摘要:

AbstractThis lecture discusses the low‐frequency variability of surface temperature using a coupled ocean‐atmosphere‐land‐surface model developed at the Geophysical Fluid Dynamics Laboratory/NOAA. Despite the highly idealized parametrization of various physical processes, the coupled model simulates reasonably well the variability of local and global mean surface temperature. The first half of the lecture explores the basic physical mechanisms responsible for the variability. The second half examines the trends of local surface temperature during the last half century in the context of decadal variability simulated by the couple

ISSN:0035-9009    

DOI:10.1002/qj.49712757102

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractA number of recent studies have examined the statistics of tropical storms simulated by general circulation models (GCMs) forced by observed sea surface temperatures. Many GCMs have demonstrated an ability to simulate some aspects of the observed interannual variability of tropical storms, in particular, variability in storm frequency. This has led to nascent attempts to use GCMs as part of programs to produce operational seasonal forecasts of tropical‐storm numbers.In this study, the sensitivity of the statistics of GCM‐simulated tropical storms to changes in the model's physical parametrizations is examined. After preliminary results indicated that these statistics were most sensitive to details of the convective parametrization, GCM simulations with identical dynamical cores but different convective parametrizations were created. The parametrizations examined included moist convective adjustment, two variants of the Arakawa‐Schubert scheme, and several variants of the relaxed Arakawa‐Schubert (RAS) scheme; the impact of including a shallow‐convection parametrization was also examined.The simulated tropical‐storm frequency, intensity, structure, and interannual variability were all found to exhibit significant sensitivities to changes in convective parametrization. A particularly large sensitivity was found when the RAS and Arakawa‐Schubert parametrizations were modified to place restrictions on the production of deep convection.Climatologies of the GCM tropical atmosphere and composites of tropical storms were examined to address the question of whether the tropical‐storm statistics were directly impacted on by changes in convection associated with tropical storms, or if they were indirectly affected by parametrization‐induced changes in the tropical mean atmosphere. A number of results point to the latter being the primary cause. A regional hurricane model, initialized with mean states from the GCM simulation climatologies, is used to further investigate this point. Particularly compelling is the fact that versions of the RAS scheme that produce significantly less realistic simulations of tropical storms nevertheless produce a much more realistic interannual variability of storms, apparently due to an improved tropical mean climate.A careful analysis of the background convective available potential energy (CAPE) is used to suggest that this quantity is particularly relevant to the occurrence of tropical storms in the low‐resolution GCMs, although this may not be the case with observations. If the tropical CAPE is too low, tropical storms in the low‐resolution GCMs cannot form wit

ISSN:0035-9009    

DOI:10.1002/qj.49712757103

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractA method for estimating lateral entrainment rate in clouds is suggested, linking its magnitude to buoyant production of vertical kinetic energy within a cloud updraught. In single‐column model studies the formulation captures the high values of entrainment seen in shallow convection and the values an order of magnitude lower in cases of deep convection, as suggested by observational studies and large‐eddy model simulations, together with a realistic vertical variation. Comparison is made with other recent attempts to parametrize entrainm

ISSN:0035-9009    

DOI:10.1002/qj.49712757104

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe effect of the potential vorticity (PV) anomalies formed by mesoscale convective systems (MCSs) on the downstream flow is investigated using a numerical weather prediction (NWP) model. Four case studies of North American MCSs from June and July 1999 are selected in which the MCS PV anomalies are not present in the model analysis. Idealized MCS PV anomalies are introduced into the model analyses and their impact on the NWP model forecast ascertained.In two of the four cases the presence of the MCS PV anomalies leads to significant perturbation in the forecast evolution. Sensitivity studies show that it is the presence of the mid‐level cyclonic vortex (MCV) that has the biggest impact on the NWP forecast. This positive PV anomaly is shown to cause areas of ascent and descent, as has been postulated following other studies. This can lead to the development of a growing mesoscale disturbance if the MCV becomes situated in a strong baroclinic gradient. These disturbances are shown to have direct and indirect impacts on the depths of synoptic‐scale depressions.The presence of the MCS PV anomalies leads to some reduction in the forecast errors. It is suggested that attempting to assimilate the MCS PV anomalies into the NWP model analysis may be of some benefit to forecast accuracy. However, uncertainties in the properties of the MCV may mean that a more practical application may be in perturbing the initial states of ensemble foreca

ISSN:0035-9009    

DOI:10.1002/qj.49712757105

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe pressure‐potential‐temperature covariance in free and rotating turbulent convection with no mean velocity shear is analysed, using a dataset generated with a large‐eddy simulation (LES) model. The pressure field is resolved into turbulence‐turbulence, buoyancy and Coriolis components, and the contributions from these components to the pressure‐gradient‐potential‐temperature covariance in the budget equation for the potential‐temperature flux are examined.In non‐rotating convection, the buoyancy contribution compensates for about one half of the buoyant production term in the flux budget equation, and the turbulence‐turbulence contribution is well approximated by the Rotta‐type return‐to‐isotropy model with the relaxation time‐scale set proportional to the turbulence energy dissipation time‐scale.In convection with rotation, neither the simplest Rotta‐type model with the relaxation time‐scale proportional to the energy dissipation time‐scale nor the more sophisticated two‐component‐limit (TCL) nonlinear model are able to accurately describe the LES data. A somewhat better agreement is found when a limitation is imposed on the relaxation time‐scale due to the background rotation. The simplest model for the buoyancy contribution, where it is set proportional to the buoyant production term in the flux budget equation, fares poorly. The TCL model shows better agreement with LES data although some uncertainties remain. However, the relative importance of the buoyancy contribution to the pressure‐gradient‐potential‐temperature covariance decreases with increasing rotation rate.In contrast, the Coriolis contribution becomes more important as the rotation rate increases. Neither the simplest linear model for the Coriolis contribution nor the much more complex nonlinear TCL model are found to be adequate. Neither model appropriately accounts for the component of the angular velocity of rotation that is parallel to the component of the pressure gradient in question. In the seemingly simplest case considered in the present paper, when the rotation vector is aligned with the vector of gravity, no Coriolis contribution to the vertical‐pressure‐gradient‐potential‐temperature covariance is predicted by these models. This results in a strong underestimation of the pressure term in the flux budget equation and may lead to an erroneous prediction of the vertical potential‐temperature flux in convection with rotation. In an attempt to remedy the situation, an extension of the TCL model that contains only one extra empiri

ISSN:0035-9009    

DOI:10.1002/qj.49712757106

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractA three‐time‐level semi‐Lagrangian global spectral model was introduced operationally at the European Centre for Medium‐Range Weather Forecasts in 1991. This paper first documents some later refinements to the three‐time‐level scheme, and then describes its conversion to a two‐time‐level scheme. Experimental results are presented to show that the two‐time‐level scheme maintains the accuracy of its three‐time‐level predecessor, while being considerably more co

ISSN:0035-9009    

DOI:10.1002/qj.49712757107

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractWave propagation from a localized source into a non‐uniform medium is analysed using a hybrid method. This consists of an integral representation in the near‐field and a numerical ray tracing in the far‐field. The ray tracing includes the calculation of wave‐action density and is initialized from the stationary‐phase approximation to the near‐field integral. The method is tested on the problem of mountain waves in a turning wind, as studied by Shutts. A ray calculation with rotation added to the Shutts model is also presented and is interpreted with a simple analytical model in which the horizontal divergence of rays balances the vertical convergence of rays. In the example presented here, the inclusion of rotation speeds up the approach to wave‐breaki

ISSN:0035-9009    

DOI:10.1002/qj.49712757108

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe transformed Eulerian‐mean mass stream function gives an estimate of the mass flux across isentropic surfaces in terms of eddy fluxes evaluated on pressure surfaces. This is convenient both for data analysis and for the interpretation of theoretical results which are sometimes more easily expressed in pressure coordinates. A generalization is derived which describes the mass flux across an arbitrary iso‐surface, provided that the quantity defining the iso‐surface has a clear vertical gradient. The generalization is applied to iso‐surfaces of Ertel's potential vorticity, including the dynamical tro

ISSN:0035-9009    

DOI:10.1002/qj.49712757109

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractObservations of föhn in the Alps and other mountainous regions suggest that the underlying dynamics is often affected by gap‐like features in elongated ridge‐like topography. To assess the dynamics of these flows, idealized numerical experiments are conducted with a hydrostatic numerical model, usingf‐plane geometry and a free‐slip lower boundary condition. The topography is taken to be a two‐dimensional ridge oriented in the west/east direction with a valley transect of depth ΔHacross it. The upstream flow is westerly, with a constant wind speedUand constant Brunt‐Väisälä frequencyN. The control parameters defined by this setting are a dimensionless gap depthNΔH/U, the ratio between ridge height and gap depthH/ΔH, a Rossby number describing the south‐north width of the ridge, and additional parameters associated with the shape of the gap. With intermediate Rossby numbers (Ro≈︁1) the setting resembles that of shallow Alpine south‐föhn cases, which are characterized by a cross‐Alpine flow essentially confined to valley transects. For small dimensionless gap depths and large Rossby numbers, the flow follows the predictions of linear theory and takes on an approximately symmetric pattern with respect to the ridge line. ForNΔH/U≳ 1, flow separation and splitting takes place upstream and downstream of the gap, respectively. The flow within the gap decouples from the flow aloft and is driven by the geostrophic south‐north pressure gradient to yield a föhn‐like flow. It is demonstrated that the limitf→ 0 is singular (i.e. the flow solution does not converge towards the symmetricf= 0 solution), and that there exist multiple stationary solutions forf= 0 (two with northerly and southerly flow across the gap, respectively, and one with north/south symmetry). The existence of these multiple steady states is related to a wake instability, yet vortex shedding is suppressed by the presence of the ridge downstream of the gap. Additional simulations are presented which demonstrate that a transient external forcing can induce transitions between the multiple flow solutions. The relationship of the idealized setting to Alpine shallow föhn is discussed, and additional experiments are conducted to assess the effects of surface friction and of an inv

ISSN:0035-9009    

DOI:10.1002/qj.49712757110

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractWe introduce a new graphical representation for data‐assimilation methods which allows the visualization of the following items: control spaces, dual spaces, elementary operators and flow charts, for the computation of the gradient of the cost function. It is illustrated on a large variety of data‐assimilation methods used in meteorology and oceanography: three‐dimensional variational, four‐dimensional (4‐D) variational, 4D‐dual, Kalman filters, incremental formulations, formulations with imperfect models, addition of weak constraints, and reductions of the control space, etc.The flow‐charting approach is a powerful tool for decomposing a data‐assimilation method into elementary steps, for pointing out the elementary steps common to several methods or for building new and more efficient algorithms. In addition to the use of a common system of notation for assimilation methods, flow charting allows different algorithms to be easily compared, and is a necessary step prior to the development of any operational suite coupling data and models through one or several assimilation methods. This approach has been successfully followed to design a universal coupler called PALM (Projet d'Assimilation par Logiciel Multi‐méthodes) for the purpose of operational oceanography in the framework of th

ISSN:0035-9009    

DOI:10.1002/qj.49712757111

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY