ISSN:0035-9009    

DOI:10.1002/qj.49711347502

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractDuring the 1978 JASIN experiment radiosonde and tethered balloon measurements were performed from theR. V. Meteor.On 1 September, two cloud layers (cumulus below stratocumulus) were observed below an inversion at about 900m. Such a boundary layer situation occurred relatively often during the experiment. Cumulus clouds were not present all the time but alternately appeared and disappeared. When they occurred on 1 September, a relatively large vertical shear (about 0.006 s−1) together with an inflection point in the profile of the wind component normal to the mean wind direction was observed in the cumulus cloud layer. the airsea temperature difference was small (δT= −0.5 K) and there was a relatively large dynamical contribution to cloud generation. This may suggest the idea of an interaction between large‐scale processes generating a favourable mean wind shear profile and dynamically forced convection.Significant vertical transport of heat and momentum in the cloud layer occurred only in connection with cumulus clouds. Although in parts of some clouds the transports were locally as large as several 100W m−2and 1N m−2, there were only a few events with such large excursions so that the transports averaged over the whole field of cumulus clouds were small.The relative air flow in and around active cumulus clouds was characterized by updraughts within the cloud and downdraughts at the cloud edges. the thermodynamic properties of updraughts and downdraughts are

ISSN:0035-9009    

DOI:10.1002/qj.49711347503

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractA one‐dimensional numerical model designed to study the formation and growth of radiation fog is described. the realism of the model in simulating the formation of fog is assessed by critical comparisons with detailed observations made on two nights. the comparisons show that the model is unable to reproduce the very light winds which occur near the surface in stable conditions. As a consequence the formation of radiation fog is inhibited in the model unless the afternoon relative humidity is very high. It is suggested that the discrepancy is caused by the use of a turbulence formulation tuned to match observations over an ideally flat site. the influence of turbulence, radiative cooling, advection and frost deposition on the formation of fog during the two nights is also discusse

ISSN:0035-9009    

DOI:10.1002/qj.49711347504

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractA new model has been developed to investigate turbulence in flows over two‐dimensional hills: the von Mises transformation was applied in mean momentum equations, and turbulence equations (second‐order‐closure type) were solved in a coordinate system aligned with streamlines; the hill perturbation pressure was calculated by means of potential flow theory. the model predictions were compared with some experimental results from the Askervien Hill project with good agreement. It was found that apart from rapid distortion the curvature of streamlines has an important dynamical effect on turbulence. the curvature effect is shown to be particularly pronounced in the vicinity of the inner layer height on the hill top, where it attenuates turbulence. Overall, the salient features of the hill top turbulence profile were successfully explained by analysing the turbulence conservation equations in streamline coordi

ISSN:0035-9009    

DOI:10.1002/qj.49711347505

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractA formula for the relative diffusion of a puff is proposed in which the rate of growth is related to common, one‐dimensional velocity spectra obtainable, for instance, from direct measurements in the atmosphere. Often, an equation of this type is in demand for modelling of instant or short‐term release of potentially harmful gases or smoke, for example, in connection with a puff diffusion model.A simple expression like Taylor's equation for single‐particle diffusion is not obtainable in general for relative diffusion. Therefore, the present approach is based on a kinematic‐statistical model in which a Gaussian approximation has been applied to the relative displacement process of the fluid particles. This is along lines suggested previously, where the expansion of a Gaussian puff has been related to the kinematic energy spectrum in three‐dimensional, isotropic turbulence. Here, however, we attempt to relate the growth rate directly to the one‐dimensional velocity spectra (or their corresponding correlations) most commonly available in the atmosphere. In particular, this is of value for the surface layer, where diffusions in the horizontal and vertical directions are different (anisotropy) due to the presence of the ground.Experimental evaluation is based on a series of smoke release experiments carried out in the surface layer over homogeneous terrain in Denmark. Solutions of the puff growth rate equation with the measured correlations are found to compare well with the experimental data of cross‐wind relative diffusion over the limited scale of th

ISSN:0035-9009    

DOI:10.1002/qj.49711347506

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractTo study the dispersion of scalar trace constituents such as heat, water vapour or CO2from extensive sources within vegetation canopies, a Lagrangian approach is essential because of the need to account for the influence of travel time (as a fraction of the Lagrangian time scale) on the dispersion properties of marked fluid particles. A simple, analytic Lagrangian theory is developed for a canopy in which the turbulence is homogeneous. For a given scalar source distribution, predictions are obtained for mean scalar concentration, vertical flux, terms in the balance equation for vertical flux, and vertical diffusivities at first, second and third order. the predictions agree remarkably well with measurements of dispersion from an elevated plane source within a model plant canopy in a wind tunnel. the theory also explains the countergradient heat and CO2fluxes observed in real forest canopies.

ISSN:0035-9009    

DOI:10.1002/qj.49711347507

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractThe two‐dimensional dynamics of multicell and unicell squall lines are examined using a numerical model. Both types feature a convective jump updraught with extensive, deep, rearward‐extending anvils. the multicell case contains travelling convection cells and is unsteady but persistent, whereas the unicell case is quasi‐steady.In the multicell case, an extensive density current is produced by the evaporation and drag of rain falling from a succession of convection cells which travel rearwards relative to the density current head. the circulation within the density current is complex and consists of three mutually interactive branches: a rotor, a cold downdraught, and a subsidence region.In the unicell case, a density current is not produced and the updraught is forced by a propagating wave of elevation, generated as an interaction between the updraught, water loading and evaporation in a very localized area.The squall line structure is compared with other models and it is shown that certain propagating two‐dimensional tropical and mid‐latitude squall lines are dynamically similar, their detailed structure being controlled by the inflow wind profile.The multicell case is compared with a quasi‐two‐dimensional West African squall line which occurred on 22 June 1981 during the Convection Profonde Tropicale (COPT 81) experiment performed in the northern part of the Ivory Coast. Good agreement is found between the model and the observations, and hence the dynamics of the observed squall line is quantified. the unicell case, in which a density current and conventional downdraughts are absent, has yet to be identified in atmospheric

ISSN:0035-9009    

DOI:10.1002/qj.49711347508

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractWe explore the simultaneous use of potential radius and potential temperature coordinates in the Eliassen balanced vortex model of tropical cyclones. This results in a simple flux form for the equation governing the inverse potential vorticity. the fluxes are accomplished byRand θ, which are given in terms of the angular momentum and heat sources. If the sources are specified functions ofRand θ, analytic solutions for the time evolution of the potential vorticity distribution can be obtained. the solutions illustrate how latent heat release generates potential vorticity at low levels, destroys it at upper levels, and through vertical advection causes a deepening of the low‐level potential vorticity maximum and a pinching off of the upper‐level potential vorticity minimum. to recover the tangential wind and mass fields from the known potential vorticity, one must solve a second‐order nonlinear partial differential equation. Numerical solutions of this equation result in tangential wind fields which are similar in many ways to observed tropical c

ISSN:0035-9009    

DOI:10.1002/qj.49711347509

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractExisting semi‐geostrophic theories of flow over a two‐dimensional orographic ridge are extended using a geometrical interpretation of the equations. the method of solution allows information to be gained even when there are local violations of the scaling assumptions. the results show that the occurrence of many observed aspects of the flow, such as downslope winds, drag and flow separation, can be predicted. Quantitative assessment requires a three‐dimensional version of the

ISSN:0035-9009    

DOI:10.1002/qj.49711347510

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractSouthern hemisphere blocking during the Global Weather Experiment has been studied. Two different data sets, and two different methods to identify blocking have been used. One data set consists of maps analysed at the Australian Bureau of Meteorology and the other of FGGE level III‐b data. the results show that one method, which tends to emphasize blocks accompanied by cut‐off lows, is more sensitive to data accuracy than the other, which is based on a count of persistent positive anomalies exceeding a certain threshold va

ISSN:0035-9009    

DOI:10.1002/qj.49711347511

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY