bluuebunny/crossref_metadata_2025_split huggingface dataset:
Monte Carlo Simulation of the Formation of Snowflakes
Authors: Maruyama Ken-ichi , Fujiyoshi Yasushi 
Published: June 2005

Abstract
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

Abstract
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 &lt; 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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

AbstractEnsembles 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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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.