摘要:

AbstractThe conventional formulation of the wave‐CISK mechanism involves a quasi‐equilibrium assumption whereby the instantaneous large‐scale diabatic heating that is induced by cloud‐scale motions is assumed to be related to the instantaneous distribution of the large‐scale flow, and in particular to the low‐level moisture convergence of the large‐scale flow. Thus in this context the deep convective clouds of the tropical atmosphere are viewed as instantaneous processors of the large‐scale moisture supply.Herein a conventional, inviscid wave‐CISK model is developed and an examination undertaken of the properties of the analytically determined solutions. The quasi‐equilibrium assumption is then discarded by incorporating into the CISK model's formulation a simple time‐dependent model of the dynamics of a cumulus cloud ensemble. The latter model has a finite adjustment time (TR) to changes in the large‐scale forcing field. Hence the apparent diabatic heat source experienced by the large‐scale fields becomes a function of the time history of that same forcing field.Neutral solutions of the refined wave‐CISK model are derived as functions of the diabatic heating amplitude and the adjustment time. These solutions illustrate the frequency‐selective stabilizing effect of the hypothesized cloud‐scale dynamics. Again, the marginally unstable waves of the refined model are shown to exhibit properties that overcome some of the debilitating features of conventional wave‐CISK. These solutions also reproduce some of the observed features of tropical wave motions. In particular, for Tr ∼ 12 h and with a physically reasonable value of the cumulus diabatic heating amplitude, it is shown that all the inertia‐gravity modes are stable, and that the most unstable Kelvin wave, mixed Rossby‐gravity wave and Rossby (n = 1 and n = 2) waves have finite preferred spatial length scales corresponding respectively to zonal wavenumbers (m) of 2, 4, 6 and 8. The periods of these four waves correspond approximately to 10, 4/1/2 12/1/2 and 15 days but the growth rates are comparatively small. The agreement with tropical wave data is not unreasonable when allowance is made for the Doppler shift of the Rossby waves.Also the vertical structures of these preferred modes are different: the Kelvin and mixed Rossby‐gravity waves are associated respectively with the shortest and longest vertical wavelengths, and the meridional wind field of the Rossby wave (n = 1) has a maximum in both the upper and lower troposphere. The degree of mutual compensation in the model solutions between diabatic heating due to cumulus clouds and cooling due to large‐scale ascent is not a

ISSN:0035-9009    

DOI:10.1002/qj.49710544402

出版商:John Wiley&Sons, Ltd    

年代:1979

数据来源: WILEY

摘要:

AbstractA mixed layer ocean model, which includes the effects of advection and salinity, is coupled to an atmospheric general circulation model of the southern hemisphere. The response of the ocean model is compared with observations and shown to be successful in simulating the pattern of heating occurring during the summer season, in subtropical and middle latitudes.The sensitivity of the model to changes in atmospheric circulation, advection and salinity is assessed.

ISSN:0035-9009    

DOI:10.1002/qj.49710544403

出版商:John Wiley&Sons, Ltd    

年代:1979

数据来源: WILEY

摘要:

AbstractIn this paper we consider 15 cases of line convection at ana‐cold fronts which have been observed by radar. Although line convection can be a simple two‐dimensional feature, it is usually broken up into line elements having lengths from a few to many tens of kilometres separated by somewhat smaller gaps. The passage of a line element is accompanied by a short burst of heavy precipitation, a temperature drop of about 2degC, a sudden veer and decrease in wind speed, and a pressure jump of 1 to 2 mb. If a gap between line elements passes over a station, however, the changes can occur gradually over a period of up to an hour. Despite the variable size of the line elements, they are found to share common characteristics. They are always oriented slightly clockwise with respect to the synoptic cold front. They all move with the same velocity as their neighbours on a given front, with a component parallel to the front in the direction of the strong low‐level flow which occurs ahead of the front. Moreover, the line elements tend to travel in a fairly predictable manner, largely unaffected by topography and with the major features having lifetimes of several

ISSN:0035-9009    

DOI:10.1002/qj.49710544404

出版商:John Wiley&Sons, Ltd    

年代:1979

数据来源: WILEY

摘要:

AbstractA two‐dimensional theory of Jackson and Hunt for turbulent flow over a shallow ridge is extended to three‐dimensional topography. The results are compared both with theoretical results for a ridge and with surface wind observations from a nearly circular isolated hill. Agreement between theory and observations is encourag

ISSN:0035-9009    

DOI:10.1002/qj.49710544405

出版商:John Wiley&Sons, Ltd    

年代:1979

数据来源: WILEY

摘要:

AbstractDuring August 1974 the Co‐operative Atmospheric Boundary Layer Experiment (CABLE) took place in which several experimental groups combined resources to study spatial and temporal characteristics of the convective atmospheric boundary layer over a fairly uniform area of SE England. The results presented concentrate on the four‐day period 19–22 August. Mesoscale variations in boundary layer development and the influence of synoptic scale features on boundary layer structure are described and discussed. Profiles of sensible heat flux, obtained by several independent methods, are examined in some detail and are shown to imply significant heating due to absorption of solar radi

