bluuebunny/crossref_metadata_2025_split huggingface dataset:
Sensitivity of tropical storms simulated by a general circulation model to changes in cumulus parametrization
Authors: Vitart F. , Anderson J. L. , Sirutis J. , Tuleya R. E. 
Published: February 2007

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 with realistic frequency.
Boundary‐layer convection and diurnal variation of vertical‐velocity characteristics in the free troposphere
Authors: Muschinski Andreas , Chilson Phillip B. , Hooper David A. , Palmer Robert D. , Schmidt Gerhard , Steinhagen Hans 
Published: February 2007

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−1 and 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 of w show 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. The w spectra, which have been calculated from 8 h long w time series (sampling period 1 min), have a noise floor of 10−3 m2s−2Hz−1 or less. This corresponds to an uncorrelated noise with a standard deviation of 3 mm s−1 for the 1 min samples. The concept of gravity‐wave intermittency is introduced, in analogy to a well‐established concept in turbulence physics. It is demonstrated how this concept can be used to distinguish turbulence from gravity waves empirically.
Hierarchical modelling of tropical convective systems using explicit and parametrized approaches
Authors: Liu Changhai , Moncrieff Mitchell W. , Grabowski Wojciech W. 
Published: February 2007

AbstractCloud systems observed during 1–7 September of GATE are examined in a hierarchical approach, namely: a two‐dimensional cloud‐resolving simulation using a 2 km grid length; two‐ and three‐dimensional simulations using the Kain‐Fritsch convective parametrization and 10, 15 and 25 km grid lengths; and coarse‐grid simulations without any convective parametrization. All simulations are forced by the same objectively analysed time‐varying large‐scale advection of temperature and moisture. The domain‐mean winds are relaxed to the observed wind profiles.Both the cloud‐resolving modelling and the lower‐resolution modelling with parametrized convection realize the three observed cloud system categories (squall line, non‐squall cluster and scattered convection) and transitions among them. In particular, the well‐organized fast‐moving squall‐type cloud system observed on 4 September is realized in a three‐dimensional experiment with parametrized convection. In contrast, the lower‐resolution modelling without any convective parametrization fails to produce the squall‐type convective system during the weakly forced period but successfully represents the non‐squall cluster during strong forcing. This lack of success is mostly attributed to convective triggering and the absence of resolved downdraught‐enhanced surface fluxes. These issues are not as critical during strong large‐scale forcing.The observed evolutions of temperature, water vapour mixing ratio, precipitation and surface moisture flux are realized in all simulations. A common deficiency is the overprediction of upper‐level relative humidity. The simulation with parametrized convection features a comparatively large low‐level water vapour mixing ratio, a surface and upper‐level cold temperature bias and a mid‐tropospheric warm bias. This is mainly attributed to deficiencies in how the Kain‐Fritsch scheme represents convective mass flux, detrainment and entrainment by cumulus congestus.
Reconsideration of the physical and empirical origins of Z–R relations in radar meteorology
Authors: Jameson A. R. , Kostinski A. B. 
Published: February 2007

AbstractThe rainfall rate, R, and the radar reflectivity factor, Z, are represented by a sum over a finite number of raindrops. It is shown here and in past work that these variables should be linearly related. Yet observations show that correlations between R and Z are often more appropriately described by nonlinear power laws. In the absence of measurement effects, why should this be so?In order to justify this observation, there have been many attempts to create physical ‘explanations’ for power laws. However, the present work argues that, because correlations do not prove causation (an accepted fact in the statistical sciences), such explanations are suspect, particularly since the parametric fits are not unique and because they exhibit fundamental physical inconsistencies. So why, then, do so many correlations fit power laws when physical arguments show that Z and R should be related linearly?It is shown in the present work that physically based, linear, relations between Z and R apply in statistically homogeneous rain. (Note that statistical homogeneity does not mean that the rain is spatially uniform.) In contrast, nonlinear power laws are empirical fits to correlated, but statistically inhomogeneous data. This conclusion is proven theoretically after developing a ‘generalized’ Z–R relation based upon physical consideration of R and Z as random variables. This relation explicitly incorporates details of the drop microphysics as well as the variability in measurements of Z and R. In statistically homogeneous rain, this generalized expression shows that the coefficient relating Z and R is a constant resulting in a linear Z–R relation. In statistically inhomogeneous rain, however, the coefficient varies in an unknown fashion so that one must resort to statistical fits, often power laws, in order to relate the two quantities empirically over widely varying conditions. This conclusion is independently verified using Monte Carlo simulations of rain from earlier work and is also corroborated using disdrometer observations. Thus, the justification for nonlinear power‐law Z–R relations is not physical, but rather statistical, in that they provide convenient parametric fits for estimating mean R from measured mean Z in statistically inhomogeneous rain.Finally, examples based upon disdrometer data suggest that such generalized relations between two variables defined by such sums are potentially useful over a wide range of remote‐sensing problems and over a wide range of scales. The examples also offer hope that data collected over disparate sampling‐volumes and sampling‐frequencies can still be combined to yield meaningful estimates. Although additional testing is required, this allows us to write programs which combine estimates of R using remote‐sensing techniques with sparse but direct rainfall observations.
Influence of the assimilation scheme on the efficiency of adaptive observations
Authors: Bergot Thierry 
Published: February 2007

AbstractThe influence of assimilation schemes on the efficiency of adaptive observations is investigated, with three‐and four‐dimensional variational data assimilation (3D‐Var and 4D‐Var) systems, for 20 cases of cyclogenesis during the Fronts and Atlantic Storm‐Track Experiment (FASTEX). During FASTEX, adaptive observations were made in such a way that the forecast of synoptic‐scale systems characterized by a weak predictability was improved.This study shows that, on average, adaptive observations significantly influence the forecast for both assimilation schemes. However, with 3D‐Var, the inclusion of adaptive observations leads to a blending of improvements and worsenings, resulting in a very weak mean improvement. In contrast, with 4D‐Var the forecasts are, on average, significantly improved at the verification time (a mean improvement of about 10% and a maximum improvement of more than 50%) without producing large variability at medium‐range.Results confirm that the positive impact of 4D‐Var primarily occurs as a result of a change in the initial conditions in the most unstable directions. This positive impact is particularly obvious when adaptive observations are mainly located at the border of the sensitive area. In such cases, the known deficiencies of 3D‐Var may be detrimental. This study also demonstrates that the impact of sampling of the sensitive area depends on the assimilation scheme.
The diurnal cycle and African easterly waves: A land surface perspective
Authors: Taylor Christopher M. , Clark Douglas B. 
Published: February 2007

AbstractFluxes from the land surface into the atmosphere exhibit strong variability on diurnal and daily time‐scales during the Sahelian wet season. The influence of this variability on the atmosphere is examined here using a general circulation model. A land‐surface scheme with an improved description of the sparse vegetation which characterizes the Sahel is introduced, and compared with simulations using existing parameters. With smaller evaporation rates, the sparse vegetation results in a warmer and deeper planetary boundary layer (PBL) in better agreement with observations. The diurnal cycle in simulated rainfall is much weaker, due to less frequent triggering of daytime convection. The damping of diurnal variability is accompanied by enhanced daily variability, with a deep moist PBL developing over several days. Above the sparsely vegetated surface, there is also greater African easterly wave (AEW) activity, and more long‐lived rain events associated with it.A mechanism is proposed to explain the increased frequency of AEWs above sparser vegetation, based on wave composites in the model. Surface heating of the lower atmosphere is modulated strongly by AEWs. This occurs via a surface‐convection‐radiation feedback. Estimates of the boundary‐layer heat budget indicate that variations in surface heating can, to a large extent, account for the low‐level temperature signal characteristic of AEWs. By reinforcing the low‐level temperature anomalies, the surface may influence the meridional wind signature of the wave above the PBL. Further evidence of the close coupling between the land surface and atmosphere is found by examining the modulation of precipitation by AEWs. Surface heat and moisture fluxes influence the build‐up of convective available potential energy ahead of the wave trough. In this way, the properties of the surface can affect the timing and intensity of rainfall associated with, an AEW.
Surface fluxes under shear‐free convection
Authors: Akylas E. , Tombrou M. , Lalas D. , Zilitinkevich S. S. 
Published: February 2007

AbstractIn this study, theoretical models of Schumann, Sykeset al., Beljaars, and Zilitinkevichet al.are examined, compared with data, and evaluated with regard to the calculation of the minimum friction velocity and the heat transfer coefficient. All data employed in earlier papers, namely those from meteorological campaigns SCOPE, TOGA COARE and BOREX‐95, and the Schmidt and Schumann and Sykes and Henn large‐eddy simulations (LESs), are considered. To achieve objective comparison between different formulae, empirical coefficients are recalculated by fitting theoretical curves separately for field data and for data from LESs. Despite essential differences in the shapes of the vertical profiles and the surface‐layer height formulations applied in different models, practically all of them perform rather similarly and in fairly good correspondence with the chosen data set. However, a remarkable systematic difference is observed between data from measurements, on the one hand, and LES data, on the other. It is argued that this difference results from a contribution from uncounted mean‐wind shear to the friction velocity in the field experiments. By this expedient, applicability of LESs to the resistance and heat‐mass transfer problem is confirmed, and empirical coefficients in the resistance and heat transfer formulations are refined.
Relationship between singular vectors and transient features in the background flow
Authors: Reynolds C. A. , Gelaro R. , Doyle J. D. 
Published: February 2007

AbstractThe relationships between singular vectors (SVs) and transient features in the background flow are examined through both composite techniques and case‐studies. The SVs are derived using the NOGAPS forward and adjoint tangent models based on 48‐hour forecasts during the NORPEX period. Composite results and case‐studies both reveal significant spatial relationships between the SVs and transient features in the background flow. The SV perturbations often occur below distinctive high potential‐vorticity (PV) features in the middle‐to‐upper troposphere. Case‐studies reveal that the SVs propagate upward rapidly and have an impact on these PV features through the end of the optimization interval. In order to investigate how these small, initial perturbations have such a large impact on future development, the relationship between SVs and the quasi‐geostrophic forcing is examined through the use of Q‐vectors. The SV perturbations, while scaled to have a very small impact on the temperature and wind fields, have a very large impact on the mid‐tropospheric Q‐vectors. In contrast, the impact of the SV on the Q‐vectors is negligible when added to an analysis field for which it is not optimal (e.g. the analysis field from another day). These findings establish a significant relationship between the SV perturbations and dynamically active regions in the middle troposphere, and point toward an integral link between SVs and upper‐level PV precursors in synoptic development.
Ensembles using multiple models and analyses
Authors: Richardson D. S. 
Published: February 2007

AbstractThe performance of the European Centre for Medium‐Range Weather Forecasts (ECMWF) Ensemble Prediction System (EC EPS) is compared with that of a number of alternative configurations which incorporate information from additional analyses or an additional model or both. Each configuration is approximately equivalent in size, resolution and computational cost and could therefore in principle provide an alternative operational EPS. A multi‐centre ensemble system (MC EPS) is constructed by replacing half of the EC EPS with integrations of the Met Office (UKMO) model perturbed about the UKMO analysis. Two additional configurations are constructed using the ECMWF model only, but adding information available from four other operational analyses. A ‘consensus’ ensemble (CONS EPS) is generated by adding the operational EC EPS perturbations to the mean of the available analyses (the consensus analysis); a multi‐analysis ensemble (MA EPS) is generated by combining smaller ensembles of ECMWF‐model integrations perturbed about each of the available operational analyses. The different systems are compared over a large sample of 60 cases covering the period October 1998 to July 1999.All the alternative configurations are found to improve over the EC EPS in terms of ensemble‐mean skill, ensemble spread, and probabilistic evaluation. For 500 hPa height, the MC EPS is consistently the most skilful configuration, but for 850 hPa temperature the MC and MA systems are equivalent. The CONS EPS is generally less skilful than the other two alternatives. The benefit of the MC EPS over the MA EPS for 500 hPa height can be attributed to the combination of different models within the MC EPS. However, it is shown that application of a simple bias correction to the ensemble members removes the advantage of the MC EPS. It is also noted that, by construction, the MC and MA systems have larger initial spread than the EC and CONS configurations. It is demonstrated that increasing the amplitude of the initial perturbations of the CONS EPS improves the performance of this configuration to be comparable with the MC and MA systems. It is concluded that the benefit of the MC EPS can be realized using the single‐model EC EPS incorporating information from additional operational analyses (provided that the issue of model bias is addressed), thus avoiding the technical difficulties of maintaining more than one model in an operational EPS.
A strategy for high‐resolution ensemble prediction. I: Definition of representative members and global‐model experiments
Authors: Molteni F. , Buizza R. , Marsigli C. , Montani A. , Nerozzi F. , Paccagnella T. 
Published: February 2007

AbstractIn the last few years, tens of alternative weather forecasts have been made available to forecasters by operational ensemble prediction systems. In many forecasting applications, it is useful to identify (possibly in an objective way) a few representative ensemble members, deemed to represent the most interesting weather scenarios. In this paper, a strategy to select representative members (RMs hereafter) from an ensemble prediction is developed, and applied to four cases of medium‐range ensemble forecasts performed with the European Centre for Medium‐Range Weather Forecasts (ECMWF) Ensemble Prediction System (EPS). The four case‐studies correspond to events of very intense rainfall (leading to localized floods) in the Alpine region, selected as benchmarks for numerical simulations in the Mesoscale Alpine Programme. The RM selection procedure uses a cluster analysis of the ensemble forecasts as its first step. For each cluster, an RM is defined to be the member with the smallest ratio between its average distance from the members of its own cluster and its average distance from the members of the other clusters. Distances are computed either using an L2‐norm applied to 700 hPa geopotential height fields or an L1‐norm to precipitation fields.RMs are compared with cluster centroids in the four case‐studies of extreme rainfall. By definition, RMs are characterized by a synoptic‐scale atmospheric flow similar to the flow of the corresponding cluster centroid, but they contain more small‐scale features, especially in the prediction of weather parameters such as precipitation.RM initial conditions can be used to initiate higher‐resolution global forecasts; alternatively, RMs may be used to define initial and boundary conditions for nested high‐resolution forecasts with limited‐area models. Integrations of RMs with the ECMWF global model at T1 319 horizontal resolution (compared with the T1 159 resolution used in the EPS) were performed. Results indicate that each higher‐resolution forecast, started from RM initial conditions, remains closer to the low‐resolution RM than to other ensemble members, but provides a more detailed forecast of weather parameters, especially in regions of complex topography. Experiments with a nested limited‐area model, started from the same set of RMs, are described in a companion paper.
Microphysical properties of continental clouds from in situ measurements
Authors: Korolev A. V. , Isaac G. A. , Mazin I. P. , Barker H. W. 
Published: February 2007

AbstractIt is important to gain knowledge about the microphysical characteristics of continental clouds in order to properly understand their formation, their radiative properties and their ability to produce precipitation. Satellites to remotely sense cloud properties, and cloud interactions with aircraft that fly through them require knowledge of cloud microphysics. Moreover, numerical simulations of global climate are sensitive to small systematic changes in cloud optical properties, but very few large datasets are available that document the characteristics of continental clouds. This paper summarizes 9 x 104 km of in‐cloud measurements made by the Central Aerological Observatory over the former USSR during 1977‐84. Statistical characteristics of total water content (W), extinction coefficient (β), effective diameter (Deff) and effective concentration (Neff) are summarized as functions of temperature and cloud type, for measurements archived at 700 m horizontal resolution. The effect of threshold sensitivity, or cloud definition, on the statistical distributions is discussed. Decreases of W, β and Neff with colder temperatures are consistent with our general knowledge of cloud formation. For all temperature intervals and cloud types, correlations between pairs of W, β and Deff are too small thereby barring any hope of simple linear parametrizations. Differences in W, β, Deff and Neff for the various cloud types indicate changes in cloud formation mechanisms. These data can assist in verification studies of cloud parametrization schemes in general‐circulation models, numerical weather‐prediction models, and cloud‐resolving models. The problems of cloud type, scale averaging, variability over a model grid cell, distribution of water between the liquid and ice phase, and the lumping together of precipitating and non‐precipitating clouds must be considered in these studies.