摘要:

AbstractThe classical problem of the response of a balanced, axisymmetric vortex to thermal and mechanical forcing is re‐examined, paying special attention to the lower boundary condition. The correct condition is DΦ/Dt = 0, where Φ is the geopotential and D/Dt the material derivative, which explicitly accounts for a mass redistribution as part of the mean‐flow response. This redistribution is neglected when using the boundary condition Dp/Dt = 0, which has conventionally been applied in this problem. It is shown that applying the incorrect boundary condition, and thereby ignoring the surface pressure change, leads to a zonal wind acceleration δū/δt that is too strong, especially near the surface. The effect is significant for planetary‐scale forcing even when applied at tropopause level.A comparison is made between the mean‐flow evolution in a baroclinic life‐cycle, as simulated in a fully nonlinear, primitive‐equation model, and that predicted by using the simulated eddy fluxes in the zonally‐symmetric response problem. Use of the correct lower boundary condition is shown to lead to

ISSN:0035-9009    

DOI:10.1002/qj.49711549002

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractA two‐dimensional mesoscale cloud model is used to simulate an ensemble of deep tropical convective clouds using GATE data. Random heating perturbations near the sea surface initiate the development of a population of clouds which grow under the influence of specified large‐scale, radiative and surface forcing. Budgets of convective heating and moistening are presented and the model found to be a reasonable representation of the atmosphere.The model is used to investigate the parametrization of cumulus convection in large‐scale models. Equations governing the effect of convection upon the large‐scale atmosphere are derived in full and compared with approximate forms used by previous workers. These forms are the basis of many parametrization schemes. Simple representations of cumulus heating (Q1), cumulus moistening (Q2), and dynamical transports for use in parametrization schemes in large‐scale atmospheric models are studied. The importance of shallow convection in determining the vertical structure of Q2 is recognized. The dependence of convective rainfall upon atmospheric structure and large‐scale processes (the closure problem) is investigated.The importance of downdraughts is emphasized and difficulties in the calculation of Q2 identified. Study of parametric forms for cumulus momentum transports indicates that the ‘cumulus friction’ hypothesis is inadequate. It is suggested that present closure schemes are

ISSN:0035-9009    

DOI:10.1002/qj.49711549003

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractThe acoustically‐filtered, nonhydrostatic flow model formulated by M. J. Miller and R. P. Pearce in 1974 may be modified so as to remove dependence on a certain thermodynamic reference state. The modified model seems appropriate for mesoscale integrations. It implies an elliptic diagnostic equation for the height field, a satisfactory energy equation and an analogue of Ertel's potential vorticity theorem. These properties are also possessed by Miller and Pearce's original model, but in different forms (which have been obtained previously by various authors

ISSN:0035-9009    

DOI:10.1002/qj.49711549004

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractA modified set of ECMWF FGGE level III‐b analyses is used to examine the four‐dimensional structure of the region containing the South Pacific convergence zone (SPCZ) during a period when two cyclones form in the tropics and propagate south‐eastward along the SPCZ into middle latitudes. During this period. 10–18 January 1979, the atmosphere is characterized by a highly baroclinic state which, together with the existence of an upper tropospheric subtropical jet, supports the growth and development of the observed cyclone activity. In a previous study, it was found that convective activity was prominent during the life cycle of each cyclone.The goal of the present study is to examine the significance of kinematic and thermodynamic processes in maintaining the upper baroclinic region coincident with the subtropical jet. This baroclinic zone lies just poleward of the warm core associated with the convectively‐active SPCZ. A partitioned form of the fronto‐genetical function is used to diagnose the adiabatic and diabatic contributions to the maintenance of baroclinicity in the vicinity of the SPCZ. A major balance is found between the frontogenetical contribution by differential diabatic heating and the opposing diabatic tilting processes. In contrast to the results of most mid‐latitude cyclone studies, the adiabatic contributions from the deformation and tilting terms appear to be of lesser importance. Also, the contribution from the deformation associated with the horizontal branch of the diabatically forced circulation is found to be relatively unimportant. The latter finding is in disagreement with some previously reported numerical results concerned with middle latitude moist frontogenesis. Possible reasons for these differences

ISSN:0035-9009    

DOI:10.1002/qj.49711549005

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractQuantitative theoretical estimates are made of the reflection and radiative damping of the Kelvin and Rossby‐gravity (RG) waves propagating upward in the 20–90 km altitude region. An atmosphere with realistic temperature, wind and cooling rate coefficient values is approximated by a model of vertically stacked, thin, homogeneous layers. The amplitude reflection coefficients are shown for different wave periods (T) and zonal wavenumbers (kx). Combining the attenuation due to radiative cooling with the growth due to density decrease, the net changes of wave amplitudes with height in the 20–90 km region are shown for various wave modes.For Kelvin waves the reflected wave energy is generally less than 15% of the incident wave energy, but the reflected wave amplitude can be 20%‐30% of the incident wave amplitude for many modes, at 20 km height. In contrast, many RG wave modes suffer strong or total reflection at various heights if they are not severely damped at lower heights. Calculations for January and July wind conditions reveal a good deal of sensitivity of RG wave reflection to wind conditions.Radiative damping is found to be severe in the mesosphere for Kelvin waves withT⩾ 6–8 days, depending on wind conditions; it is severe even in the lower stratosphere for RG waves withT>4 days. Considerable changes in the damping rates and wave amplitudes at various heights take place from January to July owing to wind changes. Kelvin waves in the 2–8‐day period range and RG waves in the 1.5‐3‐day range can propagate into the upper mesosphere/lower thermosphere without large attenuation, depending upon wind conditions.It is shown that our theoretical results can explain several observed characteristics of equatorial waves in the stratosphere and mesosphere, including the recent results on wave disturbances in satellite‐measured temperatures. Further modelling studies based on our methodology can lead to a good physical insight and quantitative interpretations of the SAO‐ and the Q

ISSN:0035-9009    

DOI:10.1002/qj.49711549006

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractObserved tropical intraseasonal oscillations exhibit a preferential zonal scale of wavenumber one or two and a baroclinic structure with largest flow field amplitude in the upper troposphere. Unstable interaction of boundary layer friction‐induced moisture convergence with condensational heating could cause the scale selection and result in the asymmetric baroclinic structure by means of coupling of vertical normal modes. A vertically continuous linear model describing two‐dimensional equatorial motion demonstrates this feedback mechanism and permits detailed study of vertical structure of unstable moist Kelvin wa

ISSN:0035-9009    

DOI:10.1002/qj.49711549007

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractThe Sawyer‐Eliassen (S‐E) equation for frontal circulation is extended to include the effects of eddy viscosity and small negative moist potential vorticity (MPV) under the condition of weak or, at least, not very strong frontogenetic forcing. When MPV is positive, viscosity can be neglected and this equation becomes the conventional S‐E equation. When forcing is absent and MPV is strongly negative, this equation degenerates into the equation of linear viscous conditional (moist) symmetric instability (CSI). By using an idealized distribution of frontogenetic forcing which attenuates exponentially away from the position of maximum forcing, the extended S‐E equation is solved both analytically and numerically for frontal circulations where frontogenetic forcing and negative MPV coexist (but the negative MPV is not so strong as to initiate viscous CSI). The solutions contain slantwise banded substructures on the warm side of the region of maximum forcing. The intensity, structure and scale of the bands are controlled by the competition between the frontogenetic forcing, negative MPV and eddy viscosity with the following characteristics:(a)For a horizontally concentrated forcing whose attenuation length Lb(i.e. the length over which the forcing is reduced by a factor of eπ= 23.1 away from the region of maximum forcing) is less thanLs2 (whereLsis the semi‐geostrophic Rossby radius of deformation), the substructure is single‐banded. The band occurs even before MPV becomes negative. As MPV decreases and becomes negative, the band becomes narrow and intense.(b)For a widespread forcing whose attenuation lengthLbis larger than Ls, the substructure is multi‐banded. The multiple bands may occur only if MPV is negative enough, and the first occurring multi‐bands are characterized by wide bands of moist ascent with narrow and weak dry descents embedded between the moist bands. As MPV becomes further negative, the moist bands become narrow, intense and widely spaced.(c)For a moderately widespread forcing whose attenuation length Lb is between Ls/2 and Ls, the substructure changes from wide and weak multiple bands to a narrow and strong single band as MPV becomes increasingly negative.(d)As the coefficient of eddy viscosity increases within a moderately wide range (5‐100m2s−1), the band (or bands) become weak and wide.The theoretical findings are explained physically and compar

ISSN:0035-9009    

DOI:10.1002/qj.49711549008

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractUsing the synoptic ALPEX (Alpine Experiment) data, a diagnostic study of vertical SSMF (subsynoptic momentum flux) is presented. The errors in the data are partially eliminated by the diagnostic model. It is found that the vertical SSMF has the order of magnitude of 1 N/m2in the troposphere with its maximum at about 600 to 400hPa. In the synoptic momentum budgets, SSMF is closely related to ageostrophic components. Systematic distributions and changes with time of the SSFM in a cyclogenesis process are found. Slant convective activity is a possible mechanism responsible for the intensive SSMF found in the cold area of the cyclone.

ISSN:0035-9009    

DOI:10.1002/qj.49711549009

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractIn deep water, there is a growing observational consensus that supports an ƒ4variation for the Kitaigorodskii subrange of the high‐frequency energy spectra. We therefore re‐examine and extend the TMA (Texel Marsen Arsloe) similarity and concepts for shallow water waves, which were originally derived for an ƒ5Phillips subrange. The similarity for an ƒ4a subrange is found to closely resemble that of the ƒ5subrange. The former exists when nonlinear transfer due to wave‐wave interactions dominates over other source terms, such as wind input, in the energy balance.Starting with a standard second generation deep water coupled discrete spectral wave model, we implement the TMA similarity. To evaluate our modifications, we perform the real and synthetic shallow water tests of the recent SWIM study. Thus we intercompare our model with HYPAS, an analogously modified deep water coupled hybrid model, and with other wave models which were not based on the TMA similarity. Our model and HYPAS are found to behave quite similarly in the sloping and flat‐bottom SWIM tests, compared to the other wave models of the SWIM intercomparison. The more difficult and complex North Sea SWIM hindcast extends this property, in s

ISSN:0035-9009    

DOI:10.1002/qj.49711549010

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractObservations were made of a thunderstorm over Langmuir Laboratory for Atmospheric Research on 2 August 1984. Four Doppler radars were closely spaced, allowing observations to be taken at high elevation angles. As a result, the vertical component of the precipitation particle velocity was accurately measured. In conjunction with the vertical air velocity, obtained by integration of the continuity equation, the particle terminal velocity field was computed.Backward integration of particle trajectories showed that most precipitation embryos originated near the tops of thunderstorm precursor clouds when these cloud tops were 7–8 km above sea level. Early and mature phase precipitation grew by accretion at levels below 7 km, and resulted in relatively high density graupel or hail. Late phase precipitation underwent growth mainly above 8 km, and was sufficiently small in diameter and low in density to undergo rapid melting at the freezing leve

ISSN:0035-9009    

DOI:10.1002/qj.49711549011

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY