Vertical Shear Research Articles

An Observational Study on the Rapid Intensification of Typhoon Chanthu (2021) near the Complex Terrain of Taiwan

Abstract Intensification of Typhoon Chanthu (2021) along the eastern coast of Taiwan was accompanied by pronounced asymmetry in eyewall convection dominated by wavenumber-1 features, as observed by a dense radar network in Taiwan. The maximum wind speed at 3-km altitude, retrieved from radar observations, exhibited a rapid increase of approximately 18 m s−1 within an 11-h period during the intensification stage, followed by a significant decrease of approximately 19 m s−1 within 8 h during the weakening stage. Namely, Chanthu underwent both rapid intensification (RI) and rapid weakening (RW) within the 24-h analyzed period, posing challenges for intensity forecasts. During the intensifying stages, the region of maximum eyewall convection asymmetry underwent a sudden cyclonic rotation from the eastern to the northern semicircle immediately after the initiation of terrain-induced boundary inflow from the south of the typhoon, as observed by surface station data. This abrupt rotation of eyewall asymmetry exhibited better agreement with radar-derived vertical wind shear (VWS) than that derived from global reanalysis data. This finding suggests that the meso-β-scale VWS is more representative for tropical cyclones than meso-α-scale VWS when the terrain-induced forcing predominates in the environmental conditions. Further examination of the radar-derived VWS indicated that the VWS profile pattern provided a more favorable environment for typhoon intensification. In summary, Chanthu’s RI was influenced by the three factors: 1) terrain-induced boundary inflow from the south of the typhoon, observed by surface station data; 2) low-level flow pointing toward the upshear-left direction; and 3) weak upper-level VWS. Significance Statement Tropical cyclone intensity change has been an important issue for both real-time operation and research, but the influence of terrain on intensity change has not been fully understood. Typhoon Chanthu (2021) underwent a significant intensity change near the complex terrain of Taiwan that was observed by a dense radar network. This study analyzes 24 h of radar and weather station data to investigate Chanthu’s evolution. The analyses indicate that the complex terrain affected the low-level flow near the TC. Such a change in flow pattern provided additional boundary inflow and a relatively favorable vertical wind shear pattern for TC intensification.

Comparative analysis of the rapid intensification of two super cyclonic storms in the Arabian Sea

A comparative analysis of the rapid intensification (RI) of super cyclonic storms Chapala (2015) and Kyarr (2019) in the Arabian Sea is conducted using the North Indian Ocean tropical cyclone data, microwave sounding images, the NOAA OISST data and the ERA5 reanalysis data. Results show that the subtropical westerly jet stream and the Southern Hemisphere anticyclonic circulation led to the formation of an obvious double-channel outflow from the northern and southern sides of the two storm centers, and the substantial inflow appeared at the eastern boundary layer of both storms. These promoted the vertical ascent motion and release of the latent heat of condensation. A warm sea surface is a necessary but not dominant factor for the RI of cyclonic storms in the Arabian Sea. During the RI of Chapala and Kyarr, the deep vertical wind shear was less than 10 m s−1; moreover, the mid-level humidity conditions favored the RI of the two cyclonic storms. Chapala had a single warm core, whereas Kyarr had double warm cores in the vertical direction. The impacts of the latent heat of fusion is more obvious for Chapala, and the potential vorticity in its inner core increases from 4.4 PVU to 8.8 PVU, whereas the potential vorticity and vorticity in the inner core of Kyarr do not change significantly. Microwave detection images show that both Chapala and Kyarr were accompanied by the formation of eyewalls during the RI phase, and the radius of maximum wind decreased and the maximum wind speed increased during the eyewall-thinning process. Both Chapala and Kyarr passed through a positive anomaly region of maximum potential intensity during the RI phase, which increases the possibility to develop to higher intensity after genesis.

