ISSN:0035-9009    

DOI:10.1002/qj.49710042502

出版商:John Wiley&Sons, Ltd    

年代:1974

数据来源: WILEY

摘要:

AbstractThe paper describes calculations of the development of droplet‐size spectra in cumulus clouds under the combined effects of condensation and coalescence. It is demonstrated that the concentration of condensation nuclei is a more important factor in controlling the onset of precipitation than is the updraught, at least for small clouds. It is shown that condensation can indirectly affect the growth rate of droplets larger than 25 μm radius by enhancing the growth of the smaller droplets which are then captured more efficiently by the larger ones. The overall result is to produce more rapid distortion of the droplet spectrum and more rapid growth of the largest drops than would occur by condensation and coalescence acting separately to produce an apparent barrier to the growth of droplets between 20 μm and 30 μm in radius. The onset of precipitation in continental clouds containing a high nucleus concentration depends rather critically on the values used for the collection efficiencies of droplets of radius<40 μm and may be considerably accelerated if the collection efficiencies are increased by small‐scale atmospheric tur

ISSN:0035-9009    

DOI:10.1002/qj.49710042503

出版商:John Wiley&Sons, Ltd    

年代:1974

数据来源: WILEY

摘要:

AbstractA stochastic model is developed which describes the increase in ice particle concentrations resulting from the splintering of supercooled drops on freezing. Analysis of the experimental evidence suggests that significant splintering occurs at least for drops with diameters in the range 50 to 200 μm. At timet= 0 one such drop freezes and ejectsrice splinters. Each of these splinters may be captured by another water drop in the same size range at any subsequent time, which in turn will freeze and eject a furtherrsplinters, and so on. Consideration of the probability that a particular capture will take place at a particular time yields an expression form(t), the estimated number of particles existing at timet,m(t)= (r2e(r‐1)t/τ− 1)/(r− 1),Where τ is the mean lifetime of an ice splinter. It is stressed that by considering the probability of a collision occurring at any time after the birth of a splinter, far higher estimates of the population are obtained than from a non‐stochastic model.Calculations show that the largest ice particle multiplication factors measured by Mossopet al.(1972), of order 104, can be produced in the available time if the value ofrfor the splintering size range is 5 or 6. Analysis of the experimental evidence suggests that such a value forri

ISSN:0035-9009    

DOI:10.1002/qj.49710042504

出版商:John Wiley&Sons, Ltd    

年代:1974

数据来源: WILEY

摘要:

AbstractA case study is presented showing the three‐dimensional structure and evolution of precipitation upwind, over, and downwind of the south Wales hills during the passage of a wintertime warm sector that gave rather heavy and prolonged rainfall. The precipitation structure was synthesized from a network of weather radars and autographic raingauges; it is interpreted within a dynamical framework derived from routine upper air soundings supplemented by serial ascents upwind and downwind of the hills.The warm sector was characterized by a fast‐moving airstream with potential instability (PI) not only at low levels due to its passage over a warm sea but also in the middle troposphere. The middle‐level PI occurred extensively and played a significant role in determining the distribution and amount of precipitation. It has not received much attention until now, mainly because the limitations of the humidity element in conventional radiosondes tend to cause its magnitude to be underestimated. The middle‐level PI was due to differential thermal advection in an intense and nearly vertical baroclinic zone that extended ahead of the surface cold front at middle levels.Large‐scale ascent was slight in the warm sector but, nevertheless, middle‐level PI was realized even over the sea in scattered ‘Mesoscale Precipitation Areas’ (MPAs) which travelled rapidly at the speed of the winds in the middle troposphere (about 120 km hr−1). Once the airstream experienced orographic uplift, fresh outbreaks of middle‐level convection occurred extensively between existing MPAs. The fresh outbreaks were observed first as middle‐level precipitation echoes over the sea up to 100 km upwind of the hills, indicating that the orographic ascent aloft began far upwind of the hills. Thus some of the precipitation generated by orographic ascent in the middle levels reached low levels over the hills despite the drift of the precipitation in the strong winds; there it seeded low‐level orographic cloud which gave heavy rain over the hills. Some of the convective precipitation generated aloft as a result of the middle‐level PI also reached the ground downwind of the hills, thereby displacing to some extent the commonly‐observed rain shadow.The importance of seeding from aloft for releasing heavy orographic rainfall has long been recognized. Sometimes the seeding is achieved by precipitation generated aloft by large scale ascent; however, the present study suggests that in certain circumstances the hills may be able to generate their own seeds when they are not being generated by large scale ascent. In the case study the occurrence of heavy warm‐sector rainfall over the hills appears to have been favoured by the existence of middle‐level PI which, although not being realized generally by large scale ascent, required only a small amount of local orographic ascent to realize it. Thus forecasting techniques that predict rainfall on the basis of the forecast large‐scale vertical ascent may fail to identify some important situations of orographic warm sector rain unless the interaction of PI and orography is

ISSN:0035-9009    

DOI:10.1002/qj.49710042505

出版商:John Wiley&Sons, Ltd    

年代:1974

数据来源: WILEY

摘要:

AbstractA weather radar has been used to measure rainfall over hilly terrain in north Wales. The radar was a standard Plessey Type 43S, which has a wavelength of 10 cm and a beam width to half power points of 2°. Measurements were compared with those based on a raingauge network consisting of 62 tipping bucket gauges distributed over 1,000 km2. This density of gauges was inadequate to define the hourly areal rainfall sufficiently accurately on some occasions and in these circumstances the radar‐derived pattern was used to interpolate between the gauges so as to obtain an ‘optimum’ estimate of the actual sub‐catchment rainfall which was then used to evaluate the accuracy of the radar measurement.Errors in the radar estimates were excessive unless the radar was calibrated hourly using raingauge measurements from one site. The accuracy was further increased when the horizontal drift of the rain in the wind between the radar beam and the calibration site was allowed for. Using these procedures, the mean percentage difference regardless of sign,Y, between the radar and optimum estimates of three‐hourly rainfall over sub‐catchments of typically 50 km2varied from about 15% close to the calibration site to about 20% at a distance of 20 km. Differences decreased as the period of integration was increased;Ywas 13% for 6 hour periods compared with 20% for 2 hour periods.Yalso varied with the area of comparison. For point measurements hourly estimates differed by 37% but over an area of 500 km2around the calibration site they differed by about 13%.The main causes of error in the radar measurements seem to be (i) variations in the drop size distribution relation, (ii) the spatial variation in the growth (or evaporation) which occurred between the beam and the ground and (iii) the horizontal drift of the rain in the wind. Probably all of these factors, but in particular (ii), are more important in the hilly terrain of this study than would be the case in flatter terrain. A further factor contributing to the differences between the radar and optimum estimates was the (unavoidable) error in each optimum estimate of the actual rainfall.The accuracy of the radar estimates decreased markedly if the beam intersected the melting layer. The increased errors were reduced by introducing an empirical correction factor which is a function of the range of the sub‐catchment from the radar and the height of the melting layer, but the spatial and temporal variations of the melting layer were such that it was not possible to obtain the same accuracy as when the beam was entire

ISSN:0035-9009    

DOI:10.1002/qj.49710042506

出版商:John Wiley&Sons, Ltd    

年代:1974

数据来源: WILEY

摘要:

AbstractThe evolution of the structure of the atmospheric boundary layer during the cooling phase of the diurnal cycle is studied with the help of a one‐dimensional numerical model. The model uses a turbulent energy equation for the determination of the eddy exchange coefficients, which also involve the local value of the Monin‐Obukhov length. The surface temperature is prescribed as a function of time and the geostrophic wind is held constant. Results are presented in nondimensional form and cover a wide range of possible values for the external parameters. After several hours of cooling the inversion layer is found to reach a maximum depth, which depends strongly upon the magnitude of the geostrophic wind and, to a lesser extent, on the cooling rate of the ground surf

ISSN:0035-9009    

DOI:10.1002/qj.49710042507

出版商:John Wiley&Sons, Ltd    

年代:1974

数据来源: WILEY

摘要:

AbstractResults of our recent investigation of spectral measurements of wind velocity in the surface layer are analysed in light of the hypothesis proposed by Kaimalet al.(1972). It is confirmed that in the inertial subrange the spectra for all the three components and varying stability converge to a −2/3 line at the high frequency end. At the low frequency end the spectra cluster together in random fashion in a certain bandwidth and do not show any ordered arrangement withz/

ISSN:0035-9009    

DOI:10.1002/qj.49710042508

出版商:John Wiley&Sons, Ltd    

年代:1974

数据来源: WILEY

摘要:

AbstractIntegrations of the shallow water equations on the sphere using a second order finite difference method and a spectral method are compared. By increasing the resolution, a good estimate of the exact solution may be made, thus allowing an estimate of the accuracy of each integration. The particular initial conditions used are a Rossby wave of zonal wavenumber 4 which moves with little change in shape or amplitude and a Rossby wave of zonal wavenumber 8 which undergoes large changes within 5 days. All the models perform reasonably for the wavenumber 4 integration. A 5° times; 3° grid for the finite difference simulation is insufficient to resolve the breakdown in wavenumber 8 despite there being 9 points per zonal wavelength. A spectral model with a truncation at wavenumber 16 uses less storage and takes the same computing time as the grid‐point model. However, it is able to predict quite accurately the breakd

ISSN:0035-9009    

DOI:10.1002/qj.49710042509

出版商:John Wiley&Sons, Ltd    

年代:1974

数据来源: WILEY

摘要:

AbstractAssuming an air layer of geometric thicknessL, a particle number densityN, particles of uniform radiusaand density ϱ, the optical depth of the particle layer is τ = πa2QNL, Qbeing the extinction efficiency factor. The total massMof the particles in a vertical air column of unit cross‐section isM= (4/3)πa3ρNL. The ratio of the two quantities isM/τ = 4aϱ/3Q.The above ratio is applied to data of τ for the observed 20% reduction in direct solar radiation over south‐east Australia about mid‐1963, reported by Dyer and Hicks. ForM, an estimate given by Deirmendjian is used. It is inferred from the above ratio that a particle radius of a few tenths of one micron leads to aQvalue such that the values for the radius and forQare mutually consistent with the Mie scattering theory. If we assume much smaller radii, we run into contradiction with the latter. We thus conclude that most of the extinction must have been effected by particles of radii of about the same magnitude as the wavelengths where the bulk of the solar radiation energy resides. We estimate that the particle concentration may have been about 5 cm−3and that absorption by the particles was prob

ISSN:0035-9009    

DOI:10.1002/qj.49710042510

出版商:John Wiley&Sons, Ltd    

年代:1974

数据来源: WILEY

摘要:

AbstractAn up‐to‐date Table is provided of the monthly and seasonal means representative of the air over central England from 1659 onward, incorporating some minor revisions and extensions of the earlier Table (Manley 1953 and 1959). Comments on its construction, and on the prospect of further extension backward in time, are ad

ISSN:0035-9009    

DOI:10.1002/qj.49710042511

出版商:John Wiley&Sons, Ltd    

年代:1974

数据来源: WILEY