Mesoscale Vorticity Research Articles

The Stochastic Spinup of Vorticity in Spontaneous Tropical Cyclogenesis

Abstract Cloud-permitting simulations have shown that tropical cyclones (TCs) can form spontaneously in a quiescent environment with uniform sea surface temperature. While several mesoscale feedbacks are known to amplify an existing midlevel vortex, how the noisy deep convection produces the initial midlevel vortex remains unclear. This paper develops a theoretical framework to understand the evolution of the midlevel mesoscale vorticity’s histogram in the first two days of spontaneous tropical cyclogenesis, which we call the “stochastic spinup stage.” The mesoscale vorticity is produced by two random processes related to deep convection: the random stretching of planetary vorticity f and the tilting of random vertical shear. With the central limit theorem, the mesoscale vorticity is modeled as the sum of three independent normal distributions, which include the cyclones produced by stretching, cyclones produced by tilting, and anticyclones produced by tilting. The theory predicts that the midlevel mesoscale vorticity obeys a normal distribution, and its standard deviation is universally proportional to the square root of the domain-averaged accumulated rainfall, agreeing with simulations. The theory also predicts a critical latitude below which tilting is dominant in producing mesoscale vorticity. Treating the magnitude of random vertical shear as a fitting parameter, the critical latitude is shown to be around 12°N. Because the magnitude of vertical shear should be larger in the real atmosphere, this result suggests that tilting is an important source of mesoscale vorticity fluctuation in the tropics.

Effect of the different stratification changes along the two sides of a typhoon track on near-inertial energy propagation

The β-effect and mesoscale vorticity are considered the two important mechanisms guiding the downward propagation of near-inertial energy (NIE), while the influence of stratification is often neglected. In this study, the “heat pumping” and “cold suction” effects of Typhoon Kalmaegi on ocean stratification were analyzed. Enhanced stratification accelerated the vertical propagation of near-inertial waves (NIWs) from the mixed layer into the ocean interior along the left side of the typhoon track. To illustrate the impact of stratification changes on both sides of typhoon tracks, the Regional Ocean Modeling System was used to simulate the NIWs generated by Typhoon Kalmaegi in 2014. The results show that, despite higher wind energy input along the right side of the typhoon track, the NIE injection into the deep ocean along the left side was stronger owing to reduced dissipation and enhanced stratification. This study improves our understanding of the generation, propagation, and dissipation mechanisms of wind-induced NIWs during typhoons.

Studying the influence of cold-core mesoscale ocean eddies on sound propagation based on the parabolic equation method

Mesoscale vorticity is an important factor that affects the non-uniform horizontal and vertical distribution of hydrologic elements in the ocean. In this paper, temperature and salinity data from measured mesoscale cold-core eddies during a voyage within a certain sea area are studied, and a synchronous sound field test was conducted on the mesoscale vorticity. Based on the temperature and salinity data, the parabolic equation (PE) method was selected to predict the acoustic field, and the results were compared with the measured propagation loss data of the acoustic field to verify the accuracy and effectiveness of the PE model. The underwater sound propagation characteristics in the mesoscale cold-core eddy environment were then analyzed using the temperature and salinity data retrieved from the voyage. It was found that the convergence area of the acoustic field gradually dispersed with an increase in the depth of the sound source. In a mesoscale eddy environment, when sound waves propagate from inside to outside the eddy, the presence of a cold-core eddy causes the convergence area to shift toward the edge of the eddy, and the deviation gradually decreases with an increase in the depth of the sound source.

Scale interactions between the meso‐ and synoptic scales and the impact of diabatic heating

AbstractFor both the meso‐ and synoptic scales, reduced mathematical models give insight into their dynamical behaviour. For the mesoscale, the weak temperature gradient approximation is one of several approaches, while for the synoptic scale the quasigeostrophic theory is well established. However, the way these two scales interact with each other is usually not included in such reduced models, thereby limiting our current perception of flow‐dependent predictability and upscale error growth. Here, we address the scale interactions explicitly by developing a two‐scale asymptotic model for the meso‐ and synoptic scales with two coupled sets of equations for the meso‐ and synoptic scales respectively. The mesoscale equations follow a weak temperature gradient balance and the synoptic‐scale equations align with quasigeostrophic theory. Importantly, the equation sets are coupled via scale‐interaction terms: eddy correlations of mesoscale variables impact the synoptic potential vorticity tendency and synoptic variables force the mesoscale vorticity (for instance due to tilting of synoptic‐scale wind shear). Furthermore, different diabatic heating rates—representing the effect of precipitation—define different flow characteristics. With weak mesoscale heating relatable to precipitation rates of , the mesoscale dynamics resembles two‐dimensional incompressible vorticity dynamics and the upscale impact of the mesoscale on the synoptic scale is only of a dynamical nature. With a strong mesosocale heating relatable to precipitation rates of , divergent motions and three‐dimensional effects become relevant for the mesoscale dynamics and the upscale impact also includes thermodynamical effects.

Turbulence and equatorial waves in moist and dry shallow‐water flow, excited through mesoscale stochastic forcing

AbstractTurbulence and large‐scale waves in the tropical region are studied using the spherical shallow‐water equations. With mesoscale vorticity forcing, both moist and dry systems show an upscale transfer of kinetic energy which is dominated by rotational modes, scales as a power law with exponent, requires eddy–eddy interactions and ranges from the forcing scale to the respective equatorial deformation radius. At larger planetary scales, the divergent component of the energy increases and we see a footprint of tropical waves. The dry system shows a signature of the entire family of equatorial waves, while the moist simulations show only low‐frequency Rossby, Kelvin and mixed Rossby–gravity waves with an equivalent depth that matches rapid condensation estimates. Initially, runs with interactive moisture exhibit a weak inverse transfer of moisture variance as well exponential growth across a range of length‐scales. This results in an equilibrium moist energy spectrum obeying a 2 power law and the formation of moisture aggregates. Once formed, aggregates propagate westward in the Tropics with speeds of the order of a few metres per second. In contrast, forcing divergence does not excite an inverse transfer, and injected energy remains trapped at the forcing scale. Height (i.e., temperature or mass) forcing results in a peak at the forcing scale, but also generates large‐scale waves and projects onto rotational modes which undergo an inverse energy transfer. Similarly, forcing the moisture field by itself produces an inverse transfer of rotational energy and a well‐formed large‐scale equatorial wave spectrum. Notably, the height‐ and moisture‐forced inverse transfers are different in nature. Specifically, they require the presence of ambient planetary rotation. In all, these experiments demonstrate that the vortical and divergent wind are inextricably linked with the evolving moisture field, and that large‐scale equatorial waves co‐exist with synoptic‐scale moist turbulence.

Near‐Inertial Wave Propagation in the Wake of Super Typhoon Mangkhut: Measurements From a Profiling Float Array

AbstractNear‐inertial internal waves (NIWs) are generated by inertially‐rotating winds under tropical cyclones (TC). Since NIWs are mostly horizontal, their vertical propagation out of the mixed layer is slow. However, mesoscale vorticity and shear increase vertical group speed by increasing near‐inertial frequency and horizontal wavenumber. To assess NIW propagation, a profiling float array under Super Typhoon Mangkhut in September 2018 made broad and persistent measurements in space and time of density in the upper 200 m and depth‐mean velocity. The TC wake was a region of positive vorticity on its southern side, displayed elevated shear, and thereby enhanced downward propagation of NIWs. The vertical energy flux is estimated as 0.04–0.11 W m−2, which is about 1%–4% (3%–8%) of the mean total (near‐inertial) wind work of 3.0 (1.3) W m−2 calculated from a high‐resolution TC model. Considerable uncertainties arise in the (a) estimated group speed based on wavelength, shear, and frequency and (b) energy density based on depth‐varying density and the NIW polarization relations, which are sensitive to frequency. Following the TC's passage, NIWs propagated southward and horizontal wavelengths decreased from 1,000 to 500 km, as time progressed. Also, we identify an interfacial wave at the mixed layer base, which displaces isopycnals vertically. The following process is suggested. As the North Equatorial Current flows over these displacements, which act as topographic obstacles, secondary NIWs propagate up‐/downward into the mixed layer/thermocline. These waves are 180° out of phase in the mixed layer and thermocline, which can enhance shear at the mixed layer base.

Evidence of Tornadoes and Microbursts in São Paulo State, Brazil: A Synoptic and Mesoscale Analysis

Synoptic and mesoscale analyses indicate environmental conditions favorable to the development of tornados and downbursts in Eastern Sao Paulo State, Brazil, between 16 May and 06 June 2016 under El Nino Sothern Oscillation (ENSO) condition. A subtropical jet stream strengthened by the 2015/2016 El Nino event over southeast Brazil and low-level moisture advection from the Amazon induced mid-level mesoscale vorticity and strong updrafts, respectively. These synoptic summer-like conditions yielded high dynamic and thermodynamic instability, producing deep convection and supercells with high lightning and precipitation rates with golf ball-sized hail stones and high winds at the surface, causing damages to trees, houses, towers and other structures with debris features associated with tornadoes and microbursts. Cellular phone photos and movies of severe weather events in Campinas and Janiru cities on 05 and 06 June 2016 suggest wind damage features caused by tornadoes ranked in the enhanced Fujita (EF) wind scale as EF1 (38–49 m s−1) and EF2 (49–60 m s−1), respectively. Similarly, cellular phone movies of Embu-Guacu City severe weather event on 16 May 2016 suggest a microburst case. The present study is based on quantitative and qualitative records of three high-impact rainfall, hail, lightning and wind gust episodes in the extra tropics during late fall 2016.

Sun glitter imagery of surface waves. Part 2: Waves transformation on ocean currents

Under favorable imaging conditions, the Sentinel-2 Multi-Spectral Instrument (MSI) can provide spectacular and novel quantitative ocean surface wave directional measurements in satellite Sun Glitter Imagery (SSGI). Owing to a relatively large-swath with high-spatial resolution (10 m), ocean surface roughness mapping capabilities, changes in ocean wave energy, and propagation direction can be precisely quantified at very high resolution, across spatial distances of 10 km and more. This provides unique opportunities to study ocean wave refraction induced by spatial varying surface currents. As expected and demonstrated over the Grand Agulhas current area, the mesoscale variability of near-surface currents, documented and reconstructed from satellite altimetry, can significantly deflect in-coming south-western swell systems. Based on ray-tracing calculations, and unambiguously revealed from the analysis of Sentinel-2 MSI SSGI measurements, the variability of the near-surface current explains significant wave-current refraction, leading to wave-trapping phenomenon and strong local enhancement of the total wave energy. In addition to its importance for wave modeling and hazard prediction, these results open new possibilities to combine different satellite measurements and greatly improve the determination of the upper ocean mesoscale vorticity motions.

Non-equilibrium processes in ash-laden volcanic plumes: new insights from 3D multiphase flow simulations

In the framework of the IAVCEI (International Association of Volcanology and Chemistry of the Earth Interior) initiative on volcanic plume models intercomparison, we discuss three-dimensional numerical simulations performed with the multiphase flow model PDAC (Pyroclastic Dispersal Analysis Code). The model describes the dynamics of volcanic and atmospheric gases (in absence of wind) and two pyroclastic phases by adopting a non-equilibrium Eulerian–Eulerian formulation. Accordingly, gas and particulate phases are treated as interpenetrating fluids, interacting with each other through momentum (drag) and heat exchange. Numerical results describe the time-wise and spatial evolution of weak (mass eruption rate: 1.5×106 kg/s) and strong (mass eruption rate: 1.5×109 kg/s) plumes. The two tested cases display a remarkably different phenomenology, associated with the different roles of atmospheric stratification, compressibility and mechanism of buoyancy reversal, reflecting in a different structure of the plume, of the turbulent eddies and of the atmospheric circulation. This also brings about different rates of turbulent mixing and atmospheric air entrainment. The adopted multiphase flow model allows to quantify temperature and velocity differences between the gas and particles, including settling, preferential concentration by turbulence and thermal non-equilibrium, as a function of their Stokes number, i.e., the ratio between their kinetic equilibrium time and the characteristic large-eddy turnover time of the turbulent plume. As a result, the spatial and temporal distribution of coarse ash in the atmosphere significantly differs from that of the fine ash, leading to a modification of the plume shape. Finally, three-dimensional numerical results have been averaged in time and across horizontal slices in order to obtain a one-dimensional picture of the plume in a stationary regime. For the weak plume, the results are consistent with one-dimensional models, at least in the buoyant plume region, and allow to reckon a variable, effective entrainment coefficient with a mean value around 0.1 (consistently with laboratory experiments). For the strong plume, analysis of the results reveals that the two most critical assumptions of one-dimensional integral models are the self-similarity and the pressure equilibrium. In such a case, the plume appears to be controlled by the dynamics in the jet stage (below the buoyancy reversal) and by mesoscale vorticity associated with the development of the umbrella.

Refraction of swell by surface currents

Using recordings of swell from pitch-and-roll buoys, we have reproduced the classic observations of long-range surface wave propagation originally made by Munk et al. (1963) using a triangular array of bottom pressure measurements. In the modern data, the direction of the incoming swell fluctuates by about ±10∘ on a time scale of one hour. But if the incoming direction is averaged over the duration of an event then, in contrast with the observations by Munk et al. (1963), the sources inferred by great-circle backtracking are most often in good agreement with the location of large storms on weather maps of the Southern Ocean. However there are a few puzzling failures of great-circle backtracking e.g., in one case, the direct great-circle route is blocked by the Tuamoto Islands and the inferred source falls on New Zealand. Mirages like this occur more frequently in the bottom-pressure observations of Munk et al. (1963), where several inferred sources fell on the Antarctic continent. Using spherical ray tracing we investigate the hypothesis that the refraction of waves by surface currents produces the mirages. With reconstructions of surface currents inferred from satellite altimetry, we show that mesoscale vorticity significantly deflects swell away from great-circle propagation so that the source and receiver are connected by a bundle of many rays, none of which precisely follow a great circle. The ±10∘ directional fluctuations at the receiver result from the arrival of wave packets that have travelled along the different rays within this multipath. The occasional failure of great-circle backtracking, and the associated mirages, probably results from partial topographic obstruction of the multipath, which biases the directional average at the receiver.

Method for determining atmospheric motion characteristics using measurements on geostationary meteorological satellites

The method for determining the dynamic characteristics of the atmosphere using the data of sounding from geostationary meteorological satellites developed by the authors and based on using inhomogeneities of the conservative admixture concentration field as tracers and on applying the correlation extreme algorithms is described in detail. The accuracies in calculating the horizontal wind velocity vector (V) and coefficient of horizontal mesoscale turbulent diffusion (Kd) are estimated on the basis of processing the data of sounding the atmosphere with a SEVIRI (Spinning Enhanced Visible and Infrared Imager) radiometer on the Meteosat-8 and Meteosat-9 European geostationary meteorological satellites in the water vapor channels centered at 6.2 and 7.3 μm and on comparing the results with the data of independent observations and theoretical models. It is indicated that the accuracy in calculating V using the developed method almost coincides with the accuracy of the commonly used foreign methods. In contrast to the methods applied abroad, the developed method makes it possible to determine not only the wind velocity vector field but also the coefficient of mesoscale turbulent diffusion and vorticity on one scale of air mass motion.

Restratification of the Surface Mixed Layer with Submesoscale Lateral Density Gradients: Diagnosing the Importance of the Horizontal Dimension

Abstract A depth-cycling towed conductivity–temperature–depth (CTD) and vessel-mounted acoustic Doppler current profiler (ADCP) were used to obtain four-dimensional measurements of the restratification of the surface mixed layer (SML) at a submesoscale lateral density gradient near the subtropical front. With the objective of studying the role of horizontal processes in restratification, the thermohaline and velocity fields were monitored for 33 h by 16 small-scale (≤15 km2) surveys centered on a drogued float. Daytime warming by insolation caused a unidirectional displacement of the initially vertical isopycnals toward increasing density. Across the entire SML (50-m vertical scale), solar insolation accounted for 60% of observed restratification, but over 10-m scales, the percentage decreased with depth from 80% at 25–35 m to ≤25% at 55–65 m. Below 35 m, stratification was enhanced by the vertically sheared horizontal advection of the lateral density gradient due to a near-inertial wave of ∼100-m vertical wavelength that rotated anticyclonically at the inertial frequency. The phase and similar period (25.4 h) of the local inertial period to the diurnal cycle ensured constructive interference with isopycnal displacements due to insolation. Restratification by sheared advection matched that predicted due to vertically sheared inertial oscillations generated during the geostrophic adjustment of a density front, but direct wind forcing may also have generated the wave that was subsequently modified by interaction with mesoscale vorticity associated with a nearby large-scale front. By further including the effects of lateral uncompensated thermohaline inhomogeneity, the authors can account for 100% ± 20% of the observed N 2 during daytime restratification. No detectable restratification due to the slumping of horizontal density gradients under gravity alone was found.

