摘要:

AbstractThe behaviour of two raindrops of opposite polarity interacting in an electric field has been investigated numerically with a view to eventually identifying the microphysics and electrical conditions favourable for the natural triggering of cloud discharges. In the past, a few studies have been carried out on the subject of distortion and disintegration of drop‐pairs but in none of them has the aerodynamic pressure featured in the equation for the pressure balance between the two drops. Consequently in what follows we have expanded and modified the earlier model for the distortion of an isolated raindrop (Coquillat and Chauzy 1993) in which the effect of the aerodynamic pressure is taken into account. the action of this pressure on each drop leads to oblate shapes, whereas in the earlier studies both drops became prolate.The mutual approach of two drops of opposite polarity and/or an increase in the ambient electric field do not always have the same effect on the distortion of each drop, so revealing the highly nonlinear character of the interaction processes between two hydrometeors. In fact the local electric fields do not necessarily increase (they can even decrease) as the distance,d, between them decreases and/or as the ambient field increases. Therefore it does not follow that the corona emission may be triggered more easily from a drop‐pair than from an isolated drop.We show that a short air gap between the two drops is not equivalent to a strong ambient field. In particular, a pair of closely separated drops may coexist in the weakest and in the strongest intensities of the ambient electric field, but not in the mid intensities. We finally discuss the consequences of cloud electrificat

ISSN:0035-9009    

DOI:10.1002/qj.49712152402

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractRecent work on the influence of moist processes at synoptic fronts has shown how condensation heating can be a strong and even a dominant factor in their development. At mesoscale lines of convection the heating is again dominant, but it is thought that cooling by the evaporation of precipitation is also a crucial factor in their development and propagation. Although most simple models of synoptic fronts do not attempt to simulate diabatic cooling accurately, data from the FRONTS 87 experiment have identified evidence of a strong mesoscale downdraught below a synoptic frontal surface. There has been speculation that such a downdraught is forced by the sublimation of falling snow.Since snow is more efficient at cooling the atmosphere, for a given precipitation rate, than evaporation of rain, rainfall parametrizations are likely to underestimate the influence of such cooling. In this paper, a suitable parametrization of diabatic cooling, based on the assumption of small stability to slantwise convection, is used to simulate a band of snow sublimation in developing fronts. the semi‐geostrophic equations are employed in the study of fronts which are formed by deformation or horizontal shear of the geostrophic winds.It is found that the cooling has little influence on the large scales of motion, and does not greatly accelerate the formation of frontal singularity. However, the cross‐frontal flows in the vicinity of the cooling are modified strongly, with the formation of a narrow downdraught of comparable dimensions and intensity to that suggested by the experimental data. the cooling has a weak influence on all the fields at a larger distance from the front; this is discussed in the context of the large‐scale, or geostrophic, response to lines of conve

ISSN:0035-9009    

DOI:10.1002/qj.49712152403

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA set of balanced and unbalanced slab‐symmetric linear 2‐layer models is analysed. the stability properties depend on a parametrized moisture budget dominated by the value of the underlying sea surface temperature. Convective heating occurs within an inner region characterizing the size of the convective disturbance. the heating is equal to the sum of two terms, representing the mid‐level and lower‐level upward mass‐fluxes.A fast and a slow mode of instability can be distinguished. the slow mode shows finite maximum growth for infinitesimally small disturbances and is the instability traditionally associated with Conditional Instability of the Second Kind (CISK). the fast mode reveals infinitely large positive growth‐rates for finite‐size disturbances.The transition of the model solution from slow to fast mode occurs at a threshold value of sea surface temperature. For the parameters chosen in this study, the threshold occurs at approxi

ISSN:0035-9009    

DOI:10.1002/qj.49712152404

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractSimulations are made of the long‐term mean clear‐sky and total‐scene components of the earth's radiation budget (ERB), using detailed solar and thermal infrared radiative transfer schemes. These employ as input a wide range of multi‐year observations of atmospheric, surface and cloud parameters. For clear skies, differences between the simulations and results from the Earth Radiation Budget Experiment (ERBE) are generally within the expected accuracy of the observations and calculations, with the most notable disagreement at high latitudes. For cloudy skies, the agreement is less good, but still generally within the systematic error bounds. the absorbed shortwave radiation (ASR) is simulated least well over oceanic regions, with the pattern of disagreement resembling the pattern of response when low cloud cover or liquid water path is perturbed; this suggests that the representation of low cloud cover or liquid water path dominates the ASR discrepancy between the simulation and ERBE. the simulated outgoing longwave radiation (OLR), which is sensitive to the high‐cloud emissivity, differs most from the ERBE OLR in deep convective regions; whilst assuming the high cloud to be black lessens agreement between the simulation and observations in some areas, agreement is substantially improved in others. This indicates that better knowledge of high‐cloud emissivity remains an important issue. the spatial and temporal variations of the sensitivity of the ERB to perturbations to low, mid and high cloud properties are investigated. Principal results include (a) the dominant importance of cloud amount, as compared to cloud temperature, in determining the sensitivity of the ERB to high‐cloud properties, and (b) the greater sensitivity of the net ERB to low‐cloud properties than to those of mid and high cloud. A major uncertainty in the calculations is the degree of cloud overlap; the inclusion of overlapping clouds is found to be capable of reversing the net high‐cloud sensitivities in certain locations. Global mean sensitivity to low cloud perturbations are compared with previous results obtained using a GCM. the simulated sensitivity of the ERB to low cloud cover is substantially larger than in the previous results, although the effects of changes in liquid water

ISSN:0035-9009    

DOI:10.1002/qj.49712152405

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe behaviour of initially‐barotropic vortices in vertically‐sheared environmental flows is investigated. the strength and structure of the vortices used are representative of tropical cyclones. the calculations are performed using a primitive‐equation numerical model on anf‐plane. It is found that the initial response of the vortex to the vertical shear is to tilt in the plane of the shear. As soon as a tilt is established, the upper‐ and lower‐level centres begin to rotate cyclonically about the mid‐level centre. This rotation can be understood in terms of upper‐ and lower‐level potential‐vorticity anomalies which are displaced in the horizontal relative to one another. the flow associated with the vertical projection of each anomaly advects the other anomaly, leading to the observed cyclonic rotation. the rotation rate decreases with time, so that the direction of tilt becomes constant, but the magnitude of the tilt continues to increase. We argue that the observed rotation acts to oppose the destructive action of the vertical shear on the vortex, even in the absence of diabatic processes.The role of the vertical circulation is considered in detail. It is shown that the vertical circulation develops in a manner which is consistent with the model flow remaining balanced. It is found that the mesoscale nature of the vertical circulation leads to a distortion of the axisymmetric vortex. This results in the inner core having a smaller vertical tilt than the outer region. the vertical circulation does not act on a large enough scale to explain why the vortex is not destroyed by the vertical shear.The behaviour of the vortex is found to depend on various parameters. Results are presented where the vertical shear, the strength and size of the vortex, the Coriolis parameter, and the static stability are varied. With the exception of the vertical shear, altering any of these parameters alters the vertical penetration of a potential‐vorticity anomaly. the results show that increasing the penetration depth leads to an increase in the rotation rate of the upper‐ and lower‐level vortex centres about the mid‐level centre, and to a reduction in the mag

ISSN:0035-9009    

DOI:10.1002/qj.49712152406

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA fairly simple stratospheric model of the three longest planetary waves is constructed, which includes interference terms between the three wave components, and a diffusive parametrization of wave breaking in regions where the local meridional potential vorticity gradient is negative. In order to allow comparison with the observed behaviour of planetary waves the model is forced near the tropopause with observed wave amplitudes and the model waves propagate over the observed zonal‐mean state. the model is able to mimic the behaviour of the observed waves fairly accurately, with the thermal damping, interference and wave breaking terms all being important to its success. Good results are obtained with a value of local wave‐breaking diffusion coefficient between 106and 107m2s−1. It is shown that the deficiencies in Matsuno's (1970) model are probably due mainly to the neglect of wave‐wave interactions and an over‐simplified zonal‐mean temperature structure.It is shown that Garcia's (1991) idea that wave breaking completely absorbs any build‐up of zonal‐mean wave activity is probably too severe, but that better results are obtained if the local nature of the wave‐breaking is taken into account.The zonal‐mean diffusion coefficient sometimes has large values within the vortex, as well as in the midlatitude surf zone, with the two regions of diffusion being separated by the vortex edge.The difficulty in estimating the diffusion coefficient required for a tracer is discussed, as well as the behaviour of the zonal‐mean potential vorticity flux during a wave‐breaking episode. the assumption of turbulent diffusion in reversed‐gradient regions implies a positive diffusive contribution to the potential vorticity flux.It was also found that changes in the equatorial zonal wind result in a modulation of the extra‐tropical potential vorticity flux similar to that found by Dunkerton and Baldwin (1991), but that the modulation did not depend strongly on a movement of

