bluuebunny/crossref_metadata_2025_split huggingface dataset:
Entrainment in Cumulus Clouds: What Resolution is Cloud-Resolving?
Authors: Craig George C. , Dörnbrack Andreas 
Published: June 2008

AbstractSystematic numerical experiments were conducted to determine the spatial resolution required to resolve a moist thermal show convergence at a scale proportional to the smaller of the initial thermal diameter D0 and a buoyancy length scale Lbuoy. The buoyancy length scale Lbuoy = ΔT0/ΔΓ (ΔT0 is the initial buoyancy excess of the thermal and ΔΓ is the ambient stratification) describes the maximum vertical displacement that can be induced against the stratification in the environment by buoyancy-driven pressure perturbations in the cloud and, thus, the maximum scale of eddies that cross the cloud boundary. For typical atmospheric conditions in which the cloud size D0 is larger than Lbuoy, numerical simulations of the mixing processes in cumulus clouds must resolve Lbuoy.
A Method to Find Perturbations That Trigger Blocking Onset: Conditional Nonlinear Optimal Perturbations
Authors: Mu Mu , Jiang Zhina 
Published: June 2008

Abstract
In this paper the following question is addressed: assuming that some information is available about initial perturbations (e.g., that they belong to an ensemble, which consists of the perturbations whose magnitudes are less than a given value), how can one determine which perturbations belong to this ensemble and trigger the blocking onset? The applicability of linear singular vectors (LSVs) and conditional nonlinear optimal perturbations (CNOPs) is investigated by a T21L3 quasigeostrophic (QG) model and its tangent linear and adjoint versions. Particular attention is focused on the roles of nonlinear processes and the importance of choosing a proper objective function.
LSVs are the fastest-growing perturbations when the evolutions of the initial perturbations are well described by the tangent linear version of the nonlinear model. CNOPs are a natural generalization of LSVs into the nonlinear category, that is, the initial perturbation whose nonlinear evolution attains the maximum of the objective function at a prescribed forecast time under some initial constraint conditions. The results of this research show that in some cases for the given initial ensemble perturbations CNOPs trigger a transition to a blocking regime (whereas LSVs may not generate such a transition), which shows that nonlinear advection processes are fundamental for studying the weather regime transitions from zonal flow to blocking in the medium range.
By choosing two objective functions and investigating the resulting CNOPs, it is found that CNOPs obtained from the objective function of the blocking-index form (type-1 CNOPs) may trigger a transition to a blocking regime under some circumstances, whereas CNOPs related to the streamfunction squared norm (type-2 CNOPs) fail to yield such a transition. This demonstrates the importance of selecting a proper objective function when aiming at finding the perturbations yielding such a transition.
The mechanism of blocking onset triggered by perturbations is also explored. It is shown that the approach of type-1 CNOP remains a viable tool to capture the spatial structure of initial perturbations that trigger a blocking onset. The planetary-scale projection of the nonlinear interaction of such initial perturbations contributes to the amplification of the blocking downstream and then triggers a blocking onset.
Sensitivity of a Cloud-Resolving Model to Bulk and Explicit Bin Microphysical Schemes. Part I: Comparisons
Authors: Li Xiaowen , Tao Wei-Kuo , Khain Alexander P. , Simpson Joanne , Johnson Daniel E. 
Published: June 2008

Abstract
A two-dimensional cloud-resolving model is used to study the sensitivities of two microphysical schemes, a bulk scheme and an explicit spectral bin scheme, in simulating a midlatitude summertime squall line [Preliminary Regional Experiment for Storm-Scale Operational and Research Meteorology (PRE-STORM), 10–11 June 1985]. In this first part of a two-part paper, the developing and mature stages of simulated storms are compared in detail. Some variables observed during the field campaign are also presented for validation. It is found that both schemes agree well with each other, and also with published observations and retrievals, in terms of storm structures and evolution, average storm flow patterns, pressure and temperature perturbations, and total heating profiles. The bin scheme is able to produce a much more extensive and homogeneous stratiform region, which compares better with observations.
However, instantaneous fields and high temporal resolution analyses show distinct characteristics in the two simulations. During the mature stage, the bulk simulation produces a multicell storm with convective cells embedded in its stratiform region. Its leading convection also shows a distinct life cycle (strong evolution). In contrast, the bin simulation produces a unicell storm with little temporal variation in its leading cell regeneration (weak evolution). More detailed, high-resolution observations are needed to validate and, perhaps, generalize these model results. Interactions between the cloud microphysics and storm dynamics that produce the sensitivities described here are discussed in detail in Part II of this paper.
Sensitivity of a Cloud-Resolving Model to Bulk and Explicit Bin Microphysical Schemes. Part II: Cloud Microphysics and Storm Dynamics Interactions
Authors: Li Xiaowen , Tao Wei-Kuo , Khain Alexander P. , Simpson Joanne , Johnson Daniel E. 
Published: June 2008