Study of Microphysical Signatures Based on Spectral Polarimetry during the RELAMPAGO Field Experiment in Argentina

Abstract Weather radars with dual-polarization capabilities enable the study of various characteristics of hydrometeors, including their size, shape, and orientation. Radar polarimetric measurements, coupled with Doppler information, allow for analysis in the spectral domain. This analysis can be leveraged to reveal valuable insight into the microphysics and kinematics of hydrometeors in precipitation systems. This paper uses spectral polarimetry to investigate precipitation microphysics and kinematics in storm environments observed during the Remote Sensing of Electrification, Lightning, and Mesoscale/Microscale Processes with Adaptive Ground Observations (RELAMPAGO) field experiment in Argentina. This study uses range–height indicator scan measurements from a C-band polarimetric Doppler weather radar deployed during the field campaign. In this work, the impact of storm dynamics on hydrometeors is studied, including the size sorting of hydrometeors due to vertical wind shear. In addition, particle microphysical processes because of aggregation and growth of ice crystals in anvil clouds, as well as graupel formation resulting from the riming of ice crystals and dendrites, are also analyzed here. The presence of different particle size distributions because of the mixing of hydrometeors in a sheared environment and resulting size sorting has been reported using spectral differential reflectivity (sZdr) slope. Spectral reflectivity sZh and sZdr have also been used to understand the signature of ice crystal aggregation in an anvil cloud. The regions of pristine ice crystals are identified from vertical profiles of spectral polarimetric variables in anvil cloud because of sZh < 0 dB and sZdr values around 2 dB. It is also found that the growth process of these ice crystals causes a skewed bimodal sZh spectrum due to the presence of both pristine ice crystals and dry snow. Next, graupel formation due to riming has been studied, and it is found that the riming process produces sZh values of about 10 dB and corresponding sZdr values of 1 dB. This positive sZdr indicates the presence of needle/columnar secondary ice particles formed by ice multiplication processes in the riming zones. Last, the temporal evolution of a storm is investigated by analyzing changes in hydrometeor types with time and their influence on the spectral polarimetric variables.

Comparison study of mechanical characteristics between conventional shield tunnel and shield tunnel with mortise-groove cross-section in ground fissure site

ABSTRACT Under the condition of the shield tunnel intersecting the ground fissure section at a right angle, a physical model test was conducted at a geometric scale ratio of 1:30. This test aimed to compare and analyze the force and deformation of the conventional shield tunnel and shield tunnel after adopting measures to mitigate the effects of ground fissure dislocation. Meanwhile, a finite-element numerical simulation of a conventional shield tunnel under the same working conditions was performed. The physical model test results align closely with the outcomes predicted by numerical calculations. The deformation observed in the shield tunnel primarily consisted of vertical shear deformation near the ground fissure, accompanied by a minor torsional deformation. In grade IV ground fissure sites, both the conventional shield tunnel and shield tunnel with mortise-groove cross-section after taking measures exhibit a basic level of safety. However, only the shield tunnel, after taking measures, is safe when the ground fissure develops to grade III. By contrast, the conventional shield tunnel generates significant shear deformation at the first circumferential seam in the footwall of the ground fissure, which easily causes the joints inside the circumferential seam to yield shear. When the ground fissure grade was raised to II, the conventional shield tunnel and shield tunnel with mortise-groove cross-section after taking measures experienced relatively serious damage.

Imaging faults and fractures with the difference of fast and slow shear-wave splitting reflectivity, Δγ(S): 3D/9C survey in Midland Basin, West Texas, and 3D/3C survey in Washakie Basin, Wyoming

One of the most important applications of shear-wave (S-wave) seismic exploration has been in reservoir fracture characterization. While many advancements have been made over the past 30 years to compute and correct for the long-wavelength kinematics of S-wave splitting (SWS) (fast S-wave polarization directions and slow S-wave time delays), practically no progress has been made in imaging the short-wavelength reflectivity of fractures directly with Δγ(S). This property is the contrast in the SWS anisotropy parameter, γ(S), and represents the reflection amplitude at vertical incidence for changes in fracture density and orientation across an interface. In this article, we examine the Lupin nine-component survey in Midland Basin, Texas, for the technical reliability of imaging fractures in depth with converted P to S waves (PS waves) guided by pure-mode horizontal shear waves and vertical shear waves. Final Δγ(S) amplitude maps for each mode show sensitivity to fractures, faults, and the maximum horizontal stress direction. These maps are computed from the difference between fast and slow S-wave stacks (after SWS analysis and correction) and P-wave amplitude variation with offset gradient stacks. The S-wave difference maps identify an east–west lineament, possibly a strike-slip fault or fracture corridor, that is not observed by P-wave depth slices. Pure-mode S waves and PS waves are orders of magnitude more sensitive to Δγ(S) than P waves. We also review the development of Δγ(S) and find that it has been relatively unexploited by the exploration industry. In addition, we demonstrate that Δγ(S) can be obtained directly from the objective function of the transverse energy to correct for SWS, and show a four-component Alford rotation example from a previous PS-wave survey in the Washakie Basin, Wyoming.

