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1.
INSTABILITY PROPERTIES OF ATMOSPHERIC DISTURBANCES   总被引：1，自引：0，他引：1       下载免费PDF全文
There are some basic problems in previous theoretical studies of baroclinic instability.The derived criticalbaroclinity was considerably lower than the time averaged mean meridional temperature gradient,especially in the lowertroposphere.Also,the linear mechanism of baroclinic disturbance development which is noted restricted by the criticalbaroclinity was not studied sufficiently.The realistic critical baroclinity and disturbance development are discussed inthis study.It will be shown that the critical condition of instability and typical time and space scales of disturbances de-pend on three-dimensional structures of atmosphere and sphericity of the earth,other than the horizontal temperaturegradient alone.The variant behaviour of atmospheric disturbances depends highly on their specific scales that may bedescribed by the same theoretical model.Thus,there would be no substantial differences in the basic instability mecha-nism of many disturbances including the polar lows and explosive cyclones.  相似文献   

2.
Based on a non-frictional and non-divergent nonlinear barotropic vorticity equation and its solutions oftravelling waves,the criteria for linear and nonlinear barotropic instability are gained respectively at an equilibriumpoint of the equation on a phase plane.The linear and nonlinear analytical solutions to instability waves arealso found.The computational results show that if their amplitudes are equal at the initial time,the amplitudeincrements of nonlinear instable barotropic wave are always less than those of linear instable barotropic wave.The nonlinear effects can slow down the exponential growth of linear instability.The time needed for makingthe amplitude double that of initial time by instabilities,is about 6h for linear instability and about 18h fornonlinear instability,the latter is in agreement with the observations in the real atmosphere.  相似文献   

3.
This is the second part of “Mesoscale Instability of a Baroclinic Basic Flow” which discusses theinstability of a basic flow against mesoscale perturbations of transversal type.A bi-mode instability spectrum is obtained by generalizing the Eady model to ageostrophic regime inan f-plane:Eady modes present at the synoptic and subsynoptic scales,while the ageostrophic baroclinicmesoscale modes present at the inertial scales of a few tens to hundreds kilometers.The mesoscale modeis featured by an asymmetric "eat eyes" pattern in the vertical cross section and by an alternative distri-bution of divergence and vorticity in the horizontal direction.The growth rates of the mesoscale modesare about four times larger than those of Eady modes in magnitudes for the same wind profile.The majorenergy source for development both Eady mode and mesoscale mode is the baroclinic available energy storedin the rotational basic flow.  相似文献   

4.
This is the first part of“Mesoscale Instability of a Baroclinic Basic Flow”,which deals with the stabilityof a baroclinic flow against mesoscale perturbations parallel to it and the possibility for occurrence of abandwise mesoscale disturbance along with its effects on triggering and organizing deep convections in astratified atmosphere of weak static stability.The atmospheric perturbation equations with anelastic assumption are formed in the way that theyare suitable to be solved by generalized matrix method and by shooting method.The mesoscaIe instabilityspectra and the structures of the characteristic perturbations are calculated for linear,hypertangent profilesand realistic wind profile sounded before a squall line event.By energy analysis the effects of baroclinityof the basic flow on generating such kind of ageostrophic inertial convection are also discussed.  相似文献   

5.
This is the first part of "Mesoscale Instability of a Baroclinic Basic Flow",which deals with the stability of a baroclinic flow against mesoscale perturbations parallel to it and the possibility for occurrence of a bandwise mesoscale disturbance along with its effects on triggering and organizing deep convections in a stratified atmosphere of weak static stability.The atmospheric perturbation equations with anelastic assumption are formed in the way that they are suitable to be solved by generalized matrix method and by shooting method.The mesoscaIe instability spectra and the structures of the characteristic perturbations are calculated for linear,hypertangent profiles and realistic wind profile sounded before a squall line event.By energy analysis the effects of baroclinity of the basic flow on generating such kind of ageostrophic inertial convection are also discussed.  相似文献   

6.
This paper describes a geopotential thickness difference method for computing instability energy E. EP1P2 =g0 ( ΔHsP1P2-ΔHP1P2 , where ΔHP1P2 is the geopotential thickness of P1-P2 level; ΔHsP1P2 is called adia-batic geopotential thickness, based on which a computational method for high resolution of instability energy is proposed. E(x,y)≈g0(A(x,y) - B(x,y)), where A is interpolating polynomial of ΔHSP2P2 and it is afunction ofθe, of surface observing stations (x, y); B(x,y) is the thickness over corresponding stations (x, y) obtained using surface fitting method. Therefore, data of METAR can be used by computer to produce hourly horizontal distribution chart of E of surface observing station density. With the result that the temporal and spatial resolution of stability analysis has been improved. Practical use has shown that this method is an effective tool for very short range forecast of severe convective storms.  相似文献   

7.
By using the linearized barotropic vorticity equation in polar coordinates the stability of pertur-bations on a large scale circular basic flow is transformed into a generalized eigenvalue problem,yielding the relationship between the growth rate of the amplitude of perturbations and the az-imuthal wave number. Then, numerical experiments whose integration time is 60 model hours areperformed in terms of a quasi-geostrophic barotropic model in Cartesian coordinates using the per-turbation stream function field of unstable mode superimposed on a strong and weak circular basicflows as the initial fields. The experimental results reveal that the amplitudes of the initial pertur-bations in the model atmosphere grow with time. The amplitude of the perturbations superimposedon the strong circular basic flow grows quicker and forms a spiral-band-like structure.  相似文献   

8.
A surface wind (seabreeze), thermally generated by differential sea surface temperature, is introduced to Gill-Clarke's model (1974) through wind stress for investigating the effects of seabreeze on coastal upwel-ling. A coupled air-sea system is treated as an eigenvalue problem. The solutions show that the thermally forced local winds break down the coastal Kelvin wave into three parts: small-scale (L<100 km) growing and stationary modes, mesoscale (100 km200 km) coastal Kelvin modes. The consistency of the length scale between the most growing mode predicted by this model and the observed cold/warm alternation pattern of surface water near the Peruvian Coast (around 15 °S) implies that seabreeze may play some role in coastal upwelling.  相似文献   

9.
The concept of wet bulb potential vorticity and SCAPE (slantwise convective available potentialenergy) is used to calculate the horizontal and vertical distribution of moist symmetric instabilityincluding convective symmetric instability (CSI) and local symmetric instability (LSI) in the processof a storm snowfall in North China.The potential contribution of moist symmetric instability to anarrow storm snowfall belt occurring in the Hetao and Lindong,Linxi region of Inner Mongolia andthe relationship between moist symmetric instability and direction of the basic flow,wind shear andmoisture are discussed.It is found that the strong snowfall belt is almost parallel with negative valuearea of wet bulb potential vorticity at low levels in the vicinity of snow area.And the dynamicalmechanism of the strong snowfall center in the Lindong,Linxi region is different from that in theHetao region.The former is induced by frontogenetical forcing with weak symmetric instability in thewarm section of frontal area and the latter triggered by obvious moist symmetric instability.  相似文献   

10.
Using the wind data at mountain stations, gravity waves on the axis of low level jet and their relation to rain clusters have been studied by means of the spectrum method. It is found that the spectrum densities of both wind and rainfall reach their maximums at the period of 8 h and the moduli between wind and rainfall also reach the maximum in that range. Both observation and theoretical study give the same conclusion that the unstable development of gravity waves depends not only on the atmospheric stratification but also on the vertical shear of the environmental wind. The appearencc of the jet-shape vertical profile of wind is favourable for the unstable development of gravity waves. The lower the height of the axis, the larger the growth rate of amplitude.  相似文献   

11.
In this paper,the simultaneous effects of boundary layer and topography on the instability of Eady waveare investigated by using a new parameterization of the vertical velocity at the top of PBL and the influencesof the stratification of the PBL,roughness and the slope of terrain are shown.Furthermore,the effects of theboundary layer friction and topography on generalized Eady wave are also investigated.  相似文献   

12.
By using the β-plane, two-layer quasi-geostrophic baroclinic model, this paper discusses the baroclinic instability problem concerning the effects of friction and heating of convective condensation. By linear analysis it is shown that the combination of β effect, friction and convective heating brings about the asymmetric phenomenon of margin curves. The convective heating plays a role in the increased baroclinic instability. As the heating increases (m*→1), the short wave cutoff can increase infinitely. Besides, the numerical integration of the finite-amplitude equations shows that the trajectory on the phase plane oscillates periodically in the case of non-dissipation. When the friction dissipation is considered, the trajectory of phase decays and oscillates to the equilibrium. The stronger convective heating not only makes the unstable wave length shorter and the amplitude of the equilibrium decrease, but also makes multiple equilibrium into single equilibrium.  相似文献   

13.
Under the influence of the synoptic-scale environmental flows, the implicit nonthermal windvorticity imbalance with ξ_T>0 may arise when the change in the vertical shear vorticity of the flows,which is caused by the change of vorticity advection with altitude, is larger than that in the thermalwind vorticity of temperature fields, which is caused by the horizontal inhomogeneity of temperatureadvection. This kind of implicit imbalance will cause ascending motion to develop in the adjustmentprocess. If the static stratification is horizontally inhomogeneous with the presence of its mesoscalehigh-value regions or the conditionally unstable region where moisture can be fully provided, thedevelopment of mesoscale disturbance with scale L_m may result from the coupling of the ascendingmotion on synoptic scale L_1 with the parameter field of static stratification on mesoscale L_2. The disturbance possesses such a vertical structure as ascending motion in the center, horizontaldivergence in the upper and convergence in the lower atmosphere. This developmental process of insta-bility may be one of the most important processes responsible for the growing of heavy rains in summerhalf year in China.  相似文献   

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