scholarly journals Reversed Lateral Circulation in a Sharp Estuarine Bend with Weak Stratification

2019 ◽  
Vol 49 (6) ◽  
pp. 1619-1637 ◽  
Author(s):  
Wouter M. Kranenburg ◽  
W. Rockwell Geyer ◽  
Adrian Mikhail P. Garcia ◽  
David K. Ralston
Keyword(s):  
Cross Section ◽  
Outer Part ◽  
Streamwise Velocity ◽  
Water Levels ◽  
Secondary Circulation ◽  
Vertical Shear ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Topographic Effects ◽  
Curvature Effects

AbstractAlthough the hydrodynamics of river meanders are well studied, the influence of curvature on flow in estuaries, with alternating tidal flow and varying water levels and salinity gradients, is less well understood. This paper describes a field study on curvature effects in a narrow salt-marsh creek with sharp bends. The key observations, obtained during times of negligible stratification, are 1) distinct differences between secondary flow during ebb and flood, with helical circulation as in rivers during ebb and a reversed circulation during flood, and 2) maximum (ebb and flood) streamwise velocities near the inside of the bend, unlike typical river bend flow. The streamwise velocity structure is explained by the lack of a distinct point bar and the relatively deep cross section in the estuary, which means that curvature-induced inward momentum redistribution is not overcome by outward redistribution by frictional and topographic effects. Through differential advection of the along-estuary salinity gradient, the laterally sheared streamwise velocity generates lateral salinity differences, with the saltiest water near the inside during flood. The resulting lateral baroclinic pressure gradient force enhances the standard helical circulation during ebb but counteracts it during flood. This first leads to a reversed secondary circulation during flood in the outer part of the cross section, which triggers a positive feedback mechanism by bringing slower-moving water from the outside inward along the surface. This leads to a reversal of the vertical shear in the streamwise flow, and therefore in the centrifugal force, which further enhances the reversed secondary circulation.

scholarly journals Evolution of Low-Level Angular Momentum in the 2 June 1995 Dimmitt, Texas, Tornado Cyclone

2007 ◽  
Vol 64 (4) ◽  
pp. 1365-1378 ◽  
Author(s):  
Erik N. Rasmussen ◽  
Jerry M. Straka
Keyword(s):  
Angular Momentum ◽  
Life Cycle ◽  
Axisymmetric Flow ◽  
Tangential Velocity ◽  
Secondary Circulation ◽  
Cloud Base ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Low Level ◽  
Debris Cloud

Abstract The life cycle of the 2 June 1995 Dimmitt, Texas, tornado cyclone, observed during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX), is described. The tornado cyclone here is defined as a significantly axisymmetric flow larger than the visible tornado and characterized by increasing angular momentum with increasing radius. Its life cycle included three phases with somewhat differing evolution of angular momentum, herein called intensifying, transition, and weakening. During the intensifying stage, the funnel and debris cloud gradually increased in size. The azimuthally averaged secondary circulation of the larger-scale tornado cyclone, as determined using high-resolution single-Doppler data obtained by a mobile radar, was primarily inward and upward, consistent with the presence of a wall cloud outside the tornado. The azimuthally averaged angular momentum increased monotonically away from the tornado, so inward advection allowed the angular momentum to increase slowly with time in part of the tornado cyclone. During the transition phase, downdrafts began to occur within the tornado cyclone. The transport of angular momentum by the secondary circulation nearly was offset by eddy flux convergence of angular momentum so that the azimuthally averaged angular momentum tendency was only weakly negative at most radii. The tornado was visually impressive during this stage, featuring a 400-m diameter debris cloud extending to cloud base, while the surrounding wall cloud shrank and eroded. During the weakening phase, the funnel and debris cloud gradually shrank, and the funnel went through a rope stage prior to disappearing. The weakening phase was characterized by extensive downdrafts at all radii outside the tornado, and large-scale near-ground outflow as observed by mobile mesonet systems in a portion of the tornado cyclone. The secondary circulation acted to transport smaller angular momentum downward from aloft, and outward along the ground. All terms of the angular momentum budget became negative throughout most of the low-level (0–800-m AGL) tornado cyclone during the weakening phase. Several hypotheses for this evolution are evaluated, including changes in water loading in the tornado cyclone, cooling of the near-ground air, and the distribution of tangential velocity with height with its concomitant influence on the nonhydrostatic vertical pressure gradient force.

Potential Vorticity Diagnosis of a Simulated Hurricane. Part II: Quasi-Balanced Contributions to Forced Secondary Circulations

2006 ◽  
Vol 63 (11) ◽  
pp. 2898-2914 ◽  
Author(s):  
Da-Lin Zhang ◽  
Chanh Q. Kieu
Keyword(s):  
Potential Vorticity ◽  
Large Scale ◽  
Inner Core ◽  
Vertical Shear ◽  
Mature Stage ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Latent Heating ◽  
Tangential Wind ◽  
Secondary Circulations

Abstract Although the forced secondary circulations (FSCs) associated with hurricane-like vortices have been previously examined, understanding is still limited to idealized, axisymmetric flows and forcing functions. In this study, the individual contributions of latent heating, frictional, and dry dynamical processes to the FSCs of a hurricane vortex are separated in order to examine how a hurricane can intensify against the destructive action of vertical shear and how a warm-cored eye forms. This is achieved by applying a potential vorticity (PV) inversion and quasi-balanced omega equations system to a cloud-resolving simulation of Hurricane Andrew (1992) during its mature stage with the finest grid size of 6 km. It is shown that the latent heating FSC, tilting outward with height, acts to oppose the shear-forced vertical tilt of the storm, and part of the upward mass fluxes near the top of the eyewall is detrained inward, causing the convergence aloft and subsidence warming in the hurricane eye. The friction FSC is similar to that of the Ekman pumping with its peak upward motion occurring near the top of the planetary boundary layer (PBL) in the eye. About 40% of the PBL convergence is related to surface friction and the rest to latent heating in the eyewall. In contrast, the dry dynamical forcing is determined by vertical shear and system-relative flow. When an axisymmetric balanced vortex is subjected to westerly shear, a deep countershear FSC appears across the inner-core region with the rising (sinking) motion downshear (upshear) and easterly sheared horizontal flows in the vertical. The shear FSC is shown to reduce the destructive roles of the large-scale shear imposed, as much as 40%, including its forced vertical tilt. Moreover, the shear FSC intensity is near-linearly proportional to the shear magnitude, and the wavenumber-1 vertical motion asymmetry can be considered as the integrated effects of the shear FSCs from all the tropospheric layers. The shear FSC can be attributed to the Laplacian of thermal advection and the temporal and spatial variations of centrifugal force in the quasi-balanced omega equation, and confirms the previous finding of the development of wavenumber-1 cloud asymmetries in hurricanes. Hurricane eye dynamics are presented by synthesizing the latent heating FSC with previous studies. The authors propose to separate the eye formation from maintenance processes. The upper-level inward mass detrainment forces the subsidence warming (and the formation of an eye), the surface pressure fall, and increased rotation in the eyewall. This increased rotation will induce an additional vertical pressure gradient force to balance the net buoyancy generated by the subsidence warming for the maintenance of the hurricane eye. In this sense, the negative vertical shear in tangential wind in the eyewall should be considered as being forced by the subsidence warming, and maintained by the rotation in the eyewall.

scholarly journals Responses of estuarine circulation to the morphological evolution in a convergent, microtidal estuary

2021 ◽  
Author(s):  
Rui Zhang ◽  
Bo Hong ◽  
Lei Zhu ◽  
Wenping Gong ◽  
Heng Zhang
Keyword(s):  
Human Activities ◽  
Morphological Evolution ◽  
Estuarine Circulation ◽  
Longitudinal Vortex ◽  
Vertical Shear ◽  
Momentum Balance ◽  
Dominant Factor ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Lateral Circulation

Abstract. The Huangmaohai Estuary (HE) is a funnel-shaped microtidal estuary in the west of the Pearl River Delta (PRD) in southern China. Since China's reform and opening up in 1978, extensive human activities have occurred and greatly changed the estuary's topography, and modified its hydrodynamics. In this study, we examined the morphological evolution by analyzing remote sensing data with ArcGIS tools and studied the responses of hydrodynamics to the changes in topography from 1977 to 2010 by using the Delft3d model. We took the changes in estuarine circulation during neap tides in dry seasons as an example. The results show that human reclamation caused a narrowing of the estuary, and channel dredging deepened the estuary. These human activities changed both the longitudinal and lateral estuarine circulations. The longitudinal circulation was observed to increase with the deepening and narrowing of the estuary. The lateral circulation experienced changes in both the magnitude and pattern. The momentum balance analysis shows that when the depth and width changed simultaneously, the longitudinal estuarine circulation was modulated by both the channel deepening and width reduction, in which the friction, pressure gradient force, and advection terms were altered. The analysis of the longitudinal vortex dynamics indicates that the changes in the vertical shear of the longitudinal flow, lateral salinity gradient, and vertical mixing were responsible for the change in the lateral circulation. The changes in water depth are the dominant factor affecting lateral circulation intensity. This study has implications for sediment transport and morphological evolution in estuaries heavily impacted by human interventions.

A Dissection of the Topographic Effects from Eurasia and North America on the Isentropic Meridional Mass Circulation in Northern Winter

2021 ◽  
Author(s):  
Yueyue Yu ◽  
Rongcai Ren ◽  
Xin Xia ◽  
Ruxue Liang ◽  
Jian Rao
Keyword(s):  
North America ◽  
Stationary Flow ◽  
Independent Effect ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Stationary Waves ◽  
Topographic Effects ◽  
Mean Flow ◽  
Meridional Mass Circulation ◽  
Mass Circulation

Abstract The topographic dynamical effect from Eurasia (EA_Topo) and North America (NA_Topo) on the winter isentropic meridional mass circulation (IMMC) is investigated using the WACCM. The independent effect of EA_Topo and that of NA_Topo, with the former much stronger, are both to strengthen the IMMC that is composed of the lower equatorward cold air branch (CB) and the upper poleward warm air branch in the extratropical tropopshere (WB_TR) and stratosphere (WB_ST). Further investigation of the individual contributions from changes in stationary vs. transient and zonal-mean flow vs. waves reveals that, due to the topography-forced mass redistribution, changes in the low-level meridional pressure gradient force a zonal-mean counter-clockwise/ clockwise meridional cell in the southern/northern side of topography. This weakens/strengthens the IMMC south/north of 30°N from the troposphere to lower stratosphere, acting as a dominant contributor to the IMMC changes south of 50°N. Meanwhile, the EA/NA_Topo-forced amplification of stationary waves constructively interacts with those determined by land-sea contrast, making the dominant/minor contributions to the strengthening of CB and WB_TR north of 50°N. The related increase in the upward wave propagation further dominates the WB_ST strengthening in the subpolar region. Meanwhile, transient eddy activities are depressed by EA/NA_Topo along with the weakened background westerly, which partly-offset/dominate-over the contribution from stationary flow in midlatitudes and subpolar region. The coexistence of the other topography (NA/EA_Topo) yields destructive mutual interferrence, which can weaken/offset the independent-EA/NA_Topo-forced meridional mass transport mainly via changing the zonal-mean as well as the downstream wave pattern of mass and meridional wind.

A Three-Dimensional Anelastic Model Based on the Vorticity Equation

2008 ◽  
Vol 136 (1) ◽  
pp. 276-294 ◽  
Author(s):  
Joon-Hee Jung ◽  
Akio Arakawa
Keyword(s):  
Elliptic Equation ◽  
Three Dimensional ◽  
Vertical Component ◽  
Momentum Equation ◽  
Vertical Shear ◽  
Vorticity Equation ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Acceleration Process ◽  
Vorticity Dynamics

Abstract A three-dimensional anelastic model has been developed using the vorticity equation, in which the pressure gradient force is eliminated. The prognostic variables of the model dynamics are the horizontal components of vorticity at all heights and the vertical component of vorticity and the horizontally uniform part of the horizontal velocity at a selected height. To implement the anelastic approximation, vertical velocity is diagnostically determined from the predicted horizontal components of vorticity by solving an elliptic equation. This procedure replaces solving the elliptic equation for pressure in anelastic models based on the momentum equation. Discretization of the advection terms uses an upstream-weighted partially third-order scheme. When time is continuous, the solution of this scheme is quadratically bounded. As an application of the model, interactions between convection and its environment with vertical shear are studied without and with model physics from the viewpoint of vorticity dynamics, that is, the deceleration/acceleration process of the basic flow in particular. The authors point out that the process is purely three-dimensional, especially when the convection is relatively localized, involving the twisting terms and the horizontal as well as vertical transports of vorticity. Finally, it is emphasized that parameterization of cumulus friction is a resolution-dependent problem of vorticity dynamics associated with cumulus convection.

