摘要:

AbstractConventional parametrization schemes in weather and climate prediction models describe the effects of subgrid‐scale processes by deterministic bulk formulae which depend on local resolved‐scale variables and a number of adjustable parameters. Despite the unquestionable success of such models for weather and climate prediction, it is impossible to justify the use of such formulae from first principles. Using low‐order dynamical‐systems models, and elementary results from dynamical‐systems and turbulence theory, it is shown that even if unresolved scales only describe a small fraction of the total variance of the system, neglecting their variability can, in some circumstances, lead to gross errors in the climatology of the dominant scales. It is suggested that some of the remaining errors in weather and climate prediction models may have their origin in the neglect of subgrid‐scale variability, and that such variability should be parametrized by non‐local dynamically based stochastic parametrization schemes. Results from existing schemes are described, and mechanisms which might account for the impact of random parametrization error on planetary‐scale motions are discussed. Proposals for the development of non‐local stochastic‐dynamic parametrization schemes are outlined, based on potential‐vorticity diagnosis, singular‐vector analysis and a simple stochastic c

ISSN:0035-9009    

DOI:10.1002/qj.49712757202

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractA mechanism is proposed by which the energy flow through a turbulent medium might be constrained to maximize its dissipation or (equivalently) its thermodynamic efficiency. The mechanism may provide a physical basis for the various findings over the years that the earth‐atmosphere system has adopted a format which maximizes its overall rate of entropy production. The qualitative picture is of a system which, because of the asymmetry of its turbulent fluctuations about the locus of possible steady states determined by energy balance, moves to a preferred steady state and therefore to a preferred turbulent transfer coefficien

ISSN:0035-9009    

DOI:10.1002/qj.49712757203

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe relationships between decadal (>5‐year period) observed winter precipitation (PP) variability over Europe and atmospheric circulation and sea surface temperature (SST) during the period 1950–1995 are investigated. More than 48% of the decadal precipitation variability is described by two modes: a meridional dipolar pattern (high PP over northern Europe and low over southern Europe, the Mediterranean and most of central Europe) explaining about 32% of the variance, and an alternating pattern in latitude over Europe (high in central Europe and low in far north‐western Europe and the Mediterranean) explaining about 16% of the variance. The first mode is strongly related to the North Atlantic Oscillation.A canonical correlation analysis between regional precipitation and sea‐level pressure fields shows that a PP pattern similar to the first decadal PP mode is associated with a north‐westerly circulation anomaly pattern while a PP pattern similar to the second decadal PP mode is associated with a monopolar circulation anomaly pattern over most parts of Europe.The correlation map between the time series of expansion coefficients of the first PP mode and sea surface temperature emphasizes a global pattern which resembles the linear trend coefficients pattern of SST for the analysed period. The second PP mode is associated with an SST anomaly pattern similar to that of the SST anomalies characterizing ‘decadal El Niňo Southern Oscillation’ SST pattern.Composite maps of global 500 mb geopotential height based on the time series of expansion coefficients of PP modes emphasize global patterns compatible with SST patterns. After removing the linear trend from the data, the first PP pattern remains almost unchanged but it emphasizes mainly regional connections. The second spatial pattern of the second PP mode and its connections are not affected significantly by remo

ISSN:0035-9009    

DOI:10.1002/qj.49712757204

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe El Niňo Southern Oscillation (ENSO) strongly influences the interannual and seasonal atmospheric circulation over the North Pacific. The present study shows that the meridional displacement of the time‐mean upper‐level jet associated with ENSO modulates the time‐mean barotropic (meridional) wind shear over the central and eastern North Pacific storm track. Earlier theoretical and observational studies established the influence of barotropic wind shear on the life cycles of extratropical cyclones and their upper‐level potential vorticity (PV) waves. The present study suggests that differences in the time‐mean flow associated with the 1997–99 ENSO cycle had a similar impact on tropopause PV structure, meridional eddy fluxes of momentum and temperature, and pr

ISSN:0035-9009    

DOI:10.1002/qj.49712757205

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe primary goals of ensemble prediction are the identification of particularly unpredictable situations, and the prediction of forecast error. In the medium range, major forecast failures are associated with regime transitions. The predictions of linear theories on error dynamics are inadequate to describe these situations, characterized by the rapid growth of errors which are far from being infinitesimal. Little is known about the efficiency of ensemble forecasting methods when applied to highly nonlinear situations like those associated with a change of regime. This problem is examined in an atmospheric model with two regimes and a small number of degrees of freedom by comparing current ensemble forecasting methods with ensemble forecasts that span all the degrees of freedom of the system. The study, performed in a perfect model environment, shows how the statistical properties of errors are affected by regime transitions. Results obtained selecting initial perturbations in the direction of the leading Lyapunov vectors (LVs) and singular vectors (SVs) are compared with those obtained with initially random vectors (RVs) spanning the whole phase space. It is found that the LV ensembles do not necessarily capture those particular perturbations that eventually lead to a regime transition, but closely reproduce the average RV distribution. On the other hand, the SV ensembles reproduce very closely the distribution obtained by selecting the maximum amplitude of random perturbations, although their consistency with the distribution of the full RV ensemble is poorer than in the LV ensembles.When the spread of individual RV ensembles is analysed by an empirical orthogonal function (EOF) decomposition, one finds that the variations in the amplitude of the first EOF (which carries the bimodal signature of regime transitions) is better estimated by the SV than by the LV ensembles. In verification‐based contingency tables, incorrect categorical forecasts of transition probability are more frequent in the LV than in the SV ensemble

ISSN:0035-9009    

DOI:10.1002/qj.49712757206

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractDry air descending within the circulation of a polar low was observed to give a striking signature in the echo return from an upward‐looking VHF radar. Data from this radar, along with weather‐radar data, satellite imagery and output from limited‐area and mesoscale versions of the Met Office Unified Model, are synthesized to depict the mesoscale structure of the weather system. The polar low had a compact vortex centred near 3 km, a cold core below this level, and a warm core above. Attribution of quasi‐geostrophic forcing showed that on the larger scale the polar low was dominated by upper‐level forcing. The detailed mesoscale analysis showed that the polar low, during its mature phase, was affected by a stratospheric intrusion that brought air from near tropopause level down to 3 km to give the striking signature in the VHF radar echo pattern. The echo pattern also suggested that thin tongues of this stratospheric, or near‐stratospheric, air were penetrating slantwise down a further several hundred metres, above the shallow cloud constituting the southern end of an archetypal cloud head. It is argued that this may be a locally important region of mixing between air originating near the tropopause and the boundary layer. In more general terms, the study demonstrates the ability of the operational mesoscale model to represent various small‐scale features. The study also helps in developing the interpretation of the patterns of clear‐air radar echo observed

ISSN:0035-9009    

DOI:10.1002/qj.49712757207

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThis paper provides an eight‐year high‐resolution climatology of Sahelian mesoscale convective systems (MCSs) during the summer. MCSs are defined as convective cloud clusters larger than 5000 km2. They are extensively tracked from METEOSAT full‐resolution infrared images (time resolution 0.5 h and spatial resolution about 5 km). The method enables every MCS to be tracked throughout its entire lifetime. For each time step, the MCS location and its morphological and radiative characteristics are computed for three different brightness temperature thresholds. The methodology is presented, evaluated and compared with previous studies using low‐resolution data.Statistical MCS distributions, diurnal cycle and spatial variability of MCS characteristics are analysed on the basis of this high‐resolution tracking. It is shown that a few large and long‐lived cloud clusters contribute most of the total cloud cover. Sahelian cloud clusters propagate westward at a greater speed when very deep convection is well developed. The diurnal organization of the convection has been analysed, and has proved that the merging of MCSs is partly explained by the actual merging of independent convective entities, whereas the splitting of MCSs is mostly associated with weakening of convection. The importance of mesoscale convective complexes for the total MCS coverage has also b

ISSN:0035-9009    

DOI:10.1002/qj.49712757208

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractA closure for the fluxes of mass, heat, and moisture at cloud base in the cumulus‐capped boundary layer is developed. The cloud‐base mass flux is obtained from a simplifed turbulence kinetic energy (TKE) budget for the sub‐cloud layer, in which cumulus convection is assumed to be associated with a transport of TKE from the sub‐cloud layer to the cloud layer.The heat and moisture fluxes are obtained from a jump model based on the virtual‐potential‐temperature equation. A key part of this parametrization is the parametrization of the virtual‐temperature flux at the top of the transition zone between the sub‐cloud and cloud layers.It is argued that pressure fluctuations must be responsible for the transport of TKE from the cloud layer to the

ISSN:0035-9009    

DOI:10.1002/qj.49712757209

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractDuring the field experiment SOMARE‐99 (Sounding System Multifrequency Atmospheric Radar Experiment 1999), the troposphere above the Harz Mountains in Northern Germany was observed with the SOUSY VHF radar, with four microbarographs, and with 26 radiosondes launched every 3 h from nearby the radar site. This paper examines the 72 h period between 0000 UTC 25 May and 0000 UTC 28 May. Time series, root‐mean‐square values, and power spectra of radar‐observed vertical‐wind,w, fluctuations in the free troposphere and of pressure,p, fluctuations at the surface are presented and discussed. A pronounced diurnal cycle was observed: standard deviations (from 1 h periods) were about 10 cm s−1(w) and 2 Pa (p) at night and up to 60 cm s−1and 6 Pa during daytime. This is seen as evidence for the relevance of boundary‐layer convection for short‐period gravity waves in the free troposphere. Frequency spectra ofwshow a power‐law drop‐off beyond the Brunt‐Väisälä frequency with logarithmic slopes close to ‐5/3 at moderate and strong wind speeds and down to ‐3 or ‐4 at weak winds (smaller than 5 m s−1), supporting earlier work on the effects of Doppler shift on Eulerian gravity‐wave spectra. Thewspectra, which have been calculated from 8 h longwtime series (sampling period 1 min), have a noise floor of 10−3m2s−2Hz−1or less. This corresponds to an uncorrelated noise with a standard deviation of 3 mm s−1for the 1 min samples. The concept ofgravity‐wave intermittencyis introduced, in analogy to a well‐established concept in turbulence physics. It is demonstrated how this concept can be used to disti

ISSN:0035-9009    

DOI:10.1002/qj.49712757210

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe large‐scale organization of tropical deep convection is investigated in idealized two‐dimensional cloud‐resolving simulations. A 20 000 km periodic computational domain allows interactions among moist convection, mesoscale organization, and surface exchange on a wide range of scales. A uniform 10 m s−1easterly wind and a uniform sea surface temperature of 30°C are assumed. A prescribed temperature tendency mimicking the mean radiative cooling of the tropical troposphere is used in this pilot study. The large‐scale organization of convection resembles the observed tropical atmosphere as well as results from some previous numerical studies using parametrized convection. The simulation highlights mesoscale convective systems with horizontal scales of several hundred kilometres moving east to west with speeds similar to the mean flow, and an envelope of convection with a horizontal extent of a few thousand kilometres and propagating west to east. The propagation speed of the large‐scale envelope compares well with the phase velocitites of convectively coupled Kelvin waves observed in the equatorial waveguide. Convective momentum transport and the impact of convective systems on temperature and moisture near the surface are key processes responsible for the large‐scale organization of convection. These results are discussed in the context of recent observations and numerical studies of convection organization

ISSN:0035-9009    

DOI:10.1002/qj.49712757211

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY