摘要:

AbstractDuring an investigation into convective cloud at a single temperature level near −6°C, observations by aircraft have found ice crystal concentrations significantly in excess (up to 600 L−1) of those to be expected from primary ice formation (i.e. ice multiplication). Despite the fact that the main conditions for the operation of the Hallet‐Mossop secondary ice multiplication process were fulfilled, the corresponding rate of ice production, as calculated, appeared to be lower than was observed in the cloud. This suggests that other ice‐enhancement processes might be at work during the ice‐forming stage.A small‐scale (100m) analysis of the data revealed that ice‐multiplication processes operated only in the fringes of the updraught regions of growing cells. It was also observed that a sudden occurrence of the ice usually coincided with the decay of the updraught. From these observations and from an evaluation of the amount of heat released by riming and vapour deposition processes, we suggest that the cell cores are auto‐seeded by the mixing or recycling of numerous ice crystals and that the rapid glaciation tends to be ineffective in increasing the buoy

ISSN:0035-9009    

DOI:10.1002/qj.49711850402

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThis paper describes a model of the effects of low hills upon scalar fields, particularly the temperature and humidity fields, in the planetary boundary layer under near‐neutral conditions. the approach is to use linearized equations and two‐layer asymptotic matching, similar to methods which are now well‐established for the velocity field. In theinner regionnear the surface the scalar fields are affected by turbulent transfer from the surface and are represented using an eddy diffusivity. In theouter regionthe response of both the flow and the scalar fields to the hill is essentially inviscid. By matching these layers, hill‐induced perturbations in the concentrations and fluxes of an arbitrary scalar can be determined in terms of upwind and surface conditions, and the calculated mean and turbulent wind fields over the hill. the analysis separates these perturbations into components associated with (1) the convergence and divergence of the streamlines, (2) vertical stress gradients, and (3) changes in boundary conditions (surface stress and scalar flux density) over the hill. In order to describe coupled heat and water‐vapour transfer with this model, it is necessary to decouple the scalars by using two new scalar entities, essentially energy per unit volume and potential saturation deficit. This is a flow‐independent technique previously applied to sensible‐and latent‐heat fluxes in local advection and in convective boundary layers. an extension given here accounts for the effects of change in elevation on the evaporation process.Perturbations in the temperature and humidity fields and fluxes over the hill arise physically from slope‐induced radiative effects, aerodynamic effects induced by changes in the mean wind and stress fields, and elevation effects induced by adiabatic cooling on ascent. Three dimensionless parameters control these effects: the hill slope,H/L; the unperturbed Priestley‐Taylor ratio, α, (the ratio of surface latent‐heat flux density to available energy flux density, far from the hill); and an ‘elevation parameter’, Pelev, proportional to the change in saturation deficit induced by adiabatic cooling as air ascends through the hill height,H, normalized by the turbulence specific‐humidity scale. For the surface latent‐heat flux, radiatively‐induced perturbations scale on(H/L)α1−1aerodynamically‐induced perturbations on (H/L)(α1‐1‐αeq−1) (where αeqis the thermodynamic equilibrium value of α 0.688 at 20°C); and elevation‐induced perturbations on Pelev.In general terms, the strongest influences on the perturbations of heat and water‐vapour fields and fluxes are aerodynamic effects associated with changes in surface shear stress, and elevation effects. For a hill of height 100m, surface resistance 50 s m−1and slopeH/L= 0.2, typical changes in potential temperature and specific humidity are of order 1 degC and 1 gkg−1, respectively, while typical perturbations in sensible‐ and latent‐heat fluxes are of order 20 per cent.The linear analysis predicts that spatial averages of heat and water‐vapour fluxes are independent of l

ISSN:0035-9009    

DOI:10.1002/qj.49711850403

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThe importance of three‐dimensional effects in a flow with negative potential vorticity is considered here; this is the three‐dimensional counterpart of symmetric instability. the term symmetric instability refers to a two‐dimensional flow with negative potential vorticity which develops roll circulations aligned along the thermal wind. the requirement for exact two‐dimensionality is here relaxed by using three‐dimensional numerical simulations. Two classes of simulation are described; both take a two‐dimensional basic flow.In the first part we explore the response of a flow with uniform negative potential vorticity to localized initial perturbations. It is found that circulations become established which elongate at an angle to the thermal wind. This angle, it is shown, is determined by viscous effects and is in accord with the structure of linear viscous tilted modes for this flow. If, as happens on occasions, such an angle formed by rainbands to the thermal wind is observed in nature, it gives an indication of the importance of viscous effects. In the cases presented here the orientation is such that the rolls tilt towards the warm air as viewed in the direction of the thermal wind; i.e. they are rotated anticyclonically relative to the front. It is shown that these structures, when at finite amplitude, are stable to perturbations imposed along their length.In the second part localized regions of instability are produced which are confined along the thermal wind direction by prescribing in these zones a reduced static stability for vertical motion. This represents the effects of latent‐heat release and it is only in these zones that the effective potential vorticity is negative. It is found that if the length of the zone along the thermal wind direction is smaller than about twice the roll wavelength across that direction then the growth rate is substantially reduced; otherwise it is relatively unaffected by this confinement. It therefore appears that a flow with negative potential vorticity is not only unstable in the pathological two‐d

ISSN:0035-9009    

DOI:10.1002/qj.49711850404

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractTime series of surface pressure from an array of microbarographs in the Balearic Islands (western Mediterranean) are used to obtain wavelength, phase speed and direction of propagation of gravity waves on the Balearic Islands. These pressure waves are commonly found during the summer and they force the large seal‐level oscillations observed in the inlet of Ciutadella; a phenomenon locally known as ‘rissaga’. Four events are described here, each of which has a duration of between 30 and 48 hours. Radiosonde data from Palma de Mallorca are used to relate the waves to the vertical structure of the atmosphere. the various source mechanisms of these waves and also the possibility of the existence of a duct layer trapping the wave energy near the surface are discussed. the waves are found to have no significant variation of phase speed with frequency and so they are basically non‐dispersive. This is an indicator pointing either to a local generation of gravity waves by wind‐shear instability or to the existence of neutral trapped waves, in the lower tr

ISSN:0035-9009    

DOI:10.1002/qj.49711850405

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractA preliminary morphology of convective systems observed in the vicinity of Darwin, Australia is presented. Several types of tropical convection during monsoonal and break‐season flow are identified using specific examples and compared with a range of systems observed worldwide in the general context of the bulk Richardson number. A significant spectrum of convective activity ranging from low shear and low Convective Available Potential Energy (CAPE), to high shear and moderate to high CAPE, typical of mid‐latitude severe storms, is identified. Significant differences between monsoonal flow and break‐season systems are apparent. the formation of convective lines perpendicular to the low‐level shear and the apparent balance between advancing shallow cold pools and the shear are ubiquitous features. Convective‐scale downdraughts and mesoscale descent appear to be responsible for redistribution of 700 mb momentum to the planetary boundary layer (PEL).The motion of the break‐season squall lines appears to result from an equilibrium between the cold pool and the undisturbed environmental PEL flow. Deviations from this balance are hypothesized, to result in the observed ‘propagating’ and ‘non‐propagating’ or slow‐moving modes. Examples of these modes are given, showing the classical continuous development of new cells in a balanced state along the leading edge of long‐lived squalls and, in the propagating case, the discontinuous development of new cells on the cold

ISSN:0035-9009    

DOI:10.1002/qj.49711850406

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractA potential vorticity (PV) index is defined as a measure of the zonally averaged, raid‐latitude PV gradient on the 300 K isentropic surface in the northern hemisphere, and use is made of the 1978/79 wińter ECMWF FGGE Hl‐b data set to study the evolution of that index and its relation to teleconnection patterns of 500 mb geopotential‐height anomaly. This index vacillates with a dominant period of about two weeks. For some time lags, the isoline patterns of the cross‐correlation function between the PV index and 500mb geopotential‐height anomalies are similar to some teleconnection patterns studied by several investigators using large climatological data sets. Scandinavia, Greenland and the Pacific are important centres of ‘action’ in the northern hemisphere in the 1978/79 winter. the formation of blocking highs at these locations occurs at rather specific phases during the vacillatory evolution of the PV index, especially as the flow transits from a low to a high PV index.When the above study is duplicated using a zonal mean wind gradient at 500mb as a zonal index, the results do not reflect the teleconnection patterns as crisply as the PV index does for

ISSN:0035-9009    

DOI:10.1002/qj.49711850407

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractUsing the high‐resolution 2 × 2.5 degree version of the Goddard Laboratory for Atmospheres general circulation model, experiments were performed for a 46‐day summer period from 1 July to 15 August 1979. During the simulation, sea‐surface‐temperature (s.s.t.) anomalies were updated daily with observed s.s.t.s to provide a realistic surface heating field. A parallel control run used climatology s.s.t.s. in lieu of observed s.s.t.s.Both the s.s.t. updating experiment and control run were able to simulate the persistent blocking patterns in Eurasia and the eastern Pacific during July. However, only the s.s.t. updating experiment was able to generate the realistic blocking episodes in the last 10‐day period during the 46‐day simulation. Forecast skill was examined in terms of 500 mb anomaly correlation and error kinetic energy. When the blocking patterns develop or become reinforced, a high forecast skill is apparent in nearly half of the northern hemisphere including the area of blocking and its downstream region. Unlike the major winter blockings, which may be treated as the manifestation of wave number one and two, the summer blockings are localized, and have a smaller scale than that of winter blockings. However, summer blockings treated in this study are persistent, and possess a considerable pr

ISSN:0035-9009    

DOI:10.1002/qj.49711850408

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractTropical wind fields from the European Centre for Medium‐range Weather Forecasts operational analyses are analysed to search for equatorially trapped waves in the upper troposphere. Data are studied for 1980‐87. Zonal wind spectra show a predominance of power at low zonal wave numbers and low frequencies; these data do not show distinct spectral peaks corresponding to eastward‐propagating Kelvin waves or westward‐propagating equatorially‐trapped Rossby waves. the zonal wind spectra do show episodic evidence for westward‐propagating 5‐day zonal wave‐1 features with maxima over the equator; these are likely related to Rossby normal‐mode oscillations previously reported.Meridional wind spectra show a predominance of westward‐propagating power in the 6‐ to 10‐day‐period range, primarily concentrated in zonal waves 4‐7. Enhanced meridional wind activity at the equator occurs episodically in time, with events persisting for one to several months duration. Wave packets are longitudinally localized, with preferred occurrence over the eastern Pacific Ocean. Most cases show evidence of meridional wind maxima in the tropics, suggesting a degree of equatorial trapping. the horizontal structure of these oscillations is that of mixed Rossby‐gravity waves, although they are not necessarily centred directly over the equator. These waves have largest amplitudes in the upper troposphere, showing weak coherence with the surface; they appear to be fundamentally distinct from fast 4‐ to 5‐day Rossby‐gravity waves observed in the western Pacific.A detailed study of a group of intense Rossby‐gravity waves observed over the eastern Pacific during August‐September 1985 is presented; the reality of these features is confirmed in direct rawinsonde observations. Correlations are found between bursts of equatorial wave activity and enhanced meridional momentum fluxes from the subtropics, suggesting extratropical forcing as at least one mechanism for their generation. the waves are also strongly coupled to the upper tropospheric zonal wind field, which likely accounts for thei

ISSN:0035-9009    

DOI:10.1002/qj.49711850409

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractDyeet al., in a 1984 thunderstorm study, identified regions containing electric charge in two growing clouds from measurements of the atmospheric electric fields taken, during penetrations of these clouds, by an instrumented aeroplane. There are, however, problems concerning the locations they assigned to these charges relative to the radar echoes from the clouds. In one case the authors identified the pressure altitude reported by the aeroplane pilot with the altitude of the radar echo top; this introduced an error of about 450m. In another case, the location given for the charged region relative to the cloud echoes is not compatible with the pilot's report of an updraught. While the authors measurements add to our knowledge of cloud electrification, some of their interpretation is questionable, and other explanations for the charged regions are possible.

ISSN:0035-9009    

DOI:10.1002/qj.49711850410

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractIn response to the comments of Moore and Vonnegut we have thoroughly re‐examined the aircraft and radar measurements made in two small thunderstorms in New Mexico originally reported byDye et al.(1988). Our conclusions remain the same. Although pressure altitude was used instead of geometric altitude in one case and distance labels were interchanged for a vertical section used to illustrate the other case, improved calculations for the particle image measurements and the ice particle collision rate show that the two regions of charge were in or near locations with appreciable supercooled liquid water, ice particle concentrations and graupel, and also near the top of reflectivity cores as originally stated. As before, we conclude that, given this spatial correlation of the inferred charge with ice particles and liquid water, the observations are consistent with what is expected from the non‐inductive ice‐collision charge separation mechanism of thunderstorm electrification. However, because the data are incomplete we cannot preclude the possibility that one of the regions of charge (on 15 August 1984) was the result of positive ions being attracted to the cloud by a downward‐directed field, as suggested by Moore and V

ISSN:0035-9009    

DOI:10.1002/qj.49711850411

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY