Cyclonic Vorticity Research Articles

Surface Circulation and Vertical Structure of Upper Ocean Variability Around Fernando de Noronha Archipelago and Rocas Atoll During Spring 2015 and Fall 2017

Using current, hydrographic and satellite observations collected off Northeast Brazil around the Fernando de Noronha Archipelago and Rocas Atoll during two oceanographic cruises (spring 2015 and fall 2017), we investigated the general oceanic circulation and its modifications induced by the islands. In spring 2015, the area was characterized by lower SST (26.6°C) and deep mixed-layer (∼90 m). At this depth, a strong current shear was observed between the central branch of the eastward flowing near-surface South Equatorial Current and the westward flowing South Equatorial Undercurrent. In contrast, in fall 2017, SST was higher (∼28.8°C) and the mixed-layer shallower (∼50 m). The shear between the central South Equatorial Current and the South Equatorial Undercurrent was weaker during this period. Interestingly, no oxygen-rich water from the south (retroflection of the North Brazil undercurrent) was observed in the region in fall 2017. In contrast, we revealed the presence of an oxygen-rich water entrained by the South Equatorial Undercurrent reaching Rocas Atoll in spring 2015. Beside these global patterns, island wake effects were noted. The presence of islands, in particular Fernando de Noronha, strongly perturbs central South Equatorial Current and South Equatorial Undercurrent features, with an upstream core splitting and a reorganization of single current core structures downstream of the islands. Near islands, flow disturbances impact the thermohaline structure and biogeochemistry, with a negative anomaly in temperature (−1.3°C) and salinity (−0.15) between 200 and 400 m depth in the southeast side of Fernando Noronha (station 5), where the fluorescence peak (>1.0 mg m–3) was shallower than at other stations located around Fernando de Noronha, reinforcing the influence of flow-topography. Satellite maps of sea-surface temperature and chlorophyll-a confirmed the presence of several submesoscale features in the study region. Altimetry data suggested the presence of a cyclonic mesoscale eddy around Rocas Atoll in spring 2015. A cyclonic vortex (radius of 28 km) was actually observed in subsurface (150–350 m depth) southeast of Rocas Atoll. This vortex was associated with topographically induced South Equatorial Undercurrent flow separation. These features are likely key processes providing an enrichment from the subsurface to the euphotic layer near islands, supplying local productivity.

Tropical cyclone impacts on the Western Australian coast and extreme wind speeds in Region D

ABSTRACT This paper reviews the numbers of tropical cyclones in the southern Indian Ocean, and the land-falling cyclones that have impacted the Pilbara and Gascoyne coastlines of Western Australia since 1970, with particular emphasis on those of Category 4 strength and above. It shows reductions in impacts of severe cyclones on that coastline in the most recent two decades. Nearly as many severe cyclones have impacted the coast outside the current northern boundary of Region D in AS/NZS 1170.2 as have occurred within that boundary, suggesting the limits of Region D may need reviewing. Recorded and corrected extreme wind gusts exceeding 22 m/s from eight coastal stations have been processed as a group, and individually for 3 stations with long records. This indicates that the extreme value distribution in the draft standard DR AS/NZS 1170.2:2020 is quite adequate, without any additional factors, but the predicted extreme wind speeds for Carnarvon are well under the Region D specifications. The calculated wind direction multipliers for Region D show higher values from east and northeast; this can be explained by the wind directions generated by the clockwise rotations created by the cyclonic vortices, as the storms cross the coastline, or as they pass along the coastline at near full strength.

Evolution of convective characteristics during tropical cyclogenesis

AbstractFour idealized, high‐resolution (500 m horizontal grid spacing), numerical simulations are used to investigate the evolution of convective structures during tropical cyclogenesis. The simulations all begin with a weak initial axisymmetric cloud‐free vortex in a quiescent environment, but differ in the moisture level of the initial sounding and whether or not ice microphysical processes are considered. Irrespective of experimental setup, there is only a short period where shallow or congestus clouds dominate. The shallow cloud phase is slightly extended with the drier initial environmental sounding. The composite structure of the convective elements sampled changes markedly throughout the genesis period. For much of the genesis phase, vertical profiles of the mean convective cell show significant amounts of anticyclonic vorticity produced in cells in the inner core. Towards the end of the genesis phase, there is a large increase in the production of cyclonic vertical vorticity in inner‐core convection, and cyclonic vorticity becomes dominant at low‐mid levels. The evolution from roughly equal strength vertical profiles of cyclonic/anticyclonic vorticity at low‐mid levels to profiles where cyclonic vorticity dominates occurs at relatively low system wind speeds (Vmax less than 10 m·s−1). This finding indicates a change in the structure of vortical convection prior to rapid intensification. In outer‐core convection, there are roughly equal strength vertical vorticity dipoles produced throughout the genesis period.

Structure of Submesoscale Fronts of the Mississippi River Plume

AbstractSubmesoscale currents (SMCs), in the forms of fronts, filaments, and vortices, are studied using a high-resolution (~150 m) Regional Oceanic Modeling System (ROMS) simulation in the Mississippi River plume system. Fronts and filaments are identified by large horizontal velocity and buoyancy gradients, surface convergence, and cyclonic vertical vorticity with along-coast fronts and along-plume-edge filaments notably evident. Frontogenesis and arrest/destruction are two fundamental phases in the life cycle of fronts and filaments. In the Mississippi River plume region, the horizontal advective tendency induced by confluence and convergence plays a primary role in frontogenesis. Confluent currents sharpen preexisting horizontal buoyancy gradients and initiate frontogenesis. Once the fronts and filaments are formed and the Rossby number reaches O(1), they further evolve frontogenetically mainly by convergent secondary circulations, which can be maintained by different cross-front momentum balance regimes. Confluent motions and preexisting horizontal buoyancy gradients depend on the interaction between wind-induced Ekman transport and the spreading plume water. Consequently, the direction of wind has a significant effect on the temporal variability of SMCs, with more active SMCs generated during a coastally downwelling-favorable wind and fewer SMCs during an upwelling-favorable wind. Submesoscale instabilities (~1–3 km) play a primary role in the arrest and fragmentation of most fronts and filaments. These instabilities propagate along the fronts and filaments, and their energy conversion is a mixed barotropic–baroclinic type with horizontal-shear instabilities dominating.

Hindcasting of tropical cyclone winds and waves

This paper describes a series of hindcast simulations of 17 tropical cyclones over the northwest shelf region of Australia. Tropical cyclone track and vortex details were obtained from the Bureau of Meteorology “Best Track” database. Wind fields were simulated using a dynamic boundary layer model referred to as the Kepert-Wang model. Surface wind and pressure fields were blended with background fields from the ERA-Interim dataset. These blended wind fields were then used as forcing for the WAVEWATCH III wave model to generate wave fields. Several different configurations of wave model source terms were trialled. Wind and wave models were validated against available observations from the Bureau of Meteorology and our industry partner. The mean absolute error for peak significant wave height (Hs) for all configurations was mostly less than 0.90 m, compared to a mean observed peak Hs of 4.8 m, and the bias was less than around 0.41 m. It was found that while in general the ST6 wave model source term physics package with NL3 non-linear interactions performs well overall, there is no single configuration that performs best for all 17 tropical cyclones.

Transient generation of spiral inertia-gravity waves from a geostrophic vortex

The emission of inertia-gravity waves (IGWs) from an exact geostrophic vortex in a rotating and stratified fluid is investigated by three-dimensional numerical modeling. An initially balanced geostrophic vortex inevitably generates IGWs with spiral patterns within a short transient time period through an instability mechanism. This result reinforces the nonexistence of exactly invariant slow manifolds. The direction of the rotation of spiral IGWs is clockwise for both anticyclonic and cyclonic geostrophic vortices, which is consistent with the theoretical prediction. Spiral patterning can be regarded as a universal feature of IGWs, which occurs in the transient generation process. In the vertical direction, the energy of IGWs is dominated by mode-1 in the generation and propagation processes, leading to weak dissipation and long-distance propagation. A comparison of barotropic and baroclinic vortices suggests that horizontal nonzero strain and vorticity are essential for the occurrence of this instability mechanism, while the presence of vortex baroclinicity increases the intensity of the IGWs. The amplitude of the IGWs increases linearly with the Rossby number in the range of 0.04–0.1. Additionally, the IGWs emitted from an anticyclonic vortex are stronger than those radiated from a cyclonic vortex. Anticyclonic and cyclonic geostrophic vortices transfer approximately 0.54% and 0.41% of their kinetic energy to IGWs in this transient generation process, respectively. This transient generation of IGWs can supply an energy pathway from mesoscale eddies to diapycnal mixing processes in the interior of the oceans.

The merger of two three-dimensional quasi-geostrophic baroclinic tripolar eddies

We investigate the strong interaction between two baroclinic tripolar eddies in a three-dimensional, rapidly-rotating, continuously stratified flow under the quasi-geostrophic approximation. Each tripolar eddy consists of an anticyclonic central vortex with two oblate cyclonic vortices located above and below the anticyclone. The interaction depends on the vertical and horizontal offsets between the two tripolar eddies. For small and low PV oblate cyclones, each tripolar eddy alone is only weakly unstable to a baroclinic mode. The instability puts the three vortices out of alignment. Most of the eddy however survives the instability. When two tripolar eddies interact, their constituent vortices may merge. Merger occurs when the eddies are close enough together, and shows similarities with the merger of monopolar vortices. Vertically separated eddies do not align vertically. This suggests the importance of an external flow for the alignment, observed in the oceans, to occur. We finally show that the interaction between two tripolar eddies with intense oblate cyclones is very different and show similarities with the dynamics of dipolar baroclinic eddies known as hetons.

A mechanism for formation of the western North Pacific monsoon trough: nonlinear upscale cascade

Linear and nonlinear barotropic vorticity model frameworks are constructed to understand the formation of the monsoon trough in boreal summer over the western North Pacific. The governing equation is written with respect to specified zonal background flows, and a wave perturbation is prescribed in the eastern boundary. Whereas a uniform background mean flow leads no scale contraction, a confluent background zonal flow causes the contraction of zonal wavelength. Under linear dynamics, the wave contraction leads to the development of smaller scale vorticity perturbations. As a result, there is no upscale cascade. Under nonlinear dynamics, cyclonic (anticyclonic) wave disturbances shift northward (southward) away from the central latitude due to the vorticity segregation process. The merging of small-scale cyclonic and anticyclonic perturbations finally leads to the generation of a pair of large-scale cyclonic and anti-cyclonic vorticity gyres, straddling across the central latitude. The large-scale cyclonic circulation due to nonlinear upscale cascade can be further strengthened through a positive convection-circulation feedback.

Dissimilar heat transfer enhancement by introduction of a vortex tube in plane Couette flow

Recent studies have suggested that a cyclonic vortex, which has the same signed vorticity as that of background shear, can lead to dissimilar heat transfer enhancement, i.e. more heat transfer than momentum transfer. However, it has not yet been known how the cyclonic vortex achieves dissimilarity, or if other types of vortices would also lead to dissimilarity. In order to tackle these problems, we introduce a straight vortex tube into laminar plane Couette flow under various conditions and numerically investigate heat and momentum transfer therein. The essential process for dissimilarity is isolated from complex ingredients of realistic flow to consider the interaction of a vortex with background shear. It is found that introduction of a strong spanwise anti-cyclonic vortex, which has the opposite signed vorticity to that of background shear, can realize more dissimilar heat transfer than the known cyclonic vortex as a consequence of the long-term interaction with the background flow, whereas the spanwise cyclonic vortex achieves larger dissimilarity in the early stage of its time evolution. The physical mechanisms of dissimilarity due to these vortices are elucidated by interpreting the distribution of the streamwise pressure gradient and the resulting difference between temperature and streamwise velocity around the vortex.

Effect of a Low-Frequency Vortex on the Size of Typhoon Lan (2017)

AbstractTyphoon Lan (2017) was one of the largest tropical cyclones (TC) in the western North Pacific Ocean (WNP), and it was developed in a low-frequency (10–90-day filtered) large-scale cyclonic vortex environment. The physical mechanism responsible for the TC’s unusual size was investigated through idealized numerical experiments with the Weather Research and Forecasting Model. Sensitivity experiments showed that the low-frequency cyclonic circulation played an important role in modulating the TC size through the following three processes. First, it weakened the background vertical wind shear and provided a favorable condition for a more rapid growth of Lan. Second, it strengthened a vorticity aggregation process through enhanced background vorticity. As a result, a stronger and more organized TC core was quickly set up, which strengthened the TC intensity and expanded its size. Third, it enhanced the total surface wind speed and surface latent heat flux, strengthening convective instability in the outer region through increased moisture. The development of the outer rainband expanded the radial profile of diabatic heating, leading to greater low-level inflow and tangential wind acceleration in the outer region and thus a large TC size.

On the vertical extending of the explosive extratropical cyclone: A case study

AbstractExplosive extratropical cyclone (EEC) is the main disastrous weather system over the ocean and offshore areas in the cold season. As a type of vertically deep system, after decades of studies, key features of EECs' vertical extents still remain vague. Based on a reasonably simulated entire‐troposphere‐thick EEC, this study analyzes variation of the EEC's vertical extent and investigates governing mechanisms for its vertical extending. Main findings are as follows: (a) the EEC's vertical extent showed consistent variation features with its central sea level pressure and lower‐level vorticity (correlation coefficients were ~0.9), whereas its relationship with EEC's maximum surface wind was not significant; (b) EEC's upward extending featured strong ascending motion and rapid cyclonic‐vorticity enhancement at the top level of the cyclone and obvious inflow (convergence) in the lower troposphere. (c) vorticity budget at the EEC's top level shows that net import transport of cyclonic vorticity (by westerly and northwesterly winds) from the trough west of the cyclone dominated its upward extending, and upward transport of cyclonic vorticity from levels below the cyclone's top level acted as the second dominant factor. In contrast, divergence‐related vertical shrinking around the EEC's top level was the most detrimental factor for the cyclone's upward extending.

On the Relationship of a Low-Level Jet and the Formation of a Heavy-Rainfall-Producing Mesoscale Vortex over the Yangtze River Basin

Dabie vortices (DBVs) are a type of heavy-rainfall-producing mesoscale vortices that appear with a high frequency around the Dabie Mountain over the Yangtze River Basin. For a long time, scholars have found that DBVs tend to form when a low-level jet (LLJ) appears in their neighboring regions. However, the underlying mechanisms of this phenomenon still remain vague. This study furthers the understanding of this type of event by conducting detailed analyses on a long-lived eastward-moving DBV that caused a severe flood in the 2020 summer. It is found that the LLJ in this event was belonged to a nocturnal LLJ type, with its maximum/minimum appeared around 2100/0600 UTC. The diurnal cycle of LLJ affected precipitation and intensity of the DBV notably: As the LLJ intensified, vortex’s precipitation and intensity both enhanced, and vice versa. The LLJ exerted two effects on the DBV’s formation that are opposite to each other. The more important effect is that the LLJ caused intense lower-level convergence around its northern terminus. This convergence directly produced cyclonic vorticity through vertical stretching, which dominates the DBV’s formation and enhances the convection-related upward cyclonic vorticity transport that acted as another favorable factor. The less important effect is that (i) the LLJ induced import of anticyclonic vorticity into the vortex’s central region, which decelerated the DBV’s formation; and (ii) the LLJ-related to strong ascending motions tilted horizontal vorticity into negative vertical vorticity, which reduced the growth rate of cyclonic vorticity.

Characteristics and mechanism of vertical coupling in the genesis of tropical cyclone Durian (2001)

The vertical coupling (VC) process and mechanism during the genesis of a tropical cyclone (TC) implied by the weak vertical shear of horizontal wind, one of the key factors impacting TC genesis, constitute important but unanswered fundamental scientific problems. This paper carried out a targeted investigation of this problem through numerical simulation and theoretical analyses. The main conclusions are as follows. Even if TC genesis occurs in a barotropic environment, a VC process still occurs between the trough (vortex) at the middle level and that at the lower level in the TC embryo area. VC mainly occurs at the tropical disturbance (TDS) stage. Only after the VC is accomplished can the tropical depression (TD) organize further by itself and develop into the tropical storm (TS) stage or the stronger tropical typhoon (TY) stage through the WISHE (wind-induced surface heat exchange) mechanism. In the VC process, vortical hot towers (VHTs) play vertical connecting roles and are the actual practitioners of the VC. Through the VHTs’ vertical connections, the middle- and lower-troposphere trough axes move towards each other and realize the VC. VHTs can produce intensive cyclonic vorticity in the lower troposphere, which is mainly contributed by the stretching term. The tilting term can produce a single dipole or double dipole of vorticity, but the positive and negative vorticity pairs offset each other roughly. While the stretching term ensures that the cyclonic rotations of the wind field in the middle and lower levels tend to be consistent, the tilting term acts to uniformly distribute the horizontal wind in the vertical direction, and both terms facilitate the VC of the wind field. With the latent heat of condensation, VHTs heat the upper and middle troposphere so that the 352 K equivalent potential temperature contour penetrates vertically into the 925–300 hPa layer, realizing the VC of the temperature field. While forming cloud towers, VHTs make the ambient air become moist and nearly saturated so that the 95% relative humidity contour penetrates vertically into the 925–400 hPa layer, realizing the VC of the humidity field. Due to the collective contributions of the VHTs, the embryo area develops into a warm, nearly saturated core with strong cyclonic vorticity. The barotropic instability mechanism may also occur during TC genesis over the Northwest Pacific and provide rich large-scale environmental vorticity for TC genesis. The axisymmetric distribution of VHTs is an important sign of TC genesis. When a TC is about to form, there may be accompanying phenomena between the axisymmetric process of VHTs and vortex Rossby waves.

Filaments, Fronts and Eddies in the Cabo Frio Coastal Upwelling System, Brazil

We investigate the dynamics of meso- and submesoscale features of the northern South Brazil Bight shelf region with a 500-m horizontal resolution regional model. We focus on the Cabo Frio upwelling center, where nutrient-rich, coastal waters are transported into the mid- and outer shelf, because of its importance for local and remote productivity. The Cabo Frio upwelling center undergoes an upwelling phase, from late September to March, and a relaxation phase, from April to early September. During the upwelling phase, an intense front around 200 km long and 20 km wide with horizontal temperature gradients as large as 8 ∘C over less than 10 km develops. A surface-intensified frontal jet of 0.7 ms−1 in the upper 20 m and velocities of around 0.3 ms−1 reaching down to 65 m depth makes this front a preferential cross-shelf transport pathway. Large vertical mixing and vertical velocities are observed within the frontal region. The front is associated with strong cyclonic vorticity and strong variance in relative vorticity, frequently with O(1) Rossby numbers. The dynamical balance within the front is between the pressure gradient, Coriolis and vertical mixing terms, which are induced both by the winds, during the upwelling season, and by the geostrophic frontal jet. Therefore, the frontal dynamics may be largely described as sum of Ekman and turbulent thermal wind balances. During the upwelling phase, a mix of barotropic and baroclinic instabilities dominates in the upwelling center. However, these instabilities do not lead to the local formation of coherent eddies when the front is strong. In the relaxation phase, the front vanishes, and the water column becomes less stratified. The interaction between eastward coastal currents generated by sea level variability, coastal intrusions of the Brazil Current, and sporadic wind-driven, coastal upwelling events induce the formation of cyclonic eddies with diameters of, approximately, 20 km. They are in gradient-wind balance and propagate along the 100-m isobath on the shelf. During this phase baroclinic instability dominates. Cold filaments with widths of 2 km are formed due to straining and stretching of cold, coastal temperature anomalies. They last for a few days and are characterized by downwelling as large as 1 cms−1. The turbulent thermal wind balance provides a good first order estimate of the dynamical balance within the filament, but vertical and horizontal advection are shown to be important. To our knowledge, this is the first account of these smaller scale features in the region. Because these meso- and submesoscale features on the shelf heavily affect the water properties crucial to productivity of the South Brazil Bight, it is important to take these features into account for a better understanding of the functioning of this ecosystem and its resilience to both direct human activities as well as to climate change.

Iba: The First Pure Tropical Cyclogenesis Over the Western South Atlantic Ocean

AbstractIn March 2019, the tropical western South Atlantic Ocean (∼16.5°S) registered the first pure tropical cyclogenesis since the beginning of the satellite era, which the Brazilian Navy named Iba. This study presents a description of the synoptic features and meteorological mechanisms contributing to Iba's initial cyclonic vorticity as well as the anomalous synoptic patterns linked to this unprecedented tropical cyclogenesis. Moreover, a general description of Iba's lifecycle is provided. The genesis of Iba was as a pure tropical cyclone (00:00 UTC on March 22), and it underwent subtropical (18:00 UTC on March 25) and extratropical (06:00 UTC on March 29) transitions. Anomalies for a 5‐day period prior to the genesis relative to the March climatology show environmental features appropriate for tropical cyclogenesis: (a) vertical wind shear in the 850–200 hPa lower than 10 m s−1, (b) intense initial cyclonic relative vorticity at 925 hPa (−10 × 10−5 s−1), (c) latent and sensible heat fluxes near the climatological values (∼150 W m−2), (d) warmer sea surface temperature (+2°C), and (e) relative humidity at 700 hPa 20% higher than the climatology. These features were associated with an anomalous wave pattern at 500 hPa, resembling a blocking structure over the southeastern South Pacific Ocean, and an anticyclonic circulation at upper levels over southeastern Brazil. This environment propitiated a weak vertical wind shear and convective cloud organization. Near the surface, the horizontal wind shear between the postfrontal southwesterly winds and the northeasterly winds of the South Atlantic subtropical anticyclone helped to generate Iba's initial cyclonic vorticity.

Cutoff low over the southeastern Pacific Ocean: a case study

Cutoff lows (COLs) are infrequent events in the tropics that can cause extreme rainfall, flash flooding and landslides in arid areas, such as western South America. In this study, the life cycle of a COL in the southeastern Pacific at the beginning of April 2012 was analysed using the ERA-Interim reanalysis dataset. This paper examines: (1) the precursor flow evolution prior to the COL, its development and dissipation by applying the quasi-geostrophic and vorticity equations; and (2) the influence of the COL in the heavy precipitation events over the western Peruvian Andes. During April 2012, the highest amount of precipitation was recorded in Chosica (850 masl) with 37 mm on 5 April. Days prior to the formation of the COL, a subtropical trough deepened by the amplification of a ridge over the tropical Pacific and the incursion of cold air from medium and low levels into the trough. The strong cyclonic vorticity advection was intensified by a short-wave trough embedded inside a long-wave one that strengthened the system on 5 April 2012. In the dissipation stage, warm vertical advection predominated, resulting in the reabsorption of the COL by a new trough. Understanding the behaviour COL systems is important for reducing the impact of these extreme weather events on lives and infrastructure in densely populated areas.

MECHANISMS OF FORMING OF DYNAMIC STRUCTURES IN DEEP WATERS OF MARMARA SEA

Based on the numerical calculation using the eddy-resolving thermohydrodynamic model of the MHI, the fields of hydrodynamic and energy characteristics of the deep waters of the Marmara Sea averaged for the hydrological seasons of 2008 were obtained and analyzed. It is established that in winter period, at horizons from 20 to 75 m, the system consisting of mesoscale anticyclones and cyclone, as well as eddies near the northwestern and northeastern coastal regions and in the Dardanelles region, are determined by the work of the buoyancy force. In the winter-autumn period, the main contribution to the dynamic processes at the indicated depths is made by the work of the wind force. Thus, in spring and summer, the waters of the basin are involved in a mesoscale central anticyclonic vortex. In autumn, the central anticyclone, decreasing in radius, shifts to the west, and in the eastern part there is a mesoscale cyclonic vortex. In the indicated range of depths, the radii of the eddies vary from 5 to 35 km. Below 75 m the generation of dynamic structures is mainly determined by the work of the buoyancy force. In this case, the radii of the eddies vary from 7 to 16 km

Numerical and Experimental Analysis of Flow Pattern, Pressure Drop and Collection Efficiency in a Cyclone with a Square Inlet and Different Dimensions of a Vortex Finder

The paper presents results of numerical simulations and experimental investigations of a cyclone with a square inlet and different dimensions of a vortex finder. Investigations were conducted for five, different cyclone models. The main goal of the research was to determine the influence of cyclone vortex finder geometry (diameter, length) on key parameters for a cyclone operation, such as: gas flow pattern, gas velocity and pressure distribution, pressure drop and collection efficiency. The analysis of flow pattern inside the cyclone was carried out with the use of CFD (Computational Fluid Dynamics) simulations, verified using CTA (Constant Temperature Anemometry). Similarly, pressure distributions, pressure drop and collection efficiency for cyclones were determined numerically and with measurements. The study demonstrated that dimensions of the vortex finder have a significant effect on a cyclone performance. The numerical visualisations of flow showed some unfavorable and beneficial effects and phenomena that may occur in cyclones. Moreover, the smallest pressure drop (305 Pa) was predicted for the cyclone with a maximum diameter of the vortex finder (De = 0.105 m), the largest (358 Pa) when this diameter was the smallest (De = 0.075 m). The tests did not show any significant influence of the vortex finder length on the pressure drop in a cyclone. A different tendency was observed in the case of collection efficiency. The maximum value of this parameter (89.5%) was predicted for the cyclone with the shortest vortex finder (s = 0.060 m), minimum (85.5%) when the vortex finder was the longest (s = 0.220 m). In this case, however, tests did not show a significant impact of an outlet diameter on the collection efficiency. Results and conclusions presented in this paper can be practically used for design optimization of cyclone separators, and also to select their operating parameters.

On the formation and wind enhancement mechanisms of a Mongolian cyclone that caused a transmission line galloping trip in Gansu province: 导致甘肃输电线路舞动的蒙古气旋的形成及增风机理研究

Influenced by strong winds associated with a southeastward-moving Mongolian cyclone, a severe transmission line galloping occurred in Baiyin City, Gansu Province, on 14 April 2020. This caused a tripping incident of the transmission line in this region. Based on the hourly, 0.5° × 0.5°, ECMWF ERA5 reanalysis data, this study investigated the formation mechanisms of the Mongolian cyclone and its associated strong winds. Results from the vorticity budget indicate that the convergence-related vertical stretching and the upward transport of cyclonic vorticity governed formation of the Mongolian cyclone in this event; whereas, tilting and export of cyclonic vorticity from the central region of the cyclone mainly decelerated the cyclone's formation. The kinetic energy (KE) budget shows that the wind associated with the Mongolian cyclone was mainly enhanced by the positive work of the pressure gradient force. Unlike some typical strong wind events in Northwest China, during this event, no significant downward momentum transportation from the upper troposphere was found. The vertical transport of KE exerted a slightly favorable effect on the KE increase around the location where the transmission line galloping trip appeared. In contrast, the horizontal transport mainly caused an export of KE from this region, which applied an overall negative effect on the wind enhancement associated with the Mongolian cyclone.摘要受一次向东南方向移动蒙古气旋强风的影响, 2020年4月18日甘肃省白银市的高压输电线路出现了严重的舞动, 这造成了该地区的输电线路跳闸事故.基于0.5度逐小时分辨率的ERA5再分析数据, 本文研究了这次蒙古气旋及其强风的形成机理.涡度收支的结果表明, 辐合相关的垂直伸展以及垂直运动向上的正涡度输送是本次蒙古气旋形成的主因, 而倾斜项的负涡度制造以及从气旋中心区域向外的正涡度输送在一定程度上不利于蒙古气旋的形成.动能收支的结果表明, 蒙古气旋所伴随的风场主要由于气压梯度力的做功而迅速增强.与我国西北部其它的典型强风事件不同的是在本次事件中, 未发现显著的来自于对流层高层的动量下传现象, 垂直的动能输送在本次事件中仅对强风的形成有些许的促进作用.此外, 水平风场对动能的输送作用使得蒙古气旋大风区的动能存在向外的净输出, 这对蒙古气旋强风的生成有一定的延缓作用.

Upper Tropospheric Cyclonic Vortex and Brazilian Northeast Jet Stream over Alagoas State: Circulation Patterns and Rainfall

Abstract Interaction between Brazilian Northeast Jet Stream (BNEJS) and Upper Tropospheric Cyclonic Vortexes (UTCV) isn't still well known, as well as its seasonal variability and relation with rainfall pattern. This study's aim is to determine the air circulation patterns in UTCV events with BNEJS in La Niña and El Niño years and its connections with rainfall in the Alagoas State. These connections were analyzed for 9 years between 1988 and 2000 in El Niño, La Niña and Neutral years. NCEP Reanalysis data, rainfall data from 71 meteorological stations and satellite images were used. Fifty-eight percent of 140 observed UTCVs were associated with BNEJS events located in the Brazilian Northeast and 26%, located over the Alagoas State. Three types of BNEJS patterns, denominated as Meridional (M), Zonal (Z) and Transversal (T), have been identified. The M-type BNEJS was the most frequent in Alagoas State, especially in El Niño years. Z and T-types occurred less frequently and were rarely seen in La Niña years. The results showed that in the M-type BNEJS events in 48% of days were not registered precipitation, while Z and T events recorded rainfall in almost 96% of the days.