bluuebunny/crossref_metadata_2025_split huggingface dataset:
Zonal Momentum Budget of the Madden–Julian Oscillation: The Source and Strength of Equivalent Linear Damping
Authors: Lin Jia-Lin , Zhang Minghua , Mapes Brian 
Published: August 2005

Abstract
Linear, dissipative models with resting base states are sometimes used in theoretical studies of the Madden–Julian oscillation (MJO). Linear mechanical damping in such models ranges from nonexistent to strong, since an observational basis for its source and strength has been lacking. This study examines the zonal momentum budget of a composite MJO over the equatorial western Pacific region, constructed using filtering and regression techniques from 15 yr (1979–93) of daily global reanalysis data. Two different reanalyses (NCEP–NCAR and ERA-15) give qualitatively similar results for all major terms, including the budget residual, whose structure is consistent with its interpretation as eddy momentum flux convergence (EMFC) in convection.
The results show that the MJO is a highly viscous oscillation, with a 3–5-day equivalent linear damping time scale, in the upper as well as lower troposphere. Upper-level damping is mainly in the form of large-scale advection terms, which are linear in MJO amplitude but involve horizontal and vertical background flow. Specifically, the leading terms are the advection of time-mean zonal shear by MJO vertical motion anomalies and advection of MJO wind anomalies by time-mean ascent. This upper-level damping in the western Pacific is mostly confined between 10°N and 10°S. In contrast, zonal wind damping in the lower troposphere involves EMFC (budget residual) and zonal mean linear meridional advection.
Stated another way, the strong upper-level damping necessitates upper-level geopotential height gradients to maintain the observed zonal wind anomalies over the time scales implied by the MJO’s low frequency. The existence of the background flow thus tends to shift MJO temperature perturbations westward so that the warm anomaly ahead (east) of the convective center is shifted back into the convection. This shifting effect is fully realized only for anomalies with a period much longer than the 3–5-day damping time.
Coupling Diffuse Sky Radiation and Surface Albedo
Authors: Pinty Bernard , Lattanzio Alessio , Martonchik John V. , Verstraete Michel M. , Gobron Nadine , Taberner Malcolm , Widlowski Jean-Luc , Dickinson Robert E. , Govaerts Yves 
Published: August 2005

Abstract
New satellite instruments have been delivering a wealth of information regarding land surface albedo. This basic quantity describes what fraction of solar radiation is reflected from the earth’s surface. However, its concept and measurements have some ambiguity resulting from its dependence on the incidence angles of both the direct and diffuse solar radiation. At any time of day, a surface receives direct radiation in the direction of the sun, and diffuse radiation from the various other directions in which it may have been scattered by air molecules, aerosols, and cloud droplets. This contribution proposes a complete description of the distribution of incident radiation with angles, and the implications in terms of surface albedo are given in a mathematical form, which is suitable for climate models that require evaluating surface albedo many times. The different definitions of observed albedos are explained in terms of the coupling between surface and atmospheric scattering properties. The analytical development in this paper relates the various quantities that are retrieved from orbiting platforms to what is needed by an atmospheric model. It provides a physically simple and practical approach to evaluation of land surface albedo values at any condition of sun illumination irrespective of the current range of surface anisotropic conditions and atmospheric aerosol load. The numerical differences between the various definitions of albedo for a set of typical atmospheric and surface scattering conditions are illustrated through numerical computation.
Baroclinic Multiple Zonal Jets on the Sphere
Authors: Lee Sukyoung 
Published: August 2005

Abstract
Multiple zonal jets are investigated with a two-level primitive equation model on the sphere in which both baroclinicity and planetary radius are varied. As in the case for a two-layer quasigeostrophic model on a β-plane channel, it is found both that the Rhines scale successfully predicts the meridional scale of the multiple zonal jets, and that these jets are maintained in part by an eddy momentum flux divergence associated with slow baroclinic waves at the interjet minimum.
A scaling analysis suggests that njets∝ (a/θm)1/2, with the constraints ζe ≡ 8 sin2f (θm/▵θ ) &gt; 1 and njets ≥ 1, where njets is the number of the jets, a the planetary radius, θm one-half of the pole-to-equator potential temperature difference, ξe the supercriticality of the two-layer Phillips model, Δθ the potential temperature difference between the two levels, and ϕ the latitude. The number of jets simulated by the model agrees with this scaling, provided that Ljet ≤ a, where Ljet is the jet scale.
In model runs with a large planet where multiple zonal jets exist, the time–mean eddy heat flux is found to be consistent with the diffusive picture of Held and Larichev. In contrast, for the model runs with the planetary size equal to that of Earth, baroclinic adjustment is found to be more relevant. These results are consistent with the finding that in the large-planet (Earth-like) model runs, the jet/eddy scale is smaller than (comparable to) the corresponding planetary radius.
The Effects of Stratification on Flow Separation
Authors: Ambaum Maarten H. P. , Marshall David P. 
Published: August 2005

Abstract
Separation of stratified flow over a two-dimensional hill is inhibited or facilitated by acceleration or deceleration of the flow just outside the attached boundary layer. In this note, an expression is derived for this acceleration or deceleration in terms of streamline curvature and stratification. The expression is valid for linear as well as nonlinear deformation of the flow. For hills of vanishing aspect ratio a linear theory can be derived and a full regime diagram for separation can be constructed. For hills of finite aspect ratio scaling relationships can be derived that indicate the presence of a critical aspect ratio, proportional to the stratification, above which separation will occur as well as a second critical aspect ratio above which separation will always occur irrespective of stratification.
Tropical Zonal Momentum Balance in the NCEP Reanalyses
Authors: Dima Ioana M. , Wallace John M. , Kraucunas Ian 
Published: August 2005

Abstract
The seasonal cycle of the zonal-mean zonal momentum balance in the Tropics is investigated using NCEP reanalysis data. It is found that the climatological stationary waves in the tropical upper troposphere, which are dominated by the equatorial Rossby wave response to tropical heating, produce an equatorward eddy flux of westerly momentum in the equatorial belt. The resulting westerly acceleration in the tropical upper troposphere is balanced by the advection of easterly momentum associated with the cross-equatorial mean meridional circulation. The eddy momentum fluxes and the cross-equatorial flow both tend to be strongest during the monsoon seasons, when the maximum diabatic heating is off the equator, and weakest during April–May, the season of strongest equatorial symmetry of the heating. The upper-level Rossby wave pattern exhibits a surprising degree of equatorial symmetry and follows a similar seasonal progression. Solutions of the nonlinear shallow water wave equation also show a predominantly equatorially symmetric response to a heat source centered off the equator.
Small-Scale Drop Size Variability: Impact on Estimation of Cloud Optical Properties
Authors: Knyazikhin Y. , Myneni R. B. , Marshak A. , Wiscombe W. J. , Larsen M. L. , Martonchik J. V. 
Published: August 2005

Abstract
Most cloud radiation models and conventional data processing techniques assume that the mean number of drops of a given radius is proportional to volume. The analysis of microphysical data on liquid water drop sizes shows that, for sufficiently small volumes, this proportionality breaks down; the number of cloud drops of a given radius is instead proportional to the volume raised to a drop size–dependent nonunit power. The coefficient of proportionality, a generalized drop concentration, is a function of the drop size. For abundant small drops the power is unity as assumed in the conventional approach. However, for rarer large drops, it falls increasingly below unity. This empirical fact leads to drop clustering, with the larger drops exhibiting a greater degree of clustering. The generalized drop concentration shows the mean number of drops per cluster, while the power characterizes the occurrence frequency of clusters. With a fixed total number of drops in a cloud, a decrease in frequency of clusters is accompanied by a corresponding increase in the generalized concentration. This initiates a competing process missed in the conventional models: an increase in the number of drops per cluster enhances the impact of rarer large drops on cloud radiation while a decrease in the frequency suppresses it. Because of the nonlinear relationship between the number of clustered drops and the volume, these two opposite tendencies do not necessarily compensate each other. The data analysis suggests that clustered drops likely have a stronger radiative impact compared to their unclustered counterpart; ignoring it results in underestimation of the contribution from large drops to cloud horizontal optical path.
Bimodality of the Planetary-Scale Atmospheric Wave Amplitude Index
Authors: Christiansen Bo 
Published: August 2005

Abstract
The evidence for multiple flow regimes in the planetary-scale atmospheric wave amplitude index (WAI) is studied using the 56 winters from the NCEP reanalysis data. The regimes are identified by bimodality in the probability density estimates. Both the probability density of the WAI alone and the probability density in the two-dimensional space spanned by the WAI and its temporal rate of change are examined. The latter procedure allows us to exploit the quasi stationarity of the regimes and increase the statistical significance. The statistical significance of bimodality in the probability densities is tested by a Monte Carlo approach using surrogate time series that preserve the full autocorrelation spectrum of the original WAI. By using a longer dataset and including the rate of change, some of the questions raised in previous studies about the robustness and statistical significance of the bimodality of the WAI are resolved.
Statistically significant bimodality is found in the WAI based on the 500-hPa height. The probability density of the WAI shows considerable low-frequency variability on decadal scales. However, the bimodality is reproduced in all decadal subperiods although without statistical significance. The last decade has been dominated by a strong (disturbed) regime while a weak (zonal) regime dominates the previous decades. This recent change toward the disturbed regime is statistically significant. Imprints of the regimes are found at other tropospheric levels including the sea level. In particular, the regimes are found with statistical significance in the WAI based on the sea level pressure for the subperiod 1979–2003. Systematically varying the upper and lower boundaries of the latitudinal interval over which the geopotential height is averaged shows that the bimodality of the WAI is rather sensitive to these parameters, but also that statistically significant bimodality is found for a range of intervals with the lower boundary at 45°–50°N.
Theoretical Formulation of Collision Rate and Collision Efficiency of Hydrodynamically Interacting Cloud Droplets in Turbulent Atmosphere
Authors: Wang Lian-Ping , Ayala Orlando , Kasprzak Scott E. , Grabowski Wojciech W. 
Published: August 2005

Abstract
A methodology for conducting direct numerical simulations (DNSs) of hydrodynamically interacting droplets in the context of cloud microphysics has been developed and used to validate a new kinematic formulation capable of describing the collision rate and collision efficiency of cloud droplets in turbulent air. The theoretical formulation is formally the same as the formulation recently developed for geometrical collision rate of finite-inertia, nonsettling particles. It is shown that its application to hydrodynamically interacting droplets requires corrections because of a nonoverlap requirement. An approximate method for correcting the kinematic properties has been developed and validated against DNS data. The formulation presented here is more general and accurate than previously published formulations that, in most cases, are some extension to the description of hydrodynamic–gravitational collision. General dynamic and kinematic representations of the properly defined collision efficiency in a turbulent flow have been discussed. In addition to augmenting the geometric collision rate, air turbulence has been found to enhance the collision efficiency because, in a turbulent flow, hydrodynamic interactions become less effective in reducing the average relative radial velocity. The level of increase in the collision efficiency depends on the flow dissipation rate. For example, the collision efficiency between droplets of 20 and 25 μm in radii is increased by 59% and 10% by air turbulence at dissipation rates of 400 and 100 cm2 s−3, respectively. It is also shown that hydrodynamic interactions lead to higher droplet concentration fluctuations. The formulation presented here separates the effect of turbulence on collision efficiency from the previously observed effect of turbulence on the geometric collision rate.
Evolution of a Florida Cirrus Anvil
Authors: Garrett T. J. , Navarro B. C. , Twohy C. H. , Jensen E. J. , Baumgardner D. G. , Bui P. T. , Gerber H. , Herman R. L. , Heymsfield A. J. , Lawson P. , Minnis P. , Nguyen L. , Poellot M. , Pope S. K. , Valero F. P. J. , Weinstock E. M. 
Published: August 2005

Abstract
This paper presents a detailed study of a single thunderstorm anvil cirrus cloud measured on 21 July 2002 near southern Florida during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL-FACE). NASA WB-57F and University of North Dakota Citation aircraft tracked the microphysical and radiative development of the anvil for 3 h. Measurements showed that the cloud mass that was advected downwind from the thunderstorm was separated vertically into two layers: a cirrus anvil with cloud-top temperatures of −45°C lay below a second, thin tropopause cirrus (TTC) layer with the same horizontal dimensions as the anvil and temperatures near −70°C. In both cloud layers, ice crystals smaller than 50 μm across dominated the size distributions and cloud radiative properties. In the anvil, ice crystals larger than 50 μm aggregated and precipitated while small ice crystals increasingly dominated the size distributions; as a consequence, measured ice water contents and ice crystal effective radii decreased with time. Meanwhile, the anvil thinned vertically and maintained a stratification similar to its environment. Because effective radii were small, radiative heating and cooling were concentrated in layers approximately 100 m thick at the anvil top and base. A simple analysis suggests that the anvil cirrus spread laterally because mixing in these radiatively driven layers created horizontal pressure gradients between the cloud and its stratified environment. The TTC layer also spread but, unlike the anvil, did not dissipate—perhaps because the anvil shielded the TTC from terrestrial infrared heating. Calculations of top-of-troposphere radiative forcing above the anvil and TTC showed strong cooling that tapered as the anvil evolved.
A Topographic Drag Closure Built on an Analytical Base Flux
Authors: Garner Stephen T. 
Published: August 2005

Abstract
Topographic drag schemes depend on grid-scale representations of the average height, width, and orientation of the subgrid topography. Until now, these representations have been based on a combination of statistics and dimensional analysis. However, under certain physical assumptions, linear analysis provides the exact amplitude and orientation of the drag for arbitrary topography. The author proposes a computationally practical closure based on this analysis.
Also proposed is a nonlinear correction for nonpropagating base flux. This is patterned after existing schemes but is better constrained to match the linear solution because it assumes a correlation between mountain height and width. When the correction is interpreted as a formula for the transition to saturation in the wave train, it also provides a way of estimating the vertical distribution of the momentum forcing. The explicit subgrid height distribution causes a natural broadening of the layers experiencing the forcing. Linear drag due to simple oscillating flow over topography, which is relevant to ocean tides, has almost the same form as for the stationary atmospheric problem. However, dimensional analysis suggests that the nonpropagating drag in this situation is mostly due to topographic length scales that are small enough to keep the steady-state assumption satisfied.
A Simulation of a Supercell Thunderstorm with Emulated Radiative Cooling beneath the Anvil
Authors: Markowski Paul M. , Harrington Jerry Y. 
Published: August 2005

Abstract
This note reports the preliminary results of an ongoing numerical study designed to investigate what effects, if any, radiative transfer processes can have on the evolution of convective storms. A pair of idealized three-dimensional simulations are conducted to demonstrate the potential dynamical importance of shortwave radiation reductions within the large shadows cast by storms. One of the simulations (the control) is run without surface physics and radiation. In the other simulation, radiative cooling due to cloud shading is emulated by prescribing a cooling rate to the skin temperature at any grid point at which cloud water was present overhead. The imposed skin cooling rate is consistent with past observations. Low-level air temperatures are coupled to the skin cooling in this second simulation by the inclusion of surface sensible heat fluxes using simple bulk aerodynamic drag laws (latent and soil heat fluxes are not included). Significant differences are observed between the two simulated storms, particularly in the evolution of the vertical vorticity field and gust fronts. The storm simulated with emulated cloud shading develops substantially weaker low-level rotation than the storm in the control simulation.
Sensitivity of the Simulated Monsoons of 1987 and 1988 to Convective Parameterization Schemes in MM5
Authors: Ratnam J. Venkata , Kumar K. Krishna 
Published: August 2005

AbstractIn this study the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) version 3.5.2 was used to simulate the Indian summer monsoon during the two contrasting years of 1987 and 1988, a dry year and a wet year, respectively. Three different convection parameterization schemes of Betts–Miller–Janjic, Kain–Fritsch, and Grell were used to study the sensitivity of monsoon to cumulus effects. The model was integrated for a period of 6 months, starting from three different initial conditions of 0000 UTC on 1, 2, and 3 May of each year using the NCEP–NCAR reanalysis data as input. The 6-hourly reanalysis data were used to provide the lateral boundary conditions, and the observed weekly Reynolds sea surface temperature, linearly interpolated to 6 h, was used as the lower boundary forcing. The results show that all three cumulus schemes were able to simulate the interannual and intraseasonal variabilities in the monsoon with reasonable accuracy. However, the spatial distribution of the rainfall and its quantity were different in all the schemes. The Grell scheme underestimated the rainfall in both the years. The Kain–Fritsch scheme simulated the observed rainfall well during July and August, the peak monsoon months, of the year 1988 but overestimated the rainfall in June and September of 1988 and throughout the monsoon season of 1987. The Betts–Miller–Janjic scheme simulated less rainfall in the drought year of 1987 and overestimated the rainfall in June and July of 1988. The circulation patterns simulated by the Betts–Miller–Janjic and Kain–Fritsch schemes are comparable to the observed patterns.
High Clouds over Oceans in the ECMWF 15- and 45-Yr Reanalyses
Authors: Chevallier Frédéric , Kelly Graeme , Simmons Adrian J. , Uppala Sakari , Hernandez Angeles 
Published: August 2005

Abstract
The reanalysis programs of numerical weather prediction (NWP) centers provide global, comprehensive descriptions of the atmosphere and of the earth’s surface over long periods of time. The high realism of their representation of key NWP parameters, like temperature and winds, implies some realism for less emblematic parameters, such as cloud cover, but the degree of this realism needs to be documented.
This study aims to evaluate the high clouds over open oceans in the ECMWF 15- and 45-yr reanalyses. The assessment is based on a new 23-yr climatology of monthly frequencies of high-cloud occurrence retrieved from the infrared radiances measured by operational polar satellites. It is complemented by data from the International Satellite Cloud Climatology Project.
It is shown that the 45-yr ECMWF reanalysis dramatically improves on the previous 15-yr reanalysis for the realism of seasonal and interannual variations in high clouds, despite remaining systematic errors. More than 60% of the observed anomalies during the January 1979–February 2002 period over large oceanic basins are captured by the latest reanalysis. However the realism of the analyses in the areas and in the years with sparse observations appears to be poor. Consequently, the interannual variations may not be reliable before January 1979 in most parts of the world. Possible improvements of the handling of assimilated satellite observations before and after this date are suggested.
Impact of the Diurnal Cycle of Solar Radiation on Intraseasonal SST Variability in the Western Equatorial Pacific
Authors: Shinoda Toshiaki 
Published: August 2005

Abstract
The mechanism by which the diurnal cycle of solar radiation modulates intraseasonal SST variability in the western Pacific warm pool is investigated using a one-dimensional mixed layer model. SSTs in the model experiments forced with hourly surface fluxes during the calm–sunny phase of intraseasonal oscillation are significantly warmer than those with daily mean surface fluxes. The difference in two experiments is explained by upper-ocean mixing processes during nighttime. Surface warming during daytime creates a shallow diurnal warm layer near the surface (0–3 m), which can be easily eroded by surface cooling during nighttime. Further cooling, however, requires a substantial amount of energy because deeper waters need to be entrained into the mixed layer. Since the shallow diurnal layer is not formed in the experiment with daily mean surface fluxes, the SST for the hourly forcing case is warmer most of the time due to the diurnally varying solar radiation.
Sensitivity of the intraseasonal SST variation to the penetrative component of solar radiation is examined, showing that the diurnal cycle plays an important role in the sensitivity. Solar radiation absorbed in the upper few meters significantly influences intraseasonal SST variations through changes in amplitude of diurnal SST variation.