bluuebunny/crossref_metadata_2025 huggingface dataset:
Optics, Pictures and Evidence: Leonardo's Drawings of Mirrors and Machinery
Authors: Dupré Sven
Published: June 2005

Leonardo's drawings of optical machinery have been used (by David Hockney and others) as evidence for the claim that Leonardo built machines to make concave mirrors with which he could project images. This paper argues that Leonardo's drawings cannot be used as evidence for this claim. It will be shown that Leonardo used the drawings to communicate with his patrons and craftsmen, to experiment on paper, to record trials with models, and to think about 'theoretical' problems in optics. At both the theoretical and the practical level, Leonardo was only concerned with the burning properties of concave mirrors, not with their imaging properties. The paper will conclude that the drawings of optical machinery allowed Leonardo to differentiate himself from the ordinary mirror-makers in his workshop. The same drawings, however, also forced him to remain within the conceptual framework of perspectivist optics.
Early Conceptualizations of the Telescope as an Optical Instrument
Authors: Malet Antoni
Published: June 2005

This article focuses on some theoretical developments prompted by the use and construction of telescopes in the first half of the seventeenth century. It argues that today's notion of "scientific instrument" cannot be used to categorize these optical devices or explain their impact on natural philosophy. The article analyzes in historical terms the construction of conceptual references for the telescope as an instrument of a new kind, which possessed capabilities and working principles unlike those of traditional "mathematical instruments." It shows that through the 1650s, in both rhetorical and explanatory terms, first-rank telescope makers, theoreticians, and astronomers found it useful to equate the telescope with the eye, suggesting that the data the telescope produced was as reliable as that obtained in naked-eye vision. Kepler's and Descartes' theory of the telescope will be shown to dovetail uncannily with this understanding of the telescope.
Reflections on the Hockney-Falco Thesis: Optical Theory and Artistic Practice in the Fifteenth and Sixteenth Centuries
Authors: Smith A. Mark
Published: June 2005

One problem facing Hockney and Falco is the lack of evidence among optical sources to support their claim that artists used image-projection by the early 1400s. After all, if quattrocento artists knew about image-projection, they must have learned about it from experts in the field, and no one was more expert at the time than Perspectivist opticians. As I argue in this paper, however, Perspectivist reflection-analysis posed certain theoretical and conceptual constraints that would have prevented Perspectivist opticians from recognizing, much less understanding, image-projection. Their silence on this matter is therefore not evidence against the Hockney-Falco thesis.
Hockney's Secret Knowledge, Vanvitelli's Camera Obscura
Authors: Lüthy Christoph
Published: June 2005

This article opens with a distinction between David Hockney's strong and weak theses. According to the strong thesis, in the period 1430-1860, optical tools (mirrors, lenses, the camera obscura, etc.) were used in the production of paintings; according to the weak thesis, mirrors and lenses merely inspired their naturalistic look. It will be argued that while for the fifteenth and sixteenth centuries, there is little evidence in favor of the strong thesis, the case is different for the seventeenth century, for which the use of optical instruments by painters is a documented fact. In this article, an early case is examined. The extant preparatory drawings of Gaspare Vanvitelli (Gaspar van Wittel, 1652-1736) suggest that this cityscape painter relied on a camera obscura. But even here, the strong thesis must be tempered. The fact that several stages of artistic transformation separate the camera obscura projection from the finished painting undermines Hockney's analogy between optically assisted painting and 'naturalistic' photography.
Between Knowing and Doing: Mirrors and Their Imperfections in the Renaissance
Authors: Schechner Sara
Published: June 2005

Inspection of surviving mirrors and related objects shows that they were too crude to offer the early Renaissance painter an optical short-cut to a naturalistic image of his subject. The craftsmanship of mirror makers was independent of and inferior to the quality of theories of image formation of the day.
Ensemble Forecasting in a System with Model Error
Authors: Orrell David
Published: June 2005

Error in weather forecasting is due to inaccuracy both in the models used and in the estimate of the current atmospheric state at which the model is initiated. Because weather models are thought to be chaotic and, therefore, sensitive to initial conditions, the technique of ensemble forecasting has been developed in part to address the latter effect. An ensemble of forecasts is made with perturbed initial conditions, with the aim being to produce an estimate of the probability distribution function for the future state of the weather. Some ensemble schemes also include changes to the model. While the ensemble approach is quite widely adopted, however, its verification is complicated, and the effect of model error on ensemble performance is not clear. In this paper, the effect of model error on ensemble behavior for a version of the Lorenz ’96 system is investigated. It is shown that estimates of the model’s ability to shadow the observations, obtained using the model drift, are robust to observational error and smoothing schemes, such as four-dimensional variational data assimilation (4DVAR), and help reveal the effect of model error on ensemble performance. Comparisons are made with full weather models. The aim is to provide a study of ensemble error in the context of the Lorenz ’96 system, which may be useful in formulating questions and experiments for weather models.
The Vertical Structure of Tropical Convection and Its Impact on the Budgets of Water Vapor and Ozone
Authors: Folkins Ian , Martin Randall V.
Published: June 2005

Convective clouds in the Tropics that penetrate the boundary layer inversion preferentially detrain into a shallow outflow layer (2–5 km) or a deep outflow layer (10–17 km). The properties of these layers are diagnosed from a one-dimensional model of the Tropics constrained by observed mean temperature and water vapor profiles. The mass flux divergence of the shallow cumuli (2–5 km) is balanced by a mass flux convergence of evaporatively forced descent (downdrafts), while the mass flux divergence of deep cumulonimbus clouds (10–17 km) is balanced by a mass flux convergence of clear-sky radiative descent. The pseudoadiabatic temperature stratification of the midtroposphere (5–10 km) suppresses cloud outflow in this interval. The detrainment profile in the deep outflow layer is shifted downward by about 1.5 km from the profile one would anticipate based on undilute pseudoadiabatic ascent of air from the boundary layer. The main source of water vapor to most of the tropical troposphere is evaporative moistening. Below 12 km, evaporatively forced descent plays an important role in the vertical mass flux budget of the Tropics. This gives rise to a coupling between the water vapor and mass flux budgets, which, between 5 and 10 km, provides a constraint on the variation of relative humidity with height. Between 12 and 15 km, the observed relative humidity profile can be reproduced by assuming a simple first-order balance between detrainment moistening and subsidence drying. The mean ozone profile of the Tropics can be reproduced using a simple one-dimensional model constrained by the cloud mass flux divergence profile of the diagnostic model.
Simulations of Moist Nearly Neutral Flow over a Ridge
Authors: Miglietta M. M. , Rotunno R.
Published: June 2005

Although a fairly common atmospheric condition in orographic-rain scenarios, there is relatively little known about moist neutral flows over a ridge from theory and modeling. Presented in this paper are numerical simulations of the orographic-flow modification occurring for a two-dimensional moist nearly neutral flow over a ridge in the regime where the Coriolis force can be neglected. If an initially saturated moist neutral flow were to remain everywhere saturated as it flows over an obstacle, then the expected solution would be the linear solution because the condition for linearity (hill height less than the ambient wind velocity/static stability) is always met. However, for higher mountains, the solutions indicate the development of areas of unsaturated air, with correspondingly larger values of local static stability. This internal switching from small to large values of static stability is an inherent nonlinearity, which has far-reaching consequences for understanding the orographic-flow modification in this regime. The sensitivity of the solution to the mountain height and to the initial cloud water content is analyzed here. The authors find that the solutions fall into three basic categories. If the mountain height is small enough, a saturated flow can be maintained everywhere given sufficient initial cloud water; for tall mountains the atmosphere upwind of the mountain is maintained in a saturated state and transitions to an unsaturated downslope flow on the lee side, which has characteristics associated with downslope windstorms; for mountains of intermediate height, the solutions show the existence of an upwind-propagating disturbance that has the effect of desaturating the atmosphere above the mountain.
Refinements in the Treatment of Ice Particle Terminal Velocities, Highlighting Aggregates
Authors: Mitchell David L. , Heymsfield Andrew J.
Published: June 2005

Recent work on the terminal velocity of ice crystal aggregates suggests that their “Re–X” relationship may not be well predicted by current theory. This study examines possible reasons for this departure from theory, and develops a new Re–X relationship appropriate for ice crystal aggregates. The methodology of Khvorostyanov and Curry was applied to this new relationship to formulate power-law expressions for all ice particle types.

Fall speed differences between the Khvorostyanov and Curry approach and the approach described here were as large as 50% for aggregates and 30% for single crystals. This was primarily due to the following: 1) surface roughness coefficients used in the former were appropriate for rigid spheres and liquid drops but not for ice crystals and 2) the relationship between Reynolds number Re and Best number X at high Re is better described for aggregates by adding a second term to the Re–X governing equation, as done in this work.

The corrections and improvements described here may be critical to the calculation of snowfall rates, to the modeling of the aggregation process, and for interpreting Doppler radar measurements during snowfall events. Since most of the size distribution mass is generally associated with aggregates below cloud base, an accurate treatment of aggregate fall speeds is needed for determining snowfall rates.
Life Cycle of Numerically Simulated Shallow Cumulus Clouds. Part I: Transport
Authors: Zhao Ming , Austin Philip H.
Published: June 2005

This paper is the first in a two-part series in which the life cycles of numerically simulated shallow cumulus clouds are systematically examined. The life cycle data for six clouds with a range of cloud-top heights are isolated from an equilibrium trade cumulus field generated by a large-eddy simulation (LES) with a uniform resolution of 25 m. A passive subcloud tracer is used to partition the cloud life cycle transport into saturated and unsaturated components; the tracer shows that on average cumulus convection occurs in a region with time-integrated volume roughly 2 to 3 times that of the liquid-water-containing volume. All six clouds exhibit qualitatively similar vertical mass flux profiles with net downward mass transport at upper levels and net upward mass flux at lower levels. This downward mass flux comes primarily from the unsaturated cloud-mixed convective region during the dissipation stage and is evaporatively driven. Unsaturated negatively buoyant cloud mixtures dominate the buoyancy and mass fluxes in the upper portion of all clouds while saturated positively buoyant cloud mixtures dominate the fluxes at lower levels. Small and large clouds have distinct vertical profiles of heating/cooling and drying/moistening, with small clouds cooling and moistening throughout their depth, while larger clouds cool and moisten at upper levels and heat and dry at lower levels. The simulation results are compared to the predictions of conceptual models commonly used in shallow cumulus parameterizations.
Life Cycle of Numerically Simulated Shallow Cumulus Clouds. Part II: Mixing Dynamics
Authors: Zhao Ming , Austin Philip H.
Published: June 2005

This paper is the second in a two-part series in which life cycles of six numerically simulated shallow cumulus clouds are systematically examined. The six clouds, selected from a single realization of a large-eddy simulation, grow as a series of pulses/thermals detached from the subcloud layer. All six clouds exhibit a coherent vortical circulation and a low buoyancy, low velocity trailing wake. The ascending cloud top (ACT), which contains this vortical circulation, is associated with a dynamic perturbation pressure field with high pressure located at the ascending frontal cap and low pressure below and on the downshear side of the maximum updrafts. Examination of the thermodynamic and kinematic structure, together with passive tracer experiments, suggests that this vortical circulation is primarily responsible for mixing between cloud and environment. As the cloud ACTs rise through the sheared environment, the low pressure, vortical circulation, and mixing are all strongly enhanced on the downshear side and weakened on the upshear side. Collapse of the ACT also occurs on the downshear side, with subsequent thermals ascending on the upshear side of their predecessors. The coherent core structure is maintained throughout the ACT ascent; mixing begins to gradually dilute the ACT core only in the upper half of the cloud's depth. The characteristic kinematic and dynamic structure of these simulated ACTs, together with their mixing behavior, corresponds closely to that of shedding thermals. These shallow simulated clouds, however, reach a maximum height of only about four ACT diameters so that ACT mixing differs from predictions of self-similar laboratory thermals.
Monte Carlo Simulation of the Formation of Snowflakes
Authors: Maruyama Ken-ichi , Fujiyoshi Yasushi
Published: June 2005

A stochastic microphysical model of snow aggregation that combines a simple aggregation model with a Monte Carlo method was developed. Explicit treatment of the shape of individual snowflakes in the new model facilitates examination of the structure of snowflakes and the relationships between the parameters of the generated snowflakes, such as mass versus diameter, in addition to comparisons with observations. In this study, complexities in the shape of snowflakes are successfully simulated, and the understanding of the evolution of their size distribution is advanced. The mean diameter of snow particles evolves more rapidly in the aggregate model than in the sphere model. However, growth rates of the aggregates greatly depend on the collision section of particles in aggregation. The mean mass of snowflakes in the aggregate model grows more slowly than the mass in the sphere model when the sum of the particle cross section is used as the collision cross section. The mean mass grows more quickly when a circle is used whose radius is the sum of the radii of two particles. Sensitivity experiments showed that aggregation also depends on the mean and standard deviation of the initial distribution, and on the density of constituent particles.
What Causes the Divergences in Local Second-Order Closure Models?
Authors: Canuto V. M. , Cheng Y. , Howard A. M.
Published: June 2005

It has been known for three decades that in the case of buoyancy-driven flows the widely used second-order closure (SOC) level-2.5 turbulence models exhibit divergences that render them unphysical in certain domains. This occurs when the dimensionless temperature gradient Gh (defined below) approaches a critical value Gh(cr) of the order of 10; thus far, the divergences have been treated with ad hoc limitations of the typewhere τ is the eddy turnover time scale, g is the gravitational acceleration, α is the coefficient of thermal expansion, T is the mean potential temperature, and z is the height. It must be noted that large eddy simulation (LES) data show no such limitation. The divergent results have the following implications. In most of the ∂T/∂z < 0 portion of a convective planetary boundary layer (PBL), a variety of data show that τ increases with z, −∂T/∂z decreases with z, and Gh decreases with z. As one approaches the surface layer from above, at some zcr (∼0.2H, H is the PBL height), Gh approaches Gh(cr) and the model results diverge. Below zcr, existing models assume the displayed equation above. Physically, this amounts to artificially making the eddy lifetime shorter than what it really is. Since short-lived eddies are small eddies, one is essentially changing large eddies into small eddies. Since large eddies are the main contributors to bulk properties such as heat, momentum flux, etc., the artificial transformation of large eddies into small eddies is equivalent to underestimating the efficiency of turbulence as a mixing process.

In this paper the physical origin of the divergences is investigated. First, it is shown that it is due to the local nature of the level-2.5 models. Second, it is shown that once an appropriate nonlocal model is employed, all the divergences cancel out, yielding a finite result. An immediate implication of this result is the need for a reliable model for the third-order moments (TOMs) that represent nonlocality. The TOMs must not only compare well with LES data, but in addition, they must yield nondivergent second-order moments.
Southern Hemisphere Blocking Onsets Associated with Upper-Tropospheric Divergence Anomalies
Authors: Sáez de Adana F. Javier , Colucci Stephen J.
Published: June 2005

Upper-tropospheric divergence anomalies and divergence tendencies prior to and during the onset of blocking have been investigated for selected cases over the Southern Hemisphere in search of links between the upper-tropospheric response to tropical convective activity and the onset of blocking in midlatitudes. Climatologies of blocking, defined by an objective index, and divergence are established for the Southern Hemisphere and the southern Pacific, respectively. Relative blocking frequency versus longitude reveals a region of maximum blocking activity between 160°E and 75°W. Blocking frequencies for each ENSO phase indicate a shift toward the late austral fall and early winter during the warm phase, whereas during the cold and neutral phases the highest frequencies are in June and July, respectively.

Composites of area-averaged divergence anomalies for the selected blocking cases reveal more anomalous divergence than during nonblocking periods over the blocking regions and the immediate upstream regions in midlatitudes. A full divergence tendency equation is utilized to diagnose the local development of divergence preceding the onset of blocking. Results indicate that divergence tendencies over midlatitudes in the block-onset region were forced primarily by horizontal advection, ageostrophic relative vorticity, and a nonlinear effect. In the region directly upstream from the block-onset region, ageostrophic relative vorticity had the greatest contribution followed by the horizontal advection. In the Tropics, divergence tendencies appear to be driven primarily by horizontal advection. Correlations of calculated divergence tendencies with the blocking index suggest that ageostrophic vorticity may locally generate divergence that in turn may force anticyclonic vorticity associated with blocking. Lag correlations with a blocking index during blocking reveal the importance of horizontal advection in driving divergence anomalies, implying divergence-induced vorticities, toward the incipient block.
Entropy Sources in Equilibrium Conditions over a Tropical Ocean
Authors: Warner Charles
Published: June 2005

Confusion has existed as to sources of entropy due to irreversible processes in the atmosphere, the total of which matches the export of entropy by radiation. What is the mechanical efficiency of convection? For an ideal tropical oceanic system in radiative–convective equilibrium, relative magnitudes of sources of entropy are reviewed—from both observations and numerical model results. Recycling of moisture is shown to be important. Leading terms are those relating to evaporation of precipitation, water loading by falling precipitation, and mixing of unsaturated parcels of air, contributing roughly 37%, 30%, and 15% of the total irreversible production of entropy, respectively. Evaporation from the surface accounts for 11%. The remaining 7% is due to turbulent kinetic energy, generation of gravity waves, and sensible heating at the surface.

A mechanical efficiency of conversion of heat supply at the surface into kinetic energy of the direct circulation, ≈2.0%, is obtained after the budget study. The leading contribution to the conversion is due to the effect of hydrometeors. Drag of hydrometeors is split into two components based on relative contributions of form drag plus water loading (50%) and frictional drag (50%); however, only the former contributes to the direct circulation. The contribution of turbulent kinetic energy is found to be small.

Results from the budget study are found to correspond with the finding of a threshold in values of convective available potential energy by Roff and Yano, and with numerical results from a three-dimensional model of convective equilibrium by Shutts and Gray.
Diurnal Cycle of Convective Instability around the Central Mountains in Japan during the Warm Season
Authors: Sato Tomonori , Kimura Fujio
Published: June 2005

Convective rainfall often shows a clear diurnal cycle. The nighttime peak of convective activity prevails in various regions near the world's mountains. The influence of the water vapor and convective instability upon nocturnal precipitation is investigated using a numerical model and observed data. Recent developments in GPS meteorology allow the estimation of precipitable water vapor (PWV) with a high temporal resolution. A dense network has been established in Japan. The GPS analysis in August 2000 provides the following results: In the early evening, a high-GPS-PWV region forms over mountainous areas because of the convergence of low-level moisture, which gradually propagates toward the adjacent plain before midnight. A region of convection propagates simultaneously eastward into the plain. The precipitating frequency correlates fairly well with the GPS-PWV and attains a maximum value at night over the plain. The model also provides similar characteristics in the diurnal cycles of rainfall and high PWV. Abundant moisture accumulates over the mountainous areas in the afternoon and then advects continuously toward the plain by the ambient wind. The specific humidity greatly increases at about the 800-hPa level over the plain at night, and the PWV reaches its nocturnal maximum. The increase in the specific humidity causes an increase of equivalent potential temperature at about the 800-hPa level; as a result, the convective instability index becomes more unstable over the plain at night. These findings are consistent with the diurnal cycle of the observed precipitating frequency.
Cloud Properties Simulated by a Single-Column Model. Part I: Comparison to Cloud Radar Observations of Cirrus Clouds
Authors: Luo Yali , Krueger Steven K. , Moorthi Shrinivas
Published: June 2005

This study describes and demonstrates a new method for identifying deficiencies in how cloud processes are represented in large-scale models. Kilometer-scale-resolving cloud radar observations and cloud-resolving model (CRM) simulations were used to evaluate the representation of cirrus clouds in the single-column model (SCM) version of the National Centers for Environmental Prediction Global Forecast System model for a 29-day period during June and July 1997 at the Atmospheric Radiation Measurement Program site in Oklahoma.

To produce kilometer-scale cirrus statistics from the SCM results, synthetic subgrid-scale (SGS) cloud fields were generated using the SCM’s cloud fraction and hydrometeor content profiles, and the SCM’s cloud overlap and horizontal inhomogeneity assumptions. Three sets of SCM synthetic SGS cloud fields were analyzed. Two NOSNOW sets were produced in which clouds did not include snow; one set used random overlap, the other, maximum/random. In the SNOW set, clouds included snow and random overlap was used. The three sets were sampled in the same way as the cloud-radar-detected cloud fields and the CRM-simulated cloud fields.

The mean cirrus cloud occurrence frequency for the SCM NOSNOW cloud fields agrees with the observed value as well as the CRM’s does, while that for SCM SNOW cloud fields is only half that observed. In most aspects, the SCM’s cirrus properties differ significantly from the cloud radar’s and the CRM’s, which generally agree.

In comparison, there are too many physically thin SCM NOSNOW cirrus layers (most occupy only a single model layer) and too many physically thick SCM SNOW cirrus layers (most are thicker than 4 km). For the optically thin subset of cirrus layers, 1) the mean, mode, and median ice water path, and layer-mean ice water content (IWC) values for the SCM are significantly larger than the observed and CRM values; 2) the SCM layer-mean IWCs decrease with cloud physical thickness, opposite to the observations and CRM results; and 3) the range of layer-mean effective radii in the SCM thin cirrus is too narrow.
Designing Chaotic Models
Authors: Lorenz Edward N.
Published: June 2005

After enumerating the properties of a simple model that has been used to simulate the behavior of a scalar atmospheric quantity at one level and one latitude, this paper describes the process of designing one modification to produce smoother variations from one longitude to the next and another to produce small-scale activity superposed on smooth large-scale waves. Use of the new models is illustrated by applying them to the problem of the growth of errors in weather prediction and, not surprisingly, they indicate that only limited improvement in prediction can be attained by improving the analysis but not the operational model, or vice versa. Additional applications and modifications are suggested.
Using Bayesian Model Averaging to Calibrate Forecast Ensembles
Authors: Raftery Adrian E. , Gneiting Tilmann , Balabdaoui Fadoua , Polakowski Michael
Published: June 2005

Ensembles used for probabilistic weather forecasting often exhibit a spread-error correlation, but they tend to be underdispersive. This paper proposes a statistical method for postprocessing ensembles based on Bayesian model averaging (BMA), which is a standard method for combining predictive distributions from different sources. The BMA predictive probability density function (PDF) of any quantity of interest is a weighted average of PDFs centered on the individual bias-corrected forecasts, where the weights are equal to posterior probabilities of the models generating the forecasts and reflect the models' relative contributions to predictive skill over the training period. The BMA weights can be used to assess the usefulness of ensemble members, and this can be used as a basis for selecting ensemble members; this can be useful given the cost of running large ensembles. The BMA PDF can be represented as an unweighted ensemble of any desired size, by simulating from the BMA predictive distribution.

The BMA predictive variance can be decomposed into two components, one corresponding to the between-forecast variability, and the second to the within-forecast variability. Predictive PDFs or intervals based solely on the ensemble spread incorporate the first component but not the second. Thus BMA provides a theoretical explanation of the tendency of ensembles to exhibit a spread-error correlation but yet be underdispersive.

The method was applied to 48-h forecasts of surface temperature in the Pacific Northwest in January–June 2000 using the University of Washington fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) ensemble. The predictive PDFs were much better calibrated than the raw ensemble, and the BMA forecasts were sharp in that 90% BMA prediction intervals were 66% shorter on average than those produced by sample climatology. As a by-product, BMA yields a deterministic point forecast, and this had root-mean-square errors 7% lower than the best of the ensemble members and 8% lower than the ensemble mean. Similar results were obtained for forecasts of sea level pressure. Simulation experiments show that BMA performs reasonably well when the underlying ensemble is calibrated, or even overdispersed.
Stability of Leapfrog Constant-Coefficients Semi-Implicit Schemes for the Fully Elastic System of Euler Equations: Case with Orography
Authors: Bénard P. , Mašek J. , Smolíková P.
Published: June 2005

The stability of constant-coefficients semi-implicit schemes for the hydrostatic primitive equations and the fully elastic Euler equations in the presence of explicitly treated thermal residuals has been theoretically examined in the earlier literature, but only for the case of a flat terrain. This paper extends these analyses to a case in which an orography is present, in the shape of a uniform slope. It is shown, with mass-based coordinates, that for the Euler equations, the presence of a slope reduces furthermore the set of the prognostic variables that can be used in the vertical momentum equation to maintain the robustness of the scheme, compared to the case of a flat terrain. The situation appears to be similar for systems cast in mass-based and height-based vertical coordinates. Still for mass-based vertical coordinates, an optimal prognostic variable is proposed and is shown to result in a robustness similar to the one observed for the hydrostatic primitive equations system. The prognostic variables that lead to robust semi-implicit schemes share the property of having cumbersome evolution equations, and an alternative time treatment of some terms is then required to keep the evolution equation reasonably simple. This treatment is shown not to modify substantially the stability of the time scheme. This study finally indicates that with a pertinent choice for the prognostic variables, mass-based vertical coordinates are equally suitable as height-based coordinates for efficiently solving the compressible Euler equations system.
Geostrophic Adjustment for Reversibly Staggered Grids
Authors: McGregor John L.
Published: June 2005

A technique is presented for meteorological modeling in which all variables are held on an unstaggered grid, but the winds are transformed to a staggered C grid for the gravity wave calculations. An important feature is the use of a new reversible interpolation procedure for the staggering–unstaggering of the winds. This reversible procedure has excellent dispersion properties for geostrophic adjustment of the linearized shallow-water equations, being generally superior to those of the A, B, and C grids. Its dispersion behavior is generally similar to that of the unstaggered Z grid of Randall, which carries divergence and vorticity as primary variables. The scheme has fewer computational overheads than the Z grid.
Diversity in Interpretations of Probability: Implications for Weather Forecasting
Authors: de Elía Ramón , Laprise René
Published: June 2005

Over the last years, probability weather forecasts have become increasingly popular due in part to the development of ensemble forecast systems. Despite its widespread use in atmospheric sciences, probability forecasting remains a subtle and ambiguous way of representing the uncertainty related to a future meteorological situation. There are several schools of thought regarding the interpretation of probabilities, none of them without flaws, internal contradictions, or paradoxes. Usually, researchers tend to have personal views that are mostly based on intuition and follow a pragmatic approach.

These conceptual differences may not matter when accuracy of a probabilistic forecast is measured over a long period (e.g., through the use of Brier score), which may be useful for particular objectives such as cost/benefit decision making. However, when scientists wonder about the exact meaning of the probabilistic forecast in a single case (e.g., rare and extreme event), the differences of interpretation become important.

This work intends to describe this problem by first drawing attention to the more commonly accepted interpretations of probability, and then, the consequences of these assumptions are studied. Results suggest that without agreement on the interpretation, the usefulness of the probability forecast as a tool for single events—which include record-breaking events—remains unknown. An open discussion of this topic within the community would be useful to clarify the communication among researchers, with the public and with decision makers.
Generation of African Easterly Wave Disturbances: Relationship to the African Easterly Jet
Authors: Hsieh Jen-Shan , Cook Kerry H.
Published: June 2005

The relationship between African easterly waves and the background climatology in which they form is studied using a regional climate model. The surface and lateral boundary conditions in the model are manipulated to modify the background climatology, especially the African easterly jet and the ITCZ, and the behavior of the waves in these different settings is evaluated.

Three climate simulations are presented, with monthly mean lateral and surface boundary conditions. One has a strong jet and a strong ITCZ, the second has a strong jet and a weak ITCZ, and the third has a weak jet and a strong ITCZ. In these simulations, the presence of wave activity is more closely associated with the concentration of the ITCZ than the strength of the African easterly jet. In particular, the simulation with a strong jet accompanied by a weak ITCZ does not produce significant wave activity, but a weak jet with a strong ITCZ has realistic wave disturbances. Both the Charney–Stern and the Fjörtoft necessary conditions are satisfied in all three simulations, suggesting that combined barotropic and baroclinic instability contributes to the generation of waves. Near the origin of the waves, meridional gradient reversals of isentropic potential vorticity result from potential vorticity anomalies generated by convective heating within the ITCZ, implying that the unstable zonal flow may be caused by cumulus convection within the ITCZ and not by shear instability associated with the jet.

Two additional simulations with 1988 lateral boundary conditions demonstrate that 3–5-day wave disturbances can be generated in the absence of the African easterly jet, but with unrealistically small wavelengths. These results suggest that African easterly waves are initiated by cumulus convection within the ITCZ, and not by barotropic instability associated with the jet.
Eastern North Pacific Hurricane Season of 2003
Authors: Beven John L. , Avila Lixion A. , Franklin James L. , Lawrence Miles B. , Pasch Richard J. , Stewart Stacy R.
Published: June 2005

The tropical cyclone activity for 2003 in the eastern North Pacific hurricane basin is summarized. Activity during 2003 was slightly below normal. Sixteen tropical storms developed, seven of which became hurricanes. However, there were no major hurricanes in the basin for the first time since 1977. The first hurricane did not form until 24 August, the latest observed first hurricane at least since reliable satellite observations began in 1966. Five tropical cyclones made landfall on the Pacific coast of Mexico, resulting in 14 deaths.
Wintertime Nonbrightband Rain in California and Oregon during CALJET and PACJET: Geographic, Interannual, and Synoptic Variability
Authors: Neiman Paul J. , Wick Gary A. , Ralph F. Martin , Martner Brooks E. , White Allen B. , Kingsmill David E.
Published: June 2005

An objective algorithm presented in White et al. was applied to vertically pointing S-band (S-PROF) radar data recorded at four sites in northern California and western Oregon during four winters to assess the geographic, interannual, and synoptic variability of stratiform nonbrightband (NBB) rain in landfalling winter storms. NBB rain typically fell in a shallow layer residing beneath the melting level (<∼3.5 km MSL), whereas rainfall possessing a brightband (BB) was usually associated with deeper echoes (>∼6 km MSL). The shallow NBB echo tops often resided beneath the coverage of the operational Weather Surveillance Radar-1988 Doppler (WSR-88D) scanning radars yet were still capable of producing flooding rains.

NBB rain contributed significantly to the total winter-season rainfall at each of the four geographically distinct sites (i.e., 18%–35% of the winter-season rain totals). In addition, the rainfall observed at the coastal mountain site near Cazadero, California (CZD), during each of four winters was composed of a significant percentage of NBB rain (18%–50%); substantial NBB rainfall occurred regardless of the phase of the El Niño–Southern Oscillation (which ranged from strong El Niño to moderate La Niña conditions). Clearly, NBB rain occurs more widely and commonly in California and Oregon than can be inferred from the single-winter, single-site study of White et al.

Composite NCEP–NCAR reanalysis maps and Geostationary Operational Environment Satellite (GOES) cloud-top temperature data were examined to evaluate the synoptic conditions that characterize periods of NBB precipitation observed at CZD and how they differ from periods with bright bands. The composites indicate that both rain types were tied generally to landfalling polar-cold-frontal systems. However, synoptic conditions favoring BB rain exhibited notable distinctions from those characterizing NBB periods. This included key differences in the position of the composite 300-mb jet stream and underlying cold front with respect to CZD, as well as notable differences in the intensity of the 500-mb shortwave trough offshore of CZD. The suite of BB composites exhibited dynamically consistent synoptic-scale characteristics that yielded stronger and deeper ascent over CZD than for the typically shallower NBB rain, consistent with the GOES satellite composites that showed 20-K warmer (2.3-km shallower) cloud tops for NBB rain. Composite soundings for both rain types possessed low-level potential instability, but the NBB sounding was warmer and moister with stronger low-level upslope flow, thus implying that orographically forced rainfall is enhanced during NBB conditions.
Forecast Model Bias Correction in Ocean Data Assimilation
Authors: Chepurin Gennady A. , Carton James A. , Dee Dick
Published: June 2005

Numerical models of ocean circulation are subject to systematic errors resulting from errors in model physics, numerics, inaccurately specified initial conditions, and errors in surface forcing. In addition to a time-mean component, the systematic errors include components that are time varying, which could result, for example, from inaccuracies in the time-varying forcing. Despite their importance, most assimilation algorithms incorrectly assume that the forecast model is unbiased. In this paper the authors characterize the bias for a current assimilation scheme in the tropical Pacific. The characterization is used to show how relatively simple empirical bias forecast models may be used in a two-stage bias correction procedure to improve the quality of the analysis.
Contraction Rate and Its Relationship to Frontogenesis, the Lyapunov Exponent, Fluid Trapping, and Airstream Boundaries
Authors: Cohen Robert A. , Schultz David M.
Published: June 2005

Although a kinematic framework for diagnosing frontogenesis exists in the form of the Petterssen frontogenesis function and its vector generalization, a similar framework for diagnosing airstream boundaries (e.g., drylines, lee troughs) has not been constructed. This paper presents such a framework, beginning with a kinematic expression for the rate of change of the separation vector between two adjacent air parcels. The maximum growth rate of the separation vector is called the instantaneous dilatation rate and its orientation is called the axis of dilatation. Similarly, a maximum decay rate is called the instantaneous contraction rate and its orientation is called the axis of contraction. These expressions are related to the vector frontogenesis function, in that the growth rate of the separation vector corresponds with the scalar frontogenesis function, and the rotation rate of the separation vector corresponds with the rotational component of the vector frontogenesis function.

Because vorticity can rotate air-parcel pairs out of regions of deformation, the instantaneous dilatation and contraction rates and axes may not be appropriate diagnostics of airstream boundaries for fluid flows in general. Rather, the growth rate and orientation of an airstream boundary may correspond better to the so-called asymptotic contraction rate and the asymptotic axis of dilatation, respectively. Expressions for the asymptotic dilatation and contraction rates, as well as their orientations, the asymptotic dilatation and contraction axes, are derived. The asymptotic dilatation rate is related to the Lyapunov exponent for the flow. In addition, a fluid-trapping diagnostic is derived to distinguish among adjacent parcels being pulled apart, being pushed together, or trapped in an eddy. Finally, these diagnostics are applied to simple, idealized, steady-state flows and a nonsteady idealized vortex in nondivergent, diffluent flow to show their utility for determining the character of air-parcel trajectories and airstream boundaries.
Applying Horizontal Diffusion on Pressure Surface to Mesoscale Models on Terrain-Following Coordinates
Authors: Juang Hann-Ming Henry , Lee Ching-Teng , Zhang Yongxin , Song Yucheng , Wu Ming-Chin , Chen Yi-Leng , Kodama Kevin , Chen Shyh-Chin
Published: June 2005

The National Centers for Environmental Prediction regional spectral model and mesoscale spectral model (NCEP RSM/MSM) use a spectral computation on perturbation. The perturbation is defined as a deviation between RSM/MSM forecast value and their outer model or analysis value on model sigma-coordinate surfaces. The horizontal diffusion used in the models applies perturbation diffusion in spectral space on model sigma-coordinate surfaces. However, because of the large difference between RSM/MSM and their outer model or analysis terrains, the perturbation on sigma surfaces could be large over steep mountain areas as horizontal resolution increases. This large perturbation could introduce systematical error due to artificial vertical mixing from horizontal diffusion on sigma surface for variables with strong vertical stratification, such as temperature and humidity. This nonnegligible error would eventually ruin the forecast and simulation results over mountain areas in high-resolution modeling.

To avoid the erroneous vertical mixing on the systematic perturbation, a coordinate transformation is applied in deriving a horizontal diffusion on pressure surface from the variables provided on terrain-following sigma coordinates. Three cases are selected to illustrate the impact of the horizontal diffusion on pressure surfaces, which reduces or eliminates numerical errors of mesoscale modeling over mountain areas. These cases address concerns from all aspects, including unstable and stable synoptic conditions, moist and dry atmospheric settings, weather and climate integrations, hydrostatic and nonhydrostatic modeling, and island and continental orography.

After implementing the horizontal diffusion on pressure surfaces for temperature and humidity, the results show better rainfall and flow pattern simulations when compared to observations. Horizontal diffusion corrects the warming, moistening, excessive rainfall, and convergent flow patterns around high mountains under unstable and moist synoptic conditions and corrects the cooling, drying, and divergent flow patterns under stable and dry synoptic settings.
Diagnostic Comparison of Meteorological Analyses during the 2002 Antarctic Winter
Authors: Manney Gloria L. , Allen Douglas R. , Krüger Kirstin , Naujokat Barbara , Santee Michelle L. , Sabutis Joseph L. , Pawson Steven , Swinbank Richard , Randall Cora E. , Simmons Adrian J. , Long Craig
Published: June 2005

Several meteorological datasets, including U.K. Met Office (MetO), European Centre for Medium-Range Weather Forecasts (ECMWF), National Centers for Environmental Prediction (NCEP), and NASA’s Goddard Earth Observation System (GEOS-4) analyses, are being used in studies of the 2002 Southern Hemisphere (SH) stratospheric winter and Antarctic major warming. Diagnostics are compared to assess how these studies may be affected by the meteorological data used. While the overall structure and evolution of temperatures, winds, and wave diagnostics in the different analyses provide a consistent picture of the large-scale dynamics of the SH 2002 winter, several significant differences may affect detailed studies. The NCEP–NCAR reanalysis (REAN) and NCEP–Department of Energy (DOE) reanalysis-2 (REAN-2) datasets are not recommended for detailed studies, especially those related to polar processing, because of lower-stratospheric temperature biases that result in underestimates of polar processing potential, and because their winds and wave diagnostics show increasing differences from other analyses between ∼30 and 10 hPa (their top level). Southern Hemisphere polar stratospheric temperatures in the ECMWF 40-Yr Re-analysis (ERA-40) show unrealistic vertical structure, so this long-term reanalysis is also unsuited for quantitative studies. The NCEP/Climate Prediction Center (CPC) objective analyses give an inferior representation of the upper-stratospheric vortex. Polar vortex transport barriers are similar in all analyses, but there is large variation in the amount, patterns, and timing of mixing, even among the operational assimilated datasets (ECMWF, MetO, and GEOS-4). The higher-resolution GEOS-4 and ECMWF assimilations provide significantly better representation of filamentation and small-scale structure than the other analyses, even when fields gridded at reduced resolution are studied. The choice of which analysis to use is most critical for detailed transport studies (including polar process modeling) and studies involving synoptic evolution in the upper stratosphere. The operational assimilated datasets are better suited for most applications than the NCEP/CPC objective analyses and the reanalysis datasets (REAN/REAN-2 and ERA-40).
A Wavelet Approach to Representing Background Error Covariances in a Limited-Area Model
Authors: Deckmyn Alex , Berre Loïk
Published: June 2005

The use of orthogonal wavelets for the representation of background error covariances over a limited area is studied. Each wavelet function contains both information on position and information on scale: using a diagonal correlation matrix in wavelet space thus gives the possibility of representing the local variations of correlation scale. To this end, a generalized family of orthogonal Meyer wavelets that are not restricted to dyadic domains (i.e., powers of 2) is introduced. A three-bases approach is used, which allows one to take advantage of the respective properties of the spectral, wavelet, and gridpoint spaces. While the implied local anisotropies are relatively small, the local changes in the two-dimensional length scale are rather well represented.