Tidal Intrusion Fronts, Surface Convergence, and Mixing in an Estuary with Complex Topography

Abstract Observations from a tidal estuary show that tidal intrusion fronts occur regularly during flood tides near topographic features including constrictions and bends. A realistic model is used to study the generation of these fronts and their influence on stratification and mixing in the estuary. At the constriction, flow separation occurs on both sides of the jet flow downstream of the narrow opening, leading to sharp lateral salinity gradients and baroclinic secondary circulation. A tidal intrusion front, with a V-shaped convergence zone on the surface, is generated by the interaction between secondary circulation and the jet flow. Stratification is created at the front due to the straining of lateral salinity gradients by secondary circulation. Though stratification is expected to suppress turbulence, strong turbulent mixing is found near the surface front. The intense mixing is attributed to enhanced vertical shear due to both frontal baroclinicity and the twisting of lateral shear by secondary circulation. In the bend, flow separation occurs along the inner bank, resulting in lateral salinity gradients, secondary circulation, frontogenesis, and enhanced mixing near the front. In contrast to the V-shaped front at the constriction, an oblique linear surface convergence front occurs in the bend, which resembles a one-sided tidal intrusion front. Moreover, in addition to baroclinicity, channel curvature also affects secondary circulation, frontogenesis, and mixing in the bend. Overall in the estuary, the near-surface mixing associated with tidal intrusion fronts during flood tides is similar in magnitude to bottom boundary layer mixing that occurs primarily during ebbs.

Climatology of turbulent kinetic energy dissipation rate in the middle troposphere using the 205 MHz S–T radar at Kochi, India

Turbulent kinetic energy dissipation rate (ɛ) in the mid-troposphere (5–12 km) using data obtained from the S–T wind profiler radar located at Kochi (10.04°N, 76.33°E) is analyzed for a period of 1595 days, ranging from March 2017 to December 2022, at a vertical resolution of 180 m. The spectral width method was employed for the estimation of ɛ after initial data filtering, and corrections for beam and shear broadening. Monthly variations in ɛ exhibited a marginal decrease with height. The background wind conditions, such as horizontal wind speed, vertical shear of horizontal wind, and wind direction, have minimal impact vertically. It is found that turbulence is more significantly affected by wind speed and wind direction. Westerly winds, despite being infrequent in the mid-troposphere, emerged as the primary contributors to elevated turbulence values. This observation was attributed to the unique topographical characteristics, creating an interplay of land–sea contrast and orography, leading to enhanced turbulence during westerly winds. During the monsoon season, enhanced turbulence is observed throughout the altitude. The presence of the tropical easterly jet was identified as a significant contributor to the increased wind speed and shear observed during the monsoon season and this alignment correlates with an increase in turbulence.

A New Rotating Axes Method for Processing High-Resolution Horizontal Velocity Measurements on EM-APEX floats

Abstract A new rotating axes method (RAM) is developed to improve the vertical resolution of the horizontal current velocity measurements u at EM-APEX floats. Unlike the traditional harmonic fitting method (HFM), which yields u averaged in 50-s intervals, RAM decodes and interprets 1-Hz measurements of horizontal seawater velocity , and averages in 12-s windows for removing wind waves with a typical peak frequency ∼ 0.12 Hz. Estimates of u from RAM agree with those from HFM but with a higher vertical resolution of ∼1.5 m, 4 times better than HFM. Note that extracting float signals due to seawater motion needs to assume slow-varying voltage offset ΔΦoffset. The typical variations of estimated ΔΦoffset do not affect the results of u significantly. Estimates of u are excluded when ΔΦoffset fluctuates strongly in time and scatter significantly. RAM is applied to float measurements taken near Mien-Hua Canyon, Taiwan. Composite vertical shear spectra Ψ computed using u from RAM exhibit a spectral slope of −1, as expected for the saturated internal waves in the vertical fine-scale range. The RAM provides EM-APEX float’s horizontal velocity measurements into fine vertical scales and will help improve our understanding of energy cascade from internal wave breaking and shear instability into turbulence mixing.

Accurate Lagrangian stochastic model on homogeneous turbulence under two shear orientations

The paper reports a numerical modelisation of homogeneous and stratified turbulence under vertical and horizontal shear by way of Lagrangian stochastic model (LSM) versus Froude number. Vertical and horizontal shear (θ = 0 and θ = π/2) are adopted. First, a comparison of predictions by LSM with results of direct numerical simulation of Jacobitz (DNSJ) is carried out. Current results obtained by LSM demonstrate the existence of asymptotic equilibrium states for various parameters of the problem particularly for the horizontal shear. Compared to the case of a vertical shear, an excellent agreement between predictions of LSM related to the case of horizontal shear and results of DNSJ is detected for growth rate, ratio kinetic energy to total energy and potential energy to total energy beyond τ = 40. For two shear orientations, results of normalized production and turbulence dissipation are comparable with different approaches for τ ≥ 70. Second, a comparative study of various thermal and dynamic parameters governing the considered problem for horizontal and vertical shear is performed versus Froude number. This study indicates that the LSM can present a significant contribution for numerical modelisation of stratified and homogeneous turbulence especially in the presence of a horizontal shear.

Comparing the atmospheric and ocean characteristics associated with two distinctly intensified pre‐monsoon tropical cyclones over the Bay of Bengal

AbstractTropical cyclone (TC) Amphan is analyzed in terms of the various factors that governed the intensification process associated with it and compared with Fani. Furthermore, the TC radial characteristics and ocean productivity are examined. Notably, both TCs formed in the Bay of Bengal during the pre‐monsoon seasons of 2020 and 2019, respectively. For this study, both ocean and atmospheric parameters from various sources including global analyses, satellite observations, and outputs from Model for Prediction Across Scales‐Atmosphere (MPAS‐A) and Advanced Research Weather Research and Forecasting (WRF‐ARW), are considered. The results indicate a gradual decrease in vertical wind shear during Fani. In the case of Amphan, the increase in mid‐tropospheric relative humidity values is found to be substantial. The sea surface cooling after the passage of Amphan was higher than in the case of Fani. The higher sea surface temperature in the Amphan case corresponds to the lower aerosol loading (partly because of lockdown measures) than that of Fani in the pre‐cyclone phase. And the decrease (increase) in aerosol loading coincides with an increase (decrease) in the direct radiative forcing at the ocean surface. The Madden–Julian Oscillation played a greater role in the cyclogenesis of Fani, but Kelvin waves offered a major support in the case of Amphan. The warmer sea surface, higher tropical cyclone heat potential, and conducive ocean and atmospheric setting together supported the further intensification of Amphan to the supercyclone stage. The difference in chlorophyll concentration showed a significant variation, with higher positive values seen in the case of Amphan implicating greater vertical mixing. The numerical modeling effort indicated superior performance of MPAS‐A compared to WRF‐ARW in simulating the radial parameters of the TCs.

Influence of Lee Wave Breaking on Far‐Field Mixing in the Deep Ocean

AbstractBreaking of internal lee waves generated by flow‐topography interaction is an important driving mechanism for abyssal mixing. By assuming that lee‐wave‐driven mixing is a local process, direct measurements in the past mainly focused on the water column over rough topography. In this study, a non‐local role of lee waves on remote mixing is investigated by using mooring observations in the northwestern Philippine Basin and a series of 2‐dimensional high‐resolution numerical simulations. We find that the breaking of lee waves can generate near‐inertial waves (NIWs) which can propagate away from their generation sites. The NIWs have strong vertical shear which can significantly elevate turbulent dissipation and mixing over remote uneven seafloor. Based on the topography perturbations numerical experiments, we find that for the remote gentle topography with inverse Froude number (Fr−1) between 0 and 0.5, the arrival of upstream NIWs can elevate the near‐bottom layer (0–1,000 m above bottom) dissipation rate and vertical diffusivity by 9.2 and 10.4 times, respectively at 30–50 km away from the NIWs source. For the remote rough topography with Fr−1 between 0 and 4, the corresponding increases are 3.5 and 4.4 times, respectively. The mechanism of upstream NIWs to elevate the downstream dissipation and mixing is through strengthening the shear instability over the far‐field topography, which is associated with NIWs' interactions with the topography and the generated lee waves therein. The results of this study will provide a foundation for improving the mixing parameterizations associated with current‐topography interactions.

On corrugation mode radial wavelengths of the vertical shear instability

Abstract The vertical shear instability (VSI) is a promising mechanism to drive turbulence in protoplanetary disks. Numerical simulations in the literature demonstrate that the VSI non-linear saturation is predominated by the linear corrugation modes. These modes possess vertical wavelengths crucially longer than radial wavelengths. This paper aims to investigate the natural radial wavelength of corrugation modes upon VSI saturation, by a series of numerical simulations conducted in Athena++ at different grid resolutions, disk aspect ratios, and viscosity parameterized by ν. We find a sign of convergence emerges at 64 cells per gas scale height for fiducial simulations. Below it a continuous reduction of wavelengths with grid resolution is observed. Synthetic ALMA molecular line observations of $^{12}\rm CO(2-1)$ are performed to inspect the observability of the corrugation modes feature, which is significantly diminished with a resolution of 32 cells per scale height or above. Flared and viscous disks exhibiting longer saturation wavelengths may mitigate the observational difficulty.

Eddy‐Mediated Turbulent Mixing of Oxygen in the Equatorial Pacific

AbstractIn the tropical Pacific, weak ventilation and intense microbial respiration at depth give rise to a low dissolved oxygen (O2) environment that is thought to be ventilated primarily by the equatorial current system (ECS). The role of mesoscale eddies and vertical mixing as potential pathways of O2 supply in this region, however, remains poorly known due to sparse observations and coarse model resolution. Using an eddy resolving simulation of ocean circulation and biogeochemistry, we assess the contribution of these processes to the O2 budget balance and find that vertical mixing of O2, which is modulated by the surface wind speed and the vertical shear of the eddying currents, contributes substantially to the replenishment of O2 in the upper equatorial Pacific thermocline, complementing the advective supply of O2 by the ECS and meridional circulation at depth. These transport processes vary seasonally in conjunction with the wind: mixing of O2 into the upper thermocline is strongest during boreal summer and fall when the vertical shear and eddy kinetic energy are intensified. The relationship between eddy activity and the downward mixing of O2 arises from the modulation of equatorial turbulence by Tropical Instability Waves via their impacts on the vertical shear. This interaction of processes across scales sustains a local pathway of O2 delivery into the equatorial Pacific interior and highlights the need for adequate observations and models of turbulent mixing and mesoscale processes for understanding and predicting the fate of the tropical Pacific O2 content in a warmer and more stratified ocean.

Aeolus wind lidar observations of the 2019/2020 quasi-biennial oscillation disruption with comparison to radiosondes and reanalysis

Abstract. The quasi-biennial oscillation (QBO) was unexpectedly disrupted for only the second time in the historical record during the 2019/2020 boreal winter. As the dominant mode of atmospheric variability in the tropical stratosphere and a significant source of seasonal predictability globally, understanding the drivers behind this unusual behaviour is very important. Here, novel data from Aeolus, the first Doppler wind lidar (DWL) in space, are used to observe the 2019/2020 QBO disruption. Aeolus is the first satellite able to observe winds at high resolution on a global scale, and it is therefore a uniquely capable platform for studying the evolution of the disruption and the broader circulation changes triggered by it. This study therefore contains the first direct wind observations of the QBO from space, and it exploits measurements from a special Aeolus scanning mode, implemented to observe this disruption as it happened. Aeolus observes easterly winds of up to 20 m s−1 in the core of the disruption jet during July 2020. By co-locating with radiosonde measurements from Singapore and the ERA5 reanalysis, comparisons of the observed wind structures in the tropical stratosphere are produced, showing differences in equatorial wave activity during the disruption period. Local zonal wind biases are found in both Aeolus and ERA5 around the tropopause, and the average Aeolus-ERA5 Rayleigh horizontal line-of-sight random error is found to be 7.58 m s−1. The onset of the QBO disruption easterly jet occurs 5 d earlier in Aeolus observations compared with the reanalysis. This discrepancy is linked to Kelvin wave variances that are 3 to 6 m2 s−2 higher in Aeolus compared with ERA5, centred on regions of maximum vertical wind shear in the tropical tropopause layer that are up to twice as sharp. The enhanced lower-stratospheric westerly winds which are known to help disrupt the QBO, perhaps with increasing frequency as the climate changes, are also stronger in Aeolus observations, with important implications for the future predictability of such disruptions. An investigation into differences in the equivalent depth of the most dominant Kelvin waves suggests that slower, shorter-vertical-wavelength waves break more readily in Aeolus observations compared with the reanalysis. This analysis therefore highlights how Aeolus and future DWL satellites can deepen our understanding of the QBO, its disruptions and the tropical upper-troposphere lower-stratosphere region more generally.

The Lower Atmospheric Characteristics of Dust Storms Using Ground-Based Sensor Data: A Comparative Analysis of Two Cases in Jinan, China

Two severe dust storm (DS) events (15–17 March and 28–29 March) hit northern China in 2021 consecutively. The lower atmospheric vertical dynamic and thermal structures during the two cases were compared using the ground-based sensor data from the microwave radiometer and radar wind profiler, combined with the environmental and meteorological observations data in Jinan, China. It was found that both cases occurred under the background of cold vortexes over northeastern China. The dust was transported through the cold air on the northwest route. During the dust period, 2–3 km was the west or northwest airflow, and below 2 km was the northeast wind. The variation in the dynamic structure determined the duration of the DS. During the DS maintenance phase, the vertical wind shear (VWS) below 3 km measured approximately 10 m∙(s∙km)−1. The increased VWS during the dust intrusion period facilitated the transportation of dust. In contrast, the more significant VWS was not conducive to the maintenance of DS, and the shift to south wind control in the upper middle layer indicated the weakening of DS. In both cases, we observed a cliff-like decrease in relative humidity as a prominent indicator of dust outbreaks, occurring approximately 2–5 h beforehand. The diurnal difference between the vertical temperature and relative humidity during the dust maintenance period was found to be insignificant.

Frequency-Domain 3D Computer Program for Predicting Motions and Loads on a Ship in Regular Waves

The development of an in-house computer program for determining the motions and loads of advancing ships through sea waves in the frequency domain, is described in this paper. The code is based on the potential flow formulation and originates from a double-body code enhanced with the regular part of the velocity potential computed using the pulsing source Green function. The code is fully developed in C++ language with extensive use of the object-oriented paradigm. The code is capable of estimating the excitation and inertial radiation loads or arbitrary incoming wave frequencies and incidence angles. The hydrodynamic responses such as hydrodynamic coefficients, ship motions, the vertical shear force and the vertical bending moment are estimated. A benchmark container ship and an LNG carrier are selected for testing and validating the computer code. The obtained results are compared with the available experimental data which demonstrate the acceptable compliance for the zero speed whereas there are some discrepancies over the range of frequencies for the advancing ship in different heading angles.

Understanding the Inter-Model Spread of PDO’s Impact on Tropical Cyclone Frequency over the Western North Pacific in CMIP6 Models

This work investigates the inter-model diversity of the Pacific Decadal Oscillation’s (PDO) impact on tropical cyclone frequency (TCF) over the Western North Pacific (WNP) from the historical simulation of twenty-two Coupled Model Intercomparison Project Phase 6 (CMIP6) models. The impact of the PDO is expressed as the TCF difference between the positive and negative PDO phases. The comparison between the models with high PDO skill and low PDO skill shows that the PDO-related sea surface temperature (SST) gradient between the western and central tropical Pacific plays an important role in changing the large-scale atmospheric dynamic fields for TC genesis and, thus, the TCF over the WNP. This SST gradient also significantly contributes to the inter-model spread of PDO’s impact on TCF across the 22 CMIP6 models. We, therefore, stress that the PDO-related eastward SST gradient between the western and central tropical Pacific triggers the lower troposphere westerly and eastward extending of the monsoon trough over the WNP. The moistening of the atmosphere and enhancing ascending motion in the mid-troposphere promote convection, leading to the easterly wind anomaly over the upper troposphere, which reduces the vertical wind shear. Those favorable dynamic conditions consistently promote the TC formation over the southeastern part of the Western North Pacific. Our results highlight that PDO could impact the WNP TCF through its associated tropical SST gradient.

Comparison of Cold Pool Characteristics of Two Distinct Gust Fronts over Bohai Sea Bay in China

Previous studies have demonstrated that cold pools play a pivotal role in the initiation and organization of convection, yet their influence on the evolution of gust fronts (GFs) remains inadequately understood. A destructive wind event associated with a rearward gust front (RGF; 8 grade gale after passing GF) and a prior gust front (PGF; 10 grade gale before passing GF) over the north coast of China on 10 June 2016 was analyzed. Using multiple forms of observation data, as well as the four-dimensional Variational Doppler Radar Data Assimilation System (VDRAS), we found that the depth and intensity of the cold pool in RGF are relatively shallower and weaker, leading to a correspondingly reduced strength in both outflow and convergence. In contrast, the enhanced vertical shear and boundary northeaster inflow of PGF generate intensified and more organized downdrafts, resulting in a deeper cold pool, robust outflow, and convergence. Two schematic models were proposed to explain the discrepancy between GFs and associated cold pools. We further show that there is an internal correlation between meso-γ-scale vortices (MVs) and cold pools, the collision of MVs strengthened low-level convergence and updraft between these two GFs. Moreover, the consolidation of the two cold pools exacerbates low-layer instability and rotation, generating an intense horizontal vorticity that leads to rapid convective storm intensification. These findings offer novel insights into the diversity of GFs and associated cold pools.

The stochastic spin-up 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 spin-up 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. 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. Their collective effect is calculated with the central limit theorem. The theory predicts that the standard deviation of the midlevel mesoscale vorticity is universally proportional to the square root of the domain-averaged accumulated rainfall, agreeing with simulations. The theory 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 tilting is an important source of mesoscale vorticity fluctuation in the tropics.

Energy partition between submesoscale internal waves and quasi-geostrophic vortical motion in the pycnocline

Abstract Shipboard ADCP velocity and towed CTD chain density measurements from the eastern North Pacific pycnocline are used to segregate energy between linear internal waves (IW) and linear vortical motion (quasi-geostrophy, QG) in 2-D wavenumber space spanning submesoscale horizontal wavelengths λx ∼ 1 – 50 km and finescale vertical wavelengths λz ∼ 7 – 100 m. Helmholtz decomposition and a new Burger-number Bu decomposition yield similar results despite different methodologies. Partition between IW and QG total energies depends on 𝐵𝑢. For Bu < 0.01, available potential energy EP exceeds horizontal kinetic energy EK and is contributed mostly by QG. In contrast, energy is nearly equipartitioned between QG and IW for Bu » 1. For Bu < 2, EK is contributed mainly by IW, and EP by QG, while, for Bu > 2, contributions are reversed. Vertical shear variance is contributed primarily by near-inertial IW at small λz, implying negligible QG contribution to vertical shear instability. Conversely, both QG and IW at the smallest λx ∼ 1 km contribute large horizontal shear variance, such that both may lead to horizontal shear instability. Both QG and IW contribute to vortex-stretching at small vertical scales. For QG, the relative vorticity contribution to linear potential vorticity anomaly increases with decreasing horizontal and increasing vertical scales.