摘要:

AbstractThe structure of a mature three‐dimensional baroclinic wave and its attendant cold front is examined using an idealized numerical model with a simple parametrization of dry buoyant convection resulting from diurnal heating. Relative‐flow isentropic analyses of the numerical solution and of an observed Australian summertime frontal system are compared. It is shown that the model well represents the typical synoptic environment accompanying frontogenesis in the Australian region. A diagnosis of the three‐dimensional vertical circulation is obtained using a technique involving a vector streamfunction. The circulation in a cross‐sectional plane parallel with the low‐level temperature gradient is found to be highly two‐dimensional, with geostrophic confluence constituting the principal frontogenetical forcing at low levels, while at upper levels the frontogentical forcing is dominated by geostrophic horizontal shear. In a second cross‐section, taken zonally, the frontogenetic forcing is found to consist almost solely of geostrophic horizontal shear throughout the entire troposphere. However, in this case the circulation in the plane of cross‐section is found to be only qualitatively two‐dimensional, as a significant proportion of the low‐level postfrontal subsidence is associated with circulation normal to the cross‐section. It is shown that the frontal structure analysed is sensitive to the location of the chosen cross‐sectional plane, and that this in turn provides an explanation for the observed relative shallowness of Australian summertime fronts. The effects of diurnal heating are important but confined to the surface‐based mixed layer. With the addition of diurnal heating, the prefrontal updraught amplifies almost threefold and the frontal temperature contrast over the heated region approximately doubles. The diabatic forcing term in the well‐known Sawyer‐Eliassen equation is shown to be negligible. This suggests that the increased low‐level convergence is a reflection of the model's enhanced response to frontogenetic forcing in the presence of low static stability in the surface‐based well‐mixed layer. Finally, the numerical model solution is compared with a simpler two‐dimensional model specifically developed

ISSN:0035-9009    

DOI:10.1002/qj.49711749702

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe spectral atmospheric model used for prediction at the European Centre for Medium Range Weather Forecasts (ECMWF) is modified to change the spectrally‐represented thermodynamic variable from temperature to the deviation of temperature from a reference profile which depends analytically on pressure. There is only a minor change to the form of the thermodynamic equation, and the calculation of the pressure gradient is modified to eliminate some of the cancellation between the Δ andRTΔInpterms which occurs in the standard formulation. At T42 resolution the revised scheme significantly improves southern hemispheric forecasts, with differences originating over Antarctica. There is also a small improvement over the Arctic. Sensitivity is much less at T106 resolution, but a minor advantage can still be seen in high southern latitudes, and there is a reduction in noise in the vertical velocity field near the Andes at tropical latitudes. Similar results are found for a version in which surface pressure, rather than its logarithm, is chosen as a spectrally represented prognostic variable. This gives mass conservation but the stability of the semi‐implicit time integration is somewhat reduced.The improvement seen at high latitudes is also captured by a simpler revision which retains temperature as the spectrally‐represented variable, and uses a reference temperature only in the computation of the pressure‐gradient terms in gridpoint space. Results indicate that much of the systematic difference in behaviour between T42 and T106 simulations at high latitudes can be removed by changing the pressure‐gradient calculation. In particular, the new schemes correct a systematic tendency for erroneously high pressures east of the Ross Ice Shelf over Antarctica in lower resolution simulations at medium and longer time ranges. Improvement occurs because the schemes reduce an inconsistent treatment of the Δ andRTΔInpterms in the model's semi‐implicit treatment of the vorticity equation.The reference temperature profile has also been used in data assimilation to reduce a systematic error in the calculation of first‐guess geopotential heights at standard pressure levels. The resulting height analyses agree slightly better with radiosonde measurements, and initialization causes less of a degradation of the fit to observed data. Temperatures are systematically warmer in the upper troposphere (by almost 0.5 K in the global mean at 200 hPa) and cooler in the lower troposphere, and are closer to observed values. Results from forecasts carried out after one and two days of assimilation show small improvements in the short and early medium range, and are inconclusive at l

ISSN:0035-9009    

DOI:10.1002/qj.49711749703

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractA major revision is described to the analysis component of the Meteorological Office data assimilation scheme for both the global and the regional numerical weather prediction models. Initialization by repeated insertion of data and divergence damping is retained but a more flexible modified successive‐correction scheme replaces optimum interpolation for the analysis step. The scheme is derived as an approximate iterative solution to a general equation for the optimal analysis. Within iterations, the analyses of each model variable are performed sequentially and multivariate increment fields are derived for dynamical balance at intermediate stages. The horizontal influence area of data is substantially increased, giving a beneficial impact in data‐sparse regions. All data contribute to the analysis at each gridpoint within their radius of influence. The observation weights are recalculated every model time‐step, allowing asynoptic data to be used at their true valid time, though this feature is found to have only slight impact on forecasts. Each analysis is factorized into a vertical step, then into a horizontal step. The structure of each is described in detail, with system parameters listed in appendices.Results of two kinds are presented. Sensitivity experiments highlight the effects of some individual constituents of the scheme; operational trials against the old scheme reveal the impact of the whole system. The new analyses are better balanced and lead to a better short‐period forecast, especially for the wind field. Medium‐range impact is clearly beneficial in the southern hemisphere but ambiguous in the northern hemisphere. Precipitation forecasts in the regional model are

ISSN:0035-9009    

DOI:10.1002/qj.49711749704

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractIt has previously been shown that semi‐Lagrangian schemes can be applied to spectral models of the shallow‐water equations using large time‐steps (e.g., see Ritchie 1988). The present study considers the extension of this work to a multilevel spectral primitive‐equations model.As a first step, an Eulerian vorticity‐divergence spectral model is converted to an Eulerian model based on a vector momentum form of the equation of motion. From the latter, several semi‐Lagrangian models are prepared: one using an interpolating semi‐Lagrangian treatment of advection in the horizontal (referred to as 2DISL) while retaining Eulerian advection in the vertical, another using a 3‐dimensional interpolating semi‐Lagrangian formulation (referred to as 3DISL), and another which combines the 2DISL scheme in the horizontal and a non‐interpolating semi‐Lagrangian treatment in the vertical (referred to as NISLV). Medium‐range intercomparison experiments are performed using models that include simple physical parametrizations. It is shown that the semi‐Lagrangian semi‐implicit approach can be applied accurately and stably to produce medium‐range (5‐day) forecasts using time‐steps that are far larger than those permitted by the Courant‐Friedrichs‐Lewy (CFL) stability criterion for the corresponding Eulerian model.The NISLV version is found to be more accurate than the 3DISL one which apparently has excessive damping in the vicinity of the tropopause, where all the model f

ISSN:0035-9009    

DOI:10.1002/qj.49711749705

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe motion of an initially symmetric vortex in a spatially‐varying large‐scale flow on a beta plane is investigated using a nondivergent, barotropic numerical model. The calculations extend those carried out for the case of zero basic flow in Part I. The large‐scale flows are provided by meridionally‐varying zonal flows, or single‐mode, stationary, finite‐amplitude planetary waves in a channel. Interest is focused on the evolution of vortex asymmetries and their role in determining vortex motion relative to the basic large‐scale flow.As in Part I, the calculations are used to assess averaging procedures for computing the environmental wind field of a tropical cyclone from observed wind data. It is shown that averaging over an annular region centred on the vortex is, in principle, more accurate when there is a basic flow, in comparison with the case of ze

ISSN:0035-9009    

DOI:10.1002/qj.49711749706

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractAll available, routinely collected surface data are used to document the development of the ‘Papal Front’, which crossed southern Germany on 3 May 1987 causing casualities and damage. Objective analyses on isentropic surfaces, initialized with the European aerological data, indicate that the development of the violent mesoscale front, which exhibited squall‐line characteristics, began when a distinct mid‐troposopheric anomaly of potential vorticity reached the north‐western rim of the Alpine arc. The forced uplift ahead of the associated shortwave trough combined with ageostrophic motions due to a jet streak at higher levels apparently provided a trigger for deep convection as was observed over the Alpine foreland. Numerical simulations with a dry hydrostatic model indicate the relative importance of the synoptic‐scale forcing and the Alpine orography for the generation of the ‘Papal Front’. A westerly ‘orographic jet’ over the foreland below crest height only develops in the simulation with full Alps and is corroborated by the few mountain wind observations that are available.A comparison with studies of frontal propagation along other mountain ranges reveals that the routine network in the Alpine region and its surroundings is at least of similar density, but so far hardly used for frontal case‐studies. It is concluded that a series of ‘routine‐data case‐studies’ could help to determine the variability of frontal progression along the Alps and, thus, provide guidance when assessing the significance of single events, particularly those sampled during s

