摘要:

AbstractWe present an intercomparison of analyses of the main FGGE level IIb dataset with three advanced analysis systems. the aims of the work are to estimate the extent and magnitude of the differences between the analyses; to identify the reasons for the differences; and, finally, to estimate the significance of the differences for forecast skill. We restrict ourselves primarily to a consideration of the extra‐tropical analyses. the subject of tropical analyses merits separate treatment.We discuss objective evaluations of analysis quality, such as fit to observations, statistics of analysis differences, and mean fields. In addition we place substantial emphasis on subjective evaluation of a series of case studies which were selected to illustrate the importance of different aspects of the analysis procedures, such as quality control, data selection, resolution, dynamical balance, and the role of the assimilating forecast model. In some cases the forecast models are used as selective amplifiers of analysis differences to assist in deciding which analysis was more nearly correct in the treatment of particular data. In part II we consider the overall impact of the analysis differences on forecast quality, and the implications for predictability.In general the analysis systems draw reasonably well to the data, although each system has its own characteristics in this regard. the root mean square differences between the analyses are of the expected order of magnitude, although there are clear differences in the closeness of agreement between different pairs of analyses. Systematic differences arising from particular components of the assimilation suites can be identified.The discussion of the case studies highlights those areas where differences of approach to the analysis problem have led to significant differences in the analyses. Some of the case studies suggest strongly that analysis differences in the vicinity of active baroclinic zones are of particular importance. In order to validate these suggestions in one case, we present an experiment where one analysis is transplanted, locally, into another, to show that large differences in the medium‐range forecasts are attributable to localized differences in the analyses.The results presented here, together with the forecast verification results of part II, suggest that in some cases uncertainties in the analysis are a major contributor to the loss of forecast skill in the medium ra

ISSN:0035-9009    

DOI:10.1002/qj.49711146702

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractThe purpose of our two‐part study is to assess the importance of the differences between three independent analyses of the same FGGE level IIb observational dataset. Part I is concerned with the nature and origin of the differences among the analyses. Part II of the study is concerned with the implications of the analysis differences for forecast skill, and for estimates of predictability.Our experimental material is a set of forecasts by two models from the three ensembles of analyses. the energetics of the analyses are different, and this is reflected in differences in the energetics of the forecasts based on the analyses.A study of the objective forecast verifications shows that there are clear differences in the forecast skill of the two models we used. the verifications also show a marked sensitivity to the choice of initial data.We studied in some detail how forecasts made with the same model from different analyses diverge from one another, with particular attention to three aspects: (i) the growth rate at small amplitude; (ii) the time taken for the differences to reach an asymptotic level; (iii) the amplitude of this level relative to that of persistence. the results for small amplitude growth rates are consistent with earlier results in giving a doubling time of 2·0 days in the height field in the day 1‐2 forecasts but with slower growth rates (doubling times of 2·6 days) in the wind field. the baroclinic waves show faster growth, with about 0·25 days shorter doubling times. the forecast differences reach an asymptotic level more rapidly in this than in earlier investigations. This suggests that earlier estimates of the purely dynamical potential for predictability of instantaneous weather patterns may be overoptimistic.We combined the measures of forecast skill and forecast divergence in an error budget which separated the contributions to forecast error arising from model error and analysis error, subject to some simplifying assumptions. the results show that between day 2 and day 5 the model error grows linearly in time and the analysis error grows exponentially. the model is the main source of forecast error in this time range. the analysis error is the main source of forecast error in the short‐range forecasts, and is a substantial contributor after about day 5. These results are very sensitive to the magnitude of the initial analysis errors.Taken together with the synoptic results of part I, these statistical results emphasize the importance of analysis technique for the success of forecasting, particularly in the medium range. Given identical data of good quality, three advanced analysis systems produced analyses which lead to rather different forecasts.The results are based on a small sample over a period of 21/2 days, which represent essentially the same synoptic situation; therefore the generality of the results should be tested on further exp

ISSN:0035-9009    

DOI:10.1002/qj.49711146703

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractAn observing systems experiment to assess the impact of aircraft observations on analyses and forecasts from a numerical model is described. Parallel runs with and without aircraft observations show significant analysis differences in the upper troposphere over northern hemisphere oceans, the aircraft giving a better representation of jet strengths and less reliance on poorer quality observations. This positive impact extends to the shape and position of upper tropospheric features in forecasts from these analyses though the magnitude of the improvements is somewhat less.These results are in broad agreement with those from a similar experiment carried out at the European Centre for Medium Range Weather Forecasts. Some differences attributable to the response of the different analysis schemes used are discussed in the light of geostrophic adjustment theory and are studied further using model forecasts from perturbed initial conditions. These studies suggest that beneficial effects on forecasts can be achieved by distributing single‐level data throughout several model levels and using wind observations to update the rotational component of the wind field.A simple linearized theoretical model supports these conclusions and is used to investigate the effects of the repeated insertion analysis procedure used in our experiments. This effectively suppresses the generation of high frequency waves but shows undesirable effects in the presence of a large mean advective velocit

ISSN:0035-9009    

DOI:10.1002/qj.49711146704

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractQuantitative and qualitative comparisons are made between laboratory measurements of rotating annulus flows and corresponding numerical model simulations. Two laboratory annuli, of similar dimensions but differing in instrumentation, are used. One contains a thermocouple array for temperature measurement: the other contains no sensor array but the working fluid is seeded with minute neutrally buoyant beads (600 m̈m diameter) which enable the horizontal velocity field to be measured. Each annulus has a rigid insulating lid in contact with the working fluid. the numerical model is a finite difference formulation based on the Navier‐Stokes equations for baroclinic flow of a Boussinesq liquid. Although the atmosphere and the laboratory annulus are both rotating baroclinic fluid systems, theforcing processesacting in the annulus are much simpler than those acting in the atmosphere, and may be accurately represented by established formulae: under a wide range of conditions no parametrizations of subgrid‐scale dynamical and diabatic processes are required. Comparison of numerical model results with laboratory measurements therefore enables the explicit dynamical formulation of numerical models of rotating, baroclinic flow to be verified to an extent which would be very difficult, if not impossible, to achieve using atmospheric data. Detailed quantitative comparisons for a steady wave flow reveal good agreement for major features of the temperature and horizontal flow fields, although a significant discrepancy in total heat flux is found. Qualitative comparisons are made by investigating the ability of the numerical model to reproduce the main flow types and phenomena of the laboratory system. Numerical simulations of intransitivity, hysteresis, wavenumber transitions, amplitude vacillation and a weak structural vacillation are described. Several suggestions for further comparative studies are made in conclu

ISSN:0035-9009    

DOI:10.1002/qj.49711146705

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractA diagnosis of the life cycles of three South Pacific cyclones, each tropical in origin, is presented. the study focuses on two of the cyclones which propagate south‐eastward along the western edge of the South Pacific convergence zone (SPCZ) and reach middle latitudes before decaying. Based on enhanced IR satellite imagery from GOES‐West, both cyclones appear to go through stages of development similar to those described by Troup and Streten, even though their classification system generally applies to higher latitude systems. the primary data set used for surface and upper air analyses is based on ECMWF FGGE level III‐b analyses. Variables presented include mean sea level pressure (m.s.l.p.), horizontal divergence, relative vorticity, and winds. A comparison between m.s.l.p. patterns obtained manually from station observations and those produced objectively by ECMWF shows good agreement except when the cyclonic disturbances are subgrid‐scale. Maximum low level convergence occurs in the SPCZ, north and east of the cyclone tracks; however, maxima of low level cyclonic vorticity correspond to the individual cyclones. Cyclone tracks appear to be influenced by the upper level subtropical jet. the two cyclones which propagate into middle latitudes are accompanied by lower tropospheric warm (cold) air advection ahead of (behind) the system. Both cyclones also exhibit upper tropospheric anticyclonic (cyclonic) vorticity ahead of (behind) the

ISSN:0035-9009    

DOI:10.1002/qj.49711146706

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractConvection in the lower troposphere produces a deficit in the local surface pressure caused by heating of the air in the well‐mixed convection layer. Variations in the surface heat flux can therefore induce horizontal pressure gradients in the convection layer. These pressure gradients can in turn lead to significant geostrophic flows, such as cyclonic ‘heat lows’ in the vicinity of local maxima in the surface heat flux. Turbulent mixing causes the motion in the convection layer to generate vertical velocities, which lead to some geostrophic flow being generated in the middle troposphere above the convection

ISSN:0035-9009    

DOI:10.1002/qj.49711146707

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractMicrophysical measurements made in small wintertime cumuli on five days in June 1981 are described. On all days an ice crystal multiplication process was active. the main conditions for the operation of the Hallett‐Mossop multiplication process were fulfilled—namely, the presence of both graupel particles and drops ≃ 24m̈m in diameter in the cloud zone between the ‐3 and ‐8°C levels. Shortcomings of the available theoretical models prevented any worth‐while comparisons between their predictions and the observed cloud behaviour. In these clouds, large drops (⩾300 m̈m diameter) grew by coalescence and appeared in a concentration of ≃0.1 per litre at about the same time as ice particles (graupel) of similar size. the drops reached a concentration of ≃5 per litre and then disappeared while the graupel continued to increase to concentrations of ≃10 per litre. There is evidence to indicate that the concentration of ice particles increased exponentially with time, being multiplied by a factor of 10 every 10 min between the values of 1 and 100 per litre. It is likely that the original ice nuclei activated were more numerous than the generally accepted average concentration

ISSN:0035-9009    

DOI:10.1002/qj.49711146708

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractA detailed analysis is presented of the large‐scale, mesoscale and local features of a south‐foehn event in the Alps on 8 November 1982. On this day, with a substantial cross‐mountain flow, instrumented aircraft made programmed flights back and forth across the Alps between southern Germany and northern Italy. Rawinsonde observations were used to complete the data set. A mesoscale double mountain wave with an amplitude of 1 km was found in the upper troposphere. In the mid troposphere above the Inn valley a rather pronounced wave with an amplitude of 2 km and a wavelength of 50 km was analysed. This wave was close to overturning. the foehn turned out to be accompanied by low wave drag at high levels and with strong wave drag at lower levels. Downward southerly (westerly) momentum flux was evaluated to be 0·3 (0·1) Pa. Mountain drag was estimated to be between 1·6 and 6·7 Pa. Light to moderate turbulence was observed immediately to the lee in a low‐level turbulence zone over the region of strong gusty surface winds. Finally, special attention has been devoted to the similarities and possible differences between foehn in the Alps and chinook in the Rock

ISSN:0035-9009    

DOI:10.1002/qj.49711146709

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractIn this paper the formation and structure of an atmospheric undular bore is examined with the aid of a numerical model and theoretical analysis. the numerical model is first used to simulate a density current propagating into an unstratified environment. A detailed comparison of the model results with existing theory shows that for a given pressure head the current can travel at a greater speed than that predicted by irrotational theory.When the density current is allowed to propagate into a low‐level stable layer it is found that for certain parameter regimes an undular bore forms ahead of the current. the results for two different types of stable layers are examined. For a layer with a step profile of potential temperature, comparison is made with both classical bore theory and the theory of density currents in two‐layer fluids and encouraging agreement is found. When a linear profile is specified in the stable layer it is found that energy‐conserving solutions can be obtained. the numerical fields are then compared with measurements of the morning glory, an undular bore observed in north‐eastern Australia, and again agreement is good.Finally, preliminary results of model simulations with stratification above the stable layer are di

ISSN:0035-9009    

DOI:10.1002/qj.49711146710

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY

摘要:

AbstractThe initialization of limited area models is complicated by the difficulty of determining the linear normal modes and of allowing for general boundary conditions. A new method of initialization is devised, which does not require explicit knowledge of the normal modes. the method is based on a filtering procedure which uses a modified inverse Laplace transform. the efficacy of the method is demonstrated by application to a one‐dimensional model, and the rationale for application to a general forecasting model is discussed. the method is closely related to the nonlinear normal mode method of initializatio

ISSN:0035-9009    

DOI:10.1002/qj.49711146711

出版商:John Wiley&Sons, Ltd    

年代:1985

数据来源: WILEY