bluuebunny/crossref_metadata_2025_split huggingface dataset:
A numerical model of the cloud‐topped marine boundary layer with explicit treatment of supersaturation–number concentration correlations
Authors: Svensson Gunilla , Seinfeld John H. 
Published: January 2003

AbstractA numerical model of the cloud‐topped marine boundary layer is presented. The model, based on ensemble‐averaged conservation equations and containing detailed aerosol microphysics, differs from previous models in one major respect: the mean supersaturation–cloud drop number concentration correlation term, generally neglected in previous numerical mean cloud models, is parametrized using a traditional boundary‐layer flux‐gradient relationship. The present work highlights the importance of this correlation term and shows how a simple representation of this correlation can lead to dramatically different, and improved, simulations of cloud behaviour. Simulations are presented for a well‐studied marine boundary‐layer stratocumulus case. Inclusion of the supersaturation–cloud drop number concentration correlation leads to more physically realistic cloud drop spectra and cloud drop concentrations throughout the height of the cloud. Although the parametrization presented must be viewed as a first approximation, the crudeness of the approach should be seen as a justification and starting point for future work. Copyright © 2002 Royal Meteorological Society
Principal‐component analysis for raindrops and its application to the remote sensing of rain
Authors: Meagher Jonathan P. , Haddad Ziad S. 
Published: January 2003

AbstractIn the problem of inverting remote‐sensing measurements of rain, current representations of the raindrop size distribution suffer crucially from the expedient but unjustified and empirically ill‐fitting assumption that the distribution has a known closed‐form shape, whether log‐normal or Γ‐distributed. This paper proposes an approach to avoid such unfounded a priori assumptions entirely. The resulting representation of the rain is then used to derive ‘forward’ formulae for rain remote‐sensing algorithms. Copyright © 2002 Royal Meteorological Society
Use of an explicit model of the microphysics of precipitating stratiform cloud to test a bulk microphysics scheme
Authors: Cardwell J. R. , Choularton T. W. , Wilson D. , Kershaw R. 
Published: January 2003

AbstractAn explicit microphysical model is used to study the development of nimbostratus cloud. The model contains a simple representation of embedded convection. The results show that secondary ice processes can be important in determining the ice and liquid‐water contents of the cloud, especially when embedded convection is switched on. The results are used to test the bulk microphysical parametrization that is used in a state‐of‐the‐art climate model, by running the bulk scheme within the same dynamical framework as the explicit scheme. Compared with the explicit scheme, the bulk scheme overestimates the ice and underestimates the liquid‐water content compared with the explicit scheme. To parametrize the secondary ice processes effectively, the bulk scheme will require representation of small ice crystals. Copyright © 2002 Royal Meteorological Society
An intercomparison of cloud‐resolving models with the atmospheric radiation measurement summer 1997 intensive observation period data
Authors: Xu Kuan‐Man , Cederwall Richard T. , Donner Leo J. , Grabowski Wojciech W. , Guichard Françoise , Johnson Daniel E. , Khairoutdinov Marat , Krueger Steven K. , Petch Jon C. , Randall David A. , Seman Charles J. , Tao Wei‐Kuo , Wang Donghai , Cheng Xie Shao , Yio J. John , Zhang Ming‐Hua 
Published: January 2003

AbstractThis paper reports an intercomparison study of midlatitude continental cumulus convection simulated by eight two‐dimensional and two three‐dimensional cloud‐resolving models (CRMs), driven by observed large‐scale advective temperature and moisture tendencies, surface turbulent fluxes, and radiative‐heating profiles during three sub‐periods of the summer 1997 Intensive Observation Period of the US Department of Energy's Atmospheric Radiation Measurement (ARM) program. Each sub‐period includes two or three precipitation events of various intensities over a span of 4 or 5 days. The results can be summarized as follows.CRMs can reasonably simulate midlatitude continental summer convection observed at the ARM Cloud and Radiation Testbed site in terms of the intensity of convective activity, and the temperature and specific‐humidity evolution. Delayed occurrences of the initial precipitation events are a common feature for all three sub‐cases among the models. Cloud mass fluxes, condensate mixing ratios and hydrometeor fractions produced by all CRMs are similar. Some of the simulated cloud properties such as cloud liquid‐water path and hydrometeor fraction are rather similar to available observations. All CRMs produce large downdraught mass fluxes with magnitudes similar to those of updraughts, in contrast to CRM results for tropical convection. Some inter‐model differences in cloud properties are likely to be related to those in the parametrizations of microphysical processes.There is generally a good agreement between the CRMs and observations with CRMs being significantly better than single‐column models (SCMs), suggesting that current results are suitable for use in improving parametrizations in SCMs. However, improvements can still be made in the CRM simulations; these include the proper initialization of the CRMs and a more proper method of diagnosing cloud boundaries in model outputs for comparison with satellite and radar cloud observations. Copyright © 2002 Royal Meteorological Society
Aspects of the parametrization of organized convection: contrasting cloud‐resolving model and single‐column model realizations
Authors: Gregory D. , Guichard F. 
Published: January 2003

AbstractCloud‐resolving model (CRM) simulations of organized tropical convection observed in the Tropical Ocean/Global Atmosphere Coupled Ocean–Atmosphere Response Experiment are used to evaluate versions of the European Centre for Medium‐Range Weather Forecasts convection and cloud schemes in single‐column model simulations. Emphasis is placed upon the ability of the convection scheme to represent ‘convective‐scale’ processes with typically mode‐1 heating structures through the troposphere, together with a cloud scheme representing the ‘stratiform (mesoscale) component’ with upper‐level heating and low‐level cooling due to the evaporation of precipitation. While diagnosis of convective and stratiform precipitation is sensitive to the sampling criteria applied to the CRM, vertical structures of the mass and heat budgets are robust. Using diagnostics from the CRM simulations as a guide, revisions to the convection and cloud schemes are suggested in order to enable the parametrization to represent the two scales. The study suggests that a mass‐flux convection scheme linked via detrainment to a prognostic treatment of cloud can represent organized convection, provided that the upward motion in the upper‐level stratiform cloud is considered. Copyright © 2002 Royal Meteorological Society
Some aspects of the improvement in skill of numerical weather prediction
Authors: Simmons A. J. , Hollingsworth A. 
Published: January 2003

AbstractRecent verification statistics show a considerable improvement in the accuracy of forecasts from three global numerical weather prediction systems. The improvement amounts to about a 1‐day gain in predictability of mean‐sea‐level pressure and 500 hPa height over the last decade in the northern hemisphere, with a similar gain over the last 3 years in the southern hemisphere. Differences between the initial analyses from the three systems have been substantially reduced.Detailed study of the European Centre for Medium‐Range Weather Forecasts verifications shows that identifiable improvements in the data assimilation, model and observing systems have significantly increased the accuracy of both short‐ and medium‐range forecasts, although interannual (flow‐dependent) variations in error‐growth characteristics complicate the picture. The implied r.m.s. error of 500 hPa height analyses has fallen well below the 10 m level typical of radiosonde measurement error. Intrinsic error‐doubling times, computed from the divergence of northern hemisphere forecasts started 1 day apart, exhibit a small overall reduction over the past 10 years at day two and beyond, and a much larger reduction at day one. Error‐doubling times for the southern hemisphere have become generally shorter and are now similar to those for the northern hemisphere. One‐day forecast errors have been reduced so much in the southern hemisphere that medium‐range forecasts for the region have become almost as skilful as those for the northern hemisphere.The approach to saturation of forecast error beyond the 10‐day range has been examined for sets of 21‐day forecasts. When the systematic (sample‐mean) component of the error is subtracted, forecast errors and the differences between successive forecasts both appear to level out near the end of the 21‐day range at values close to the limit set by the natural level of variance of the atmosphere for the northern hemisphere. A number of features of the model 500 hPa height fields remain quite realistic at the three‐week range. The most obvious discrepancy in mean climate is in the Pacific/North‐American sector, and variance is too high in the southern hemisphere. Copyright © 2002 Royal Meteorological Society
On shallow‐water potential‐vorticity inversion by Rossby‐number expansions
Authors: Mohebalhojeh A. R. 
Published: January 2003

AbstractFor the f‐plane shallow water (SW) equations, the basic properties of formal Rossby‐number expansions and potential vorticity (PV) inversion by the implied asymptotic balance conditions are re‐examined. An indeterminacy inherent in conventional expansions beyond leading‐order geostrophic balance is highlighted. The indeterminacy gives us the freedom to define, in principle, infinite sets of ‘balance conditions’ relating inertia–gravity modes to Rossby modes. Examples discussed include balance conditions that are obtained when one of the variables, linearized PV, vorticity, or perturbation height is regarded purely as a leading‐order geostrophic quantity.For the sake of conservation of global mass and boundary circulation, SW PV inversion is defined as one that employs the PV anomaly, i.e. the deviation of PV from its domain‐area average. SW PV inversion by means of various Rossby‐number balance conditions and the modified expansion procedure for the PV anomaly are presented. In the latter, called the ‘WBSV procedure’ in this paper, the PV anomaly is regarded purely as the leading‐order geostrophic quantity and the Rossby formula for SW PV anomalies is satisfied asymptotically. A modification of the WBSV procedure satisfying the exact SW equation for the PV anomaly is also presented.Numerical results for these PV inversion procedures, accurate up to second order in Rossby number, are presented for two SW simulations involving weakly and strongly ageostrophic flows. Also presented are results from PV inversion using the δ–γ hierarchy of balance conditions, which sets the Nth‐order time derivatives of divergence and ageostrophic vorticity to zero. It is shown that the WBSV procedure incurs a significant loss of accuracy relative to both the δ–γ hierarchy and other asymptotic procedures like the one that regards the linearized PV as a leading‐order geostrophic quantity. The modified WBSV procedure shows little improvement. The inaccuracy of this procedure stems from a poor leading‐order solution for the balanced fields. Copyright © 2002 Royal Meteorological Society
Objective analysis combining observation errors in physical space and observation space
Authors: Schyberg Harald , Breivik Lars‐Anders 
Published: January 2003

AbstractA generalization of the traditional observation‐error model in objective analysis is presented. It is suggested that for observations with nonlinear observation operators, the observation errors should be split up into one contribution in the measurement domain and one contribution in the domain of the model physical quantities interpolated to the measurement locations (physical space). In particular, errors of representativeness are better modelled in physical space. Analysis equations are derived for this generalized problem. The approach suggested has important applications in the assimilation of satellite observations. For special cases with a wide application this generalization does not introduce an inhibiting computational complexity. Copyright © 2002 Royal Meteorological Society
Wave breaking and critical levels for propagating inertio‐gravity waves in the lower stratosphere
Authors: Moldovan H. , Lott F. , Teitelbaum H. 
Published: January 2003

AbstractThis paper analyses and interprets the formation of mixed layers due to propagating inertio‐gravity waves observed during the Fronts and Atlantic Storm‐Track EXperiment. The data used are high‐vertical‐resolution soundings launched from different sites located in the North Atlantic sector. In agreement with other studies, attributing the origin of the inertio‐gravity waves to the adjustment of the jet stream near fronts, the largest waves observed are often located in the vicinity of and above frontal bands. The amplitude of these waves is large enough that they give rise to narrow mixed layers. By a detailed analysis of the waves' characteristics, we can distinguish whether such a mixing is favoured by the presence of an inertia‐critical level, where |Ω|=f, or results from wave‐induced convective instability occurring far from critical levels. Here, Ω is the wave intrinsic frequency and f is the Coriolis parameter. These observations highlight the importance of inertio‐gravity waves in the vertical exchanges that occur in the stratosphere. Copyright © 2002 Royal Meteorological Society
A semi‐empirical parametrization for total in situ specific humidity standard deviation derived from tethered balloon observations
Authors: Price Jeremy D. 
Published: January 2003

AbstractA parametrization has been derived from tethered balloon observations made between ground level and 1.8 km to predict the standard deviation of in situ specific humidity. The algorithm has possible use in cloud prediction models which use a distribution function for the total specific humidity, allowing it to be continuously scaled in each model grid‐box. It consists of two parts designed to cope both with stable and convective regimes. The average error in predicted humidity standard deviations has been estimated by comparison with observations and found to be 0.038 g kg−1, which is an order of magnitude smaller than the observed variation in the quantity. Copyright © 2002 Royal Meteorological Society
Diagnostic and prognostic equations for the depth of the stably stratified Ekman boundary layer
Authors: Zilitinkevich Sergej , Baklanov Alexander , Rost Jutta , Smedman Ann‐sofi , Lykosov Vasiliy , Calanca Pierliugi 
Published: January 2003

AbstractRefined diagnostic and prognostic equations for the depth of the stably stratified barotropic Ekman boundary later (SBL) are derived employing a recently developed non‐local formulation for the eddy viscosity. In well‐studied cases of the thoroughly neutral SBL, the nocturnal atmospheric SBL and the oceanic SBL dominantly affected by the static stability in the thermocline, the proposed diagnostic equation reduces to the Rossby–Montgomery, Zilitinkevich and Pollard–Rhines–Thompson equations, respectively. In its general form it is applicable to a range of regimes including long‐lived atmospheric SBLs affected by the near‐surface buoyancy flux and the static stability in the free atmosphere. Both diagnostic and prognostic SBL depth equations are validated against recent data from atmospheric measurements. Copyright © 2002 Royal Meteorological Society.
Density currents in shear flows: Effects of rigid lid and cold‐pool internal circulation, and application to squall line dynamics
Authors: Xue Ming 
Published: January 2003

AbstractIdealized density current models that contain a rigid lid, developed in previous studies, are evaluated. It is shown, through numerical experiments, that a realistic stable layer placed at the level of the rigid lid acts very much like a rigid lid, in confining the environmental flow to the levels below and in controlling the behaviour of density currents that propagate in the flow. When this stable layer is replaced by a neutrally stable layer, the behaviour of the density current is very different. These results support the hypothesis that in the atmosphere, the tropopause or a tropospheric inversion layer acts much like a rigid lid in the idealized density current models, and the solutions of these models are applicable to organized quasi‐two‐dimensional convective systems such as squall lines in the atmosphere.The effects of cold‐pool internal circulations on density current behaviour are also examined. When a single circulation is present initially inside the cold pool, the direction of circulation that gives rise to a smaller shear across the density interface leads to a smoother interface and a much more steady density current head. Large‐amplitude eddies develop along the interface when the direction of circulation is reversed. When two layers of shear are initially present inside the cold pool, the flow pattern inside the cold pool tends to be dominated by the circulation of the lower layer. In this case, the behaviour of the density interface and density current head depends on more than just the initial cross‐interface shear.The overall flow pattern in the density current, in which the cold‐pool circulation contains rearward flows at the ground level, bears a close similarity to that found in mature squall line systems, as shown by comparisons with a simulated squall line. When the cold pool in a squall line is defined in terms of the equivalent potential temperature, the solutions of idealized density currents in sheared flows become more readily applicable to squall lines. Copyright © 2002 Royal Meteorological Society.
Two‐stream approximations revisited: A new improvement and tests with GCM data
Authors: Räisänen P. 
Published: January 2003

AbstractTwo‐stream approximations (TSAs) are employed for multiple‐scattering calculations in most solar radiation schemes used in atmospheric general‐circulation models (GCMs). Two issues related to TSAs are addressed. First, a simple extension of the two‐stream formalism is presented that allows different parametrizations for different factors, such as water clouds, ice clouds and aerosols. A ‘tuned two‐stream approximation’ that uses this extension is then developed. Second, the accuracy of the tuned TSA and selected other TSAs is quantified using a global GCM‐generated dataset (nearly 25000 atmospheric columns with clouds, aerosols and absorbing gases).In the tests with the GCM dataset, the Practical Improved Flux Method (PIFM) provided slightly better results than the delta‐Eddington approximation. Two recent modifications of delta‐Eddington yielded no, or only slight, improvement. For PIFM, the root‐mean‐square (r.m.s.) errors in top‐of‐the‐atmosphere (TOA) and surface net short‐wave fluxes and in total column absorption were 2.22, 3.09 and 2.98 W m−2, as compared with delta‐16‐stream discrete‐ordinate method results. For the tuned TSA, which was developed using the present dataset, the TOA and surface r.m.s. errors were 33% and 48% smaller than for PIFM, with substantially larger improvements at low solar elevations. The robustness of these results is tested in several sensitivity experiments in which the properties of the GCM dataset are modified systematically.While typical delta‐TSAs performed, overall, fairly well, they all had a negative bias in atmospheric absorption and a positive bias in surface net short‐wave flux of ≈1 W m−2 in the global mean (including, also, night grid points). Furthermore, the absorption errors depended substantially on solar elevation. The reasons for these problems are discussed.The tests with GCM data also confirmed that the delta‐four‐stream method is considerably more accurate than the TSAs, especially so for atmospheric absorption. Copyright © 2002 Royal Meteorological Society
A comparison of ensemble‐transform Kalman‐filter targeting guidance with ECMWF and NRL total‐energy singular‐vector guidance
Authors: Majumdar S. J. , Bishop C. H. , Buizza R. , Gelaro R. 
Published: January 2003

AbstractOngoing adaptive observing programmes strive to improve short‐ and medium‐range forecasts of winter weather over populated areas. Aircraft equipped with Global Positioning System (GPS) dropwindsondes are directed over ordinarily data‐sparse regions (oceans) to collect observations to enhance the subsequent operational analysis–forecast cycle. Two objective techniques that have been used to identify optimal ‘target regions’ are the ensemble‐transform Kalman filter (ET KF) and the singular‐vector technique. The similarities and differences between targeting guidance based on the ET KF and total‐energy singular vectors (TESVs) are assessed for ten cases during the North Pacific Experiment (NORPEX). TESVs are computed at the European Centre for Medium‐Range Weather Forecasts (ECMWF) and the Naval Research Laboratory (NRL) using their respective global models, and the ET KF uses 25 ECMWF ensemble perturbations.Using measures based on (i) rankings of aircraft flight tracks and (ii) spatial similarities between targeting guidance maps, the main finding is that (a) the ET KF guidance is reasonably correlated with TESV guidance for at least seven of the ten NORPEX cases. Other findings include: (b) the ECMWF and NRL TESV targets sometimes differ significantly, (c) the ET KF and TESV guidance maps often display different optimal locations for targeting on smaller scales, but larger‐scale aspects are usually similar, (d) the ET KF generally identifies larger regions over which useful observations can be taken compared with the SV technique, and (e) regions that are deemed effective for targeting by both techniques often correspond to baroclinic zones.The ET KF and SV techniques may identify similar regions for targeting if locations of large ensemble‐based analysis‐error variance coincide with areas where rapid perturbation growth occurs. On the other hand, they may identify different targeting locations for the following two reasons. First, the ET KF implicitly accounts for error correlation length‐scales in its predictions of forecast‐error variance reduction produced by any set of targeted observations. Hence, it can identify locations for targeted observations that are distant from the regions of high analysis sensitivity that are selected for targeting by the SV technique. Second, ET KF estimates of analysis‐error variance are constrained to a subspace of evolved ensemble perturbations and are, therefore, rank deficient. Copyright © 2002 Royal Meteorological Society