ISSN:0035-9009    

DOI:10.1002/qj.49712152407

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe UK Universities' Global Atmospheric Modelling Programme General Circulation Model (UGCM) has been run at high horizontal resolution to investigate a tropopause‐folding event at middle latitudes. Geopotential‐height, potential‐vorticity and specific‐humidity distributions were diagnosed from the model and showed a realistic representation of the synoptic development of the cut‐off low associated with the tropopause fold. the model also successfully portrayed the frontogenesis associated with tropopause folding and represented both the upper‐level frontal region and the corresponding jet‐streaks as they progressed through the region of the developing trough. A series of vertical cross‐sections at the ends of the jet streaks demonstrated that a fold could be captured by the UGCM using potential vorticity and specific humidity, although more small‐scale features were evident in the specific‐humidity contours than in the corresponding potential‐vorticity fields. the corresponding baroclinic zone was also resolved, although both the slope and thickness were rather larger than sometimes observed. Both the spatial and temporal extent of the fold compared well with observations. In addition the model simulated the ageostrophic winds in the entrance, exit and middle‐streak regions. A direct circulation was obtained at the entrance and an indirect circulation at the exit region of the jet streak, in good agreement with theory and with observational data. A combination of confluence, cold advection and curvature was responsible for the forcing of the ageostrophic motion at the jet entrance, whereas the forcing at the jet exit was attributed mainly to diffluence and curvature. Q vectors were employed to investigate the vertical motion in the vicinity of the fold, and showed the precise positions of descent corresponding to the fold along the entire length of the jet streak. Finally a tropopause fold around a jet that was not subject to the development of strong baroclinic instabilities was reproduced by the model: it was smaller in size than the fold of the cyclonic trough. A Q‐vector analysis confirmed the position of the small fold along the jet and revealed large‐scale ascending motion that may have been responsible for the small size of fold. the possibility of using the model to yield a realistic estimate of the transfer of air from the stratosphere to the troposphere in a tropopause‐folding event

ISSN:0035-9009    

DOI:10.1002/qj.49712152408

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractStratospheric and Mesospheric Sounder (SAMS) methane (CH4) and nitrous oxide (N2O) constituent measurements were taken a decade before the Upper Atmosphere Research Satellite (UARS) observations and are generally of lesser quality. However, SAMS data are important because of the limited lifetime of the UARS and because they provide a unique, historical data series for these gases involved in greenhouse and ozone‐depletion effects. While most prior SAMS analyses focused on zonal means, this paper assesses the strengths and weaknesses of analysing zonally asymmetric perturbations in SAMS data. It is shown that wave‐1 perturbations can be successfully investigated, provided sufficient care is taken and aliasing considered.At the lowest SAMS level, 20 hPa (∼ 28 km altitude) zonal‐wave‐1 CH4and N2O data reveal similar features for latitudes 45°N–65°N during stratospheric warming events and break‐up of the polar vortex. Large wave‐1 anomalies in the upper stratosphere (2 and 0.6 hPa) were found to be out of phase with the corresponding anomalies at 20 hPa. In one episode in early 1981 (during stratospheric sudden warming) southward winds over North America transported air with low mixing ratios from polar latitudes, while northward winds over Siberia transported air with high mixing ratios from subtropical latitudes. the effect produced strong wave‐1 amplitudes in both CH4and N2O mixing ratios.Cross‐correlations between wave‐1 CH4and N2O are large and positive in middle and high latitudes (consistent with ideal tracer behaviour for both constituent gases) but weak over the tropics. the cause of the latte

ISSN:0035-9009    

DOI:10.1002/qj.49712152409

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA variational approach to the Euler‐Lagrange equations for the quasi‐hydrostatic model is presented together with an explicit Hamiltonian (Poisson bracket) formulation. the bracket acts on functionals of the zonal and meridional angular momentum. Features of the relationships between the Hamiltonian formulations of the equations for an ideal fluid, the hydrostatic primitive equations and the quasi‐hydrostatic equations are disc

ISSN:0035-9009    

DOI:10.1002/qj.49712152410

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe diabatic effects of the quasi‐biennial oscillation (QBO) in ozone on the oscillation of temperature and zonal mean winds are investigated using a 2‐dimensional model with full interaction between dynamics, radiation and photochemistry. It is found that the temperature anomaly caused by the ozone QBO makes a significant (25%) contribution to the total temperature anomaly of the QBO although the corresponding change in wind is smaller (about 10%). It is shown that the diabatic effects of the ozone QBO should not be neglected in attempts to simulate the QBO in other paramet

ISSN:0035-9009    

DOI:10.1002/qj.49712152411

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY