bluuebunny/crossref_metadata_2025_split huggingface dataset:
A Study of Two-Dimensional Dry Convective Plume Modes with Variable Critical Level Height
Authors: Kiefer Michael T. , Lin Yuh-Lang , Charney Joseph J. 
Published: February 2008

AbstractThis study investigates the impact of wind speed and critical level height on dry convection above a prescribed heat source. This is done using the Advanced Regional Prediction System (ARPS) model in its two-dimensional form with an imposed 400-K soil potential temperature perturbation. The result of these experiments is the identification of three modes of convective plumes. The first, termed multicell convective plumes, is analogous to multicell convection generated from squall-line cold pools in the moist atmosphere. The second mode, a deep wave mode, consists of disturbances with wavelengths of 7–10 km and results from the multicell plumes perturbing the dynamically unstable shear flow centered at the critical level. The third mode, termed the intense fire plume, has stronger updrafts than the multicell mode and is marked by quasi-stationary movement and substantial low-level inflow and upper-level outflow. The presence of a critical level is shown to be crucial to the development of both the deep wave and intense plume modes. The intense fire plume mode is most consistent with the so-called fire storm, or conflagration phenomenon, in which strong updrafts and low-level indrafts can produce mesocyclones and tornadic fire whirls capable of significant damage. This study marks an important step in understanding the dynamics behind the fire storm phenomenon, as well as other types of convection (multicell and deep wave) that may be generated by a fire.
Two-Dimensional Idealized Simulations of the Impact of Multiple Windward Ridges on Orographic Precipitation
Authors: Colle Brian A. 
Published: February 2008

Abstract
This paper presents two-dimensional (2D) idealized simulations at 1-km grid spacing using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) in order to illustrate how a series of ridges along a broad windward slope can impact the precipitation distribution and simulated microphysics. The number of windward ridges for a 2000-m mountain of 50-km half-width is varied from 0 to 16 over a 150-km distance using different stratifications, freezing levels, uniform ambient flows, and ridge amplitudes.
A few (200–400 m) windward ridges can enhance the precipitation locally over each ridge crest by a factor of 2–3. Meanwhile, a series of 8–16 ridges that are 200–400 m in height can increase the net precipitation averaged over the windward slope by 10%–35%. This average precipitation enhancement is maximized when the ridge spacing is relatively small (&lt;20 km), since there is less time for subsidence drying within the valleys and the mountain waves become more evanescent, which favors a simple upward and downward motion couplet over each ridge. In addition, small ridge spacing is shown to have a synergistic effect on precipitation over the lower windward slope, in which an upstream ridge helps increase the precipitation over the adjacent downwind ridge. There is little net precipitation enhancement by the ridges for small moist Froude numbers (Fr &lt; 0.8), since flow blocking limits the flow up and over each ridge. For a series of narrow ridges (∼10 km wide), the largest precipitation enhancement for a 500-mb freezing level occurs over lower windward slope of the barrier through warm-rain processes. In contrast, a 1000-mb freezing level has the largest precipitation enhancement over the middle and upper portions of a barrier for a series of narrow (∼10 km wide) ridges given the horizontal advection of snow aloft.
Topographic Instability: Tests
Authors: Egger Joseph , Hoinka Klaus-Peter 
Published: February 2008

Abstract
Theories of topographic instability predict growth of perturbations of mean flow and wave modes due to their interaction with mountains under favorable conditions. Mountain torques form an important part of this interaction. It has been suggested that topographic instabilities contribute significantly to the subseasonal variability of the atmosphere but observational tests of topographic instability mechanisms have not yet been performed. Greenland is selected as a test bed because of its isolation, simple shape, and appropriate size. The observed flow development during mountain torque events is investigated in terms of a regression analysis. Changes of axial angular momentum and zonal mean wind with respect to the torques are monitored for domains covering Greenland since the acceleration (deceleration) of the regional zonal flow in response to a positive (negative) torque is a key feature of topographic instability. In particular, southern and northern analysis domains are considered separately in order to test “dipole” instability theories in addition to “monopole” situations where the meridional extent of the pressure perturbations is similar to that of Greenland. Moreover, zonal bands are used as analysis domains. It is found that the response of the zonal wind to the torques is quite small and not systematic. There is no evidence of monopole or dipole topographic instability. A less detailed analysis for the Tibetan Plateau leads to the same result. Reasons for these negative outcomes are discussed as are shortcomings of the tests.
A New Rossby Wave–Breaking Interpretation of the North Atlantic Oscillation
Authors: Woollings Tim , Hoskins Brian , Blackburn Mike , Berrisford Paul 
Published: February 2008

Abstract
This paper proposes the hypothesis that the low-frequency variability of the North Atlantic Oscillation (NAO) arises as a result of variations in the occurrence of upper-level Rossby wave–breaking events over the North Atlantic. These events lead to synoptic situations similar to midlatitude blocking that are referred to as high-latitude blocking episodes. A positive NAO is envisaged as being a description of periods in which these episodes are infrequent and can be considered as a basic, unblocked situation. A negative NAO is a description of periods in which episodes occur frequently. A similar, but weaker, relationship exists between wave breaking over the Pacific and the west Pacific pattern.
Evidence is given to support this hypothesis by using a two-dimensional potential-vorticity-based index to identify wave breaking at various latitudes. This is applied to Northern Hemisphere winter data from the 40-yr ECMWF Re-Analysis (ERA-40), and the events identified are then related to the NAO.
Certain dynamical precursors are identified that appear to increase the likelihood of wave breaking. These suggest mechanisms by which variability in the tropical Pacific, and in the stratosphere, could affect the NAO.
Uniform Asymptotics for the Linear Kelvin Wave in Spherical Geometry
Authors: Boyd John P. , Zhou Cheng 
Published: February 2008

Abstract
The Kelvin wave is the gravest eigenmode of Laplace’s tidal equation. It is widely observed in both the ocean and the atmosphere. In the absence of mean currents, the Kelvin wave depends on two parameters: the zonal wavenumber s (always an integer) and Lamb’s parameter ε. An asymptotic approximation valid in the limit s2 + ε ≫ 1 is derived that generalizes the usual “equatorial wave” limit that ε → ∞ for fixed s. Just as shown for Rossby waves two decades ago, the width of the Kelvin wave is (ε + s2)−1/4 rather than ε−1/4 as in the classical equatorial beta-plane approximation.
Collisions of Cloud Droplets in a Turbulent Flow. Part V: Application of Detailed Tables of Turbulent Collision Rate Enhancement to Simulation of Droplet Spectra Evolution
Authors: Pinsky M. , Khain A. , Krugliak H. 
Published: February 2008

Abstract
The present study is a continuation of the series of studies dedicated to the investigation of cloud droplet collisions in turbulent flow with characteristics that are typical of real clouds. Detailed tables of collision kernels and collision efficiencies calculated in the presence of hydrodynamic interaction of droplets are presented. These tables were calculated for a wide range of turbulent parameters. To illustrate the sensitivity of droplet size distribution (DSD) evolution to the turbulence-induced increase in the collision rate, simulations of DSD evolution are preformed by solving the stochastic kinetic equation for collisions. The results can be applied to cloud modeling. The tables of collision efficiencies and collision kernels are available upon request. Some unsolved problems related to collisions of droplets and ice hydrometeors in turbulent clouds are discussed in the conclusion.
Impact of the Atmospheric Boundary Layer on Mountain Waves
Authors: Jiang Qingfang , Smith Ronald B. , Doyle James D. 
Published: February 2008

Abstract
Interaction between mountain waves and the atmospheric boundary layer (BL) has been investigated using a mesoscale model with BL parameterizations and analytical BL models. The impact of the BL diagnosed from the mesoscale model simulations is in qualitative agreement with the analytical BL models. In general, for stratified flow over a mesoscale ridge, the influence of the BL tends to shift wave patterns upstream, decrease the wave drag by up to 60% and the momentum flux above the BL top by up to 80%, and significantly delay the onset of wave breaking.
It has been demonstrated that the BL effect is governed by four nondimensional parameters, namely, the nondimensional BL depth, BL shape factor, nonhydrostatic parameter, and frictional adjustment parameter. The BL effect is stronger over a rougher surface, which creates a deeper BL with a smaller shape factor (i.e., slower BL flow). The scale dependence of the BL effect is governed by the nonhydrostatic and frictional adjustment parameters. For narrow terrain, the BL reduces mountain drag through rapid inertial adjustment and behaves like an inviscid shear layer, and the momentum reduction across the BL is negligible. The attenuation of the momentum flux across the BL is more substantial over a wider ridge because of the increasing importance of frictional adjustment. The BL effect decreases with increasing ridge height and for 3D terrain.
Growth of Cloud Droplets by Turbulent Collision–Coalescence
Authors: Xue Yan , Wang Lian-Ping , Grabowski Wojciech W. 
Published: February 2008

Abstract
An open question in cloud physics is how rain forms in warm cumulus as rapidly as it is sometimes observed. In particular, the growth of cloud droplets across the size gap from 10 to 50 μm in radius has not been fully explained. In this paper, the authors investigate the growth of cloud droplets by collision–coalescence, taking into account both the gravitational mechanism and several enhancements of the collision–coalescence rate due to air turbulence. The kinetic collection equation (KCE) is solved with an accurate bin integral method and a newly developed parameterization of turbulent collection kernel derived from direct numerical simulation of droplet-laden turbulent flows. Three other formulations of the turbulent collection kernel are also considered so as to assess the dependence of the rain initiation time on the nature of the collection kernel. The results are compared to the base case using the Hall hydrodynamical–gravitational collection kernel. Under liquid water content and eddy dissipation rate values typical of small cumulus clouds, it is found that air turbulence has a significant impact on the collection kernel and thus on the time required to form drizzle drops. With the most realistic turbulent kernel, the air turbulence can shorten the time for the formation of drizzle drops by about 40% relative to the base case, applying measures based on either the radar reflectivity or the mass-weighted drop size. A methodology is also developed to unambiguously identify the three phases of droplet growth, namely, the autoconversion phase, the accretion phase, and the larger hydrometeor self-collection phase. The important observation is that even a moderate enhancement of collection kernel by turbulence can have a significant impact on the autoconversion phase of the growth.
Aerosol Effects on Clouds, Precipitation, and the Organization of Shallow Cumulus Convection
Authors: Xue Huiwen , Feingold Graham , Stevens Bjorn 
Published: February 2008

Abstract
This study investigates the effects of aerosol on clouds, precipitation, and the organization of trade wind cumuli using large eddy simulations (LES). Results show that for this shallow-cumulus-under-stratocumulus case, cloud fraction increases with increasing aerosol as the aerosol number mixing ratio increases from 25 (domain-averaged surface precipitation rate ∼0.65 mm day−1) to 100 mg−1 (negligible surface precipitation). Further increases in aerosol result in a reduction in cloud fraction. It is suggested that opposing influences of aerosol-induced suppression of precipitation and aerosol-induced enhancement of evaporation are responsible for this nonmonotonic behavior.
Under clean conditions (25 mg−1), drizzle is shown to initiate and maintain mesoscale organization of cumulus convection. Precipitation induces downdrafts and cold pool outflow as the cumulus cell develops. At the surface, the center of the cell is characterized by a divergence field, while the edges of the cell are zones of convergence. Convergence drives the formation and development of new cloud cells, leading to a mesoscale open cellular structure. These zones of new cloud formation generate new precipitation zones that continue to reinforce the cellular structure. For simulations with an aerosol concentration of 100 mg−1 the cloud fields do not show any cellular organization. On average, no evidence is found for aerosol effects on the lifetime of these clouds, suggesting that cloud fraction response to changes in aerosol is tied to the frequency of convection and/or cloud size.
Gravity Currents in a Deep Anelastic Atmosphere
Authors: Bryan George H. , Rotunno Richard 
Published: February 2008

Abstract
This study presents analytic results for steady gravity currents in a channel using the deep anelastic equations. Results are cast in terms of a nondimensional parameter H/H0 that relates the channel depth H to a scale depth H0 (the depth at which density goes to zero in an isentropic atmosphere). The classic results based on the incompressible equations correspond to H/H0 = 0. For cold gravity currents (at the bottom of a channel), assuming energy-conserving flow, the nondimensional current depth h/H is much smaller, and nondimensional propagation speed C/(gH)1/2 is slightly smaller as H/H0 increases. For flows with energy dissipation, C/(gH)1/2 decreases as H/H0 increases, even for fixed h/H. The authors conclude that as H/H0 increases the normalized hydrostatic pressure rise in the cold pool increases near the bottom of the channel, whereas drag decreases near the top of the channel; these changes require gravity currents to propagate slower for steady flow to be maintained. From these results, the authors find that steady cold pools have a likely maximum depth of 4 km in the atmosphere (in the absence of shear). For warm gravity currents (at the top of a channel), h/H is slightly larger and C/(gH)1/2 is much larger as H/H0 increases. The authors also conduct two-dimensional numerical simulations of “lock-exchange flow” to provide an independent evaluation of the analytic results. For cold gravity currents the simulations support the analytic results. However, for warm gravity currents the simulations show unsteady behavior that cannot be captured by the analytic theory and which appears to have no analog in incompressible flow.
Toward Elimination of the Warm Bias in Historic Radiosonde Temperature Records—Some New Results from a Comprehensive Intercomparison of Upper-Air Data
Authors: Haimberger Leopold , Tavolato Christina , Sperka Stefan 
Published: February 2008

Abstract
The apparent cooling trend in observed global mean temperature series from radiosonde records relative to Microwave Sounding Unit (MSU) radiances has been a long-standing problem in upper-air climatology. It is very likely caused by a warm bias of radiosonde temperatures in the 1980s, which has been reduced over time with better instrumentation and correction software. The warm bias in the MSU-equivalent lower stratospheric (LS) layer is estimated as 0.6 ± 0.3 K in the global mean and as 1.0 ± 0.3 K in the tropical (20°S–20°N) mean. These estimates are based on comparisons of unadjusted radiosonde data, not only with MSU data but also with background forecast (BG) temperature time series from the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) and with two new homogenized radiosonde datasets. One of the radiosonde datasets [Radiosonde Observation Correction using Reanalyses (RAOBCORE) version 1.4] employs the BG as reference for homogenization, which is not strictly independent of MSU data. The second radiosonde dataset uses the dates of the breakpoints detected by RAOBCORE as metadata for homogenization. However, it relies only on homogeneous segments of neighboring radiosonde data for break-size estimation. Therefore, adjustments are independent of satellite data.
Both of the new adjusted radiosonde time series are in better agreement with satellite data than comparable published radiosonde datasets, not only for zonal means but also at most single stations. A robust warming maximum of 0.2–0.3K (10 yr)−1 for the 1979–2006 period in the tropical upper troposphere could be found in both homogenized radiosonde datasets. The maximum is consistent with mean temperatures of a thick layer in the upper troposphere and upper stratosphere (TS), derived from M3U3 radiances. Inferred from these results is that it is possible to detect and remove most of the mean warm bias from the radiosonde records, and thus most of the trend discrepancy compared to MSU LS and TS temperature products.
The comprehensive intercomparison also suggests that the BG is temporally quite homogeneous after 1986. Only in the early 1980s could some inhomogeneities in the BG be detected and quantified.
Variability of the Meridional Overturning in the North Atlantic from the 50-Year GECCO State Estimation
Authors: Köhl Armin , Stammer Detlef 
Published: February 2008

Abstract
The German partner of the consortium for Estimating the Circulation and Climate of the Ocean (GECCO) provided a dynamically consistent estimate of the time-varying ocean circulation over the 50-yr period 1952–2001. The GECCO synthesis combines most of the data available during the entire estimation period with the ECCO–Massachusetts Institute of Technology (MIT) ocean circulation model using its adjoint. This GECCO estimate is analyzed here for the period 1962–2001 with respect to decadal and longer-term changes of the meridional overturning circulation (MOC) of the North Atlantic. A special focus is on the maximum MOC values at 25°N. Over this period, the dynamically self-consistent synthesis stays within the error bars of H. L. Bryden et al., but reveals a general increase of the MOC strength. The variability on decadal and longer time scales is decomposed into contributions from different processes. Changes in the model’s MOC strength are strongly influenced by the southward communication of density anomalies along the western boundary originating from the subpolar North Atlantic, which are related to changes in the Denmark Strait overflow but are only marginally influenced by water mass formation in the Labrador Sea. The influence of density anomalies propagating along the southern edge of the subtropical gyre associated with baroclinically unstable Rossby waves is found to be equally important. Wind-driven processes such as local Ekman transport explain a smaller fraction of the variability on those long time scales.
A Method for Processing Acoustic Doppler Current Profiler Velocity Data from Towed, Undulating Vehicles
Authors: Pan Jiayi , Jay David A. 
Published: February 2008

Abstract
The utility of the acoustic Doppler current profiler (ADCP) for sampling small time and space scales of coastal environments can be enhanced by mounting a high-frequency (1200 kHz) ADCP on an oscillating towed body. This approach requires both an external reference to convert the measured shears to velocities in the earth coordinates and a method to determine the towed body velocities. During the River Influence on the Shelf Ecosystems (RISE) project cruise, a high-frequency (1200 kHz) and narrowbeam ADCP with mode 12 sampling was mounted on a TRIAXUS oscillating towfish, which steers a 3D path behind the ship. This deployment approach extended the vertical range of the ADCP and allowed it to sample near-surface waters outside the ship’s wake. The measurements from a ship-mounted 1200-kHz narrowbeam ADCP are used as references for TRIAXUS ADCP data, and a method of overlapping bins is employed to recover the entire vertical range of the TRIAXUS ADCP. The TRIAXUS vehicle horizontal velocities are obtained by removing the derived ocean current velocity from the TRIAXUS ADCP measurements. The results show that the method is practical.
A Simulation Study Using a Local Ensemble Transform Kalman Filter for Data Assimilation in New York Harbor
Authors: Hoffman Ross N. , Ponte Rui M. , Kostelich Eric J. , Blumberg Alan , Szunyogh Istvan , Vinogradov Sergey V. , Henderson John M. 
Published: February 2008

AbstractData assimilation approaches that use ensembles to approximate a Kalman filter have many potential advantages for oceanographic applications. To explore the extent to which this holds, the Estuarine and Coastal Ocean Model (ECOM) is coupled with a modern data assimilation method based on the local ensemble transform Kalman filter (LETKF), and a series of simulation experiments is conducted. In these experiments, a long ECOM “nature” run is taken to be the “truth.” Observations are generated at analysis times by perturbing the nature run at randomly chosen model grid points with errors of known statistics. A diverse collection of model states is used for the initial ensemble. All experiments use the same lateral boundary conditions and external forcing fields as in the nature run. In the data assimilation, the analysis step combines the observations and the ECOM forecasts using the Kalman filter equations. As a control, a free-running forecast (FRF) is made from the initial ensemble mean to check the relative importance of external forcing versus data assimilation on the analysis skill. Results of the assimilation cycle and the FRF are compared to truth to quantify the skill of each.The LETKF performs well for the cases studied here. After just a few assimilation cycles, the analysis errors are smaller than the observation errors and are much smaller than the errors in the FRF. The assimilation quickly eliminates the domain-averaged bias of the initial ensemble. The filter accurately tracks the truth at all data densities examined, from observations at 50% of the model grid points down to 2% of the model grid points. As the data density increases, the ensemble spread, bias, and error standard deviation decrease. As the ensemble size increases, the ensemble spread increases and the error standard deviation decreases. Increases in the size of the observation error lead to a larger ensemble spread but have a small impact on the analysis accuracy.