摘要:

AbstractThis study examines the influence of weak horizontal advections of momentum on a well‐mixed planetary boundary layer. In the simple flow system examined, the direction and magnitude of flow modifications depend on the Rossby number, the nondimensional surface drag coefficient and the orientation of the geostrophic wind gradient field. It is found that advective modification of the boundary layer vertical motion field involves dependencies on the square of the local vorticity and dependencies on vorticity gradients. The mixed‐layer assumptions are relaxed for a special flow case to show that these results do not depend crucially on the mixed‐layer cond

ISSN:0035-9009    

DOI:10.1002/qj.49710142702

出版商:John Wiley&Sons, Ltd    

年代:1975

数据来源: WILEY

摘要:

AbstractThe apparent emissivity of the atmosphere ϵ, defined as the ratio of incoming long‐wave radiation to black‐body radiation at screen temperatureTa, was measured under clear skies in the English Midlands and in the Sudan. At a zenith angle Z the emissivity was given by ϵ(Z) =a+bIn(usec Z) whereuis the reduced depth of precipitable water (cm). For a set of 46 scans in England, the mean values ofaandbwere 0·70±0·05 and 0·090 ± 0·002. Systematic deviations about these mean values could be ascribed to: (i) temperature gradients; (ii) aerosol. The Sudan measurements gavea= 0·67 ± 0·03 andb= 0·085 ± 0·002 consistent with the English results and observations already published. There is some evidence that minimum values ofahave increased over the past 50 years.Integration over the hemisphere gives the flux density (Wm−2) of atmospheric radiation as 1·06 σTa4− 119 (Tin K), where σ is Stefan's constant, or 5·5Ta+ 213 (Tin °C).Radiation records by Dines and Dines (1927) for overcast skies are analysed to show that the angular distribution is the same as for cloudless skies; that the mean temperature of cloud base at their site was UK below screen temperature; and that when the mean fraction of cloud cover isc, the apparent emissivity is ϵa(c)

ISSN:0035-9009    

DOI:10.1002/qj.49710142703

出版商:John Wiley&Sons, Ltd    

年代:1975

数据来源: WILEY

摘要:

AbstractAt the earth's surface, long‐wave radiation received from the atmosphere in the absence of cloud may be expressed as the sum of the flux density from an isotropic source and the flux density due to anisotropy of atmospheric radiation. Separation of these components facilitates calculations of the radiative flux received by obstructed horizontal surfaces (near walls or in valleys), by sloping planes, by solid cylinders, and by spheres and prisms. Radiation received from adjacent surfaces is also considered.As the angular distribution of radiation is the same for overcast skies as for clear skies, the relationships for clear skies may be extended to derive climatological mean values for the long‐wave irradiances of slopes, solids,

ISSN:0035-9009    

DOI:10.1002/qj.49710142704

出版商:John Wiley&Sons, Ltd    

年代:1975

数据来源: WILEY

摘要:

AbstractA Doppler radar technique originally proposed by Lhermitte (1968) has been used to measure the horizontal components of turbulence simultaneously at closely spaced height intervals from an altitude of 80m to 2500m. The observable range of turbulence scales was between about 200m and 5000m. Observations were made over the sea, with the radar situated on the Isles of Scilly, to minimize the effects of in‐homogeneities in terrain.The measurements reported in this paper were made during the passage of a warm frontal zone with strong surface winds; they show the change with height in the structure of turbulence simultaneously in both the planetary boundary layer and a free shear zone situated some way above it. Below 600m, in the planetary boundary layer, the two horizontal components of turbulence were rather similar in intensity, the total variance of horizontal velocity in the observed spectral range decreasing from 1·7m2s−2at 80m to less than 0·1m2s−2at 700m. For much of the observed spectral range the turbulence spectra were of the formkS(k) αk−n, withndecreasing from 1·2 below 300m to 0·7 at 700m. The lapse rate was slightly less than dry adiabatic in the planetary boundary layer with broken cloud above 300m, consistent with slight stability below 300m and some instability above. The decrease in the value ofnis thought to have been due to the decrease of stability with height. The spectral scale λmincreased with height reaching a value of 1·6km at 900m. Above the boundary layer for most of the time there was a placid layer several hundred metres deep and this was surmounted by a free shear layer in which the variance of velocity increased again to typically 0·5m2s−2. In the free shear layer the horizontal components of turbulence resolved parallel and perpendicular to the local shear vector were rather similar in intensity at scales less than 500m but were markedly unequal at scales between 500m and 2km. This is consistent with the occurrence of Kelvin‐Helmholtz billows with a wavelength of 1 to 2km.The Doppler radar measurements also enabled the vertical flux of horizontal momentum to be calculated. The quantity −u′w′was found to decrease from about 0·2m2s−2in the planetary boundary layer to about 0·01m2s−2in the placid layer, before increasing again to about 0·1m2s−2in the free shear layer. The energy dissipation rate ϵ was estimated from the turbulence spectra in those cases where the spectral slopenwas not greatly different from 2/3. Values of ϵ decreased from 10cm2s−3at 400m to 2cm2s−3in the placid layer, before increasing to a second maximum of about 20cm2s−3in the free shear zone. The rate of mechanical production of turbulence was calculated from the Doppler radar measurements of momentum flux and shear and it was found to vary w