A Modeling Study on the Development of a Bowing Structure and Associated Rear Inflow within a Squall Line over South China

2012 ◽  
Vol 69 (4) ◽  
pp. 1182-1207 ◽  
Author(s):  
Zhiyong Meng ◽  
Fuqing Zhang ◽  
Paul Markowski ◽  
Duochang Wu ◽  
Kun Zhao
Keyword(s):  
South China ◽  
Vertical Shear ◽  
Squall Line ◽  
Cold Pool ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Low Level ◽  
Vorticity Budget ◽  
Horizontal Pressure ◽  
Formation And Evolution

Abstract Through convection-permitting simulations, this study examines a large bowing structure within a squall line that occurred during the rainy season in South China. The bowing structure is closely associated with a local enhancement of (and balance between) the cold pool and the line-normal environmental low-level vertical shear. Rear inflow plays an essential role in the formation and evolution of this large bowing structure. It is found that the low-level rear inflow is largely a natural consequence of the baroclinically generated horizontal vorticity near the surface, while the midtropospheric rear inflow is forced by several pairs of bookend vortices. Vorticity budget and vortex-line analyses show that the bookend vortices form mainly through the tilting of horizontal vorticity. Consolidation of these pairs of bookend vortices forms a broad zone of contiguous rear inflow. The environmental flow and horizontal pressure gradient force associated with the midlevel pressure deficit induced by the rearward-tilting buoyant updrafts, on the other hand, are not primarily responsible for the formation of the rear inflow.

scholarly journals Sensitivity of Hurricane Intensity Forecast to Convective Momentum Transport Parameterization

2006 ◽  
Vol 134 (2) ◽  
pp. 664-674 ◽  
Author(s):  
Jongil Han ◽  
Hua-Lu Pan
Keyword(s):  
Wind Shear ◽  
Vertical Wind Shear ◽  
Momentum Transport ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Momentum Exchange ◽  
Forecast System ◽  
Hurricane Intensity ◽  
Convective Momentum Transport ◽  
The Many

Abstract A parameterization of the convection-induced pressure gradient force (PGF) in convective momentum transport (CMT) is tested for hurricane intensity forecasting using NCEP's operational Global Forecast System (GFS) and its nested Regional Spectral Model (RSM). In the parameterization the PGF is assumed to be proportional to the product of the cloud mass flux and vertical wind shear. Compared to control forecasts using the present operational GFS and RSM where the PGF effect in CMT is taken into account empirically, the new PGF parameterization helps increase hurricane intensity by reducing the vertical momentum exchange, giving rise to a closer comparison to the observations. In addition, the new PGF parameterization forecasts not only show more realistically organized precipitation patterns with enhanced hurricane intensity but also reduce the forecast track error. Nevertheless, the model forecasts with the new PGF parameterization still largely underpredict the observed intensity. One of the many possible reasons for the large underprediction may be the absence of hurricane initialization in the models.

scholarly journals Temporal evolution of the momentum balance terms and frictional adjustment observed over the inner shelf during a storm

Ocean Science ◽  
2016 ◽  
Vol 12 (1) ◽  
pp. 137-151 ◽  
Author(s):  
M. Grifoll ◽  
A. L. Aretxabaleta ◽  
J. L. Pelegrí ◽  
M. Espino
Keyword(s):  
Coriolis Force ◽  
Wind Stress ◽  
Water Depth ◽  
Momentum Balance ◽  
Wave Radiation ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Inner Shelf ◽  
Coastal Waves ◽  
Nw Mediterranean

Abstract. We investigate the rapidly changing equilibrium between the momentum sources and sinks during the passage of a single two-peak storm over the Catalan inner shelf (NW Mediterranean Sea). Velocity measurements at 24 m water depth are taken as representative of the inner shelf, and the cross-shelf variability is explored with measurements at 50 m water depth. During both wind pulses, the flow accelerated at 24 m until shortly after the wind maxima, when the bottom stress was able to compensate for the wind stress. Concurrently, the sea level also responded, with the pressure-gradient force opposing the wind stress. Before, during and after the second wind pulse, there were velocity fluctuations with both super- and sub-inertial periods likely associated with transient coastal waves. Throughout the storm, the Coriolis force and wave radiation stresses were relatively unimportant in the along-shelf momentum balance. The frictional adjustment timescale was around 10 h, consistent with the e-folding time obtained from bottom drag parameterizations. The momentum evolution at 50 m showed a larger influence of the Coriolis force at the expense of a decreased frictional relevance, typical in the transition from the inner to the mid-shelf.

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