A numerical analysis of a deep Mediterranean lee cyclone: sensitivity to mesoscale potential vorticity anomalies

A 12–15 November 2004 cyclone on the lee side of the Atlas Mountains and the related occurrence of severe bora along the eastern Adriatic coast are numerically analyzed using the MM5 mesoscale model. Motivated by the fact that sub-synoptic scales are more sensitive to initialization errors and dominate forecast error growth, this study is designed in order to assess the sensitivity of the mesoscale forecast to the intensity of mesoscale potential vorticity (PV) anomalies. Five sensitivity simulations are performed after subtracting the selected anomalies from the initial conditions, allowing for the analysis of the cyclone intensity and track, and additionally, the associated severe bora in the Adriatic. The results of the ensemble show that the cyclone is highly sensitive to the exact details of the upper-level dynamic forcing. The spread of cyclone intensities is the greatest in the mature phase of the cyclone lifecycle, due to different cyclone advection speeds towards the Mediterranean. However, the cyclone tracks diffluence appears to be the greatest during the cyclone movement out of the Atlas lee, prior to the mature stage of cyclone development, most likely due to the predominant upper-level steering control and its influence on the thermal anomaly creation in the mountain lee. Furthermore, it is quantitatively shown that the southern Adriatic bora is more sensitive to cyclone presence in the Mediterranean then bora in the northern Adriatic, due to unequal influence of the cyclone on the cross-mountain pressure gradient formation. The orographically induced pressure perturbation is strongly correlated with bora in the northern and to a lesser extent in the southern Adriatic, implying the existence of additional controlling mechanisms to bora in the southern part of the basin. In addition, it is shown that the bora intensity in the southern Adriatic is highly sensitive to the precise sub-synoptic pressure distribution in the cyclone itself, indicating a close relationship between the skillful forecasting of Mediterranean cyclones and bora in the southern Adriatic.

Mechanisms for the Generation of Mesoscale Vorticity Features in Tropical Cyclone Rainbands

Abstract A high-resolution tropical cyclone model with explicit cloud microphysics has been used to investigate the dynamics and energetics of tropical cyclone rainbands. Analysis of the vorticity interactions that occur within the simulated rainbands demonstrates that couplets of cyclonic–anticyclonic mesovortices can be produced in outer bands. The primary source of this vorticity is the upward tilting of system-generated horizontal vorticity by diabatic heating gradients. The vertical heating gradient in the stratiform cloud also creates a potential vorticity (PV) dipole that accelerates the tangential flow and develops a midlevel jet. The strength of the jet is enhanced by the vortex pair that is oriented radially across the rainband. The Fourier decomposition of the absolute vorticity field shows two counterpropagating vortex Rossby waves associated with the rainband. The wave located on the inner side of the band transports energy toward the vortex center. The outer wave is made up of high wavenumbers and uses the vorticity gradients generated by the rainband. The results support the hypothesis that the heating profile in the stratiform regions of rainbands generates cyclonic PV across the freezing level, which develops a midlevel jet. This mechanism creates a vorticity gradient that enables the propagation of vortex Rossby waves that could allow the rainbands to interact with the mean flow and potentially influence the evolution of the storm by contributing to the symmetric component of vorticity and the development of secondary eyewalls.

Estimating vertical velocity and radial flow from Doppler radar observations of tropical cyclones

AbstractThe mesoscale vorticity method (MVM) is used in conjunction with the ground‐based velocity track display (GBVTD) to derive the inner‐core vertical velocity from Doppler radar observations of tropical cyclone (TC) Danny (1997). MVM derives the vertical velocity from vorticity variations in space and in time based on the mesoscale vorticity equation. The use of MVM and GBVTD allows us to derive good correlations among the eye‐wall maximum wind, bow‐shaped updraught and echo east of the eye‐wall in Danny. Furthermore, we demonstrate the dynamically consistent radial flow can be derived from the vertical velocity obtained from MVM using the wind decomposition technique that solves the Poisson equations over a limited‐area domain. With the wind decomposition, we combine the rotational wind which is obtained from Doppler radar wind observations and the divergent wind which is inferred dynamically from the rotational wind to form the balanced horizontal wind in TC inner cores, where rotational wind dominates the divergent wind. In this study, we show a realistic horizontal and vertical structure of the vertical velocity and the induced radial flow in Danny's inner core. In the horizontal, the main eye‐wall updraught draws in significant surrounding air, converging at the strongest echo where the maximum updraught is located. In the vertical, the main updraught tilts vertically outwards, corresponding very well with the outward‐tilting eye‐wall. The maximum updraught is located at the inner edge of the eye‐wall clouds, while downward motions are found at the outer edge. This study demonstrates that the mesoscale vorticity method can use high‐temporal‐resolution data observed by Doppler radars to derive realistic vertical velocity and the radial flow of TCs. The vorticity temporal variations crucial to the accuracy of the vorticity method have to be derived from a high‐temporal‐frequency observing system such as state‐of‐the‐art Doppler radars. Copyright © 2006 Royal Meteorological Society

Internal wave variability in the Beaufort Sea during the winter of 1993/1994

Acoustic Doppler current profiler observations were obtained at the Sea Ice Mechanics Initiative Ice Camp as it drifted through the Beaufort Sea over the winter of 1993–1994. In this paper, the variability of the internal wave field is examined. Initially (December 1993–January 1994), the camp drifted westward through the central Canada Basin (75°N, 139°–153°W). Numerous anticyclonic baroclinic eddies were encountered. Internal wave energy levels averaged 1/25 of the Garrett‐Munk (GM) standard. Shear variance averaged 1/17 GM. During February–March 1994, the camp drifted meridionally along 156°W (near the Northwind Ridge), reversing direction several times. Large baroclinic vortices were not detected. Internal wave energy levels were significantly greater during this period, peaking at 1/6 GM. Shear variance peaked at 1/3 GM. The waves were predominantly near inertial, resulting from three distinct surface generation events. Downward vertical energy fluxes ranged from typical values of 0.02 mW/m2 to a peak value of 0.15 mW/m2. Two thirds of the downward flux was associated with waves of vertical scale greater than 55 m; the remainder with smaller‐scale waves. Upward energy fluxes ranged from 0.02 mW/m2 (typical) to 0.10 mW/m2 (peak), supported almost entirely by the larger‐scale waves. High ice‐water relative velocities (atmospheric storms), accompanied by ice deformation, appear to be necessary, but not sufficient, to assure a downward propagating wave generation event. The highly variable mesoscale has a significant effect on the near‐inertial wave field propagation. The data suggest that the vertical propagation of near‐inertial waves is strongly affected by the depth variation of the vertical component of mesoscale vorticity. The large Beaufort Sea vortices appear to have a dual effect on the wave field. Enhanced vertical propagation speed (and decreased energy density) is found in the negative vorticity cores. Propagation into the positive vorticity regions surrounding the cores is inhibited. The increase in wave energy experienced as the ice camp left the central Beaufort Sea is jointly associated with the passage of storms and the absence of large baroclinic vortices over the Northwind Ridge.

A Scale-Discriminating Vorticity Budget for a Mesoscale Vortex in a Midlatitude, Continental Mesoscale Convective System

The authors employ data from the NOAA Wind Profiler Network to present a scale-discriminating vorticity budget for a mesoscale convective vortex (MCV) that was generated by a mesoscale convective system (MCS) in Oklahoma and Kansas on 1 August 1996. A spatial bandpass filter was used to divide observed wind into mesoscale and synoptic components. Then the authors sought sources and sinks of vorticity in those two components over 9 h of the MCV's lifetime. The vorticity budget reveals that both the mesoscale and synoptic winds supplied significant vorticity to the MCV. The vortex's origin could not be proved, but data weakly suggest that tilting may have been mostly responsible. Convergence of absolute vorticity by the mesoscale wind was the reason the MCV grew deeper and stronger as the MCS weakened. Finally, tilting of synoptic and mesoscale vorticity by gradients in mesoscale vertical motion was responsible for a secondary deepening of the MCV as the MCS dissipated. The budget suggests that, if the MCV of 1 August 1996 is representative, completely realistic simulations of MCVs should include planetary vorticity and a plausible, three-dimensionally heterogeneous synoptic wind.

The vorticity method: Extension to mesoscale vertical velocity and validation for tropical storms

AbstractThe vorticity method infers vertical velocity from vorticity variations in space and in time based on the vorticity equation. The viability of the large‐scale vorticity method has been demonstrated in previous studies based on the quasi‐geostrophic vorticity equation. However, the extension of the vorticity method to the mesoscale vorticity equation, including the vertical stretching and tilting terms, is hindered by inseparable variables when formulated in Cartesian coordinates. In this study, the separation of variables for the mesoscale vorticity method is achieved by formulating the mesoscale vorticity equation in Lagrangian coordinates in terms of the characteristic line composed of three nonlinear ordinary differential equations. A numerical method based on backward integration is also presented to solve the vertical velocity along the characteristic of the mesoscale vorticity equation. Observing‐system simulation experiments were undertaken to quantify both the numerical accuracy of the vorticity method, and the sensitivities of the kinematic and vorticity methods to errors present in horizontal wind fields. With perfect horizontal winds, the vertical velocities derived from the kinematic and vorticity methods are identical to the degree of accuracy governed by the truncational errors. When error perturbations are included, the vertical motions derived from the vorticity method are less susceptible to errors in horizontal winds than the kinematic method. This conclusion is consistent with the analytical analysis and numerical results with the large‐scale vorticity method shown in previous studies. The mesoscale vorticity method with the tilting term is shown to be more accurate than the large‐scale vorticity method in deriving vertical velocities in tropical storms. Numerical experiments demonstrate that the mesoscale vorticity method can benefit from high‐temporal‐resolution data observed by Doppler radars to infer the vertical velocity in tropical storms, where the inner circulation is dominated by the rotational winds. Copyright © 2003 Royal Meteorological Society

Interactions between Near-Inertial Mixed Layer Currents and the Mesoscale: The Importance of Spatial Variabilities in the Vorticity Field*

Abstract Satellite-tracked mixed-layer drifter data from the 1987 Ocean Storms Experiment in the northeast Pacific are used to study the temporal and spatial variability in the near-inertial currents of the surface mixed layer. The data suggest that the near-inertial currents interact with the mesoscale vorticity. To explore the exact nature of this relationship, an asymptotic analysis is extended to include the mesoscale and it is shown that not only gradients in the planetary vorticity (β) but also the gradients in the mesoscale vorticity affect the generation and evolution of near-inertial mixed layer currents. While the β effect is relevant for the large-scale evolution of the near-inertial currents, the vorticity gradients of the mesoscale field determine the local behavior of near-inertial currents. Using a three-dimensional model it is shown that areas of large gradients in the mesoscale vorticity coincide with areas of low near-inertial energy. The smaller spatial scales in the near-inertial energ...

Frontal-Wave-like Evolution in Some Mesoscale Convective Complexes

Abstract In some mesoscale convective complexes (MCCs) the convective regions assume a configuration that resembles a frontal wave, while in other systems the configuration is not classifiable. We examined the evolution of the internal structure and flow of two MCCs that were part of an episode of systems that propagated along a stationary front, and in which the convective activity evolved from a chaotic to a frontal-wave-like pattern. The development of mesoscale vorticity and its possible relationship to the spatial patterns are explored. Each system developed two dissimilar convective bands that together formed an open-wave pattern. The greatest number of convective clusters and the greatest midlevel convergence and high-level divergence were found in a core region (the “apex” of the wave) where the bands intersected. The north-south convective line was less enduring than the core convection or the cast-west band; it evolved from outflow boundaries of the early storms preceding the mesosystem. A conce...