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Xiaofan Li and Bin Wang

Abstract

The movement of a symmetric vortex embedded in a resting environment with a constant planetary vorticity gradient (the beta drift) is investigated with a shallow-water model. The authors demonstrate that, depending on initial vortex structure, the vortex may follow a variety of tracks ranging from a quasi-steady displacement to a wobbling or a cycloidal track due to the evolution of a secondary asymmetric circulation. The principal part of the asymmetric circulation is a pair of counterrotating gyres (referred to as beta gyres), which determine the steering flow at the vortex center. The evolution of the beta gyres is characterized by development/decay, gyration, and radial movement.

The beta gyres develop by extracting kinetic energy from the symmetric circulation of the vortex. This energy conversion is associated with momentum advection and meridional advection of planetary vorticity. The latter (referred to as “beta conversion”) is a principal process for the generation of asymmetric circulation. A necessary condition for the development of the beta gyres is that the anticyclonic gyre must be located to the east of a cyclonic vortex center. The rate of asymmetric kinetic energy generation increases with increasing relative angular momentum of the symmetric circulation.

The counterclockwise rotation of inner beta gyres (the gyres located near the radius of maximum wind) is caused by the advection of asymmetric vorticity by symmetric cyclonic flows. On the other hand, the clockwise rotation of outer beta gyres (the gyres near the periphery of the cyclonic azimuthal wind) is determined by concurrent intensification in mutual advection of the beta gyres and symmetric circulation and weakening in the meridional advection of planetary vorticity by symmetric circulation. The outward shift of the outer beta gyres is initiated by advection of symmetric vorticity by beta gyres relative to the drifting velocity of the vortex.

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Bin Wang and Xiaofan Li

Abstract

Tropical cyclone propagation (the beta drift) is driven by a secondary circulation associated with axially asymmetric gyres (beta gyres) in the vicinity of the cyclone center. In the presence of the beta effect, the environmental flow may interact with the symmetric circulation and beta gyres of the cyclone, affecting the development of the gyres and thereby the cyclone propagation. An energetics analysis is carried out to elucidate the development mechanism of the beta gyres and to explain variations in propagation speed of a barotropic cyclone embedded in a meridionally varying zonal flow on a beta plane. Two types of zonal flows are considered: one with a constant meridional shear resembling those in the vicinity of a subtropical ridge or a monsoon trough, and the other with a constant relative vorticity gradient as in the vicinity of an easterly (westerly) jet.

Zonal flow with a constant meridional shear changes the generation rate of the gyre kinetic energy through an exchange of energy directly with the gyres. The momentum flux associated with gyres acting on the meridional shear of zonal flow accounts for this energy conversion process. Zonal flow with an anticyclonic (cyclonic) shear feeds (extracts) kinetic energy to (from) the gyres. The magnitude of this energy conversion is proportional to the strength of the meridional shear and the gyre intensity. As a result, the gyres are stronger and the beta drift is faster near a subtropical ridge (anticyclonic shear) than within a monsoon trough (cyclonic shear).

Zonal flow with a constant relative vorticity gradient affects gyre intensity via two processes that have opposing effects. A southward vorticity gradient, on the one hand, weakens the gyres by reducing the energy conversion rate from symmetric circulation to gyres; on the other hand, it enhances the gyres by indirectly feeding energy to the symmetric circulation, whose strengthening in turn accelerates the energy conversion from symmetric circulation to gyres. The effect of the second process tends to eventually become dominant.

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Xiaofan Li and Bin Wang

Abstract

An energetics analysis is presented to reveal the mechanisms by which the environmental flows affect hurricane beta-gyre intensity and beta-drift speed. The two-dimensional environmental flow examined in this study varies in both zonal and meridional directions with a constant shear.

It is found that a positive (negative) shear strain rate of the environmental flow accelerates (decelerates) beta drift. The horizontal shear of the environmental flow contains an axially symmetric component that is associated with vertical vorticity and an azimuthal wavenumber two component that is associated with shear strain rate. It is the latter that interacts with the beta gyres, determining the energy conversion between the environmental flow and beta gyres. A positive shear strain rate is required for transfering kinetic energy from the environmental flow to the beta gyres. As a result, the positive shear strain rate enhances the beta gyres and associated steering flow that, in turn, accelerates the beta drift.

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Bin Wang Xiaofan Li

Abstract

The beta effect on translation of cyclonic and anticyclonic vortices with height-dependent circulation (the beta-drift problem) is investigated via numerical experiments using a dry version of a multilevel primitive equation model (Florida State University model).

The vertical structure of vortex circulation influences steady translation in a manner similar to that of the horizontal structure. Both spatially change the mean relative angular momentum (MRAM) of the vortex. The translation speed and its meridional component are both approximately proportional to the square root of the magnitude of MRAM of the initial (or quasi-steady-state) symmetric circulation. The latitude is another important factor controlling the speed of the beta drift. The meridional component decreases by about 45% when the central latitude of the vortex increases from 10° to 30°N.

The beta-drift speed is intimately related to the axially asymmetric pressure field. During quasi-steady vortex translation the asymmetric pressure field maintains a stationary wavenumber 1 pattern in azimuthal direction with a high in the northeast and a low in the southwest quadrant of a Northern Hemisphere cyclone. The beta-drift velocity is approximately equal to the geostrophic flow implied by the asymmetric pressure gradient at the vortex center. If the Rossby number associated with the asymmetric flow is small, to the lowest order, the asymmetric pressure gradient force at the vortex center is balanced by the Coriolis force associated with the beta drift of the vortex.

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Huiyan Xu and Xiaofan Li

Abstract

In this study, the 2D and 3D cloud-resolving model simulations of the Tropical Rainfall Measuring Mission (TRMM) Kwajalein Experiment (KWAJEX) are compared to study the impact of dimensionality on barotropic processes during tropical convective development. Barotropic conversion of perturbation kinetic energy is associated with vertical transport of horizontal momentum under vertical shear of background horizontal winds. The similarities in both 2D and 3D model simulations show that 1) vertical wind shear is a necessary condition for barotropic conversion, but it does not control the barotropic conversion; 2) the evolution of barotropic conversion is related to that of the vertical transport of horizontal momentum; and 3) the tendency of vertical transport of horizontal momentum is mainly determined by the covariance between horizontal wind and the cloud hydrometeor component of buoyancy. The differences between the 2D and 3D model simulations reveal that 1) the barotropic conversion has shorter time scales and a larger contribution in the 2D model simulation than in the 3D model simulation and 2) kinetic energy is generally converted from the mean circulations to perturbation circulations in the 3D model simulation. In contrast, more kinetic energy is transferred from perturbation circulations to the mean circulations in the 2D model simulation. The same large-scale vertical velocity may account for the similarities, whereas the inclusion of meridional winds in the 3D model simulation may be responsible for the differences in barotropic conversion between the 2D and 3D model simulations.

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Xiaofan Li, Zeng-Zhen Hu, and Bohua Huang

Abstract

Evolutions of oceanic and atmospheric anomalies in the equatorial Pacific during four strong El Niños (1982/83, 1991/92, 1997/98, and 2015/16) since 1979 are compared. The contributions of the atmosphere–ocean coupling to El Niño–associated sea surface temperature anomalies (SSTA) are identified and their association with low-level winds as well as different time-scale variations is examined. Although overall SSTA in the central and eastern equatorial Pacific is strongest and comparable in the 1997/98 and 2015/16 El Niños, the associated subsurface ocean temperature as well as deep convection and surface wind stress anomalies in the central and eastern equatorial Pacific are weaker during 2015/16 than that during 1997/98. That may be associated with a variation of the wind–SST and wind–thermocline interactions. Both the wind–SST and wind–thermocline interactions play a less important role during 2015/16 than during 1997/98. Such differences are associated with the differences of the low-level westerly wind as well as the contribution of different time-scale variations in different events. Similar to the interannual time-scale variation, the intraseasonal–interseasonal time-scale component always has positive contributions to the intensity of all four strong El Niños. Interestingly, the role of the interdecadal-trend time-scale component varies with event. The contribution is negligible during the 1982/83 El Niño, negative during the 1997/98 El Niño, and positive during the 1991/92 and 2015/16 El Niños. Thus, in addition to the atmosphere–ocean coupling at intraseasonal to interannual time scales, interdecadal and longer time-scale variations may play an important and sometimes crucial role in determining the intensity of El Niño.

Open access
Xiaofan Li, Zeng-Zhen Hu, and Bohua Huang

Abstract

Based on observational data, this work examines the multi-time-scale feature of the sea surface temperature (SST) variability averaged in the whole North Atlantic Ocean (to be referred to as NASST), as well as its time-scale-dependent connections with El Niño–Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). Traditionally, the NASST index is used to characterize the SST trend and multidecadal variability in the North Atlantic. This study found that superimposed on a prominent long-term trend, NASST is nonnegligible at subannual and interannual time scales, compared with that at decadal to multidecadal time scales. Spatially, the interannual variation of NASST is characterized by a horseshoe-like pattern of the SST anomaly (SSTA) in the North Atlantic. It is mainly a lagged response to ENSO through the atmospheric bridge, and NAO plays a secondary role. At the subannual time scale, both ENSO and NAO play a role in generating the fluctuations of NASST and a horseshoe-like pattern in the North Atlantic. Nevertheless, both the ENSO- and NAO-driven variations only explain a small fraction of the variances in both the interannual and subannual time scales. Thus, other factors unrelated to ENSO or NAO may play a more important role. The associated thermodynamical processes are similar at the two time scales; however, the dynamical processes have a significant contribution to the subannual component, but not to the interannual component. Thus, the SSTA averaged in the North Atlantic as a whole varies at different time scales and is associated with different mechanisms.

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Shouting Gao, Yushu Zhou, and Xiaofan Li

Abstract

Effects of diurnal variations on tropical heat and water vapor equilibrium states are investigated based on hourly data from two-dimensional cloud-resolving simulations. The model is integrated for 40 days and the simulations reach equilibrium states in all experiments. The simulation with a time-invariant solar zenith angle produces a colder and drier equilibrium state than does the simulation with a diurnally varied solar zenith angle. The simulation with a diurnally varied sea surface temperature generates a colder equilibrium state than does the simulation with a time-invariant sea surface temperature. Mass-weighted mean temperature and precipitable water budgets are analyzed to explain the thermodynamic differences. The simulation with the time-invariant solar zenith angle produces less solar heating, more condensation, and consumes more moisture than the simulation with the diurnally varied solar zenith angle. The simulation with the diurnally varied sea surface temperature produces a colder temperature through less latent heating and more IR cooling than the simulation with the time-invariant sea surface temperature.

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Bin Wang, Xiaofan Li, and Liguang Wu

Abstract

The impacts of linear environmental shears on beta drift direction are assessed through numerical experiments with a single-layer, primitive equation model. It is found that cyclonic (anticyclonic) shears turn the beta drift more westward (northward) in the Northern Hemisphere. In addition, the longitudinal shear of meridional flows (∂V/∂x) is much more effective than the meridional shear of zonal flows (∂U/∂y) in deflection of the beta drift.

A theoretical model, the beta gyre dynamic system, describing evolution of the beta gyre amplitude and phase angle is advanced to interpret the numerical model results. In this model, the nonlinear energy transfer from the beta gyres to the primary vortex and higher asymmetric modes was partially parameterized by linear damping. The semi-empirical theory predicts that 1) beta drift direction is independent of the planetary vorticity gradient; 2) in a quiescent environment, the drift angle is primarily determined by the outer azimuthal flows of the vortex; and 3) in a sheared environmental flow, the deflection of beta drift induced by environmental shears depends mainly on the longitudinal shear of meridional flows. The authors show that the environmental shear changes beta drift angle by advection of beta gyre vorticity and planetary vorticity, which affects beta gyre orientation.

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Huiyan Xu, Guoqing Zhai, and Xiaofan Li

Abstract

In this study, the WRF Model is used to simulate the torrential rainfall of Typhoon Fitow (2013) over coastal areas of east China during its landfall. Data from the innermost model domain are used to trace trajectories of particles in three major 24-h accumulated rainfall centers using the Lagrangian flexible particle dispersion model (FLEXPART). Surface rainfall budgets and cloud microphysical budgets as well as precipitation efficiency are analyzed along the particles’ trajectories. The rainfall centers with high precipitation efficiency are associated with water vapor convergence, condensation, accretion of cloud water by raindrops, and raindrop loss/convergence. The raindrop loss/convergence over rainfall centers is supported by the raindrop gain/divergence over the areas adjacent to rainfall centers. Precipitation efficiency is mainly determined by hydrometeor loss/convergence. Hydrometeor loss/convergence corresponds to the hydrometeor flux convergence, which may be related to the increased vertical advection of hydrometeors in response to the upward motions and upward decrease of hydrometeors, whereas hydrometeor gain/divergence corresponds to the reduction in hydrometeor flux convergence, which may be associated with the decreased horizontal advection of hydrometeors in response to the zonal decrease in hydrometeors and easterly winds and the meridional increase in hydrometeors and southerly winds. The water vapor convergence and associated condensation do not show consistent relationships with orographic lifting all the time.

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