ISSN:0035-9009    

DOI:10.1002/qj.49710142705

出版商:John Wiley&Sons, Ltd    

年代:1975

数据来源: WILEY

摘要:

AbstractAn empirical orthogonal function analysis is performed on monthly mean sea level pressure (SLP) deviations from normal (anomalies) in the Australasian area and monthly mean sea surface temperature (SST) anomalies in the Tasman Sea for 14 years of data.Only the five components of each that explain most of the variance appear to be significant. Spectral and cross‐spectral analysis of the time series of the coefficients reveals significant relations at a quasi‐biennial periodicity. An empirical model of the SLP and SST fields for this period is constructed and the amplitudes of the oscillation are found to be about 2 1/2 mb in the monthly mean SLP anomaly field and 0·4 degC in the monthly mean SST anomaly field. Physical and statistical indications are that the SST field is largely caused by the SLP field but some interdependence is present.The quasi‐biennial SLP oscillation is related to Pittock'sL‐index and appears to correspond to a change in wavenumber 3 in the Southern Hemisphere atmospheric circulation. A phase change occurred at the end of 1970, but in even years prior to 1971, and in odd years since 1970, there were increased meridional transports throughout the troposphere and lower stratosphere by the large‐scale eddies. A suggestion is made that changes in land–sea heating contrasts may be an important part of t

ISSN:0035-9009    

DOI:10.1002/qj.49710142706

出版商:John Wiley&Sons, Ltd    

年代:1975

数据来源: WILEY

摘要:

AbstractThe temperature field in the Southern Hemisphere stratosphere from 100 to 1mb during August to October 1971 is described using observations from the Selective Chopper Radiometer on the Nimbus IV spacecraft.Zonal and monthly mean wind fields for September and October 1971 deduced from these temperatures are presented and the behaviour of the zonal temperature waves with wavenumbers 1, 2 and 3 is discussed. The largest amplitudes of wavenumbers 1 and 2 in the horizontal coincide broadly with the maximum westerlies. The amplitudes of wavenumber 2 are generally smaller at around 20mb than above or below. The amplitudes of both waves are small north of 30°S.Wavenumber 1 exhibits little vertical coherence throughout August and September. In October, however, the slope of the wave with height is much less variable, in spite of a slow eastward movement in early October and a westward movement in late October.Wavenumber 2 travelled eastwards with a fairly uniform rate of approximately 15·5° longitude per day between 15 August and 7 October but less uniformly at other times. Changes in wave amplitude at strato‐pause levels are associated with comparable changes near the tropopause. A maximum of wavenumber 2 amplitude on 26 September 1971 is associated with falling ZPE in the lower stratosphere and rising ZPE in the higher stratosphere. The dominant contribution to the eddy available potential energy is in the lower stratosphere.In the low stratosphere there is an inverse relationship in zonal and eddy available potential energies, rises in one form being associated with falls in the other. This inverse relationship is not well marked in the high stratosp

ISSN:0035-9009    

DOI:10.1002/qj.49710142707

出版商:John Wiley&Sons, Ltd    

年代:1975

数据来源: WILEY

摘要:

AbstractValues of the total vertical flux of sensible and latent heat over a level forested region, obtained from aerodynamic (profile‐gradient) formulae appropriate to airflow over relatively smooth surfaces, are found to fall consistently short of independent energy‐balance estimates by a factor of 2 to 3 in unstable and near‐neutral conditions (Richardson number,Ri, in the range −0·4 to +0·01), whereas forRi>+ 0·02 no similar discrepancy is detected. These results, based on tangents drawn to (semi‐logarithmic) profiles at a height of about nine aerodynamic roughness parameters (z0) above the zero plane displacement level (d) of the forest, rely on the basic assumption that the value ofdestablished in very nearly neutral conditions (|Ri|<0·003), namely 0·76 mean tree heights, holds under all conditions of thermal stability.Wake diffusion and thermal seeding effects are discussed as possible additional transfer mechanisms acting to reduce profile gradients immediately over aerodynamically rough surfaces. In terms of the former mechanism (assumed to operate belowd+ 25z0, or so), approximate empirical formulae are derived which attempt to quantify the observed discrepancy in terms ofRiand the proximity of the surface.It is concluded that aerodynamic equations ought not to be used to give independent flux estimates close to aerodynamically

ISSN:0035-9009    

DOI:10.1002/qj.49710142708

出版商:John Wiley&Sons, Ltd    

年代:1975

数据来源: WILEY

摘要:

AbstractA new study has been made of the Risø profiles with the following conclusions:(i)roughness lengths determined from neutral wind profiles selected by gradient Richardson number are biased towards high roughness lengths. Selecting the profiles by using a bulk Richardson number criterion yields average profiles with lower roughness lengths. This roughness length is relatively independent of stability while that determined from profiles using a gradient Richardson number is greater than the roughness lengths for either stable or unstable cases;(ii)it is doubtful whether ‘double kinks’ in some of the Risø wind profiles are caused by local terrain inhomogeneities, as has previously been suggested;(iii)three‐dimensionality and unknown mesoscale and baroclinic effects make it difficult if not impossible to compare current change‐of‐terrain theory with the Riø 123

ISSN:0035-9009    

DOI:10.1002/qj.49710142709

出版商:John Wiley&Sons, Ltd    

年代:1975

数据来源: WILEY

摘要:

AbstractThe infrared emissivity pattern of a large‐scale jet‐stream cirrus system has been mapped from Nimbus 4 satellite THIR (10·5‐12·5μm) data using simultaneous groundbased lidar (0·694μm) observations.The average cirrus emissivity was 0·28. Dense cells in the cirrus canopy were evident in both the infrared map and lidar backscatter time‐height profiles. The cells were about 10km to 50km in horizontal diameter and 1km deep and had liquid water contents of 0·2gm m−3or greater. On occasion, the cells were ‘black’ to infrared radiation.Temporal variations of infrared emissivity (10–12μm) measured from the lidar site were in good agreement with spatial variations in the Nimbus THIR emissivity map. The frequency of occurrence,N, of a particular THIR cirrus emissivity ϵ followed the approximate distri

ISSN:0035-9009    

DOI:10.1002/qj.49710142710

出版商:John Wiley&Sons, Ltd    

年代:1975

数据来源: WILEY

摘要:

AbstractOn the assumption that the concentration of air bubbles formed in freezing water is a function of the freezing rate, an expression for the concentration of air bubbles in accreted ice has been deduced. This expression has been confirmed by measurements of the bubble concentrations and size distributions in frozen bulk water and in ice accreted on rotating cylinders. Consequently, in both the wet and dry growth regimes it is the rate of freezing which essentially determines the bubble concentration. The size distribution of bubbles is determined by the amount of air available for bubble growth. This depends on the rate at which air diffuses out of the individual droplets during the freezing process. In the wet growth regime the freezing rate depends on the rate at which heat is dissipated from the accreting object by forced convection and also, depending on the fraction of unfrozen water, on the way in which the spongy deposits are finally frozen. In the dry growth regime the freezing rate of the accreted droplets depends on their size and speed of impact, since these determine the extent to which the droplets spread, and on the temperature of the deposit.The air bubble concentrations and size distributions were determined using ordinary optical microscopy so that only bubbles of radius greater than 1μm were counted and sized. In the dry growth regime the visible bubble concentrations range from about 106to 108cm−3, depending primarily on the deposit temperature. The mean radii of the bubbles in the deposits range from 1 to 4μm. In the wet growth regime the bubble concentrations generally lie between 105and 106cm−3while the mean radii of the bubbles range from 10 to 50μm. Consequently, the two growth regimes may be readily disting

ISSN:0035-9009    

DOI:10.1002/qj.49710142711

出版商:John Wiley&Sons, Ltd    

年代:1975

数据来源: WILEY