Mesovortices Research Articles

Using Radiosonde Observations to Assess the “Three Ingredients Method” to Forecast QLCS Mesovortices

Abstract A technique used widely to forecast the potential for QLCS mesovortices is known as the “Three Ingredients Method” (3IM). The 3IM states that mesovortices are favored where 1) the QLCS cold pool and ambient low-level shear are said to be nearly balanced or slightly shear dominant, 2) where the component of the 0–3-km wind shear normal to the convective line is ≥30 kt (1 kt ≈ 0.51 m s−1), and 3) where a rear-inflow jet or enhanced outflow causes a surge or bow along the convective line. Despite its widespread use in operational settings, this method has received little evaluation in formal literature. To evaluate the 3IM, radiosonde observations are compared to radar-observed QLCS properties. The distance between the gust front and high reflectivity in the leading convective line (the “U-to-R distance”), the presence of rear-inflow surges, and mesovortices (MVs) were each assessed across 1820 line segments within 50 observed QLCSs. Although 0–3-km line-normal wind shear is statistically different between MV-genesis and null segments, values are ≤30 kt for 44% of MV-genesis segments. The 0–6-km line-normal wind shear also shows strong discrimination between MV-genesis and null segments and displays the best linear relationship of the U-to-R distance (a measure of system balance) among layers tested, although the scatter and overlap in distributions suggest that many factors can impact MV genesis (as expected). Overall, most MVs occur where the U-to-R distance lies between −5 and 5 km in the presence of a rear-inflow surge, along with positive 0–1-km wind shear, 0–3-km wind shear > 10 kt, and 0–6-km wind shear > 20 kt (all line-normal). Significance Statement Near the leading edge of thunderstorm lines, areas of rotation that can produce tornadoes and strong winds (“mesovortices”) often develop rapidly. Despite advances in understanding mesovortices, few operational guidelines exist to anticipate their genesis. One popular method used to forecast mesovortices—the “Three Ingredients Method”—is evaluated in this study. Our work confirms the importance of two of the ingredients—a surge of outflow winds and thunderstorms that stay nearly atop the leading edge of the outflow. However, we find that many mesovortices occur below the threshold of low-level wind shear ascribed by the forecast method. Refinements to the method are suggested, including the favorable distance between the leading edge of the outflow and thunderstorm updrafts and lower bounds of wind shear over multiple layers, below which mesovortices may be unlikely.

Statistical Analysis of Mesovortices during the First Rainy Season in Guangdong, South China

Based on Doppler radar observation and reanalysis data, the statistical characteristics of mesovortices (MVs) during the first rainy season (April–June) in Guangdong, South China, from 2017 to 2019 are studied, including their spatiotemporal distributions, structural features and favorable environmental conditions. The results show that the MVs usually exhibit short lifetimes; about 70% last for less than 30 min. The intensity and horizontal scale of the MVs are proportional to their lifetime. Long-lived MVs have larger horizontal scales and stronger intensities than short-lived ones. The MVs are mainly observed over the Pearl River Delta region, followed by western Guangdong Province, but relatively fewer in both eastern and northern Guangdong Province. The uneven spatial distribution of the MVs is closely related to the differences in the topographical features and the environment conditions over South China. MVs are prone to form over flat regions. The Pearl River Delta and eastern Guangdong regions are relatively flat compared to the more mountainous western and northern Guangdong regions. Moreover, the monsoonal south-westerlies, water vapor flux, atmospheric instability and vertical wind shear over southwest Guangdong are significantly larger than those in other regions and are favorable for the formation of MVs. The occurrence frequencies of MVs in central and southern parts of Guangdong display similar diurnal variations, reaching the peak during the late afternoon and early evening while dropping to the minimum overnight. However, the situation is opposite in northern Guangdong, with the peak overnight and the minimum during the late afternoon and early evening. The regional differences in diurnal variation are likely related to the moving direction of mesoscale convective systems (MCSs) in Guangdong.

Double chamber meso scale vortex combustor for micro scale electric generator based on thermo electric generator

Research on micro/meso combustor-based small-scale power plants is developing rapidly. This rapid development is motivated by a higher energy density than lithium-ion batteries. In addition, energy needs for portable and mobile devices also drive this development. Based on the opportunities and needs of these small-scale power plants, this study focuses on developing a double-chamber mesoscale vortex combustor. This research aims to compare the temperature profile and the electrical energy generated by single and multiple combustion chambers on the meso vortex combustor. The single chamber combustor has 56 × 28 × 10 mm, and the double combustor is 56 × 56 × 10 mm. Both meso vortex combustor has dimension with a 4 mm inlet diameter. In the meso scale vortex combustor, we also installed stainless steel wire mesh to compare the temperature with and without wire mesh. Based on the numerical study results, the flame can be stabilized in the double chamber combustor configuration. The wire mesh installation can enhance the flame stabilization process. Nevertheless, the wire mesh installation reduces the wall temperature because the mixture’s exit velocity is more stable than the combustor without the wire mesh. The result shows that a double chamber meso scale vortex combustor has a higher temperature and electric energy output than a single chamber combustor. The double chamber combustor can generate 9.5 Watt electric power with 12.6% energy conversion efficiency.

A Merger‐Formation Bow Echo Caused by Low‐Level Mesovortex in South China

AbstractThis work studied a high winds‐producing bow echo that occurred over South China in the pre‐rainy season using radar observations and high‐resolution analyses from the Variational Doppler Radar Analysis System. The bow echo developed from a quasi‐linear convective system (QLCS) and acquired a well‐defined bow shape after merging with a pre‐line convective cell (CC). Interestingly, the rear‐inflow jet (RIJ), which has been well recognized to play a key role in the formation of a bow echo, was not found in this merger‐formation bow echo (MFBE). The reason is that the QLCS developed in the monsoon environment of high humidity and weak vertical wind shear, leading to a weak cold pool and line‐end vortices. A new pathway of bow echo formation was proposed which highlighted the importance of the low‐level mesovortex (MV) on the leading edge of the QLCS. Vertical vorticity budget analyses revealed that the MV originated from a weak vertical vorticity band ahead of the QLCS and grew rapidly during the merger, because of the enhanced low‐level convergence in between which led to vortexgenesis via vertical stretching of vertical vorticity. The up‐scale growth of the MV produced a RIJ‐like flow wrapping cyclonically from north of the QLCS which forced the system to bulge out and finally evolve into a bow echo. This MV contributed foremost to the near‐surface gales as well. These findings shed light on the diverse formation mechanisms of bow echoes and associated high winds in the moist monsoon environment of South China.

The Convective Features within and Surrounding Severe Tropical Cyclone Larry (2006)

A sequence of ground-based radar reflectivity images sampled in the 17 hours prior to, and during the landfall of Severe Tropical Cyclone Larry (2006) are presented and analyzed using Fourier and wavelet analysis techniques. A range of mesoscale convective anomalies were detected, with characteristics and behavior consistent with vortex Rossby wave initiation. Cyclonically propagating eye-wall kinks, elongations and mesoscale reflectivity maxima were all observed throughout the sampling period, along with intense inner spiral bands. Various deep convective maxima propagated within the eye-wall at speeds consistent with predictions derived by linear barotropic wave theory. Three eye-wall breakdown episodes were observed during the study period, along with corresponding increases in storm-core asymmetric wave power and reductions in estimated storm intensity. Vortex Rossby wave initiated radial flows are also suggested by the presence of a possible mesovortex within a broken section of the eye-wall during landfall, and the outward ejection of filaments of deep convection from an adjacent inner spiral band. The possible influence of this wave activity upon the storm intensity and integrity is discussed.

MULTI-SATELLITE OBSERVATION OF MEGH CYCLONE

Abstract. Cyclone Megh, a category-3 (Saffir-Simpson scale) cyclonic storm is regarded as the worst tropical cyclone to ever strike Yemen’s island of Socotra. In this paper, we aim to investigate the wind structure of cyclone Megh using Synthetic Aperture Radar (RISAT-1 SAR) observations. An algorithm for the cyclone wind retrieval has been applied for SAR data of Nov 8, 2015 at 0238:09 UTC in the Arabian Sea. The intensity of cyclone is 30 m/s with the 16.65 km radius of maximum wind speed from the centre of the cyclone. The high resolution SAR data analysis bring to focus the possible presence of eyewall mesovortex in case of Megh. Recent work has shown that vorticity mixing in the tropical cyclone (TC) inner core can promote mesovortex (MV) formation and impact storm intensity. This has further been corroborated using INSAT-3D and MODIS optical band observations of clouds. Analysis of these satellite derived cloud microphysical properties shows the presence of larger hydrometeors surrounding the eye due to possible embedding of stratus and stratocumulus cloud decks. Thus, this kind of study helps in understanding the microphysical processes within a TC as well estimating their impacts on cyclone intensity and lifetime.

The Role of Small-Scale Vortices in Enhancing Surface Winds and Damage in Hurricane Harvey (2017)

Strong hurricanes cause severe, but highly variable, wind damage to homes and community infrastructure. It has been speculated, but not previously shown, that damage variability is caused by tornadoes or other small-scale phenomena. Here, the authors present the first mapping and tracking of persistent tornado-scale vortices (TSVs) in the eyewall and the first documentation of the likely role of eyewall mesovortices (MVs) and TSVs in enhancing surface winds and damage. Unprecedented finescale observations in the eyewall of Hurricane Harvey (2017) were obtained by a Doppler on Wheels (DOW) radar deployed inside the eye. These observations reveal several persistent eyewall MVs revolving about the eye, as well as superimposed subkilometer-scale TSVs. Wind field perturbations associated with TSVs and MVs are less than those typical in supercell tornadoes, but since they are embedded in strong background eyewall flow, they are likely responsible for the enhancement of surface wind gusts and significant damage, including destroyed buildings and lofted vehicles. Potential climate change may result in more frequent intense and/or rapidly intensifying hurricanes; thus, understanding and forecasting the causes of hurricane wind damage is a high priority.

Kinematic Structure of Mesovortices in the Eyewall of Hurricane Ike (2008) Derived from Ground-Based Dual-Doppler Analysis

AbstractPrevious work has shown that vorticity mixing in the tropical cyclone (TC) inner core can promote mesovortex (MV) formation and impact storm intensity. Observations of MVs have largely been serendipitous but are necessary to improve understanding of these features and their role in TC dynamics. This study presents nearly 10 h of ground-based dual-Doppler analysis of MVs in the eyewall of Hurricane Ike (2008) near and during landfall. Derived 3D winds, vertical vorticity, horizontal divergence, and perturbation pressures are analyzed. Results indicate persistent kinematic field arrangements and evolving vertical structures. Perturbation pressure retrievals suggest local pressure minima associated with the MVs. Preferential updraft locations appear to transition cyclonically about the local vorticity maximum as the MVs progress around the eye. Based on published observational datasets, the dual-Doppler updraft magnitudes in Ike’s MVs are within the top 5%–10% of TC vertical velocities. The MVs are marked by peak vorticity in the lowest 2 km and contain vertically coherent vorticity structures extending to 8 km AGL. After prolonged land interaction, the MV structures deteriorate. First, the vertical extent of localized vorticity diminishes, followed by a deterioration in the prelandfall characteristic kinematic arrangements. This supports the notion that the replenishment of a high vorticity annulus contributes to MV production and maintenance, and when the elevated vorticity aloft is not maintained, MV kinematic patterns become less consistent. It is unclear whether the decay of the vertically coherent vorticity structures occurs in response to land interaction, TC inner core processes, or some combination of both.

Mesovortices within the 8 May 2009 Bow Echo over the Central United States: Analyses of the Characteristics and Evolution Based on Doppler Radar Observations and a High-Resolution Model Simulation

Abstract A derecho-producing bow-echo event over the central United States on 8 May 2009 is analyzed based on radar observations and a successful real-data WRF simulation at 0.8-km grid spacing. Emphasis is placed on documenting the existence, evolution, and characteristics of low-level mesovortices (MVs) that form along the leading edge of the bowing system. The genesis of near-surface high winds within the system is also investigated. Significant MVs are detected from the radar radial velocity using a linear least squares derivatives (LLSD) method, and from the model simulation based on calculated vorticity. Both the observed and simulated bow-echo MVs predominantly form north of the bow apex. MVs that develop on the southern bow tend to be weaker and shorter-lived than their northern counterparts. Vortex mergers occur between MVs during their forward movement, which causes redevelopment of some MVs in the decaying stage of the bow echo. MVs located at (or near) the bow apex are found to persist for a notably longer lifetime than the other MVs. Moreover, the model results show that these bow-apex MVs are accompanied with damaging straight-line winds near the surface. These high winds are mainly caused by the descent of the rear-inflow jet at the bow apex, but the MV-induced vortical flow also has a considerable contribution. The locally enhanced descent of the rear-inflow jet near the mesovortex is forced primarily by the dynamically induced downward vertical pressure gradient force while the buoyancy force only plays a minor role there.

The Genesis of Mesovortices within a Real-Data Simulation of a Bow Echo System

Abstract The genesis of two mesovortices (MVs) within a real-data, convection-resolving simulation of the 8 May 2009 central U.S. bow echo system is studied. Both MVs form near the bow apex but differ distinctively in intensity, lifetime, and damage potential. The stronger and longer-lived mesovortex, MVa, stays near the bow apex where the system-scale rear-inflow jet (RIJ) is present. The descending RIJ produces strong downdrafts and surface convergence, which in turn induce strong vertical stretching and intensification of MVa into an intense mesovortex. In contrast, the weaker and shorter-lived mesovortex, MVb, gradually moves away from the bow apex, accompanied by localized convective-scale downdrafts. Lagrangian circulation and vorticity budget analyses reveal that the vertical vorticity of MVs in general originate from the tilting of near-surface horizontal vorticity, which is mainly created via surface friction. The circulation of the material circuit that ends up to be a horizontal circuit at the foot of the MVs increases as the frictionally generated horizontal vortex tubes pass through the tilted material circuit (tilted following backward trajectories defining the material circuit) surface, especially in the final few minutes prior to mesovortex genesis. The tilted material circuit becomes horizontal at the MV foot, turning associated horizontal vorticity into vertical. The results show at least qualitatively that, in addition to baroclinicity, surface friction can also have significant contributions to the generation of low-level MVs, which was not considered in previous MV studies.

Origin and Propagation of a Disturbance Associated with an African Easterly Wave as a Precursor of Hurricane Alberto (2000)

Abstract In this study, it is proposed that mesoscale convective complexes (MCCs) and a mesovortex (MV) were embedded within a wavelike disturbance over North Africa that led to the genesis of Hurricane Alberto (2000). The wavelike disturbance observed may be classified as an African easterly wave (AEW). Based on the cloud-top area and brightness values observed from infrared satellite data, four genesis and three lysis stages are identified within a cycle of moist convection associated with the pre-Alberto disturbance. The availability of water vapor is the most essential factor controlling the convective cycle of the pre-Alberto disturbance over the African continent. The presence of significant topography also contributes to the generation or decay of the associated MCCs through regulation of the water vapor supply. Further analysis of Meteosat satellite imagery reveals that the incipient disturbances for 23 of 34 eastern Atlantic tropical cyclones originated from the Ethiopian highlands (EH) region during the period of 1990–2001. The pre-Alberto disturbance was found to have exhibited characteristics of an AEW. At the EH, there existed two modes of disturbance development: a stationary mode and a propagating mode. The stationary mode corresponded with the generation of moist convection over the EH triggered by diurnally variant sensible heating, while the propagating mode corresponded with the generation and propagation of MVs and mesoscale convective systems (MCSs) from the lee side of the EH over a period of about 2 to 3 days. These components of the disturbance propagated westward together within an AEW train at an average speed of 11.6 m s−1. The average wavelength was roughly estimated to be about 2200 km. To prove that disturbances generated at the EH are indeed AEWs, the NCAR Regional Climate Model Version 3.0 is adopted to simulate the event. The simulated fields showed that both the propagating wave and stationary mountain wave modes were present, the convection was generated over the EH, and the pre-Alberto disturbance was generated near the lee of the EH. In addition, the convective cycle detected from NCEP reanalysis data was also reflected in the simulated fields. The simulated AEW possesses similar wave characteristics as the observed pre-Alberto disturbance.

Initiation of a mesoscale convective complex over the Ethiopian Highlands preceding the genesis of Hurricane Alberto (2000)

A tropical disturbance, which would later become Hurricane Alberto (2000), was traced back in time as a mesoscale convective complex (MCC) to the Ethiopian Highlands (EH), where the MCC first developed. Meteosat‐7 imagery indicates that the MCC develops during the late afternoon and evening of 28 July 2000, and that a mesovortex (MV) was evident on the morning of 29 July 2000. A preliminary mesoscale model simulation features the development of two areas of maximum relative vorticity in the middle troposphere, with the maturation of one of the vorticity maxima into a significant MV. The higher‐level vorticity center of the simulated MV tracks westward and is nearly collocated with the vortex signature in satellite imagery, while the lower‐level center remains near the lee side of the EH. The vortex signature in satellite imagery was traceable to the cyclogenesis stage over the eastern Atlantic Ocean.