ISSN:0035-9009    

DOI:10.1002/qj.49710544406

出版商:John Wiley&Sons, Ltd    

年代:1979

数据来源: WILEY

摘要:

AbstractA Lagrangian statistical analysis is proposed for the motions of a single particle and the growth of a small cloud released from the ground in a turbulent boundary layer, essentially by considering thefluctuatingas well as themeanvertical Lagrangian velocity, by noting that its integral time scale is much smaller than the travel time of a particle from the source and by estimating (quite conventionally) the Lagrangian statistics in terms of the measured Eulerian statistics of atmospheric surface layer turbulence, in neutrally stable conditions.Comparing the results with a 1968 analysis by P. C. Chatwin based on a diffusion equation, the same value is obtained for the dispersion of the vertical displacement of a particle as a function of its mean travel time; however, a small but physically significant difference is predicted for the growth of a cloud released at ground level.The validated diffusion equation is then used to relate the concentration of a cloud to that of a continuous plume. It is shown that the index,s, used in the exponential formula exp(–Azs) to describe observations of the concentration in a plume, varies slowly with distance downwind of a source. Diffusion measurements in the atmospheric boundary layer and in wind tunnels are shown by the theory to be consistent with turbulence measurements. The theory suggests the kind of further turbulence and diffusion measurements that ought to be mad

ISSN:0035-9009    

DOI:10.1002/qj.49710544407

出版商:John Wiley&Sons, Ltd    

年代:1979

数据来源: WILEY

摘要:

AbstractEighteen rainfall series within a latitude belt 28°08′S to 30°25′S are inspected for relationships with Pittock'sLindex for the latitude of the subtropical high pressure belt (eastern Australia), and the southern oscillation index. Principal components analysis is used to reduce the dimensions of each of the three data sets, and the components for the pressure systems are used as independent variables in a step‐wise regression analysis; rainfall components are taken as the dependent variables. Useful relationships exist between rainfall and Pittock'sL, but there is no evidence to suggest that the same situation applies for the southern oscillation index. The results agree with those recently obtained for Mauritius by M. D.

ISSN:0035-9009    

DOI:10.1002/qj.49710544408

出版商:John Wiley&Sons, Ltd    

年代:1979

数据来源: WILEY

摘要:

AbstractHigh speed photography of 48 natural hailstones falling in a hailshaft near the ground has led to new results concerning the vertical, horizontal and tumbling motions of quasi‐conical hail in the size range 0.5 to 1 cm. The relation vz = 12.43 D1/2 max provides an estimate (±1.5ms−1) of the sea level fall speed of these hailstones, while deviations from this relation and variations in the horizontal velocity components indicate the possible influence of air turbulence, particle shape and density, and tumbling motion on the free‐fall hailstone kinematics. At least one third of the hailstones were tumbling or oscillating while f

ISSN:0035-9009    

DOI:10.1002/qj.49710544409

出版商:John Wiley&Sons, Ltd    

年代:1979

数据来源: WILEY

摘要:

AbstractIn this paper a statistical estimator is used to retrieve vertical temperature profiles above an area for which autocorrelation matrices of the temperature profile have been computed for each season using radiosonde data. The temperature dependence of atmospheric transmittance has been taken into account. We find that in cloudless cases the retrieved vertical temperature profiles have an average r.m.s. error of 2–3.5 K, when compared with appropriate radiosonde measurements. We note that to achieve this rather satisfactory result it is not necessary to divide the atmosphere into more than 18 levels; and we will show that it is advantageous to use the total covariance matrices instead of only the diagonal elements. To obtain further comparisons between retrieved and radiosonde profiles, the analysed gridded radiance values have been used, and in this case the r.m.s. error is 2.5–3.

ISSN:0035-9009    

DOI:10.1002/qj.49710544410

出版商:John Wiley&Sons, Ltd    

年代:1979

数据来源: WILEY

摘要:

AbstractMotions at an entraining interface of a growing mixed layer are ideally partitioned into large eddies, such as plumes and thermals which act to distort the interface, and smaller scale turbulence responsible for entrainment through the interface.Influences of horizontal entrainment and pressure effects on the depth of penetration of large eddies into the stratified free flow are examined. This penetration plays an important role in the initiation of cumulus at the mixed layer top and modifies the turbulence energy balance and entrainment rate there. For example, previous models predict the entrainment rate to depend on the cube of the fundamental velocity scale at a stratified interface and linearly on this velocity scale with vanishing stratification (free entrainment). The present development, which includes specific roles of penetrating elements, suggests a possible intermediate quadratic regime corresponding to weak inversion strength.

ISSN:0035-9009    

DOI:10.1002/qj.49710544411

出版商:John Wiley&Sons, Ltd    

年代:1979

数据来源: WILEY