ISSN:0035-9009    

DOI:10.1002/qj.49711749707

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractData from fifty‐two aircraft flight legs 100−150m above ground from HAPEX and FIFE are analysed to estimate characteristics of boundary‐layer moisture fluctuations in boundary layers with different bulk stability and surface energy regimes. In HAPEX, considerable effort was devoted to the quality of measurement of moisture fluctuations. The data include repeated 120km flight legs flown at 150m over the same relatively homogeneous terrain, allowing statistical examination of motion characteristics on horizontal scales up to 10km.This study finds two prototype boundary‐layer regimes. With significant boundary‐layer instability, relatively weak surface evaporation and drier air aloft, the entrainment‐drying boundary layer may develop. This boundary layer is characterized by vertical divergence of the moisture flux and significant diffusion of dry air from the top downwards (top‐down). In this type of boundary layer, dry air occasionally reaches the lower boundary layer, leading to negative moisture skewness in spite of positive temperature and vertical velocity skewness associated with warm moist updraughts. In contrast, the moistening boundary layer associated with greater surface evaporation is characterized by positive moisture skewness near the surface.In addition to the above turbulent moisture fluctuations, some of the above data are characterized by 10km moisture variations with horizontal gradients often concentrated in narrow zones of horizontal convergence. Since corresponding signatures of vertical velocity and temperature are weaker, these zones are referred to as ‘mesoscale’ moisture fronts. As a more general feature, moisture variations on scales of 5km and greater are negatively correlated with temperature variations associated with cool moist regions and warm dry regions. On scales of tens of kilometres and greater, such negative correlation may be due to inhomogeneity of the surface energy budget. The negative moisture‐temperature correlation leads to large mesoscale variations of relative humidity and the lifted

ISSN:0035-9009    

DOI:10.1002/qj.49711749708

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractA series of tracer experiments studying concentration fluctuations in a dispersing plume of pollutant in the atmosphere at ranges of between 50m and 1000m is described. Experiments were conducted on three different field sites in near‐neutral or slightly convective meteorological conditions. The results show time series which are characterised by the intermittent occurrence of periods of fluctuating non‐zero concentrations, interspersed by periods of essentially zero concentration. The spectrum of concentration fluctuations is found to display inertial subrange behaviour, characterised by a −2/3 power law whennS(n) is plotted against frequencyn, whereS(n) is the variance (of the fluctuation) per unit frequency interval. The spectral peak frequency varies with distance from the source. In all cases the clipped‐normal probability density function (PDF) provides a reasonable fit to the concentration PDF. The exponential PDF is less flexible in fitting a wide range of experimental conditions, but is slightly superior for some short range examples. In the alongwind direction it is found that, although there is a rapid initial decrease in fluctuation intensity with distance, the intensity seems to approach an approximately constant non‐zero value at long range. In cross‐sections of the plume the variation of fluctuation statistics is dominated by the varying proportion of time during which the concentration is essentially zero. Conditional statistics, calculated from significantly non‐zero concentrations only, show only slight variations acr

ISSN:0035-9009    

DOI:10.1002/qj.49711749709

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractIncreases in the concentrations of ice particles, as well as precipitation development, can proceed very rapidly in even quite small maritime cumuliform clouds. In the upper portions of these clouds, ice particle concentrations can increase from1 per litre. For maritime and continental cumuliform clouds with widthsD≥ 3km, the breadth of the droplet spectrum near the cloud top is a better predictor of the maximum ice particle concentrations that will develop in this region than is cloud‐top temperature.Exceptions to the general picture described above are the very narrow (D1 per litre.Several proposed mechanisms for the formation of high ice particle concentrations in clouds are discussed in the light of these field observations. In a detailed case‐study, ice particle concentrations produced by the ejection of ice splinters during riming were calculated to be about an order of magnitude less than the ice particle concentrations measured in the upper regions of the wider maritime clouds. The freezing of evaporating drops by contact nucleation could be responsible for the frozen drops that precede the high concentrations of vapour‐grown ice crystals. The latter could be produced in localized regions of unexpectedly high supersaturation that may be produced when drops begin to grow by collisions. This explanation is consistent with several of our findings concerning the conditions under which high concentrations of vapour‐grown crystals appear almost spontaneously in maritime clouds. Similarities between these natural crystals, and the conditions under which they appear, and those produced artificially by dry‐ice seeding and by aircraft fly

ISSN:0035-9009    

DOI:10.1002/qj.49711749710

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThermodynamic analysis of aircraft observations of the downdraughts at the edges of small maritime cumulus clouds is used to obtain estimates of the rate of entrainment of cloudy air into the downdraughts. The entrainment rates are around 10−4to 10−2s−1. A simple entraining parcel model is used to investigate the fate of droplets entrained at this rate into such downdraughts. It is shown that the majority of small droplets evaporate totally, resulting in a subsaturation in the downdraught of less than 1%, except close to cloud top or cloud base. However, a few droplets which contain large soluble nuclei continue to grow, or undergo only a small amount of evaporation, during their descent. The limited evaporation ensures that some droplets undergoing a cycle of ascent and descent will grow even without net ascent. The largest droplets at some levels are therefore to be found in the downdraughts although the droplet concentration and liquid water content are much lower than in the cloud at the same level. The calculated growth and the thermodynamic properties of the downdraughts are in good agreement with observations of small maritime cu

ISSN:0035-9009    

DOI:10.1002/qj.49711749711

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY