ISSN:0035-9009    

DOI:10.1002/qj.49710243102

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractThe factors which determine the crystalline and air bubble structures of accreted ice have recently been elucidated. It is now possible to ascertain the growth histories of hailstones of approximately spherical symmetry from studies of their internal structure. In principle this analysis can be extended to oblate hailstones once their aerodynamic behaviour is known. Six large hailstones 4 to 6.5cm maximum dimension, two from each of three severe storms, have been analysed. The orientation distributions and sizes of the crystals in the hailstone layers indicate that most of the growth of the hailstones took place at ambient temperatures between about −20 and −25°C, i.e. between 2.7 and 3.7km above the freezing level. The air bubble concentrations and size distributions show that all the hailstones grew with surface temperatures ranging from −1°C for the clear layers to about −11°C for opaque layers comprised of very small crystals. Heat balance considerations indicate that the effective liquid water concentrations in which the hailstones consequently grew were between 3 and 1g m−3. On the assumption that the median volume radius of the cloud droplet distribution was 10μm, the growth of most of the hailstones took place in liquid water concentrations of about the adiabatic values. However, fluctuations in the liquid water concentration of up to some 30% are required to explain the observed air bubble and crystal structures. The fact that the hailstones grew largely between the −20 and −25°C levels indicates that they were balanced in the updraught for most of their history. Since the hailstone fallspeed increases with radius, this meant that the updraught experienced by the hailstone increased steadily with time. There are two possible explanations for this: the strength of the updraught could have increased with time; alternatively, the hailstones could have moved around the main core of the updraught to encounter increasing updraught speeds, as suggested in the Browning and Foote (1975) three‐dimensional mo

ISSN:0035-9009    

DOI:10.1002/qj.49710243103

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractExperiments indicate that secondary ice crystals are produced when rime grows in a supercooled cloud containing drops ≥ 24μm in diameter. This occurs between temperatures of −3 and −8°C, the production rate being greatest at −5°C. These temperatures are constant to about ±0.5 deg C for velocities of the riming body ranging from 1.4 to 3.0m s−1. On average, one ice splinter is thrown off for every 250 drops of diameter ≥ 24μm accreted, at cloud temperature −5°C, and this value does not change much with target velocity. Various possible splintering mecha

ISSN:0035-9009    

DOI:10.1002/qj.49710243104

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractA statistical analysis has been made of long‐term rainfall records from West Africa. No established trends or periodicities can be detected, and the recent succession of drought years falls within statistical expectation. Sahelian rainfall is not clearly linked to the frequency of westerly weather over Britai

ISSN:0035-9009    

DOI:10.1002/qj.49710243105

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractIndices of the southern hemisphere circulation are derived for the Australasian area using sea level pressure data. As well as considering indices of zonal and meridional flow, indices are derived based on the empirical orthogonal function analysis of Trenberth (1975). The first two patterns, P1 and P2, of this analysis can be approximately represented by only a few station values, and in this way their time series can be extended.‘Regimes’ of weather are defined based on the short term mean behaviour of P1 and P2. These regimes are also characterized by large changes in the periodicities at which most of the variance occurs. The quasibiennial oscillation shows up as a fairly stable feature of the meridional indices and P2.Trends in both P1 and P2 since the 1940s are related to the rising temperature trend in New Zealand. There has been a trend for less westerly especially between 25 and 45°S, some increase in westerly south of 45°S and less southwesterly or more northeasterly component to the flow across the whole of New Zealand as a result of the increased tendency for anticyclones to persist to the east of New Zealand rather than over Australia and the Tasman Sea, and their preference for higher latitude blo

ISSN:0035-9009    

DOI:10.1002/qj.49710243106

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractVarious techniques for smoothing numerical forecast integrations are compared for both finite difference and finite element Galerkin methods. The difference in the accuracy of finite element and finite difference methods is analysed to illustrate the removal of ‘aliasing’ by the Galerkin approach. This is then related to the appropriate choice of smoothing technique. The extra accuracy of finite elements allows the use of more selective methods. The use of filtering is shown to be suspect when significant energy is present in high wavenumbers and the Sadourny gravity‐wave smoothing is also disc

ISSN:0035-9009    

DOI:10.1002/qj.49710243107

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractThe convergence of heat below subsidence inversions was estimated from radiosonde ascents on fine summer days. Sensible heat input from the ground was estimated from measurements of heat fluxes and net radiation over a wheat field. Absorption and back scattering of solar radiation by aerosol throughout the atmosphere were calculated from measurements of solar radiation at the ground. Radiation absorbed by aerosol in the thermal boundary layer heated the lower atmosphere at an average rate of 3.3 degC day−1(60Wm−2), about twice as fast as estimated for gaseous constituents.An equation is derived to show how the net effect of aerosol on the lower atmosphere depends on the reflection coefficient of the surface and on the aerodynamic and surface resistances to vapour transfer. Over the wheat, the net effect was almost zero when the crop was transpiring fast, but when the crop was mature, aerosol caused net cool

ISSN:0035-9009    

DOI:10.1002/qj.49710243108

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractThe semi‐geostrophic equations are derived from the primitive equations with the momentum approximated by its geostrophic value. Here these equations are used to study the stability of a zonal flow. The quasi‐geostrophic instability criterion is given a more accurate form. The semi‐geostrophic equations are also used for a numerical and analytical study of the development into the nonlinear regime of the Eady wave instability mode of a zonal flow with uniform shear in the vertical. The nonlinear tightening of the low pressure, the occlusion process and the formation of a warm frontal region are all evident in these solu

ISSN:0035-9009    

DOI:10.1002/qj.49710243109

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractThe initial stages of an investigation aimed at setting up a new system for the prediction of storm surges in the North Sea are described. The proposed scheme is based on the use of dynamical finite‐difference models of the atmosphere and of the sea, the atmospheric model providing the necessary forecasts of meteorological data. These data are converted into the form required for input to the sea model, which then computes the associated storm surge.Some preliminary results are presented and discussed and an indication given of the further development of the syste

ISSN:0035-9009    

DOI:10.1002/qj.49710243110

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractIce particle multiplication in shallow supercooled clouds is described by a stochastic process, which depends on collisions between ice particles and supercooled water droplets and drops. The multiplication is a consequence of the riming and drop splintering processes, or possibly just one of them. In the riming process, a large ice particle collects water droplets which eject splinters on freezing. In the drop splintering process, a supercooled drop which collides with a splinter, freezes and may eject splinters. Account is taken of the stronger evidence of significant drop splintering for small supercooled drops by considering those drops with diameters less than 250μm as a separate group. The probability of collision between an ice splinter and a water drop is estimated by considering their fall velocities and assuming a collection efficiency of unity. As a consequence of the small and large water drop division, the ice splinters are also divided; small ice splinters collide with all water drops, but large splinters having fall velocities comparable with those of small water drops collide only with large water drops. The growth of all ice particles by diffusion of water vapour or accretion of water droplets is taken into account, splinters not captured by water drops grow to rimer status after an appropriate time. Rimers are removed from the system at the time they fall through the 0°C isotherm. A rimer may also be formed as a result of the freezing of a large supercooled water drop, following its collision with an ice splinter. As the early production of splinters by such a rimer depends on its initial mass, which equals that of the drop, a distribution of large water drop masses is considered.Generating functions are formulated for the probabilities of particular numbers of the various categories of ice particle existing at a general timet, as a consequence of one initial ice particle. These functions lead to a set of renewal equations for the estimated mean numbers of ice particles at timet. The Laplace transforms of the renewal equations are studied and it is shown that a simple large time analysis provides a valid description of the growth mechanism during most of the cloud lifetime. A small time behaviour marching technique completes the growth description. The large time analysis predicts thatA1exp(p0t) ice particles exist at timetas a consequence of one initial small ice splinter. Values of the growth parameter,p0, are given for many combinations of the physical parameters. An integration of this large time result provides a description of ice particle multiplication initiated by the nucleating particles introduced into a cloud by a constant updraught. A thermal model is also considered and results in growth times near to those obtained for the constant updraught model. These growth times are considerably less than the growth times predicted by earlier workers.The field observations of several workers are all found to be consistent with this analysis. In many situations the most likely explanation of the multiplication involves two processes. These are: splinter production by riming; and the capture of ice splinters by drops, the drops becoming riming agents when they freez

ISSN:0035-9009    

DOI:10.1002/qj.49710243111

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY