Meso-beta scale numerical simulations of terrain drag-induced along-stream circulations Part I: Midtropospheric frontogenesis

Summary The problem of along-stream ageostrophic frontogenesis is studied by employing a numerical model at meso-alpha and meso-beta scales in simulations of the downstream circulations over the Front Range of the Rocky Mountains. Three-dimensional real data simulations at these two scales of motion are used to diagnose the transition from semigeostrophic cross-stream frontogenesis accompanying a propagating baroclinic upper-level jet streak to midtropospheric along-stream ageostrophic frontogenesis. This along-stream ageostrophic frontogenesis results from the perturbation of the jet streak by the Rocky Mountain range. The case study represents an example of internal wave dynamics which are forced by the drag of the Rocky Mountains on a strong jet streak in the presence of a low-level inversion.The simulation results indicate that, unlike semi-geostrophic frontogenesis, a front (which is alligned perpendicular to the axis of the jet stream) may form when significant adiabatic heating occurs within a stratified shear flow over horizontal length scales shorter than the Rossby radius of deformation. The mechanism responsible for the frontogenesis is the growth of the divergent along-stream wind velocity component which becomes coupled to the front's along-stream pressure gradient force. This nonlinear interaction produces hydrostatic mesoscale frontogenesis as follows: 1) vertical wind shear in the along-stream plane strengthens resulting in the increasingly nonuniform vertical variation of horizontal temperature advection as the ageostrophic wind component grows in magnitude downstream of the meso-scale terrain-induced adiabatic heating, 2) increasing along-stream differential vertical motions (i.e., along-stream thermally indirect circulation with warm air sinking to the west and cold air rising to the east) tilt the vertical gradient of isentropes into the horizontal as the vertical temperature gradient increases due to the previous process in proximity to horizontal gradients in the along-stream component of the ageostrophic wind, 3) as tilting motions act to increase the along-stream horizontal temperature gradient, the along-stream confluence acts to nonuniformly increase the along-stream frontal temperature gradient which increases the along-stream pressure gradient force resulting in further accelerations, ageostrophy, and frontal steepening as part of a scale contraction process.The evolution of the aforementioned processes results in the three-dimensional hydrostatic frontogenesis accompanying the overturning of isentropic surfaces. These adjustments act to turn air parcels to the right of the southwesterly geostrophic wind vector at successively lower atmospheric levels as the scale contraction continues. This simulated along-stream front is verified from diagnostic analysis of the profiler-derived temperature and wind fields.With 17 Figures     

Characterizing the severe turbulence environments associated with commercial aviation accidents. Part 2: Hydrostatic mesoscale numerical simulations of supergradient wind flow and streamwise ageostrophic frontogenesis

Summary Simulation experiments reveal key processes that organize a hydrostatic environment conducive to severe turbulence. The paradigm requires the juxtaposition of the entrance region of a curved jet streak, which is highly subgeostrophic, with the entrance region of a straight jet streak, which is highly supergeostrophic. The wind and mass fields become misphased as the entrance regions converge, resulting in significant spatial variation of inertial forcing, centripetal forcing, as well as along and cross-stream pressure gradient forcing over a meso- scale region. Maxima of these forces are misphased where the two dissimilar jet streaks converge and geostrophic balance is disrupted. Velocity divergence within the subgeostrophic region of largest upstream-directed pressure gradient force and velocity convergence near the region of largest downstream-directed centripetal/inertial-advective forcing act to produce a mesoscale front due to spatially varying confluent flow flanked by zones of increasing velocity divergence. This results in frontogenesis as well as the along-stream divergence of cyclonic and convergence of cyclonic ageostrophic vertical vorticity. The nonuniform centripetally forced mesoscale front becomes the locus of large gradients of ageostrophic vertical vorticity along an overturning isentrope. This region becomes favorable for streamwise vorticity gradient formation enhancing the environment for the organization of horizontal vortex tubes in the presence of buoyant forcing. This is because the mesoscale convergence of vertical vorticity on an overturning isentropic surface creates vertical rotation for the development of horizontal vorticity in regions where isentropic surfaces overturn. Vorticity, shear, and buoyancy are focused in one location by this front thus favoring an environment favorable for microvortex formation leading to turbulence.     

鲁西北一次持续性暴雨成因分析

利用地面自动气象站、NCEP 1°×1°和FY2E反演T_(BB)等资料,对2012年7月31日—8月1日发生在山东一次持续的区域性暴雨进行分析和研究。结果表明,(1)在副高稳定的形势下,低层切变线、地面冷锋是造成暴雨的主要天气系统。(2)低层锋生强迫激发了非地转垂直环流,上升气流加强,雨强增大,上升支的建立到强降水开始间隔约6 h。(3)暴雨区925 hPa湿斜压项在强降水时为负值,其增强应为低层锋生作用造成的。雨区范围和雨强随湿斜压项增强而增大。(4)对流单体的生成和加强影响降水落区和雨强,增强的单体合并时雨强最强。T_(BB)中心区位于低层切变线与锋面右侧,强降水位于两者之间。(5)地面辐合线与T_(BB)中心基本重合,辐合线左侧气旋性弯曲处与T_(BB)中心区后侧的等值线梯度大值区位置一致,强降水则出现在两者重叠处。(6)干线与辐合线是此次强降水的重要中尺度系统,干线始终落后于辐合线,其分布对暴雨落区有一定的指示意义。     

Persistent high concentration of ozone during windstorm conditions in southern Korea

Summary Prior to and following the development of a windstorm in the mountainous coastal area of southern Korea, ground level ozone (O₃)-concentrations near Kangnung city, on the lee side of the mountains, show a maximum value at approximately 1300 LST, owing to a photolytic cycle of NO₂–NO–O₃ during the day and a minimum in concentrations at night as a result of the reverse cycle. During the development period of the windstorm, ozone concentrations are generally high all day, and slightly higher during the night. This distribution pattern of ozone is very different from the typical distribution of ozone in the absence of windstorms. High daytime concentrations of ozone during the windstorm are due to both the increase in the amount of ozone from photochemical reactions involving NO_x and the increase in O₃-concentration due to a decrease in the convective boundary layer thickness under the influence of downslope windstorm conditions on the lee-side of the mountains. At night, the windstorm increases in intensity as the westerly winds combine with a katabatic wind blowing downslope toward the surface at the coast. This causes momentum transport of air parcels in the upper levels toward the surface at the coast and the development of internal gravity waves, which generate a hydraulic jump directed upward over the coast and the East sea, thereby reducing to very thin the thickness of the nocturnal surface inversion layer (NSIL). The higher O₃-concentration at night depends mainly upon the shallow NSIL and on some O₃ being transported by the momentum transfer from the upper troposphere toward the ground in windstorm conditions.     

A DIAGNOSTIC STUDY OF AN EXPLOSIVELY DEEPENING OCEANIC CYCLONE OVER THE NORTHWEST PACIFIC

The vertical motions and secondary circulation of an explosively deepening oceanic cyclone,which oc-curred over the Northwest Pacific Ocean and was in conjunction with 200 hPa-level jet stream and hascentral pressure falls of 33.9 hPa/24h,have been computed from seven-level nonlinear balance model and Saw-yer-Eliassen-Shapiro equation for the transverse ageostrophic circulation.The vertical motions are partitionedinto contributions from large-scale latent heat release,effect of cumulus heating,thermal advection,differen-tial vorticity advection,etc.,while the secondary circulation stream function is partitioned into contributionsfrom geostrophic deformation,transfer of momentum and heat in the area of cumulus and diabatic heating.The principal results are the following.Large-scale latent heat release is very crucial to the explosive de-velopment of cyclones.If there is enough transfer of moisture,the positive feedback process between ascentof air and large-scale heating would work.The cumulus heating and the transfer of momentum and heatin the area of cumulus play an important role during the explosively deepening stage.Thermal advection isthe initial triggering condition for large-scale heating and the conditional instability for the convection ofcumulus.     

The effect of spatial resolution on the simulation of upper-tropospheric frontogenesis using a sigma-coordinate primitive equation model

Summary The effects of varying horizontal and vertical grid resolution on the numerical simulation of upper-tropospheric frontal structures are examined using a, two-dimensional, dry, hydrostatic sigma-coordinate primitive equation model. These effects are illustrated with the results of 72 h model integrations in which frontogenesis is forced solely by confluence. Four different horizontal grid spacings, ranging from 100 km to 12.5 km, and four different vertical, grid resolutions, varying from 10 to 70 layers (90 mb to 13 mb), are considered.The intensity and integrity of the frontogenesis as a function of grid resolution are diagnosed in terms of time histories and spatial distributions of frontal parameters, such as the speed of the along-front jet, maxima of absolute vorticity and potential temperature gradient, and ageostrophic motions. The time histories show that, provided the vertical resolution is sufficient, increasing horizontal resolution leads to better-defined frontal structure due to the decrease in cross-frontal scale. They also indicate that for a given horizontal resolution there exists an optimal vertical resolution beyond which frontal parameters change only slightly. This optimal vertical resolution increases with increasing horizontal resolution, and apparently is related to the horizontal resolution through the slope of the frontal zone.The time histories for simulations combining low vertical resolution with high horizontal resolution exhibit substantial high-frequency variability. Cross sections show that this temporal variability appears to be manifested spatially in the form of gravity waves characterized by wavelengths on the order of 200 km and periods of 12 h, over the range of horizontal and vertical resolution that is considered. Although the source mechanism for these waves cannot be established definitively, it likely involves a grid-induced ageostrophic component of the along-front wind which disrupts thermal wind balance. This results thus demonstrates the potential risk of flawed frontal simulations, when incompatible combinations of horizontal and vertical resolution are used.With 7 Figures     

POTENTIAL VORTICITY STRUCTURE OF SUMMER CYCLONES OVER THE CHANGJIANG-HUAIHE VALLEY*

Based on the theory of Ertel potential vorticity,the isentropic potential vorticity maps and vertical profiles of potential vorticity for two summer cyclones over the Changjiang-Huaihe Valley are analysed.After discussing a possible mechanism for the genesis and development of such systems and their differences from typical extratropical cyclones,a conceptual model for their activities is proposed:A weak disturbance in the midlevel of troposphere originated from around the Qinghai-Xizang Plateau may cause heavy precipitation under favourable conditions and latent heat release in the mid-troposphere leads to downward extension of cyclonic circulation and a wave on the quasi-stationary front.This weak cyclone can develop substantially and become a typical extratropical cyclone only when air from the lower stratosphere flows downslope along isentropic surfaces into the region of interest.     

台风Lekima(1909)登陆前后动热力结构变化对浙江极端降水的影响

利用NCEPFNL 1°×1°的全球再分析资料、FY-2F卫星相当黑体亮温TBB资料、中国自动站与CMORPH降水产品融合的逐时降水资料和多普勒天气雷达资料,重点分析了台风Lekima(2019)发展演变过程中的动热力结构变化和水汽分布特征与浙江极端强降水之间的关系.台风Lekima(2019)近海急剧加强为具有特殊双...     

西北太平洋爆发性气旋的诊断分析

本文对一年中26个西北太平洋爆发性气旋形成的大尺度条件做了统计研究。结果表明:海洋上空大气层结的不稳定、高空急流出口区北侧的动力辐散,冬季副高位置偏北时其西侧的强暖平流以及中低层的强斜压区等都是气旋急剧发展的有利因素。 另外,本文利用完全的ω-方程和Sawyer-Eliassen次级环流方程对1983年1月6—9日发生在西北太平洋上的一次爆发性气旋做了诊断分析,得到:(1)大尺度加热是使气旋强烈发展的主要物理因子;积云对流加热也是重要的物理因子。(2)温度平流是使前两者起作用的先决因子,它对气旋的初期发展起了某种启动作用。     

Climatology of summer midtropospheric perturbations in the US northern plains. Part II: large-scale effects of the Rocky Mountains on genesis

Propagating convective storms across the US northern plains are often coupled with preexisting midtropospheric perturbations (MPs) initiated over the Rocky Mountains. A companion study (Part I) notes that such MPs occur most commonly at 12 UTC (early morning) and 00 UTC (late afternoon). Using a regional reanalysis and a general circulation model (GCM), this study investigates how such a bimodal distribution of the MP frequency is formed. The results point to two possible mechanisms working together while each has a different timing in terms of maximum effect. The diurnal evolutions between the midtropospheric flows over the Rockies and over the Great Plains are nearly out-of-phase due to inertial oscillation. During the nighttime, the westerly flows at 700?C500?mb over the Rockies intensify while flows at the same level over the Great Plains turn easterly. These two flows converge over the eastern Rockies and induce cyclonic vorticity through vortex stretching. After sunrise, the convergence dissipates and the cyclonic vorticity is redistributed by horizontal vorticity advection, moving it downstream. This process creates a climatological zonally propagating vorticity signal which, in turn, facilitates the early-morning MP genesis at 12 UTC. The analysis also reveals marked dynamic instability conducive to subsynoptic-scale disturbances in the midtroposphere over the Rockies. Strong meridional temperature gradients appear over the north-facing slopes of the Rockies due to terrain heating to the south and the presence of cooler air to the north. This feature, along with persistent vertical shear, creates a Charney?CStern type of instability (i.e. sign changes of the meridional potential vorticity gradient). Meanwhile, the development of terrain boundary layer reduces the Rossby deformation radius which, subsequently, enhances the likelihood for baroclinic short waves. Such effects are most pronounced in the late afternoon and therefore are supportive to the MP genesis around 00 UTC. Examination of GCM experiments with and without orography further supports the critical role of the Rocky Mountains and its associated boundary layer impacts on the formation of MPs.     

POTENTIAL VORTICITY STRUCTURE OF SUMMER CYCLONES OVER THE CHANGJIANG-HUAIHE VALLEY

Based on the theory of Ertel potential vorticity,the isentropic potential vorticity maps and vertical pro-files of potential vorticity for two summer cyclones over the Changjiang-Huaihe Valley are analysed.Afterdiscussing a possible mechanism for the genesis and development of such systems and their differences fromtypical extratropical cyclones,a conceptual model for their activities is proposed:A weak disturbance in the mid-level of troposphere originated from around the Qinghai-Xizang Plateau may cause heavy precipitation underfavourable conditions and latent heat release in the mid-troposphere leads to downward extension of cycloniccirculation and a wave on the quasi-stationary front.This weak cyclone can develop substantially and becomea typical extratropical cyclone only when air from the lower stratosphere flows downslope along isentropic sur-faces into the region of interest.     

一次梅雨锋特大暴雨过程的数值模拟研究：不同尺度天气系统的影响作用

2003年7月4~5日在江淮地区沿梅雨锋有一系列中尺度对流系统相继生成和强烈发展,导致了江淮地区特大暴雨的形成。该研究利用中尺度数值模式MM5对这次梅雨锋暴雨过程进行了数值模拟,在模拟结果的基础上重点分析了不同尺度天气系统相互作用对这次特大暴雨过程的影响作用。在这次特大暴雨过程中,位于梅雨锋北侧的东北—西南走向深厚、稳定的短波槽系统与槽前从西南移来的低涡系统相配合,加强了位于梅雨锋北侧的反气旋性扰动发展,从而导致梅雨锋北侧反气旋性涡旋的形成。该类反气旋性涡旋形成对江淮切变线的加强与维持起重要作用。中尺度对流系统的潜热释放首先导致梅雨锋低层切变线上的中尺度对流性涡旋(MCV)的形成,而中尺度对流性涡旋的形成进一步加强了切变线上的低层辐合,中尺度对流性涡旋消亡后,在切变线上形成低涡。梅雨锋附近主要存在4种不同垂直环流,它在降水的不同阶段具有不同的结构、配置与动力学作用。其中跨锋面、高层非地转两支垂直环流对锋区的对流扰动发展和暴雨形成最为重要,而降水发展可以调整锋区垂直环流的结构、配置,随降水的减弱,梅雨锋区的不同垂直环流系统又重新恢复到先前结构。梅雨锋上不同尺度、高度的天气系统之间的相互作用主要通过这些垂直环流系统调整实现。     

On the structure of supercritical western boundary currents

The structure of supercritical western boundary currents is investigated using a quasi-geostrophic numerical model. The basic flow is of meridional Munk balance, and the input boundary is perturbed by the most unstable wave solution obtained from linear spatial instability calculations. Self-preserving (or equilibrium) solutions are obtained for the model runs at Re=30, 60, 90, and 120, and their energy and vorticity budgets are analyzed. In an analogy with the laboratory turbulence of wall boundary layers, the western boundary layer is divided into inner and outer layers. In the inner layer, the mean energy is dissipated via direct viscous dissipation, while in the outer layer it is converted to the eddy energy via turbulence production. The main scenario is that the mean energy is produced in the inner layer via ageostrophic pressure work divergence, and it is partly removed due to viscous action within a narrow region near the wall, defined here as viscous sub-layer. The remaining portion is converted to the eddy energy via turbulence production in the outer layer, which is in turn transported to the inner layer, then again to the viscous sub-layer where it is ultimately dissipated. In the near-wall side, the vorticity balance of the mean flow is maintained by viscous effect and Reynolds flux divergence, while in the offshore side it is maintained by beta effect and Reynolds flux divergence. The length scale of the supercritical boundary current is roughly , where L_M is the Munk length, as observed from a dimensional analysis.     

The Effects of Strong Ageostrophic Outflows on the Formation of Surface Mesoscale Pressure Systems in Squall Lines

Based on the previous study of the streamline field triggered by singularities in a two-dimensional potential flow,the wind ficld caused by vorticity lines in an incompressible flow is deduced in this paper.The result shows an elliptic cyclonic(anticyclonic)circulation in association with a positive(negative)vorticity line.By use of the shallow-water model,the flow fields are simulated in a weak wind background under the influence of mesoscale vorticity lines.In the case of two vorticity line,one positive and the other negative,a mesoscale vortex couplet forms in the flow.When three vorticity lines are considered,three mesoscale circulations develop,and a mesohigh and two mesolows similar to the thunderstorm high,wake low and pre-squall mesolow of a mature squall line are produced.Theoretical analysis and numerical simulations show that the formation of the surface mesoscale pressure systems in squall lines may be partly attributed to the dynamical effects of the ageostrophic outflows.The strong downdrafts under the thundercloud base of the squall line lead to surface ageostrophic outflows,and produce positive-negative-positive arranged vcrtical vorticity bands(VBs)along the direction normal to the squall line,then the mesoscale circulations develop and mesoscale pressure systems form or strengthen during the geostrophic adjustment.By use of the scale separation method,this dynamic mechanism is confirmed by a case study of a severe storm passing over eastern China on 17 June 1974.     

山东省2006年4月28日飑线天气过程分析

对2006年4月28日山东省一次飑线天气过程进行诊断分析，应用湿位涡守恒理论研究了飑线的发展机制。结果表明：飑线是由500hPa西风槽影响产生的，为低层增温增湿，高层冷空气南下，低能舌叠加在高能舌之上，导致大气对流性不稳定。850hPa切变线和地面低压槽中的辐合上升运动触发对流不稳定能量释放，产生中尺度对流云团，在热力不稳定和风垂直切变的环境条件下对流云团东移发展，形成飑线。低层大气湿斜压性增强，破坏了地转平衡，倾斜涡度发展，上升运动增强，对流发展；高空高位势涡度下传使得中低层位势涡度增大，导致其垂直涡度增大，有利于对流层低层中尺度涡旋发展，对流增强。较强的上升运动与风垂直切变相互作用，促使对流系统发展形成飑线，产生雷雨大风。     

Objective cross-sectional analysis of diabatic circulations and vertical motions using ALPEX data

Summary Frontal propagation and development in the presence of orography and moisture are examined using the objective cross-section analysis method of Shapiro and Hastings (1973). The method is applied to a real data case on 4–5 March 1982 during the Alpine Experiment (ALPEX). Surface data are added to the analysis method to improve the lowlevel depiction of the frontal zone.The ageostrophic winds are computed from the isentropic representation of =dp/dt and the horizontal momentum equations. Diabatic contributions to the circulation, computed from the isentropic form of the continuity equation, are compared with observed moisture fields. Cross-sections of observed data and computed vertical motions are compared with similar fields computed from analyses on pressure surfaces. It is shown that the Shapiro and Hastings scheme provides a relatively clear depiction of the small-scale frontal features in the thermal and wind field but the analysis also exhibits more noise, which can have an adverse effect on the accurarcy of derived fields.With 8 Figures     

The synoptic setting of a thundery low and associated prefrontal squall line in western Europe

Summary A case study of a thundery low and associated prefrontal squall line in western Europe is presented. It is shown that the prefrontal squall line is linked to the vertical circulation associated with an intensifying cold front and a propagating midtropospheric jetstreak, with maximum wind speeds at levels between 700 and 500 hPa. The squall line is triggered in the updraught of the cross-frontal circulation, which can be observed at the earth's surface as a line of mass convergence or confluence stretching for more than 1000 km from southern Iberia to northern France. The intensification of the front and the destabilization of the atmosphere are interpreted by using the slope of isentropes as indicator of frontal intensity. An equation is derived for the rate of change of frontal intensity, which predicts a forward tilt of the cold front with increasing height due to the cross-frontal circulation in the area of warm air advection (the so-called Spanish plume) and, associated with this, a destabilization of the atmosphere at midtropospheric levels.With 17 Figures     

Influences of vorticity source and momentum source on atmospheric circulation

A simplified one-dimensional barotropic vorticity equation is used to study the influences of the vorticity and the momentum source on the large scale wave. Both vorticity source and momentum source can cause the formation of the large scale wave, but the former can produce large scale wave only under the condition that there is apparent basic flow acting on it, while the latter can produce the large scale wave even when ū tends to be zero. Furthermore, the amplitude of steady wave caused by the former is proportional to √ū, while the amplitude caused by the latter has no relation to ū, instead it depends only on the magnitude of the perturbation of momentum.     

致洪暴雨过程中尺度涡旋的结构和准平衡特征分析

利用2008年6月广西致洪暴雨过程高分辨率数值模式资料,分析引起暴雨的中尺度涡旋在移动和持续性发展过程中其内部结构的演变以及系统的动力学特征。研究结果表明,中尺度涡旋是导致此次广西暴雨的主要系统,其发展移动过程中伴随有强烈上升运动的深厚湿对流,并在中高层持续形成明显的暖心结构。此次涡旋系统移动过程可分为涡旋南移和涡旋东移两个阶段,由于受到环境场的影响,涡旋中心区域动量、热量和水汽的再分配为其持续性发展提供有利条件。同时诊断分析表明,涡旋内部存在具有继发特征的长时间维持的组织化深厚湿对流系统,以强非地转分量为特征的超地转流,并表现出涡散运动共存且同量级的特征,具有典型的准平衡特征。     

吉林一次极端降水发生发展动热力过程的数值模拟分析

利用ERA-Interim再分析资料、常规气象观测资料、CMORPH（CPC MORPHing technique）融合降水资料以及WRF（Weather Research and Forecasting）高分辨率数值模拟结果,对2017年7月13～14日吉林地区的极端降水天气过程的环流背景和触发机制进行了分析。结果表明:（1）东北冷涡环流控制下,副高北抬与中纬度锋区形成了有利的大尺度环流背景。降水发生在冷涡底部与副高之间的平直纬向环流中,东北冷涡南部的低槽、低空切变线、高低空急流是影响此次降水的重要天气系统;（2）在高层辐散低层辐合的有利动力条件下,极端的水汽输送与吉林地区西低东高地形的阻挡和强迫抬升是极端降水产生的重要原因;（3）中高层有干冷空气入侵,伴随高空动量下传至低空,加强了低空急流发展,低空急流发展至地面附近产生超低空急流后,加强了上升运动。南北经向动量输送交汇加强了低层风辐合切变,切变线上对流发展与永吉附近小地形的抬升作用,诱导永吉县产生极端降水。