Abstract
Part I of this paper compares two simulations, one using a bulk and the other a detailed bin microphysical scheme, of a long-lasting, continental mesoscale convective system with leading convection and trailing stratiform region. Diagnostic studies and sensitivity tests are carried out in Part II to explain the simulated contrasts in the spatial and temporal variations by the two microphysical schemes and to understand the interactions between cloud microphysics and storm dynamics. It is found that the fixed raindrop size distribution in the bulk scheme artificially enhances rain evaporation rate and produces a stronger near-surface cool pool compared with the bin simulation. In the bulk simulation, cool pool circulation dominates the near-surface environmental wind shear in contrast to the near-balance between cool pool and wind shear in the bin simulation. This is the main reason for the contrasting quasi-steady states simulated in Part I. Sensitivity tests also show that large amounts of fast-falling hail produced in the original bulk scheme not only result in a narrow trailing stratiform region but also act to further exacerbate the strong cool pool simulated in the bulk parameterization.
An empirical formula for a correction factor, r(qr) = 0.11qr−1.27 + 0.98, is developed to correct the overestimation of rain evaporation in the bulk model, where r is the ratio of the rain evaporation rate between the bulk and bin simulations and qr(g kg−1) is the rain mixing ratio. This formula offers a practical fix for the simple bulk scheme in rain evaporation parameterization.
A Simple Stochastic Model for Generating Broken Cloud Optical Depth and Cloud-Top Height Fields
Authors: Prigarin Sergei M. , Marshak Alexander 
Published: June 2008

Abstract
A simple and fast algorithm for generating two correlated stochastic two-dimensional (2D) cloud fields is described. The algorithm is illustrated with two broken cumulus cloud fields: cloud optical depth and cloud-top height retrieved from the Moderate Resolution Imaging Spectroradiometer (MODIS). Only two 2D fields are required as an input. The algorithm output is statistical realizations of these two fields with approximately the same correlation and joint distribution functions as the original ones. The major assumption of the algorithm is statistical isotropy of the fields. In contrast to fractals and the Fourier filtering methods frequently used for stochastic cloud modeling, the proposed method is based on spectral models of homogeneous random fields. To retain the same probability density function as the (first) original field, the method of inverse distribution function is used. When the spatial distribution of the first field has been generated, a realization of the correlated second field is simulated using a conditional distribution matrix. This paper serves as a theoretical justification of the publicly available software “Simulation of a two-component cloud field,” which has been recently released. Although 2D rather than full 3D, the stochastic realizations of two correlated cloud fields that mimic statistics of given fields have proven to be very useful to study 3D radiative transfer features of broken cumulus clouds for a better understanding of shortwave radiation and the interpretation of remote sensing retrievals.
Diminutive Frontal Waves—A Link between Fronts and Cyclones
Authors: Hewson Tim D. 
Published: June 2008

Abstract
A number of recent publications have dealt with cyclone identification and tracking. Following on, this paper extends the typical cyclone life cycle back in time to embrace a new feature called a “diminutive frontal wave.” One aim is to improve predictability by extending tracks. This is particularly important for small, cyclonic windstorms, which can often be missed in postprocessed output from operational, ensemble, and climate runs.
The recognition of diminutive waves requires a new, front-relative, low-level vorticity partition. The parts are labeled “frontal vorticity” and “disturbance vorticity” and are computed, respectively, from front-parallel and cross-front low-level wind components. A diminutive frontal wave then lies wherever there is a local, along-front maximum in the disturbance vorticity. Computations require local coordinates; these are conveniently provided, at all grid points, by objective front diagnostics.
Analysis of cyclone-type transitions over the North Atlantic in operational numerical model data confirms the validity of adding the diminutive wave stage to the revised cyclone life cycle. Examples then suggest that nonmodal growth of diminutive waves can occur, albeit with a sometimes complex interplay between separate cyclonic features. In all cases, model resolution is necessarily higher than the 100–500 km typically used in previous work.
Convectively Coupled Kelvin Waves over Tropical Africa during the Boreal Summer: Structure and Variability
Authors: Mekonnen Ademe , Thorncroft Chris D. , Aiyyer Anantha R. , Kiladis George N. 
Published: June 2008

Abstract
The structure and variability of convectively coupled Kelvin waves during the boreal summer are explored using satellite-observed brightness temperature data and ECMWF reanalyses. Kelvin wave activity is most prominent between the central and eastern Pacific, across Africa, and the Indian Ocean. Composite analysis shows that over sub-Saharan Africa Kelvin wave convection is peaked north of the equator, while the dynamical fields tend to be symmetric with respect to the equator. Convectively coupled Kelvin waves propagate faster over the Pacific and western Atlantic (∼24 m s−1), and slow down over tropical Africa (∼14 m s−1), consistent with stronger coupling between the dynamics and convection over tropical Africa. The Kelvin waves observed over Africa generally propagate into the region from anywhere between the eastern Pacific and the Atlantic, and decay over the eastern Indian Ocean basin.
Results show marked interannual variability of Kelvin wave activity over Africa. Anomalously high Kelvin wave variance tends to occur during dry years, while low variance occurs during wet years. African Kelvin wave activity is positively correlated with SST anomalies in the equatorial east Pacific. The same warm SST anomalies that are favorable for enhanced Kelvin wave activity suppress the mean rainfall over tropical Africa via a more slowly varying teleconnection and associated subsidence.
A brief analysis of an intense Kelvin wave in August 1987 (a dry year) shows a clear impact of the wave on convective development and daily rainfall over tropical Africa. This Kelvin wave was also associated with a series of easterly wave initiations over tropical Africa.
Anatomizing the Ocean’s Role in ENSO Changes under Global Warming*
Authors: Yang Haijun , Zhang Qiong 
Published: June 2008

Abstract
A revisit on observations shows that the tropical El Niño–Southern Oscillation (ENSO) variability, after removing both the long-term trend and decadal variation of the background climate, has been enhanced by as much as 50% during the past 50 yr. This is inconsistent with the changes in the equatorial atmosphere, which shows a slowdown of the zonal Walker circulation and tends to stabilize the tropical coupling system. The ocean role is highlighted in this paper. The enhanced ENSO variability is attributed to the strengthened equatorial thermocline that acts as a destabilizing factor of the tropical coupling system. To quantify the dynamic effect of the ocean on the ENSO variability under the global warming, ensemble experiments are performed using a coupled climate model [Fast Ocean Atmosphere Model (FOAM)], following the “1pctto2x” scenario defined in the Intergovernmental Panel on Climate Change (IPCC) reports. Term balance analyses on the temperature variability equation show that the anomalous upwelling of the mean vertical temperature gradient (referred as the “local term”) in the eastern equatorial Pacific is the most important destabilizing factor to the temperature variabilities. The magnitude of local term and its change are controlled by its two components: the mean vertical temperature gradient Tz and the “virtual vertical heat flux” −w′T ′. The former can be viewed as the background of the latter and these two components are positively correlated. A stronger Tz is usually associated with a bigger upward heat flux −w′T ′, which implies a bigger impact of thermocline depth variations on SST. The Tz is first enhanced during the transient stage of the global warming with a 1% yr−1 increase of CO2, and then reduced during the equilibrium stage with a fixed doubled CO2. This turnaround in Tz determines the turnaround of ENSO variability in the entire global warming period.
Arctic Total Water Vapor: Comparison of Regional Climate Simulations with Observations, and Simulated Decadal Trends
Authors: Rinke A. , Melsheimer C. , Dethloff K. , Heygster G. 
Published: June 2008

Abstract
Satellite-retrieved data of total water vapor (TWV) over the Arctic are patchy, with large areas of missing data because of various limitations of the retrieval algorithms. To overcome these observational difficulties, a new retrieval algorithm has been developed that allows for monitoring the TWV over the Arctic during most of the year. This method retrieves TWV from satellite microwave radiometer data [the Advanced Microwave Sounding Unit B (AMSU-B)]. These new data have been made available for 4 yr (2000–03) and have been used to evaluate high-resolution simulations with the Arctic regional atmospheric climate model HIRHAM at daily, monthly, and seasonal time scales. The strong dynamic TWV variability on the daily time scale, linked with moisture transport by weather systems, is discussed for selected case studies. Both the simulated climatological seasonal mean patterns and the variability on interannual and decadal time scales are in agreement with those of the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data. Trends in Arctic TWV for 1958–2001, broken down by season, are presented. Although an increase in the TWV is obvious in all seasons, there are also regions where a decreasing trend appears. Significant maximum positive trends are calculated over the western Arctic in summer (up to 0.06 kg m−2 yr−1), and a significant small negative trend is calculated over the East Siberian Sea in winter.
Passage of a Barotropic Vortex through a Gap
Authors: Duran-Matute M. , Fuentes O. U. Velasco 
Published: June 2008

Abstract
The flow of a vortex advected by a uniform current toward a gap in a straight barrier is studied. This is an idealization of flows observed in the world’s oceans, for example, in the Yucatan Channel and in the various passages of the Lesser Antilles. The vortex evolution and the transport properties are studied as a function of three nondimensional parameters related to the vortex intensity, the vortex initial position, and the gap’s span. The flow evolution is computed numerically with a two-dimensional inviscid model. The vortex is observed to behave in one of the following ways: it passes completely through the gap, it splits and only a fraction passes, or it stays entirely in the upstream side of the wall. In each of these regimes, transport and mixing are analyzed using the Lagrangian flow geometry, finite-size Lyapunov exponents, and residence times of fluid particles. Laboratory experiments are performed in a homogeneous, rotating fluid. In the region of parameter space that leads to total passage of the vortex, there is good agreement between the evolution observed in the laboratory and that obtained numerically. In the regions of parameter space, which lead to partial passage and full blockage, the agreement is good only until the generation of vorticity on the walls becomes important.
Quality Assessment of Weather Radar Wind Profiles during Bird Migration
Authors: Holleman Iwan , van Gasteren Hans , Bouten Willem 
Published: June 2008

Abstract
Wind profiles from an operational C-band Doppler radar have been combined with data from a bird tracking radar to assess the wind profile quality during bird migration. The weather radar wind profiles (WRWPs) are retrieved using the well-known volume velocity processing (VVP) technique. The X-band bird radar performed range–height scans perpendicular to the main migration direction and bird densities were deduced by counting and normalizing the observed echoes. It is found that the radial velocity standard deviation (σr) obtained from the VVP retrieval is a skillful indicator of bird migration. Using a threshold of 2 m s−1 on σr, more than 93% of the bird-contaminated wind vectors are rejected while over 70% of the true wind vectors are accepted correctly. For high bird migration densities the raw weather radar wind vectors have a positive speed bias of 8.6 ± 3.8 m s−1, while the quality-controlled wind vectors have a negligible speed bias. From the performance statistics against a limited area numerical weather prediction model, it is concluded that all (significant) bird contamination is removed and that high-quality weather radar wind profiles can be obtained, even during the bird migration season.
The Spatial Distribution and Evolution Characteristics of North Atlantic Cyclones
Authors: Dacre H. F. , Gray S. L. 
Published: June 2008

Abstract
A climatology of extratropical cyclones is produced using an objective method of identifying cyclones based on gradients of 1-km height wet-bulb potential temperature. Cyclone track and genesis density statistics are analyzed and this method is found to compare well with other cyclone identification methods. The North Atlantic storm track is reproduced along with the major regions of genesis. Cyclones are grouped according to their genesis location and the corresponding lysis regions are identified. Most of the cyclones that cross western Europe originate in the east Atlantic where the baroclinicity and the sea surface temperature gradients are weak compared to the west Atlantic. East Atlantic cyclones also have higher 1-km height relative vorticity and lower mean sea level pressure at their genesis point than west Atlantic cyclones. This is consistent with the hypothesis that they are secondary cyclones developing on the trailing fronts of preexisting “parent” cyclones. The evolution characteristics of composite west and east Atlantic cyclones have been compared. The ratio of their upper- to lower-level forcing indicates that type B cyclones are predominant in both the west and east Atlantic, with strong upper- and lower-level features. Among the remaining cyclones, there is a higher proportion of type C cyclones in the east Atlantic, whereas types A and C are equally frequent in the west Atlantic.
Surface Characteristics of Observed Cold Pools
Authors: Engerer Nicholas A. , Stensrud David J. , Coniglio Michael C. 
Published: June 2008

Abstract
Cold pools are a key element in the organization of precipitating convective systems, yet knowledge of their typical surface characteristics is largely anecdotal. To help to alleviate this situation, cold pools from 39 mesoscale convective system (MCS) events are sampled using Oklahoma Mesonet surface observations. In total, 1389 time series of surface observations are used to determine typical rises in surface pressure and decreases in temperature, potential temperature, and equivalent potential temperature associated with the cold pool, and the maximum wind speeds in the cold pool. The data are separated into one of four convective system life cycle stages: first storms, MCS initiation, mature MCS, and MCS dissipation. Results indicate that the mean surface pressure rises associated with cold pools increase from 3.2 hPa for the first storms’ life cycle stage to 4.5 hPa for the mature MCS stage before dropping to 3.3 hPa for the dissipation stage. In contrast, the mean temperature (potential temperature) deficits associated with cold pools decrease from 9.5 (9.8) to 5.4 K (5.6 K) from the first storms to the dissipation stage, with a decrease of approximately 1 K associated with each advance in the life cycle stage. However, the daytime and early evening observations show mean temperature deficits over 11 K. A comparison of these observed cold pool characteristics with results from idealized numerical simulations of MCSs suggests that observed cold pools likely are stronger than those found in model simulations, particularly when ice processes are neglected in the microphysics parameterization. The mean deficits in equivalent potential temperature also decrease with the MCS life cycle stage, starting at 21.6 K for first storms and dropping to 13.9 K for dissipation. Mean wind gusts are above 15 m s−1 for all life cycle stages. These results should help numerical modelers to determine whether the cold pools in high-resolution models are in reasonable agreement with the observed characteristics found herein. Thunderstorm simulations and forecasts with thin model layers near the surface are also needed to obtain better representations of cold pool surface characteristics that can be compared with observations.