧的入流依靠后部低层冷空气池的向前推进而得到支撑,产生逆环境风切变方向的主上升气流区;中层为双涡旋结构,这种深厚的内部涡旋结构可与环境风相持,形成近似刚体的风暴柱,环境风绕风暴而过,不会吹穿风暴,有利于风暴长时间维持;风暴成熟阶段表现为超级单体结构特征并伴有中气旋;三体散射(TBSS)出现在中高层,TBSS出现10~15 min后地面出现大冰雹,TBSS消失后维持10~20 min的降雹。     

中尺度对流系统的动热力特征及其演变对风垂直切变的响应

为探究环境风切变在对流系统发生发展与维持过程中的重要性,利用2009年6月5日20时（北京时）上海宝山站的探空资料生成理想试验初始场,设计了包含改变整层、中层和低层风切变在内的多组试验,对比分析各试验系统的动热力结构特征及其演变发现:(1)整层环境风切变的改变对中尺度对流系统的影响最显著,其次是中层风切变。增大整层风切变时,对流系统强度及组织性最强,生命史增长。减小整层风切变时,系统强度最弱且组织结构易发散。(2)风切变增加,水平涡度增大,其受垂直运动影响转化为垂直涡度,涡旋对与垂直运动间相互作用形成的正反馈过程是系统强度增强并可以长时间维持线状结构的重要原因。(3)风切变减小,对流系统移动速度远小于阵风锋,阵风锋移至系统前方,阻断系统前沿上升气流必需的暖输送。阵风锋后冷而稳定的环境令系统逐渐消散。      

风速垂直切变对于对流的发展和结构的影响

本文分两个部分。在第一部分中,分析了具有速度垂直切变的盛行风,对于对流发展强度的影响。指出,风速垂直切变对于对流的发展是否有利,决定于对流流场的结构。对于一种特定的流场,通过扰动增长率的计算,定量地讨论了风速垂直切变和不稳定层结对扰动发展的相对重要性。 在第二部分中,进一步求得了考虑风速垂直切变影响后的非线性对流运动方程组在边界热源作用的解,对解的数值计算表明,由于风速垂直切变的作用,对流流场的结构与静止大气中的情况不同,不再是轴对称的,而主要由两个顺时针流动的涡环组成,这种流场结构,与一般移动的雷阵雨云中的流场结构很相似。