摘要:

AbstractThe Met. Office has developed a variational assimilation for its Unified Model forecast system, which contains a grid‐point mode) that is run operationally in global, mesoscale, and stratospheric configuration. Key characteristics of the design are:•a development path from three‐dimensional to four‐dimensional variational assimilation;•global and limited‐area configurations;•variational analysis of perturbations;•and a carefully designed, well conditioned background term.The background term is implemented using a sequence of variable transforms to independent balanced and unbalanced variables, to vertical modes, and to spectral coefficients. The coefficients used are based on statistics from differences of one‐ and two‐day forecasts valid at the same time. The covariance model represents many of the features seen in the covariances of forecast differences.The three‐dimensional variational data assimilation (3D‐Var) system was implemented in the operational global forecast system on 29 March 1999. In parallel trials, the 3D‐Var system gave a 2.7% improvement in a composite skill score (verified against observations and weighted according to the

ISSN:0035-9009    

DOI:10.1002/qj.49712657002

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThe Spectral Statistical Interpolation (SSI) analysis system of the National Centers for Environmental Prediction (NCEP) is modified to include GPS/MET data (meteorological data from the Global Positioning Satellite system) using a GPS ray‐tracing operator. The new system is tested by incorporating 30 actual GPS/MET observations of refraction angles obtained during the GPS/MET experiment. This is the first time that real radio occupation refraction angles and refractivities have been incorporated into a three‐dimensional variational analysis system. We examine the magnitude and the vertical distribution of the analysis adjustments that result from using refraction‐angle observations in the NCEP SSI analysis system. The average magnitudes of the adjustments in the temperature and specific‐humidity fields are approximately 0.4 degC and 0.6 g kg−1, respectively. Individual changes can be as large as 4 degC and 4g kg−1, respectively. The greatest adjustments to the temperature occur in the middle and upper troposphere and stratosphere, while the major changes in specific humidity occur in the lower troposphere.An assessment of the impact of the GPS/MET observations on the analysis, verified by conventional (mostly radiosonde) data, is difficult because of the small number of GPS/MET data used. Nevertheless, it is found that, even over data‐rich regions (regions containing many radiosonde observations), and even when the verification data were the radiosonde data themselves, the use of GPS/MET refraction angles makes a slight improvement, overall, to the analysed temperatures and winds. The impact on the water‐vapour analyses, again as measured against radiosonde data, is mixed, with improvements shown in some layers and degradation in others. Compared with the background field, the use of refraction angles from one occultation results in an analysis whose simulated refraction angles are much closer to the withheld GPS/MET refraction angles at the two nearby occultation locations, and whose temperature and moisture profiles are also closer to those resulting from the direct assimilation of the two withheld occultations.Although the forward model used in this study, with the ray tracing being carried out in a two‐dimensional plane, is much cheaper than a more accurate three‐dimensional forward model, it is still quite expensive. In order to further reduce the computational requirement for the assimilation of GPS/MET data, we test a scheme in which the GPS/MET‐retrieved refractivities (instead of refraction angles) are used above a selected height for each occupation. These heights are determined objectively based on the departures from spherical symmetry of the model field. It is shown that the mixed use of GPS/MET refraction angles and refractivities produces an analysis result similar to the one using refraction angles alone, while the computational cost is red

ISSN:0035-9009    

DOI:10.1002/qj.49712657003

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThis paper focuses on the problem of the initialization of a fine‐scale (2.5 km resolution) model with observational data. An approach combining optimal interpolation (OI) analysis and bogussing methods was designed. The information provided by surface mesonet observations (relative humidity, temperature and winds) is incorporated by means of a mesoscale (OI) analysis. Simple cloud and precipitation analyses based on conventional radar reflectivities and infrared satellite data help to adjust the humidity and hydrometeor fields of the initial state. With this initialization procedure the resulting initial state describes the signature of mesoscale convective systems.This technique is applied to one observed mesoscale convective system to produce an initial state describing a one‐hour‐old developing convective system. From this initial state, the mesoscale nonhydrostatic model, named MESO‐NH, succeeds in simulating a convective line organization which is close to observations. Experiments to determine the sensitivity to the initial state definition and to the model characteristics are carried out and the initialization procedure is found to be instrumental in a successful simulation; both the surface‐data analysis and the cloud analysis are

ISSN:0035-9009    

DOI:10.1002/qj.49712657004

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractA one‐week convective period of the Coupled Ocean‐Atmosphere Response Experiment (10‐17 December 1992), prior to a westerly wind burst, has been simulated with a cloud‐resolving model. Large‐scale advection derived from observations is used to force the model, in the same way as usually done in single‐column models. Our aim is to evaluate this explicit simulation against observed large‐scale thermodynamic and radiative fields, and to investigate the sensitivity of model results to observational uncertainties. Precipitation, apparent heat source and moisture sink are fairly well reproduced by the model as compared to those diagnosed from observations. Temperature (T) and moisture (qv) fields are also reasonably well captured except for a moderate cold and moist bias. Simulated moist static energy is too high below 6 km and too low above, possibly because convection is slightly less active in the model than observed.In order to investigate the sensitivity of mode results to observational uncertainties, results are analysed with the moist static energy budget together with independent observational radiative datasets. This analysis suggests that the atmospheric radiative rate that is in equilibrium with the applied large‐scale advection and observed surface fluxes is too weak and that its diurnal cycle is not realistic. The most likely reason for this problem is found to be related to uncertainties in the large‐scale advection diagnosed from observations. This analysis also indicates that the simulated high‐cloud cover is too large in the model. It is greatly improved by increasing the ice‐crystal fall speed. Additional tests show a large sensitivity of the simulated moist static energy, and thusTandqv, to the range of uncertainties previously found for large‐scale advection. The vertical structure of the model bias is not significantly modified by changing the intensity of these forcings, but it is most sensitive to their vertical structures.It is argued that it is crucial to get some insights into the range of uncertainties of external forcings (large‐scale advection, surface fluxes and atmospheric radiative‐heating rate) so as to assess the relevance of any evaluation of simulated temperature and moisture when a model, either resolving clouds or parametrizing them, is forced with large‐scale advect

ISSN:0035-9009    

DOI:10.1002/qj.49712657005

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractMesoscale convective events, associated with severe weather, play a significant role in troposphere‐stratosphere coupling processes. Deep convection occurs very often at tropical latitudes. The convection events result in the transport of energy from the lower troposphere to the upper troposphere and stratosphere, and may also affect global circulation. In the Indian tropical region, meteorological convection events are a common occurrence during the months of May to December. In the present study, very‐high‐frequency radar located at Gadanki (13.45°N, 79.18°E), a tropical station in India, is used to make observations of winds, turbulence and stable‐layer structures, during the passage of convective events over the radar site under different synoptic situations.Radar observations of vertical velocity show enhanced up and down draughts during the passage of convection events. Height and time cross‐sections of radar‐observed signal‐to‐noise ratio clearly show distinct regions of enhanced signal intensity, indicating height and time Structures of enhanced atmospheric turbulence associated with the convection. It is also noted that stable‐layer structures associated with the tropical tropopause are not well defined during the passage of convection. This suggests ‘weakening’ of stable‐layer structures during the convective activity. The present observations also show enhanced mass flux, both in upward and downward directions, through the tropopause, indicating mass exchange between troposphere and stratosphere during the peri

ISSN:0035-9009    

DOI:10.1002/qj.49712657006

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractA two‐column model of the tropical atmosphere is developed. The two columns are kept in buoyancy equilibrium by mass exchange driven by the inter‐column pressure gradient, and diabatic processes are parametrized by highly simplified schemes. This model is used to investigate whether spontaneous large‐scale circulations will develop in the tropical atmosphere when (he sea surface temperature and solar radiative forcing are uniform. Such circulations do indeed develop in the model, with ascent in one column and descent in the other, when one of the columns is slightly perturbed from the initial state of radiative‐convective equilibrium. A key element of the circulation dynamics is that increased equivalent potential temperature in a column leads to enhanced convection and rainfall in that column, which further increases the equivalent potential temperature there. The latter effect arises because convection enhances surface heat fluxes and decreases outgoing long‐wave radiation by virtue of the increase in stratiform cloudiness. These results form an attractive explanation for the observed patchiness of deep convection over warm tropical oceans and suggest that further modelling and observational work be directed towards understanding the budget of equivalent potential temperature in the tropical atmosphere and its relationship to

ISSN:0035-9009    

DOI:10.1002/qj.49712657007

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThe role of large‐scale asymmetries in the evolution of a tropical‐eye lone‐like vortex in vertical shear on anf‐plane is investigated. Idealized numerical calculations using a primitive‐equation model are used to illustrate the development of the asymmetries and their role in the evolution of the vortex lilt. The asymmetries develop in the outer region of the vortex, well outside the radius of maximum wind. For the vortex profile used in most of the calculations, the region where the asymmetries develop has anomalously negative potential vorticity (PV) compared with the undisturbed environment, and the flow fields associated with the asymmetries consist of large‐scale anticyclonic gyres. The orientation of the gyres changes with height and they are located on opposite sides of the vortex at lower and upper levels. The influence of the asymmetries on the vortex evolution depends on both the structure and location of the asymmetries and on the orientation of the tilted vortex within the asymmetries.It is hypothesized that the asymmetries arise due to the distortion of the initially symmetric vortex by the horizontally sheared flow associated with the vertical projection of the tilted PV anomaly. Calculations using a barotropic model are presented in support of this hypothesis. The sensitivity of the large‐scale asymmetries to the initial vortex structure is investigated. For a vortex profile in which the tangential wind decreases more slowly with radius, the asymmetries form in a region of positive PV anomaly and the associated flow field contains large‐scale cyclonic gyres. The implications of the large‐scale asymmetries for tropical‐cyclone motion and intensity c

ISSN:0035-9009    

DOI:10.1002/qj.49712657008

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThe evolution of baroclinic tropical‐eye lone‐like vortices in environmental vertical shear on anf‐plane is investigated. Idealized numerical calculations are performed using a primitive‐equation model. The vertical structure of the initial vortex is varied by changing the strength of the upper‐level tangential wind. The baroclinic vortices develop a vertical tilt and a cyclonic rotation of the mid‐level centre about the surface centre occurs. In most of the calculations the upper‐level part of the vortex is advected away from the surface centre. The motion of the upper‐level part of the vortex can be attributed to advection by the flow associated with large‐scale asymmetries in the potential vorticity and to the vertically penetrating flow associated with the potential vorticity of the lower portion of the tilted vortex. As in the case of initially barotropic vortices the cyclonic rotation is found to depend on the parameters in the Rossby penetration depth. The height at which the environmental flow is equal to the Speed of vortex motion is found to be higher for vortices with stronger flow at upper levels. This is consistent with observations of tropical cyclones.Significant changes in the potential‐vorticity structure of the vortex occur, especially at upper levels. The vortex becomes elliptic in shape and may extrude filaments of potential vorticily. These filaments thin, and sometimes break away from the main vortex, which as a result becomes more symmetric. The separation of the filaments from the main part of the vortex is often accompanied by a decrease in the vortex tilt. A: some levels no filaments form and the elliptic vortex becomes more symmetric with time. Calculations with a barotropic model show that both the potential‐vorticity structure of the initial vortex, and the horizontally sheared flow associated with the vertical projection of the tilted potential‐vorticity anomaly, play a role in the changes in the vortex Structure.The development of potential‐temperature asymmetries and an adiabatic vertical circulation are shown to be related to the direction of the vortex tilt. This occurs even when the direction of tilt changes with height. The changes in the low‐level static stability associated with the vortex tilt are investigated and the implications for tropical‐cyclone i

ISSN:0035-9009    

DOI:10.1002/qj.49712657009

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractObservations suggest that vortex dipoles of mesoscale dimensions may provide a simple yet realistic representation of the structure and dynamics of jet streaks in the extratropical upper troposphere. Moreover, the effects of horizontal divergence in the vicinity of jet streaks often may be of secondary dynamical importance, suggesting that a nondivergent barotropic framework may provide a logical starting point for an idealized investigation of jet streaks.Analytical solutions of barotropic vortex dipoles are shown to exhibit characteristic signatures similar to those identified in observational case‐studies of jet streaks. In addition to the dipole of relative vorticity, these signatures include: (i) a localized maximum in fluid speed (i.e. a jet streak), (ii) ageostrophic flow that is directed towards lower pressure in the entrance region and towards higher pressure in the exit region of the jet streak, (iii) a four‐cell pattern of ageostrophic vorticity that is cyclonic in the entrance and exit regions and anticyclonic on the flanks of the streak, and (iv) a translation speed that is significantly slower than the maximum fluid speed. On the basis of these similarities, it is suggested that vortex dipoles provide a plausible dynamical representation of the structure and motion of jet streaks.Nevertheless, vortex dipoles in isolation are unable to account for certain observed features of jet streaks, such as the anisotropy of the wind field in the along‐stream direction and the asymmetry in the relative‐vorticity field, in which the cyclonic vortex typically is stronger than the anticyclonic vortex. Moreover, jet streaks generally are not isolated, but are embedded in a larger‐scale jet stream, which may be zonally varying or wavelike. Analytical and numerical solutions of barotropic vortex dipoles in the presence of a variety of non‐uniform background flows characteristic of the large‐scale extratropical circulation are shown to account for the above features absent from dipoles in isolation. These solutions are also shown to provide idealized depictions of the life cycles of jet streaks in the extratropical uppe

ISSN:0035-9009    

DOI:10.1002/qj.49712657010

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractDuring the Fronts and Atlantic Storm‐Track Experiment (FASTEX) two large‐scale cyclones (in intensive observation periods 11 and 17) developed in the North Atlantic. These can be classified, in their initial phase, as Petterssen Type B cyclones. Initiation of the cyclones follows transient upper‐level barotropic growth, caused by a pre‐existing upper‐level trough approaching a jet associated with a strong baroclinic zone. The cyclones later developed further due to baroclinic energy conversion. A favourable orientation of the trough relative to the baroclinic zone ('horizontal tilt against the shear) is crucial for this transient barotropic growth. This scenario is therefore proposed as a general dynamical mechanism leading to Type B cyc

ISSN:0035-9009    

DOI:10.1002/qj.49712657011

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY