ISSN:0035-9009    

DOI:10.1002/qj.49709640902

出版商:John Wiley&Sons, Ltd    

年代:1970

数据来源: WILEY

摘要:

AbstractA case study of a cold front has shown that, although appreciable ascent occurred over a deep layer, practically all precipitation growth was associated with the ascent of air which originated within the friction layer ahead of the front. Doppler radar observations showed that the ascent was accomplished in two phases; first through near‐vertical line convection to between 1 and 3 km at the surface cold front, and thence through shallow‐slope convection of the same air to between 3 and 6 km. The line convection was 2‐dimensional rather than cellular and occurred in the absence of appreciable hydrostatic instability. Horizontal convergence at low levels was very intense (10−2s−1averaged over 500 m vertically and horizontally), so that despite the shallowness of the line convection, the updraught attained a rising speed of 8 m s−1which was sufficient to generate hail and thunder. The subsequent slope convection produced a period of moderate precipitation behind the surface cold front, and was associated with a pronounced transverse circulation, with strong gradients of velocity separating the weak downdraughts in (and beneath) the sloping frontal zone from the overlying updraught. The overall efficiency of precipitation production was high, 60 per cent of the water vapour flux in the rising air reaching the ground as pr

ISSN:0035-9009    

DOI:10.1002/qj.49709640903

出版商:John Wiley&Sons, Ltd    

年代:1970

数据来源: WILEY

摘要:

AbstractAll published work on the water‐vapour continuum in the region 8 μ to 50 μ is reviewed in the light of some new experimental results in the 11 to 21 ‐μ region. This new work employed a 15.5 m multiple‐reflection cell, giving total path lengths up to 500 m, and a high‐resolution (1–2 cm−1) grating spectrograph. Control of temperature (20–40°C), and water‐vapour partial pressure (3–35 mb) allowed simulation of tropospheric conditions and their extension to rather higher humidities, but the total pressure could not be varied from atmospheric.The results suggest that two mechanisms of absorption are involved. One of them is almost certainly the foreign‐broadening of water‐vapour lines and it is responsible for most of the continuum absorption noted by previous workers. The origin of the other is not clear; it gives rise to absorption which increases with partial water‐vapour pressure, e, equalling the foreign‐broadened component at e ∼ 15 mb. This behaviour was first reported by Bignell, Saiedy and Sheppard (1963) who then attributed it to self‐broadening of H2O lines. However, the present more sensitive work casts doubt on this interpretation because the absorption is found to have a negative temperature dependence of 2 per cent per °C over the range 21° to 45°C, which is greater than that of any water‐vapour line. Absorption of the same type, though weaker, is present near 4 μ.Reinterpretation of previous open‐air measurements in atmospheric conditions ranging from warm and damp to cold and dry shows that the systematic differences they exhibit might be a manifestation of the new absorption.No firm explanation can be given, but the possibility of continuum absorption by the recently reported water di

ISSN:0035-9009    

DOI:10.1002/qj.49709640904

出版商:John Wiley&Sons, Ltd    

年代:1970

数据来源: WILEY

摘要:

AbstractNumerical experiments with a two‐level, quasi‐geostrophic model, described by Davies and Davies (1969), have been extended in several ways; the grid spacing of the previous model has been halved, greatly reducing numerical errors, and the lateral East‐West span of the flow domain is much increased.Using this model a preliminary study is made of some of the large‐scale features of the dynamics of baroclinic wave blocking by the formation of cold anticyclonic cells in high latitudes. In order to facilitate the computational work and the interpretation of the numerical results, a simple, idealized distribution of the large‐scale thermal characteristics of an ocean and a continental region is chosen. Two variants of the model are compared : model (a) is based on a heating function (independent of longitude) describing the climatological annual average over ocean surfaces : in model (b) the flow domain is divided into two longitudinal (North‐South) zones; heating functions, roughly characteristic of annual and winter averages over the ocean surfaces, are used over one zone and heating functions, roughly characteristic of similar averages over land surfaces, are employed over the other zone. Time integrations for the models have been carried out for periods of 110 days. Blocking does not take place in model (a) but in model (b) the computed flow charts and the K′ (t) function (total model eddy kinetic energy) lead to the interesting result that ‘blocking’ takes place in high latitudes of the ‘land mass’ area at broad scale minima of K′, strongly suggesting that the blocking phenomenon is associated with global scale parameters. In the model those minima of K′ associated with a large‐scale, longitudinal, land‐sea temperature differential of about 2°C led to blocking, but the minima associated with temperature differentials appreciably less than this did not produce blocking. The results also indicate an approximately linear dependence of the time scale of blocking on the large‐scale, longitudinal, land‐sea heating function differential.The prediction that blocking is associated with broad scale minima of K′ has been studied in an analysis of Northern Hemisphere synoptic charts for the winter of 1956–57, which include a well‐formed blocking situation. Estimates of eddy kinetic energy were made for both surface and 500 mb charts for the sections (a) between 50°W and 60°E, (b) between 110°W and 120°E, and for (c) the whole Northern Hemisphere. In each of these cases K′ dropped to a well marked minimum just before the incidence of a characteristic blocking situation over north‐west Europe from 21 December to 28 December, the behaviour of K′ (t) over this period being closely analogous to that found in the model blocking situations.Another objective of the study is to make a comparison of two formulations of sub‐grid scale representation. In the first instance, this is taken in the often‐used form A°2u, where u is the appropriate velocity component and A is an invariant of space and time; in the second case A is linked to model values of grid‐scale temperature gradient. A comparison indicates a much higher (and therefore more realistic) degree of variability of flow pattern leading to a 50 per cent higher val

ISSN:0035-9009    

DOI:10.1002/qj.49709640905

出版商:John Wiley&Sons, Ltd    

年代:1970

数据来源: WILEY

摘要:

AbstractThis paper considers free shear layer turbulence in the atmosphere (clear air turbulence) as a mechanism for relieving the formation of discontinuities of wind and temperature in the vertical plane by shearing and stretching deformation.An expression for the rate of change of the logarithm of the gradient Richardson number following the air motion is derived and the assumption is made that turbulence counteracts the dynamical processes which are reducing the Richardson number, maintaining it at a limiting value. The rate of working required to do this is equated to the turbulent energy dissipation rate and in the model used is given by Φ (ΔV)2/24 (where the logarithmic rate of reduction of Ri which would take place in the absence of turbulence, and ΔV is the velocity difference across the turbulent layer).Two preliminary tests of the theory as a forecasting tool using the Bushby‐Timpson 10‐level numerical model show that the dynamical processes changing Ri are largest in areas where clear air turbulence might be expected from synoptic expe

ISSN:0035-9009    

DOI:10.1002/qj.49709640906

出版商:John Wiley&Sons, Ltd    

年代:1970

数据来源: WILEY

摘要:

AbstractReflection coefficients measured over a wide variety of natural and agricultural vegetation surfaces in Nigeria (lat. 4° 40′ to 13° 30′N) were between 0.057to 0.251; and were inversely related to the vegetation height. Considerable seasonal differences were found with unexpected low values of reflectivity during the harmattan haze in the dry season. Possible reasons for these seasonal differences are discussed. The weighted mean annual reflection coefficient for Nigeria was calculated to be 0.178. This value was consistent with the relationship between surface reflection coefficient and latitude found from similar results from six other locations in the Northern Hemisphere. Urban coefficients were found to be lower than those for rural settlements or temperate zone urban ce

ISSN:0035-9009    

DOI:10.1002/qj.49709640907

出版商:John Wiley&Sons, Ltd    

年代:1970

数据来源: WILEY

摘要:

AbstractMid‐tropospheric frontogenesis is investigated for a case study in which special three‐hourly rawinsonde data are available. Isentropic analyses are made by objective means. Isentropic trajectories lead to the three‐dimensional description of the velocity field in space and time. Potential vorticity is used to check trajectories and to distinguish stratospheric from tropospheric air.Miller's frontogenesis equation is applied in two and three dimensions to an intensifying mid‐tropospheric baroclinic zone. Frontogenetical and frontolytical regions arising from twisting, horizontal confluence, vertical shear of vertical velocity and ageostrophic contributions are delineated. Typical frontogenesis magnitudes are 2°C (100 km)−1(3 hr)−1and 2°C (km)−1(3 hr)−1in two and three dimensions respectively.Previous observations of the extrusion of stratospheric air into the troposphere are confirmed.A synoptic sequence common to many occurrences of mid‐tropospheric frontogenesis is described. A simple explanation is given of the initial development of a thermally indirect circulation accompanying the early frontogenesis. Mid‐tropospheric frontogenesis seems to accompany the extrusion of stratospheric air into the troposphere during which momentum, potential vorticity, ozone and radioactivity are transported d

ISSN:0035-9009    

DOI:10.1002/qj.49709640908

出版商:John Wiley&Sons, Ltd    

年代:1970

数据来源: WILEY

摘要:

AbstractAn analysis has been made of the bubble and crystal structure and of the isotopic composition of a hailstone from a severe storm which occurred near Cardiff on 1 July 1968. A trajectory for the stone has been deduced from the isotopic data which is consistent with its crystal and bubble structures. Updraught profiles have been calculated also and are in reasonable agreement with those inferred from the extended parcel theory.

ISSN:0035-9009    

DOI:10.1002/qj.49709640909

出版商:John Wiley&Sons, Ltd    

年代:1970

数据来源: WILEY

摘要:

AbstractAn instrumented aircraft was used to sample particles in the upper regions of small supercooled cumulus clouds in maritime air off the Tasmanian coast in May 1968. 114 clouds were studied on nine days, and it was found on average that clouds had a 50 per cent chance of containing ice particles when a summit temperature of about − 8°C was reached. Not only cloud temperature but also cloud width had a profound effect on whether ice particles were found.The cloud summit temperature at which ice particles first reached detectable concentrations in narrow clouds on a particular day was related to the ice nucleus concentration as measured in a cloud chamber, being, in general, highest on days of high ice nucleus concentration. However, in some glaciated clouds ice crystal concentrations of the order of 100 1−1were found. These were greater by a factor of ∼ 104than the estimated number of active ice nuclei. The process of multiplication of ice crystals may be associated with the riming of ice particles. Riming was assisted by the presence of large droplets, up to drizzle size, in these clouds. Multiplication of the number of ice crystals by a factor of 10 took place in about 8 minutes.The various stages in the glaciation process are des

ISSN:0035-9009    

DOI:10.1002/qj.49709640910

出版商:John Wiley&Sons, Ltd    

年代:1970

数据来源: WILEY

摘要:

AbstractUsing high‐altitude balloon soundings (1961 to 1968) and other data, it is shown that the most reliable maximum value of the solar constant is at present 1.94 cal cm−2min−1, to an accuracy of 1 per cent. This value is observed with Wolf numbers between 80 and 100, below or above which the solar constant decreases. The maximum decrease does not exceed 2 to 2.5 per cent. The observations reveal the influence of nuclear explosions and volcanic eruptions which caused an anomalous turbidity of the upper atmospheric layers. These anomalies have been studied. The above conclusions require further confirmation using direct long‐term measurements of the solar constant outside the atm

ISSN:0035-9009    

DOI:10.1002/qj.49709640911

出版商:John Wiley&Sons, Ltd    

年代:1970

数据来源: WILEY