bluuebunny/crossref_metadata_2025_split huggingface dataset:
Zonal Momentum Balance, Potential Vorticity Dynamics, and Mass Fluxes on Near-Surface Isentropes
Authors: Schneider Tapio 
Published: June 2005

AbstractWhile it has been recognized for some time that isentropic coordinates provide a convenient framework for theories of the global circulation of the atmosphere, the role of boundary effects in the zonal momentum balance and in potential vorticity dynamics on isentropes that intersect the surface has remained unclear. Here, a balance equation is derived that describes the temporal and zonal mean balance of zonal momentum and of potential vorticity on isentropes, including the near-surface isentropes that sometimes intersect the surface. Integrated vertically, the mean zonal momentum or potential vorticity balance leads to a balance condition that relates the mean meridional mass flux along isentropes to eddy fluxes of potential vorticity and surface potential temperature. The isentropic-coordinate balance condition formally resembles balance conditions well known in quasigeostrophic theory, but on near-surface isentropes it generally differs from the quasigeostrophic balance conditions. Not taking the intersection of isentropes with the surface into account, quasigeostrophic theory does not adequately represent the potential vorticity dynamics and mass fluxes on near-surface isentropes—a shortcoming that calls into question the relevance of quasigeostrophic theories for the macroturbulence and global circulation of the atmosphere.
Linear and Nonlinear Signatures in the Planetary Wave Dynamics of an AGCM: Phase Space Tendencies
Authors: Branstator Grant , Berner Judith 
Published: June 2005

Abstract
To identify and quantify indications of linear and nonlinear planetary wave behavior, characteristics of a very long integration of an atmospheric general circulation model in a four-dimensional phase space are examined. The phase space is defined by the leading four empirical orthogonal functions of 500-hPa geopotential heights, and the primary investigated characteristic is the state dependence of mean phase space tendencies. Defining the linear component of planetary wave tendencies as that part which can be captured by a least squares fit linear operator driven by additive Gaussian white noise, the study finds that there are distinct linear and nonlinear signatures. These signatures are especially easy to see in plots of mean tendencies projected onto phase space planes. For some planes the mean tendencies are highly linear, while for others there are strong departures from linearity.
The results of the analysis are found to depend strongly on the lag time used to estimate tendencies with the linear component monotonically increasing with lag time. This is shown to result from the ergodicity of the system. Using the theory of Markov models it is possible to remove the lag-dependent component of the tendencies from the results. When this is done the projected mean dynamics in some planes is found to be almost exclusively nonlinear, while in others it is nearly linear.
In the four-dimensional space the linear component of the dynamics is largely a reflection of a westward propagating Northern Hemisphere pattern concentrated over the Pacific and North America. The nonlinear signature can be approximated by two linear functions, each operating in a different region of phase space. One region is centered around a Pacific blocking pattern while the other is centered on a state with enhanced zonal symmetry. It is concluded that reduced models of the planetary waves should strive to include these state-dependent dynamics.
Droplets to Drops by Turbulent Coagulation
Authors: Riemer N. , Wexler A. S. 
Published: June 2005

Abstract
This study addresses two central problems in cloud microphysics. The first is the source of large droplets, which initiates the rapid production of warm rain. The second is the broadening of the cloud droplet spectrum at both tails of the spectrum. The study explores how in-cloud turbulence can help to close the gaps in our understanding. With box model simulations, the development of cloud droplet spectra is calculated using a coagulation kernel that recently has been derived from direct numerical simulations. This kernel includes both the effect of turbulence on the relative velocities of the droplets and on the local increases in droplet concentration, the so-called accumulation effect. Under the assumption that this kernel can be extrapolated to atmospheric Reynolds numbers, the results show that for typical atmospheric conditions, the turbulent coagulation kernel is several orders of magnitude larger than the sedimentation kernel for droplets smaller then 100 μm. While for calm air after 30-min simulation time, only 7% of the total mass is found in droplets with sizes over 100 μm, this increases to 79% for a dissipation rate of 100 cm2 s−3 and 96% for 300 cm2 s−3 if a combined sedimentation and turbulent kernel is employed that assumes that the sedimentation and turbulent kernel can be added. Hence, moderate turbulence can enhance significantly the formation of large droplets. Furthermore, a time-scale analysis shows that broadening at the upper end of the spectrum is caused by turbulent coagulation whereas thermodynamic effects are responsible for broadening at the lower end.
Possible Effects of Collisional Breakup on Mixed-Phase Deep Convection Simulated by a Spectral (Bin) Cloud Model
Authors: Seifert Axel , Khain Alexander , Blahak Ulrich , Beheng Klaus D. 
Published: June 2005

Abstract
The effects of the collisional breakup of raindrops are investigated using the Hebrew University Cloud Model (HUCM). The parameterizations, which are combined in the new breakup scheme, are those of Low and List, Beard and Ochs, as well as Brown. A sensitivity study reveals strong effects of collisional breakup on the precipitation formation in mixed-phase deep convective clouds for strong as well as for weak precipitation events. Collisional breakup reduces the number of large raindrops, increases the number of small raindrops, and, as a consequence, decreases surface rain rates and considerably reduces the speed of rain formation. In addition, it was found that including breakup can lead to a more intense triggering of secondary convective cells. But a statistical comparison with observed raindrop size distributions shows that the parameterizations might systematically overestimate collisional breakup.
Numerical Simulation of Boundary Layer Structure and Cross-Equatorial Flow in the Eastern Pacific*
Authors: Small R. Justin , Xie Shang-Ping , Wang Yuqing , Esbensen Steven K. , Vickers Dean 
Published: June 2005

Abstract
Recent observations from spaceborne microwave sensors have revealed detailed structure of the surface flow over the equatorial eastern Pacific in the boreal fall season. A marked acceleration of surface wind across the northern sea surface temperature (SST) front of the cold tongue is a prominent feature of the regional climate. Previous studies have attributed the acceleration to the effect of enhanced momentum mixing over the warmer waters. A high-resolution numerical model is used to examine the cross-frontal flow adjustment. In a comprehensive comparison, the model agrees well with many observed features of cross-equatorial flow and boundary layer structure from satellite, Tropical Atmosphere Ocean (TAO) moorings, and the recent Eastern Pacific Investigation of Climate Processes (EPIC) campaign. In particular, the model simulates the acceleration across the SST front, and the change from a stable to unstable boundary layer. Analysis of the model momentum budget indicates that the hydrostatic pressure gradient, set up in response to the SST gradient, drives the surface northward acceleration. Because of thermal advection by the mean southerly flow, the pressure gradient is located downstream of the SST gradient and consequently, divergence occurs over the SST front, as observed by satellite. Pressure gradients also act to change the vertical shear of the wind as the front is crossed. However, the model underpredicts the changes in vertical wind shear across the front, relative to the EPIC observations. It is suggested that the vertical transfer of momentum by mixing, a mechanism described by Wallace et al. may also act to enhance the change in shear in the observations, but the model does not simulate this effect. Reasons for this are discussed.
The Role of Relative Humidity in Radiative–Convective Equilibrium
Authors: Sugiyama Masahiro , Stone Peter H. , Emanuel Kerry A. 
Published: June 2005

AbstractThe following conditions are derived for the existence of a radiation limit of tropospheric origin in a nongray atmosphere, extending the work on a gray atmosphere by Nakajima et al.: 1) the atmosphere must become sufficiently optically thick, and 2) the temperature must become only a function of optical depth at each frequency, independent of surface temperature. The first condition is satisfied at high temperatures even in a window region as long as there is weak but nonzero absorption, because the optical depth of the entire atmosphere roughly scales as saturation vapor pressure. At high temperatures, the pseudoadiabatic temperature structure asymptotes to the saturation vapor pressure curve, satisfying the second condition at each frequency. A rapidly decreasing vertical gradient of water vapor mixing ratio allows temperature to asymptote faster in optical depth coordinates than in pressure coordinates.Analyses using a radiative–convective model show that interactive relative humidity can give rise to a different kind of runaway greenhouse effect and multiple equilibria, if the strength of relative humidity feedback exceeds a critical value. The results suggest that this mechanism may be able to explain the runaway greenhouse effect found by Rennó et al. and radiative–convective multiple equilibria by Rennó. The framework employed in this study will serve as a diagnostic tool for further research on the runaway greenhouse effect and radiative–convective multiple equilibria.
Lagrangian Particle Dispersion Modeling of the Fumigation Process Using Large-Eddy Simulation
Authors: Kim Si-Wan , Moeng Chin-Hoh , Weil Jeffrey C. , Barth Mary C. 
Published: June 2005

Abstract
A Lagrangian particle dispersion model (LPDM) is used to study fumigation of pollutants in and above the entrainment zone into a growing convective boundary layer. Probability density functions of particle location with height and time are calculated from particle trajectories driven by the sum of the resolved-scale velocity from a large-eddy simulation (LES) model and the stochastic subgrid-scale (SGS) velocity. The crosswind-integrated concentration (CWIC) fields show good agreement with water tank experimental data. A comparison of the LPDM output with an Eulerian diffusion model output based on the same LES flow shows qualitative agreement with each other except that a greater overshoot maximum of the ground-level concentration occurs in the Eulerian model.
The dimensionless CWICs near the surface for sources located above the entrainment zone collapse to a nearly universal curve provided that the profiles are time shifted, where the shift depends on the source heights. The dimensionless CWICs for sources located within the entrainment zone show a different behavior. Thus, fumigation from sources above the entrainment zone and within the entrainment zone should be treated separately. An examination of the application of Taylor’s translation hypothesis to the fumigation process showed the importance of using the mean boundary layer wind speed as a function of time rather than the initial mean boundary layer wind speed, because the mean boundary layer wind speed decreases as the simulation proceeds.
The LPDM using LES is capable of accurately simulating fumigation of particles into the convective boundary layer. This technique provides more computationally efficient simulations than Eulerian models.
Sea Spray Impacts on Intensifying Midlatitude Cyclones
Authors: Perrie Will , Zhang Weiqing , L Andreas Edgar , Li Weibiao , Gyakum John , McTaggart-Cowan Ron 
Published: June 2005

Abstract
Air–sea transfer processes over the ocean strongly affect how hurricanes develop. High winds generate large amounts of sea spray, which can modify the transfer of momentum, heat, and moisture across the air–sea interface. However, the extent to which sea spray can modify extratropical or midlatitude hurricanes and intense cyclones has not been resolved. This paper reports simulations of extratropical Hurricanes Earl (1998) and Danielle (1998) and an intense winter cyclone from January 2000 using a mesoscale atmospheric model and a recent sea spray parameterization. These simulations show that sea spray can increase the sea surface heat flux, especially the latent heat flux, in a midlatitude cyclone and that sea spray’s impact on cyclone intensity depends on the storm structure and development and is strongest for cyclones with high winds.
A Simple Parameterization Coupling the Convective Daytime Boundary Layer and Fair-Weather Cumuli
Authors: Berg Larry K. , Stull Roland B. 
Published: June 2005

Abstract
A new parameterization for boundary layer cumulus clouds, called the cumulus potential (CuP) scheme, is introduced. This scheme uses joint probability density functions (JPDFs) of virtual potential temperature (θυ) and water-vapor mixing ratio (r), as well as the mean vertical profiles of θυ, to predict the amount and size distribution of boundary layer cloud cover. This model considers the diversity of air parcels over a heterogeneous surface, and recognizes that some parcels rise above their lifting condensation level to become cumulus, while other parcels might rise as noncloud updrafts. This model has several unique features: 1) cloud cover is determined from the boundary layer JPDF of θυ versus r, 2) clear and cloudy thermals are allowed to coexist at the same altitude, and 3) a range of cloud-base heights, cloud-top heights, and cloud thicknesses are predicted within any one cloud field, as observed.
Using data from Boundary Layer Experiment 1996 and a model intercomparsion study using large eddy simulation (LES) based on Barbados Oceanographic and Meteorological Experiment (BOMEX), it is shown that the CuP model does a good job predicting cloud-base height and cloud-top height. The model also shows promise in predicting cloud cover, and is found to give better cloud-cover estimates than three other cumulus parameterizations: one based on relative humidity, a statistical scheme based on the saturation deficit, and a slab model.
Low-Order Stochastic Mode Reduction for a Realistic Barotropic Model Climate
Authors: Franzke Christian , Majda Andrew J. , Vanden-Eijnden Eric 
Published: June 2005

Abstract
This study applies a systematic strategy for stochastic modeling of atmospheric low-frequency variability to a realistic barotropic model climate. This barotropic model climate has reasonable approximations of the Arctic Oscillation (AO) and Pacific/North America (PNA) teleconnections as its two leading principal patterns of low-frequency variability. The systematic strategy consists first of the identification of slowly evolving climate modes and faster evolving nonclimate modes by use of an empirical orthogonal function (EOF) decomposition. The low-order stochastic climate model predicts the evolution of these climate modes a priori without any regression fitting of the resolved modes. The systematic stochastic mode reduction strategy determines all correction terms and noises with minimal regression fitting of the variances and correlation times of the unresolved modes. These correction terms and noises account for the neglected interactions between the resolved climate modes and the unresolved nonclimate modes. Low-order stochastic models with only four resolved modes capture the statistics of the original barotropic model modes quite well. A budget analysis establishes that the low-order stochastic models are dominated by linear dynamics and additive noise. The linear correction terms and the additive noise stem from the linear coupling between resolved and unresolved modes, and not from nonlinear interactions between resolved and unresolved modes as assumed in previous studies.
Control Parameters for the Influence of a Mesoscale Mountain Range on Cyclone Track Continuity and Deflection
Authors: Lin Yuh-Lang , Chen Shu-Yun , Hill Christopher M. , Huang Ching-Yuang 
Published: June 2005

Abstract
In this study prospective control parameters are identified for diagnosing the continuity and deflection of cyclone tracks across a mesoscale mountain range. Based on idealized simulations of a westward-moving cyclone, it was found that the cyclone track becomes a discontinuous (continuous) track and the cyclone experiences more (less) deflection with a combination of small (large) values of Vmax/Nh, U/Nh, R/Ly, U/fLx, and Vmax/fR, and large (small) value of h/Lx. The symbols are defined as follows: Vmax the maximum tangential wind, N the Brunt–Väisälä frequency, h the mountain height, U the basic wind speed, R the radius of Vmax, f the Coriolis parameter, and Lx and Ly the horizontal scales of the mountain in x and y directions, respectively.
A conceptual model is proposed to explain track deflection and continuity for a westward-moving cyclone encountering idealized topography representative of the Central Mountain Range of Taiwan. With weak orographic blocking, a cyclone crosses over the mountain range with some northward deflection. With moderate orographic blocking, northward deflection of a cyclone is greater upstream of the mountain range and a secondary, leeside vortex forms to the southwest of the mountain range, indicative of discontinuity in the cyclone track. With strong orographic blocking, a westward-moving cyclone is deflected southward and a secondary cyclone forms to the northwest of the mountain range. The northward or southward deflection of a cyclone track is explained by the orographic blocking on the outer circulation of the cyclone.
The Planetary- and Synoptic-Scale Interactions in a Southeast Pacific Blocking Episode Using PV Diagnostics
Authors: Burkhardt John P. , Lupo Anthony R. 
Published: June 2005

Abstract
The synoptic- and planetary-scale forcing in two blocking anticyclones occurring over the southeast Pacific Ocean was examined using potential vorticity diagnostics. While many studies have examined the dynamic and thermodynamic forcing associated with blocking events in the Northern Hemisphere (NH), very few studies have examined blocking in the Southern Hemisphere (SH). Climatological analysis suggests SH blocking events in the Pacific region have similar characteristics to their NH counterparts. However, the occurrence of blocking is rare elsewhere in the SH, and these events are relatively short-lived. Some studies of NH blocking dynamics have also shown that the extent to which the planetary- and synoptic-scale and planetary–synoptic-scale interaction forcing that contribute to the genesis and maintenance of Pacific and Atlantic region events can be different. Thus, a study of the relevant atmospheric dynamics associated with blocking events in the SH was carried out in order to determine whether or not these events are associated with similar dynamic mechanisms to those in the NH. Using the National Center for Atmospheric Research and National Centers for Environmental Prediction (NCAR–NCEP) reanalyses dataset and applying a low-pass filter to the relevant variables, the authors examined the scale interactions associated with two blocking events that occurred during July and August 1986 and applied potential vorticity diagnostics. Results demonstrate that blocking in the southeast Pacific was associated with similar synoptic features, and the forcing mechanisms on the planetary, synoptic scales, and interactions were more similar to North Pacific blocking events rather than those occurring over the NH Atlantic region. However, these results also demonstrated that blocking events in the NH were associated with synergistically interacting synoptic- and planetary-scale waves, while in the SH, blocking events resulted from the superposition of synoptic and planetary waves. This result may explain the paucity of blocking occurrences and their tendency to be weaker and less persistent over much of the SH.
Impact of Cloud Cover on Solar Radiative Biases in Deep Convective Regimes
Authors: Di Giuseppe F. , Tompkins A. M. 
Published: June 2005

Abstract
Conflicting claims have been made concerning the magnitude of the bias in solar radiative transfer calculations when horizontal photon transport is neglected for deep convective scenarios. The difficulty of obtaining a realistic set of cloud scenes for situations of complex cloud geometry, while certain characteristics such as total cloud cover are systematically controlled, has hindered the attempt to reach a consensus. Here, a simple alternative approach is adopted. An ensemble of cloud scenes generated by a cloud resolving model are modified by an idealized function that progressively alters the cirrus anvil coverage without affecting the realism of the scene produced. Comparing three-dimensional radiative calculations with the independent column approximation for all cloud scenes, it is found that the bias in scene albedo can reach as much as 22% when the sun is overhead and 46% at low sun angles. The bias is an asymmetrical function of cloud cover with a maximum attained at cirrus anvil cloud cover of approximately 30%–40%. With a cloud cover of 15%, the bias is half its maximum value, while it is limited for coverage exceeding 80%. The position of the peak occurs at the cloud cover coinciding with the maximum number of independent clouds present in the scene. Increasing the cloud cover past this point produces a decrease in the number of isolated clouds because of cloud merging, with a consequential bias reduction.
With this systematic documentation of the biases as a function of total cloud cover, it is possible to identify two contributions to the total error: the geometrical consequences of the effective cloud cover increase at low sun angles and the true 3D scattering effect of photons deviating from the original path direction. An attempt to account for the former geometrical contribution to the 1D bias is made by performing a simple correction technique, whereby the field is sheared by the tangent of the solar zenith angle. It is found that this greatly reduces the 1D biases at low sun angles. Because of the small aspect ratio of the cirrus cloud deck, the remaining bias contribution is small in magnitude and almost independent of solar zenith angle.