摘要:

AbstractThe mean vorticity balances in the tropical atmosphere during December 1982 to February 1983, a season of extraordinary anomalies associated with the warming of the east equatorial Pacific Ocean, are determined using the 6‐hourly initialized and objectively analysed observations archived at the European Centre for Medium Range Weather Forecasts. the budgets are evaluated at one upper (150 mb) and one lower (850 mb) tropospheric level, with particular emphasis on the balance at 150 mb, near the level of maximum outflow associated with organized deep convection in the tropics.The balances at the upper and lower levels are found to differ qualitatively from one another. At 850 mb, the zonal flow is weak and the relative vorticity and its gradient small compared with that of the Coriolis parameter. the vorticity balance is therefore essentially a Sverdrup balance between the stretching and horizontal advection of planetary vorticity, and an assumption of linearity about a basic state at rest is not unreasonable. None of this applies, however, at 150 mb.At 150 mb, the balance is essentially nonlinear, and nearly inviscid, the primary balance obtaining between the stretching and advection of absolute vorticity by the time‐mean horizontal flow. Transients play a small though not insignificant role, but terms involving the vertical advection and twisting of vorticity are small enough to be neglected.The essentially nonlinear character of the upper tropospheric dynamics is highlighted by demonstrating the inability of a diagnostic barotropic model linearized about the observed steady zonal mean flow to simulate the anomalous flow in the central Pacific correctly. the idea of the expansion of material circuits in regions of strong convective outflow, leading to a rapid spin‐down of the local absolute vorticity, and the production of a region of almost zero absolute circulation, is of crucial importance. This is seen to be intimately related to the importance of nonline

ISSN:0035-9009    

DOI:10.1002/qj.49711146802

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractAir parcel trajectories are calculated for the mid‐stratosphere using data from the stratospheric sounding unit on board NOAA‐6. In the analysis method all the orbital data for each 24 h period are combined into a single global analysis. Three trajectory methods are used‐isobaric, isentropic and quasi‐isentropic‐and the results are compared and contrasted. For the quasi‐isentropic method a radiation model is used, via the thermodynamic equation, to imply cross‐isentrope flow at regular intervals along the trajectory. the positions of the parcels, computed using the three methods, are found in general to be in good agreement when projected on to a horizontal plane. However, the altitude varies according to the trajectory method used and changes by about 1 km along a quiescent trajectory and as much as 2 km along a disturbed trajectory, during a 10‐day period. This has important implications in the study of chemistry along parcel trajectories during disturbed periods since the temperature‐ and pressure‐dependent reactions will proceed at different rates under the different assumptions. Also, the high lapse rate in the vertical mixing ratio profile of ozone implies that the detailed photochemistry will depend critically on the height of the air parcel.Further, potential vorticity and potential temperature are used as quasi‐conservative Lagrangian tracers, to try to determine which trajectory method is most realistic. For the quiescent period studied (June 1979 in the southern hemisphere) the isobaric and isentropic methods produced similar results throughout the ten days of the trajectory but the quasi‐isentropic method was clearly superior. A disturbed period was also studied (the February 1979 stratospheric warming), but the results from the trajectories studied were consistent with the Lagrangian conservation laws for only 6 days, at most. These results illustrate the need for caution, particularly with regard to sensitivity to initial horizontal position, in using trajectories calculate

ISSN:0035-9009    

DOI:10.1002/qj.49711146803

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractBy the use of stratospheric height and temperature field data obtained from the TIROS satellite, dynamical interaction between planetary waves and mean zonal winds in the stratosphere is investigated. Special attention is paid to differences between northern and southern hemispheres. an analysis is made for pressure levels from 70 to 0.4 mb during the period from June 1981 to May 1982. Eliassen‐Palm (E‐P) flux diagnostics are used as a powerful and useful tool to investigate the wave‐mean flow interaction.From winter to summer the seasonal marches of wave activity (measured by the E‐P flux) and of mean zonal wind are different between the two hemispheres. In the NH, wave activity varies intermittently with a characteristic timescale of about two weeks. Once the core of the stratospheric westerly jet shifts poleward due to a minor warming, the subsequent wave activity breaks the westerlies dramatically to replace them by the easterlies in association with the sudden warming, whilst in the SH wave activity in mid‐winter is quiet, corresponding to small time variations of the maximum westerly speed. In late winter the core of the stratospheric westerly jet suddenly shifts poleward and downward due to a wavenumber‐2 minor warming. After the shifting of the westerly jet the wave activity of wavenumber 1 is enhanced, and continues until early summer.On the basis of this observational evidence we discuss: (i) the seasonal evolutions of wave activity and the mean zonal wind; (ii) the interannual variability of the stratospheric circulations; and (iii) the time variation of wa

ISSN:0035-9009    

DOI:10.1002/qj.49711146804

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractMaps of Ertel's potential vorticity,Q, are computed on an isentropic surface in the middle of the stratosphere. They are derived using data from stratospheric sounding units on board NOAA satellites. the reliability of the maps is demonstrated mainly by conservation ofQthat is shown in successive analyses, and by the close agreement obtained using independent data from two satellites.The maps are used to follow the movement of material during the course of strong disturbances which occurred in December 1981. They clearly show the breaking of planetary waves in the stratosphere, an inherently nonlinear process. the effect of wave breaking on the structure of the westerly vortex is considered. Any mixing ofQon isentropic surfaces during wave breaking is shown to be far from complete in the examples studied.The behaviour of diagnostics based on zonal averaging, such as the Eliassen‐Palm flux, is accounted for by local changes in the distribution ofQ. the implications of our findings for the study of stratospheric dynamics are discusse

ISSN:0035-9009    

DOI:10.1002/qj.49711146805

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractThis paper presents a case study of a wave‐mean flow interaction, or minor warming, near the southern hemisphere stratopause for 15‐30 July 1974. the analysis is based mainly on data from the selective chopper radiometer carried by Nimbus 5.A deceleration of the zonal mean wind of 70 m s‐1from 20 to 25 July is observed in middle latitudes at 0.5 mb, with an acceleration further south. the polar vortex is, however, not disrupted.Eliassen‐Palm cross‐sections are presented for the event and a good correlation found between the E‐P flux convergence and the deceleration. the channelling of the E‐P fluxes from the tropopause to the convergent region is in broad agreement with the distribution of refractive index but there is a region of E‐P flux divergence at high levels in polar latitudes suggesting a source of wave activity there.The residual circulation is calculated both from the momentum budget and from an omega equation, both methods diagnosing a region of modest equatorward flow near the region of E‐P flux divergence embedded in the stronger poleward flow elsewhere.Maps of potential vorticity on the isentropic surface of 0 = 1640 K are presented and the evolving pattern related to zonal mean and isobaric descriptions. the event is shown to be associated with an incursion of low latitude air into middle latitudes with a corresponding irreversible mixing of potential vorticity reminiscent of the ‘wave breaking’ in northern hemisphere events reported by McIntyre and Palmer (J. At. Ter. Ph.,46, 825), but of m

ISSN:0035-9009    

DOI:10.1002/qj.49711146806

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractResults are presented from an observational study of the mid‐latitude atmospheric boundary layer over the sea. the data were obtained mainly by instrumented aircraft as part of the Joint Air‐Sea Interaction Experiment (JASIN).The eight occasions chosen for study are characterized by small surface buoyancy fluxes and the conditions are well described as near neutral and barotropic. In the absence of low‐level inversions, a well‐mixed Ekman layer is observed on each occasion but is limited to a depth of approximately 0.2u */f. Accurate measurements of horizontal gradients (including pressure) together with improved wind observations and turbulent flux measurements enabled each of the terms of the momentum balance to be evaluated throughout the depth of this layer. These terms were found to balance quite closely, were well described by Ekman scaling and were consistent with the requirements of the measured turbulent kinetic energy balance. the latter suggests that the main shear production terms are dissipated locally and that little is exported to upper levels in the Ekman layer to enable significant deepening by entrainment although only a slight increase in instability appears to be needed to alter this balance. Conditions in which such boundary layer structure might be observed are suggested. Values of various coefficients used in schemes for relating surface fluxes to mean quantities (CD, CE, Cg, A, B) are derived and compared with previous measurements.Spectral analysis reveals that most of the turbulent transport is confined to a distinct high wavenumber region whose characteristics vary as a function of Ekman layer depth and stability parameters. This is superimposed on larger‐scale fluctuations which do not vary appreciably within the Ekman layer and which therefore dominate the variances in the upper regions as the intensity of the smaller‐scale turbulence decreases strongly with height.Finally, further implications of this interpretation are discussed with particular reference to the heat and water vapour balance. These imply that removal of water vapour from the Ekman layer is accomplished by transfer related to cloud activity, marking a significant change in the mechanism of turbulent transport at the top of this layer. the relationship between these and similar results obtained concurrently by different methods is als

ISSN:0035-9009    

DOI:10.1002/qj.49711146807

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractA primitive equation barotropic stability analysis of the 700 mb zonal wind profile over the Arabian Sea was performed for each day between 10 and 14 June 1979, during the onset phase of the monsoon. the flow was found to become progressively more unstable with the passage of time. the most unstable inviscid primitive equation mode has an e‐folding time of 3.1 days, a wavelength of 3500 km and a westward phase speed of 1.9ms‐1.The non‐geostrophic effect reduces the growth rate and changes the structure of the unstable mode significantly. It is noticed that when the normalized unstable mode is superposed on the basic state, the total field is closer to the observations. Momentum transports by the primitive equation mode and by the quasi‐geostrophic approximation are in opposite directions in equatorial regions. the energetics of the primitive equation unstable mode are computed and the physical processes responsible for the westward propagation of the wave are ide

ISSN:0035-9009    

DOI:10.1002/qj.49711146808

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractThe interaction of a polar trough with a polar front can lead to a characteristic satellite cloud pattern commonly referred to as an instant occlusion. the detailed structure and development of an event rather resembling an instant occlusion is analysed in this paper using routine data supplemented by additional radiosondes, soundings and cloud imagery from NOAA‐7 and Meteosat, and radar network pictures. the event presented differs from an instant occlusion as described by Anderson and others in that a band of polar front cloud developed simultaneously with the band of cloud associated with the polar trough rather than being a pre‐existing feature. Thus we refer to it under the different name of pseudo‐occlusion in order to keep this distinction in mind. A conceptual model of the pseudo‐occlusion is developed in which the key elements are (i) a moist, relatively warm, low‐level jet associated with the polar trough, referred to as the ‘polar trough conveyor belt’, and (ii) a major upper‐level jet streak associated with the polar front, referred to as the ‘polar front conveyor belt’. the two conveyor belts intersect almost at right angles. Precipitation associated with the polar front is predominantly from stratiform cloud, whereas that associated with the polar trough is a mixture of stratiform and convective. Just on the poleward side of the upper jet the cloud top drops abruptly. Here, dry air having recently descended beneath the tropopause fold, suppresses the depth of the moist air associated with the polar trough conveyor belt. This leads to a minimum in the intensity of precipitation between the rain areas associated with the trough and the front. Although the air overrunning the trough just on the poleward side of the polar front is dry as a result of earlier descent, the air in this location is likely to have begun ascending as part of the indirect circulation in the exit region of the upper level jet streak. the polar trough low‐level jet probably forms par

ISSN:0035-9009    

DOI:10.1002/qj.49711146809

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractIce particle size spectra measured by optical particle size spectrometers during airborne sampling of clouds associated with several frontal precipitation systems in the Pacific Northwest are presented and discussed. In the majority of the spectra measured, the concentration densities of particles<2 mm in diameter deviated significantly below or above the exponential curve fitting larger diameters (we term thesesub‐exponentialandsuper‐exponentialspectra, respectively). Sub‐exponential spectra were found to be dominant in regions of weak stratiform cloud, where liquid water was scarce. Super‐exponential spectra were dominant in convective regions between the ‐1 and ‐9°C levels, where supercooled liquid water was present in amounts up to 0.2 gm‐3. Exponential spectra were dominant in convective regions at the ‐11 °C level and above.Microphysical processes as well as shear‐induced particle size sorting were found to play a role in the development of spectrum shapes. Depletion of small particles due to aggregation appears to have been an important cause of sub‐exponential spectra. Secondary ice crystal production appears to have been responsible for the formation of super‐exponential spectra. the role of particle size was largely to broaden the range of spectrum shapes present, rather than to promote the dominance of a particular spectrum type.The impact of spectrum deviations on the relationship between radar reflectivity factor and snowfall rate, though observable, is found to be small with respect to other sources of error often found in rada

ISSN:0035-9009    

DOI:10.1002/qj.49711146810

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractA study of the cloud structure and precipitation in the transition zone of cold fronts over western Victoria shows that the important precipitation parameters, such as cloud top temperature and depth, are determined synoptically. Pre‐frontal clouds experience slow uplift over substantial depths; this results in cold deep clouds with lifetimes of the order of tens of hours. In the post‐frontal clouds the uplift is mesoscale and shallow and causes short‐lived cloud elements.These dynamic and thermodynamic featurea determine the cloud microphysics and largely the associated rainfall. Precipitation in pre‐frontal clouds forms mostly through lightly rimed ice crystals produced at low temperatures; the process can be very efficient in glaciating the cloud but not necessarily efficient in producing ice crystals large enough to survive the fall to the surface. In the post‐frontal systems the ice crystals are produced at much higher temperatures in the presence of higher liquid water contents and the precipitation depends very strongly on cloud depth and

ISSN:0035-9009    

DOI:10.1002/qj.49711146811

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY