摘要:

AbstractThe three‐dimensional evolution of the final warming that takes place in the stratosphere of the southern hemisphere during spring is studied using data from a satellite. The event of spring 1982 is discussed in detail, and other events from an 8‐year set are briefly surveyed. The zonal‐mean westerly jet moves poleward and downward in spring as strong, planetary‐scale disturbances develop which contribute to the weakening of the stratospheric westerly vortex. The processes governing this weakening are discussed by reference to isentropic maps of Ertel's potential vorticity and associated area diagnostics. The vortex breaks down first in the upper stratosphere and then later (and more slowly) in the middle stratosphere. This behaviour is broadly reproduced year after year. Repeating life‐cycles of growth, eastward movement and decay of anticyclones in the stratosphere are described and related to the behaviour of quasi‐stationary wave 1 and eastward‐travelling wave 2. Evidence that the topography of the southern hemisphere exerts a strong influence on the evolution of the final warming is presented. An association is found between the location of anticylones in the upper stratosphere, warm pools of air in the lower stratosphere and a climatological split of the westerly jet stream in the uppe

ISSN:0035-9009    

DOI:10.1002/qj.49711448402

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractSeasonality in planetary wave structure in the southern hemisphere stratosphere and troposphere (1000‐1 mb) is studied based on eight years (1979–1986) of daily geopotential data. Emphasis is placed on the distinctive seasonality in the stratosphere, along with tracing the stratospheric fluctuations into the troposphere. Time filtering is used to separate stationary and transient components. Maximum geopotential height variance is observed in the middle to upper stratosphere in late winter‐spring, with a distinct secondary, smaller maximum in late fall‐early winter; these maxima sandwich the strongest mid‐winter zonal winds. This sequence occurs approximately one month earlier in the upper stratosphere. Two corresponding peaks are observed in stratospheric zonal mean wind and temperature fluctuations. The two maxima in stratospheric wave variance are not in agreement with observed seasonal changes in quasi‐geostrophic refractive index of the zonal mean flow.The stratospheric geopotential variance maxima result from nearly equal amounts of stationary and transient zonal wave‐1 fluctuations, with transient wave 2 also contributing during late winter. Stationary wave 1 shows intriguing seasonal evolution: the early winter maximum is predominantly equivalent barotropic, whereas that of late winter‐spring is highly baroclinic in the stratosphere. Stratospheric poleward fluxes of heat and momentum both exhibit pronounced maxima during September‐October; these result primarily from the stationary wave‐1 contributions, and the transients assume a secondary role. Although not as pronounced as the stationary wave seasonality, seasonal differences in transient wave behaviour are also documented, including three‐dimensional propagation characteristics and wave‐mean flow

ISSN:0035-9009    

DOI:10.1002/qj.49711448403

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractIn this paper data are presented from aircraft passes through continental cumulus clouds obtained during the Cooperative Convective Precipitation Experiment. The effect of evaporation of liquid water from the cloud following the entrainment of free tropospheric air is to totally remove a fraction of the droplets of all sizes rather than partially evaporate most of the droplets. Supersaturations in the cloud are found on average to be considerably higher than those predicted for an adiabatic cloud and the supersaturation is not closely tied to the vertical wind.These observations are found to have a number of important consequences for cloud droplet growth: 1. The average lifetime of individual droplets is considerably less than the age of the cloud and is not expected to exceed about five minutes. This has important consequences for the oxidation of SO2to sulphate. 2. Statistically favoured droplets that avoid evaporation are observed to grow much larger than average droplets and to sizes larger than would occur at the same altitude in an adiabatic cloud.

ISSN:0035-9009    

DOI:10.1002/qj.49711448404

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractA general analysis is developed for turbulent shear flows over two‐ and three‐dimensional hills with low‐slopes which allows for a wide range of upwind velocity profiles, such as those caused by wakes of upwind hills, roughness changes, or changes in stratification. In this paper the atmosphere is assumed to be neutrally stable and the half‐lengths of the hills,L, are large compared with their heights,H, which are very large compared with the roughness lengthzo. The general structure of the solution is defined by dividing the flow into two regions, each of which is divided into two sublayers: an inviscid outer region composed of an upper layer in which there is potential flow when the atmosphere is neutrally stable, and a middle layer in which the wind shear dominates; and an inner region of thicknessl ≤ Lin which the effects of perturbation shear stresses are confined. The latter region is divided into two: a shear stress layer where the shear stresses, although weak, determine that the maximum of the perturbation velocity is located in this layer; and an inner surface layer of thicknesslswhere the shear stress gradient varies rapidly and the perturbation velocity tends to zero. The details of the middle layer are given here for different kinds of upwind profiles, including logarithmic, ‘power law’ and linear profiles. It is shown that the analysis can be extended to allow for nonlinear inertial effects in the middle layer. Analytical solutions are derived for the inner region as asymptotic expansions in δ = [ln(l/zo)]−1, which is assumed to be small, and this shows thatls∼ zo(l/zo)1/2.The results of the analytical model are compared with our own and with previously published numerical computations of the full equations (applying the same assumptions used for calculating the turbulent shear stresses as used in the analytical work), which have largely been validated against full‐scale measurements. These results confirm that the relative increase of surface stress is significantly greater than the increase of wind speed near the surface except when there is no upwind shear (as for example in a logarithmic boundary layer when the roughness length tends to zero).Finally, the paper shows that the outer regions of laminar (or constant eddy viscosity) and of turbulent flows over hills are broadly similar, but that the effects of the flow in the inner region on the outer regions are much smalle

ISSN:0035-9009    

DOI:10.1002/qj.49711448405

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractA lidar system has been developed to measure the mixing ratio of atmospheric water vapour using Raman scattering from water vapour and nitrogen. The system uses the third harmonic of a neodymium‐YAG laser at 354.7 nm, with interference filters to isolate the vibrational Raman bands. Humidity profiles extending from about 2 km to the tropopause have been obtained, with a resolution of 30 m vertically and 20 minutes in time below 5 km, and>300m vertically and several hours in time above 10 km. Good agreement was found between the lidar profiles and those of coincident radiosondes. Distinctive layers in the humidity profiles are presented, and measurements in the vicinity of cirrus clouds indicate sharp transitions from supersaturation to saturation with respect to ice at the cloud boundar

ISSN:0035-9009    

DOI:10.1002/qj.49711448406

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractA new design of conventional raingauge is proposed that minimizes the most important source of systematic error in point rainfall measurements: that due to the interaction of the wind with the gauge (the ‘raingauge exposure’ problem). Basic physical principles, numerical simulations and extensive field trials in England and Denmark are also used to provide a consistent quantitative description of the wind‐induced errors of conventional cylindrical gauges.It is shown that an inverted conical collector with a large semi‐vertical angle may provide the basis of an adequate solution of the raingauge exposure problem. The need to minimize outsplash demands that the collector should have a fairly large diameter and take the form of a flat champagne glass. A small diameter flat champagne glass collector has been tested for over 31/2 years against a standard 5′’ cylindrical gauge, a 5′ pit gauge and an inverted conical gauge at the Institute of Hydrology, Wallingford. The insensitivity of the losses of the ‘flat champagne glass’‐shaped collector to wind speed at low rates of rainfall is confirmed, as well as the need for a larger collector diameter to minimize outsplash. A combination of a ‘steep champagne glass’ gauge and a ‘flat champagne glass’ gauge might lead to even better estimates of rainfall in high winds, notably on ships, and could provide a low‐cost approach to improving

ISSN:0035-9009    

DOI:10.1002/qj.49711448407

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractA hydrographic section through the Gulf Stream and its recirculation to the south is interpreted in terms of an analytical model based on the baroclinic Fofonoff gyre, in which layers shielded from forcing are homogenized to a uniform value of potential vorticity. Given this simple paradigm of the circulation, the process of anticyclogenesis is studied in which seasonal changes in the volume of low potential vorticity waters accompany a steepening of the main thermocline and an intensification of the recirculating gyre. The consequences of the gyre intensification for the structure of the bowl, the circulation within it and the deep abyssal flow are investigated. It is shown that as the anticyclogenesis proceeds, there is a general southward shift of the current systems; the bowl of the circulation strikes the ocean floor further south and there is an intensification of the depth‐independent recirculation tight in to the axis of the Stream. These changes in the structure of the recirculation are broadly in accord with observed seasonal variability associated with wintertime mode water productio

ISSN:0035-9009    

DOI:10.1002/qj.49711448408

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractEstablished formulae for various rates of change of horizontal wind direction may be unified by a treatment in terms of the spin vector,S, of a vector fieldM. The spin vector measures the rate of change of direction ofM, in the appropriate osculating plane, and may be defined with respect to any infinitesimal displacement in space and time. WhenMis a horizontal vector field, the osculating plane is horizontal, andSimmediately gives the rate of change of direction relative to a fixed horizontal axis. The formulae involved are those of Hide and of Bryden for the rate of change of geostrophic wind direction with height, and of Burk and Staley and of Lecluyse and Neumann for the Lagrangian rate of change of wind direction. WhenMis a 3‐dimensional vector field, the orientation of the osculating plane is not in general constant and the Cartesian components ofSdo not represent the rate of change of direction in the most concise terms. The Lagrangian spin vector of the 3‐dimensional velocity field may nevertheless be considered as a basic kinematic quantity, on a par with the rates of change of kinetic energy, vorticity and diverge

ISSN:0035-9009    

DOI:10.1002/qj.49711448409

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractA Galerkin finite element discretization for the vertical coordinate using high‐order basis functions is introduced. The basis functions are cubic splines, assuming continuity of the spline and its first and second derivatives. Locality of the scheme is required, allowing each basis function to be different from zero on four grid intervals only. An efficient solution of the resulting Galerkin finite element equations is achieved by introducing a collocation grid.A number of test integrations were done using the ECMWF spectral model with resolution T63 for horizontal discretization. The scheme was also used in a data assimilation experiment where it reduced the first guess erro

ISSN:0035-9009    

DOI:10.1002/qj.49711448410

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

ISSN:0035-9009    

DOI:10.1002/qj.49711448411

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY