Quarterly Journal of the Royal Meteorological Society
1477-870X
0035-9009
Mỹ
Cơ quản chủ quản: WILEY , Wiley-Blackwell
Lĩnh vực:
Atmospheric Science
Phân tích ảnh hưởng
Thông tin về tạp chí
The aims of the journal are to communicate and to document the results of new research in the atmospheric sciences and associated fields. The Quarterly Journal is acknowledged as one of the world’s leading meteorological publications. Contributions may take the form of Articles, comprehensive review articles, or comments on published papers. The journal is published eight times a year with additional special issues.
Các bài báo tiêu biểu
Accurate, simple equation for saturated vapour pressure over water and ice Abstract We present and assess a simple equation for saturated vapour pressure over water and ice. The equation does not rely on an explicit integration of the Clausius–Clapeyron equation, but instead uses the equality of the Gibbs functions of the vapour and the liquid or ice in equilibrium. The resulting equation is simple, physically consistent with standard thermodynamic assumptions, uses only basic physical parameters, and is at least as accurate as commonly used empirical fits. It is further shown that the finite volume of liquid water has a negligible effect on the vapour pressure. The main variation from accurate tabulated data results from the variation of vapour and liquid isobaric heat capacities. Nevertheless, it is shown that, for the purpose of accurate calculation of saturated vapour pressure, this can usually be ignored.
Tập 146 Số 733 - Trang 4252-4258 - 2020
Covariance localisation and balance in an Ensemble Kalman Filter Abstract A major limitation of the Ensemble Kalman Filter (EnKF) is that the finite ensemble size introduces sampling error into the background covariances, with severe consequences for atmospheric and oceanographic applications. The negative effects of sampling error are customarily limited by covariance localisation, which earlier studies have suggested may introduce imbalance into the system. The deleterious effects of localisation upon balance are confirmed and detailed here, with localisation producing analyses with weaker geostrophic balance and stronger divergence than are obtained using the unlocalised covariances. These imbalances reduce as the localisation radius is increased, but are argued to be large for typical settings. An improved method for calculating local covariances from an ensemble is presented, in which the localisation is performed in streamfunction–velocity potential (ψ–χ), rather than wind component, space. Analyses using this method better preserve the balances contained within the unlocalised covariance model. This transformation further allows the option of intervariable localisation, in which the cross‐covariances involving χ, which are weak and therefore particularly subject to sampling error, are set to 0 instead of being calculated from the ensemble. The various localisations are compared in a series of identical‐twin experiments, with the new localisations producing analyses that are better balanced and significantly more accurate than the usual approach. The localisation with the χ cross‐covariances set to 0 is shown to be superior for the smaller ensemble sizes but not for the larger, implying that the larger ensembles are capable of resolving some of the true χ cross‐covariance in the test system. Copyright © 2009 Royal Meteorological Society
Tập 135 Số 642 - Trang 1157-1176 - 2009
A climatology of mesoscale convective systems over Europe using satellite infrared imagery. II: Characteristics of European mesoscale convective systems Abstract An automated method for mesoscale convective system (MCS) identification and tracking (described in part I) is applied in order to derive a sound European MCS database using Meteosat infrared channel (IR10.8) images centred over Europe, the western Mediterranean and north Africa. The database covers five warm seasons, from April to September, for the years 1993 to 1997 and includes more than 6000 MCSs reaching at least an area of 10 000 km2 . First results of the derived climatology of European MCSs are presented. They mainly address the MCS geographical location, general MCS characteristics (maximum extent, eccentricity, duration) and the diurnal cycle of the MCS. MCSs are shown to be mainly continental, but some MCS triggering is observed during the second half of August and September over the western Mediterranean Sea. Furthermore, MCS triggering is strongly correlated with orography and local maxima of MCS triggering are observed near all mountain ranges. Regions near the Alps which favour MCS triggering are described in detail. The monthly distributions of occurrence of warm‐season European MCSs are also presented. Distributions of maximum extent, eccentricity, direction of propagation, life duration and triggering and dissipation times are also derived. On average, a theoretical ‘typical European MCS’ moves to the east‐north‐east, triggers near 3 p.m. Local Solar Time (LST), lasts around 5.5 hours and dissipates near 9 p.m. LST. It typically has an eccentricity at the time of maximum extent of 0.53. The diurnal cycle is also studied and proved to be in phase with the diurnal radiative heating, except for around 20% of the MCSs. A detailed analysis of the maximum‐extent distribution shows that it can be fitted by a log‐normal distribution which leads to an average value of the MCS maximum extent of around 9000 km2 . This approximation is statistically meaningful and it is independent of the choice to study only MCSs reaching at least 10 000 km2 . Finally, a section of this article is devoted to indirect verifications of the discrimination method (see part I) for the whole geographical domain. Copyright © 2002 Royal Meteorological Society
Tập 128 Số 584 - Trang 1973-1995 - 2002
A computational study of the relationships linking lightning frequency and other thundercloud parameters Abstract In an effort to optimize the value of global‐scale measurements obtained with the NASA/MSFC satelliteborne Lightning Imaging System (LIS), a simple computational model of thundercloud electrification has been developed, from which it is possible to derive crude relationships between lightning frequency f (which LIS will measure) and cloud parameters such as radar reflectivity Z, precipitation rate P , updraught speed w , cloud radius R , ice‐crystal concentration i and graupel‐pellet concentration N g. Electric field‐growth is assumed to occur via the non‐inductive charging mechanism, for both Fletcher and Hallett‐Mossop types of glaciation mechanisms. A simple criterion is used to distinguish between cloud‐to‐ground and intracloud lightning discharges. f is found to be especially sensitive to w in situations where, as updraught speed increases, the temperature at balance level, T bal , of the upper boundary of the charging zone falls. In these circumstances N 1 and the sizes of the ice hydrometeors are significantly increased, with a corresponding enhancement of the effectiveness of charge transfer. Over a wide range of conditions, f is found to be roughly proportional to the first power of the parameters R 1 N i N g and Z and (in some circumstances) to at least the sixth power of w. the relationship between f and P depends critically on whether or not w and T bal are strongly linked. Hallett‐Mossop glaciation is capable of producing inverted‐polarity lightning from thunderclouds; Fletcher glaciation is not.
Tập 121 Số 527 - Trang 1525-1548 - 1995
The global population of mesoscale convective complexes Abstract A global set of 714 mesoscale convective complexes is compiled and some of the common properties of the convective systems are identified and examined from a global perspective. the data set includes date of occurrence, time of first storms, initiation, maximum extent, termination, duration, cold‐cloud shield areas, and tracks from initiation to termination. It is found that the typical convective complex is nocturnal, generates a cold‐cloud shield area of approximately 350 000 km2 , and persists for about 10 h. the largest systems and most persistent systems tend to occur near the summer solstices. For the globe, about 400 systems occur each year, primarily over land areas. Most systems develop in favoured zones, although some activity occurs over every continent (except Antarctica) and all major oceans. the concentration of activity into favoured zones indicates that there must be special dynamic and/or thermodynamic conditions necessary for convection to organize into convective complexes. Activity is strongly tied to the solar day, and shifts from 35°S in early January to about 50°N during the boreal summer and back to 35°S by December. Within the northern hemisphere there is a pronounced poleward migration as the jet stream shifts northward. Relatively little migration occurs in the ocean‐dominated southern hemisphere where the subtropical jet remains quasi‐stationary over the convective‐complex regions. The nocturnal life cycles, copious rainfall, large cloud shields, and great frequency of mesoscale convective complexes suggest that they may be significant contributors to the global hydrologic cycle and earth‐system energy budget.
Tập 123 Số 538 - Trang 389-405 - 1997
A new formulation of vector weights in localized particle filters Particle filters (PFs) constitute a sequential data assimilation method based on the Monte Carlo approximation of Bayesian estimation theory. Standard PFs use scalar weights derived from the likelihood of the approximate posterior probability density functions (PDFs) of observations and use resampling schemes to generate posterior particles. However, the scalar weights approach interferes with the localization algorithm and often results in filter degeneracy. Recently, a localized particle filter (LPF) was developed by extending the scalar weights of PFs to vector weights, which produces various (local) posterior PDFs for different model grids and variables. With a sampling and merging approach in the resampling, a LPF can effectively solve the filter degeneracy problem and offer a practical, efficient algorithm for localization. However, this algorithm assumes the variations in the weights of a state variable of neighbouring grids to be continuous and uses a spatially linear interpolation of PF weights to determine the local weights. In this paper, we first analyse the possible concerns associated with the linear continuity of PF weights. This assumption is found to challenge the theoretical properties of nonlinear and non‐Gaussian variations in weights and alleviate the intrinsic spatial variations of PF weights. On this basis, we propose a new algorithm to produce vector weights for PFs for neighbouring grids. Numerical experiments using the Lorenz 96 model show that our new LPF performs better than the existing LPF algorithm, indicating the advantages and potential applications of this new algorithm of vector weights in the field of data assimilation.
Tập 143 Số 709 - Trang 3269-3278 - 2017
Estimating observation impact without adjoint model in an ensemble Kalman filter Abstract We propose an ensemble sensitivity method to calculate observation impacts similar to Langland and Baker (2004) but without the need for an adjoint model, which is not always available for numerical weather prediction models. The formulation is tested on the Lorenz 40‐variable model, and the results show that the observation impact estimated from the ensemble sensitivity method is similar to that from the adjoint method. Like the adjoint method, the ensemble sensitivity method is able to detect observations that have large random errors or biases. This sensitivity could be routinely calculated in an ensemble Kalman filter, thus providing a powerful tool to monitor the quality of observations and give quantitative estimations of observation impact on the forecasts. Copyright © 2008 Royal Meteorological Society
Tập 134 Số 634 - Trang 1327-1335 - 2008
The impact of interactions between tropical and midlatitude intraseasonal oscillations around the Tibetan Plateau on the 1998 Yangtze floods This study explores the dynamic processes on intraseasonal time‐scales responsible for the devastating floods over the Yangtze Basin during June–August 1998. Wavelet analysis suggests that the unusual double Meiyu episodes over the south of the middle and lower reaches of the Yangtze River valley (SMLY) depended on the 25–60‐day intraseasonal oscillation (ISO), which has an anomalous ascending branch over the SMLY and descending branch south of 20°N over eastern China. This vertical–meridional cell was not only regulated by the tropical ISO over the Asian summer monsoon region but also modulated by the midlatitude ISO in the upper troposphere. The intraseasonal Rossby wave train along the Asian westerly jet, presenting as the anomalous anticyclone over the Tibetan Plateau (TP) sandwiched between anomalous cyclones in the upstream and downstream areas, tended to trigger another reversed vertical–meridional cell with updraught over the SMLY and downdraught to the north through vorticity advection. The phase‐lock of these ascending branches over the SMLY produced the double Meiyu episodes. The duration of the SMLY flooding was also associated with the interactions between tropical and midlatitude ISOs. During the first wet episode, as the tropical ISO‐related upper‐level divergent flows propagated northward to the southwestern TP, they tended to anchor the anticyclone over the TP through their interaction with the midlatitude rotational circulation. This favoured persistent ascent over the SMLY, and thus the prolonged Meiyu episode in June. In contrast, the northward propagation was confined to the central Bay of Bengal in July, leading to a short Meiyu episode.
Tập 144 Số 713 - Trang 1123-1139 - 2018
The omega equation and potential vorticity Abstract Two fundamental perspectives on the dynamics of midlatitude weather systems are provided by potential vorticity (PV) and the omega equation. The aim of this paper is to investigate the link between the two perspectives, which has so far received very little attention in the meteorological literature. It also aims to give a quantitative basis for discussion of quasi‐geostrophic vertical motion in terms of components associated with system movement, maintaining a constant thermal structure, and with the development of that structure. The former links with the isentropic relative‐flow analysis technique. Viewed in a moving frame of reference, the measured development of a system depends on the velocity of that frame of reference. The requirement that the development should be a minimum provides a quantitative method for determining the optimum system velocity. The component of vertical velocity associated with development is shown to satisfy an omega equation with forcing determined from the relative advection of interior PV and boundary temperature. The analysis carries through in the presence of diabatic heating provided the omega equation forcing is based on the interior PV and boundary thermal tendencies, including the heating effect. The analysis is shown to be possible also at the level of the semi‐geostrophic approximation. The analysis technique is applied to a number of idealized problems that can be considered to be building blocks for midlatitude synoptic‐scale dynamics. They focus on the influences of interior PV, boundary temperature, an interior boundary, baroclinic instability associated with two boundaries, and also diabatic heating. In each case, insights yielded by the new perspective are sought into the dynamical behaviour, especially that related to vertical motion. Copyright © 2003 Royal Meteorological Society
Tập 129 Số 595 - Trang 3277-3303 - 2003
On the use and significance of isentropic potential vorticity maps Abstract The two main principles underlying the use of isentropic maps of potential vorticity to represent dynamical processes in the atmosphere are reviewed, including the extension of those principles to take the lower boundary condition into account. the first is the familiar Lagrangian conservation principle, for potential vorticity (PV) and potential temperature, which holds approximately when advective processes dominate frictional and diabatic ones. the second is the principle of ‘invertibility’ of the PV distribution, which holds whether or not diabatic and frictional processes are important. the invertibility principle states that if the total mass under each isentropic surface is specified, then a knowledge of the global distribution of PV on each isentropic surface and of potential temperature at the lower boundary (which within certain limitations can be considered to be part of the PV distribution) is sufficient to deduce, diagnostically, all the other dynamical fields, such as winds, temperatures, geopotential heights, static stabilities, and vertical velocities, under a suitable balance condition. the statement that vertical velocities can be deduced is related to the well‐known omega equation principle, and depends on having sufficient information about diabatic and frictional processes. Quasi‐geostrophic, semigeostrophic, and ‘nonlinear normal mode initialization’ realizations of the balance condition are discussed. an important constraint on the mass‐weighted integral of PV over a material volume and on its possible diabatic and frictional change is noted. Some basic examples are given, both from operational weather analyses and from idealized theoretical models, to illustrate the insights that can be gained from this approach and to indicate its relation to classical synoptic and air‐mass concepts. Included are discussions of (a) the structure, origin and persistence of cutoff cyclones and blocking anticyclones, (b) the physical mechanisms of Rossby wave propagation, baroclinic instability, and barotropic instability, and (c) the spatially and temporally nonuniform way in which such waves and instabilities may become strongly nonlinear, as in an occluding cyclone or in the formation of an upper air shear line. Connections with principles derived from synoptic experience are indicated, such as the ‘PVA rule’ concerning positive vorticity advection on upper air charts, and the role of disturbances of upper air origin, in combination with low‐level warm advection, in triggering latent heat release to produce explosive cyclonic development. In all cases it is found that time sequences of isentropic potential vorticity and surface potential temperature charts—which succinctly summarize the combined effects of vorticity advection, thermal advection, and vertical motion without requiring explicit knowledge of the vertical motion field—lead to a very clear and complete picture of the dynamics. This picture is remarkably simple in many cases of real meteorological interest. It involves, in principle, no sacrifices in quantitative accuracy beyond what is inherent in theconcept of balance, as used for instance in the initialization of numerical weather forecasts.
Tập 111 Số 470 - Trang 877-946 - 1985