bluuebunny/crossref_metadata_2025_split huggingface dataset:
Turbulence effects on the collision kernel. I: Formation of velocity deviations of drops falling within a turbulent three‐dimensional flow
Authors: Pinsky M. B. , Khain A. P. 
Published: February 2007

AbstractThe fall of drops in three‐dimensional stationary homogeneous turbulent flow is investigated. Equations for drop motion in turbulent flow and for drop velocity relative to the air are deduced, in vector and tensor forms. Homogeneous and isotropic turbulence is described using Batchelor's approximation formula which is valid both for the inertial and viscous turbulence ranges. The mean‐square values of relative drop‐velocity are calculated as a function of the drop size and turbulence‐dissipation rate. It is shown that, for small droplets (with radit less than 50 μm) and a dissipation rate as low as 50 cm2s−3, turbulence‐induced relative velocity is of the same order as the still‐air terminal fall‐speed. For greater dissipation rates, turbulence‐induced drop‐velocity relative to the air can be several times greater than terminal velocity. The mechanisms of the formation of the relative velocity are analysed. The contribution of the inertial acceleration of the flow is found to be dominant for small drops.
Turbulence effects on the collision kernel. II: Increase of the swept volume of colliding drops
Authors: Khain A. P. , Pinsky M. B. 
Published: February 2007

AbstractAn equation is deduced which gives the relative velocity between drops falling in three‐dimensional turbulent flow. The turbulence is assumed to be homogeneous and isotropic. In describing the flow, both the inertial and viscous turbulent ranges are taken into account. It is demonstrated that the inertia of drops leads to the formation of a significant relative velocity between them, which for small droplets can be greater than the gravity‐driven velocity difference. For a turbulence dissipation rate of ϵ = 100 cm2s−3, typical of small early cumulus clouds, the turbulence‐induced relative velocity of drops with radii up to 30 μm is nearly twice the difference in their terminal velocities induced by gravity. At ϵ = 400 cm2s−3 turbulence driven relative drop velocity for drop radii up to 50 μm can be five times as great as their terminal‐velocity difference. Factors giving rise to the relative drop velocity are discussed. It is indicated that, for small drops, the inertial acceleration term dominates the terms for the vertical shear and the vertical shear of the inertial acceleration. Calculations of the evolution of the drop spectrum using the stochastic coalescence equation were carried out. In the expression widely used for collision kernels, the difference in terminal velocities was replaced by the root‐mean‐square difference in drop velocities. The results indicate that cloud turbulence influences the rate of generation of large drops significantly. For example, the formation of rain drops with radii of 100 μm starting from an initial narrow droplet‐spectrum centred at about 10 μm takes 12 minutes for ϵ = 200 cm2s−3 and about 24 minutes for ϵ = 50 cm2s−3. When, during this period, turbulence is put to zero, no rain drops form.
The transient emission of propagating gravity waves by a stably stratified shear layer
Authors: Lott François 
Published: February 2007

AbstractBy analysing the transient evolution of an initial perturbation, it is shown that a stably stratified shear layer emits gravity waves having well defined dynamical characteristics. For instance, the outgoing waves systematically have phase lines tilting against the shear, their vertical momentum flux has a sign opposite to that of the shear and their amplitude increases when the flow stability decreases. Those features are commonly observed in numerical simulations of gravity waves generated by convection. It is shown that they are related to the singular vectors of the system that have fast energy growth and fast energy decay within a finite time. In this work, the singular vectors are computed using a linear gravity‐wave model, its adjoint and an iterative Lanczos algorithm. For a given shear layer, characterized by a minimum Richardson number Ri and a depth d, these perturbations show that emission of gravity waves by a stratified shear layer essentially occurs for waves with horizontal wave number close to a critical value kc = √Ri/d. The importance of the singular vectors on the dynamics of more general initial conditions is also tested by making few ensembles of numerical simulations with stochastic initial conditions imposed inside the shear layer. The amplitude of the momentum fluxes of the outgoing waves as a function of Ri is also evaluated systematically. It gives a relationship between the efficiency of gravity‐wave emission and Ri that could be used in convective gravity‐wave parametrization schemes.
The impact of latent‐heat release on the hadley circulation
Authors: Dodd J. P. , James I. N. 
Published: February 2007

AbstractNon‐reversible latent‐heat release is introduced into a model of the zonally symmetric Hadley circulation. Including latent‐heat release does not increase the strength of the circulation when the heating is centred off the equator, in contrast with other studies. However, latent‐heat release does result in a reduction of the meridional extent of the ascent. The assumptions and simplifications made are discussed and compared with those in other simple models of the role of latent‐heat release in the low‐latitude axisymmetric circulation.
An analytical study of cumulus onset
Authors: Haiden Thomas 
Published: February 2007

AbstractA slab model of the well‐mixed daytime boundary layer is combined with a surface‐layer parametrization based on Monin‐Obukhov similarity to investigate analytically basic aspects of cumulus onset. Cumulus clouds are defined to occur when the equilibrium height of surface‐layer air equals or exceeds its lifting condensation level (LCL). For a linearized version of the model, a closed solution of onset time is derived as a function of atmospheric and surface parameters. It is found that the effect of the various parameters on the timing of cumulus onset can be encompassed by two non‐dimensional ‘cumulus’ numbers, which describe the relationship between differential variations of equilibrium height and LCL, one for the mixed layer and one for the surface layer. the most important parameter, appearing in both these numbers, is the ratio between the potential‐temperature gradient above the mixed layer and the dry‐adiabatic lapse rate of dew‐point depression (∽ 0.008 K m−1). Results from both the linearized and the nonlinear model indicate that cumulus onset is delayed by increasing the Bowen ratio in cases of moderate‐to‐high stability, whereas the opposite is found for less stable conditions. Qualitative aspects of the model results are discussed with reference to observed relations between cumulus formation and surface characteristics.
Adjoint data assimilation in coupled atmosphere‐ocean models: Determining model parameters in a simple equatorial model
Authors: Jingxilu  , Hsieh William W. 
Published: February 2007

AbstractSimple equatorial coupled atmosphere‐ocean models have been used to simulate the El Niño phenomenon. the possibility of determining the empirical parameters in these coupled models is explored by the adjoint data‐assimilation method. Identical twin experiments are conducted to retrieve six parameters (coupling and damping parameters) in a simple coupled model with the atmosphere and the ocean each represented by a single‐layer linear shallow‐water model with perfect initial conditions specified. Wind and sea level height (SLH) data, generated from a 40‐day unstable local‐growth simulation of a warm event, are assimilated to test the effects of data sparsity and noise on parameter retrieval.The temporal sparsity of both wind and SLH data is generally more detrimental for parameter estimation than spatial sparsity, and sparse wind data are more detrimental than sparse SLH data. Longer assimilation windows improve the parameter estimation, but the best window length for the wind data is not the best for the SLH data. A priori information for individual parameters as implemented in the cost function is useful in providing information for the size of the parameters and in enhancing the convexity of the cost function, but not as a substitute for inadequate data.
Extratropical forcing of tropical convection in a northern winter simulation with the UGAMP GCM
Authors: Slingo Julia M. 
Published: February 2007

AbstractThe role of the extratropics in determining the distribution of tropical convection and its inherent time‐scales has been investigated, using results from a perpetual January integration of the UK Universties Global Atmospheric Modelling Programme (UGAMP) General Circulation Model. This study has concentrated on the influence of upper tropospheric extratropical troughs in the Pacific waveguide, and of the east Asian cold surge events. The mechanisms through which the extratropics influence the model's tropical convection have been identified, and the validity of the model's behaviour has been assessed by comparing the results with previous observational studies.The model has considerable skill in representing the extratropical forcing of tropical convection. The results have shown that in the tropical east Pacific convection is initiated by the ascent and decreased static stability ahead of an upper tropospheric extratropical trough, which has penetrated the deep tropics in the region of upper‐level equatorial westerlies. Over the maritime continent, convection is enhanced by cold‐surge events, in which the increased near‐surface northerlies substantially enhance the air‐sea interaction over the South China Sea. These cold surges are triggered by the movement of the Siberian anticyclone in association with the passage of midlatitude weather systems.An important result has been the apparent coherent relationship between convective activity over different parts of the tropical Pacific Ocean, as a consequence of a well defined sequence of events involving tropical‐extratropical interctions. This emphasises that it would be seriously misleading to consider the tropics in isolation from the extratropics, and has highlighted the importance of the extratropics in determining the distribution and temporal scales of tropical convection during the northern winter. Two factors have been identified as important for a good simulation of extratropical forcing of tropical convection: firstly, the simulated mean climate should be sufficiently accurate to allow wave energy from the extratropics to influence the tropics; secondly, the convective parametrization should be formulated in such a way as to recognize and respond to extratropical phenomena.
Transport of Mt. Pinatubo aerosol by tropospheric synoptic‐scale and stratospheric planetary‐scale waves
Authors: Rogers H. L. , Norton W. A. , Lambert A. , Grainger R. G. 
Published: February 2007

AbstractThe isentropic transport of stratospheric air from the tropics into nothern hemisphere mid‐latitudes is examined using Mt. Pinatubo aerosol observed by the Improved Stratospheric and Mesospheric Sounder (ISAMS). The technique of contour advection is used to produce high‐resolution aerosol fields which are used to help diagnose transport processes. By sampling the high‐resolution contour‐advection fields at the satellite measurement points, we show that the small‐scale information present in the contour‐advection calculation is consistent with the ISAMS measurement. Result of this study indicate that transport from the tropics into mid‐latitudes is strongly dependent on altitude. In the lowest regions of the stratosphere, the upward influence of the subtropical jet and the synoptic‐scale waves that form on the storm tracks are important in transporting air from the tropics and in the distribution of aerosol in mid‐latitudes. Synodpitc‐scale spirals form in the aerosol field on the 500 K isentrope at the end of storm tracks associated with LC1 baroclinic life‐cycle behaviour. At altitudes near the 600 K isentrope, direct tropospheric effects diminish and intrusions of air from the tropics to mid‐latitudes weaken. At altitudes approaching the middle stratosphere, planetary‐wave activity becomes stronger and the associated displacements of the polar vortex to low latitudes produce trasport from the tropics, particularly during mid‐winter warming events.
The impact of diabatic initialization on stratospheric analyses, forecats, and transport experiments
Authors: Fox‐Rabinovitz Michael S. , Rood Richard B. , Lamich David J. , Govindaraju Ravi C. , Coy Lawrnce , Weaver Clark J. 
Published: February 2007

AbstractAn efficient diabatic initialization procedure has been applied to the entire troposphric and stratospheric domain within the Goddard Earth Observing System (GEOS) 4‐D data assimilation system. The initialization, or initial balancing and noise control techinique, is based on the appliction of the iterative Euler scheme as a highly selective, efficient filter. The GEOS 46‐layer General Circulation Model extending to 0.1 hPa was used to produce the first‐guess fields within the data assimilation system. This model is also used to calculate mediumrange stratospheric forecasts. Within the diabatic initialization approach, the model itslf is used to balance the resulting analysis, or initial fields.The diabatic initialization techinque appears to be a useful tool which improves stratospheric analyses and forcasts by controlling small‐scale high‐frequency non‐meteorological oscillations and shocks, or spin‐up effects resulting from initial imbalances. Changes due to initialization are cumulative with time, hence initialization affects positively the slowly varyiing components of stratospheric flows, resulting in better representation of climate characteristics.Experiments performed to assess the impact of diabatic initialization on monthly mean analyses ad diagnostic fields produced by the data assimilation system show that it provides an efficient noise control, especially for the upper stratospheric levels and in the polar regions. Stratospheric analysis errors have been significantly reduced by applying diabatic initialization.The major characteristics of stratospheric circulation, such as the residual mean meridional circulation. potential vorticity and Eliassen‐Palm or EP‐flux distributions, are improved when using diabatic initialization. The ozone transport experiments show iprovements due to using initialized winds in terms of better agreement with data. Ten‐day stratospheric forecast scores have been improved when using initialized, or balanced initial conditions.The initialization method used in the study is easy to implement and is computationally efficient. It appears to be a useful component of a stratospheric data assimilation system.
The production of high ice‐crystal concentrations in stratiform clouds
Authors: Mason B. J. 
Published: February 2007

AbstractThe appearance of ice crystals in high concentrations, of ∼ 500 1−1 in the −3 to −8°C zone of stratiform clouds, recently reported by Bower et al., is explained by the shedding of splinters by riming ice crystals at rates observed in the laboratory earlier by Hallett and Mossop. The appearance of a second maximum at −15°C, with small crystals of diameter less than 125 μm in concentrations of ∼ 1000 1−1, is attributed to the transport of small ice splinters produced in the Hallet‐Mossop zone that grow at small ice supersaturations in weak updraughts between the −8°C and −12°C levels, and thereafter at water saturation in convective cells containing liquid water and updraughts of 1 m s−1 between the −12°C and −15°C levels.
Turbulent transport in fields of warm cumulus clouds
Authors: Pasquier Jacques R. M. , Jonas Peter R. 
Published: February 2007

AbstractSimulations of fields of warm (ice‐free) cumulus clouds are made using a large‐eddy model based on observational soundings. Comparisons between numerical and experimental results show good agreement.The cumulus clouds produced by the model are similar to those observed, with cloud base, cloud top and cloud scales all in good agreement. At cloud top, vertical velocities are found to be 5 m s−1 and liquid‐water mixing ratios have a maximum of 1.6 g kg−1, which is consistent with the observations.By considering fractional cloud cover and averaging over time, the profiles of the turbulent kinetic energy and fluxes averaged over the horizontal level and in‐cloud are derived. Much of the turbulent energy and transport in the boundary layer is found to be produced in cloud. The dominant regions of turbulent kinetic energy result from the strong surface heating and the two in‐cloud buoyancy sources of latent heating through condensation at cloud base and evaporative cooling following entrainment at cloud top. This is supported by the upward transport of energy at mid‐cloud levels and downward at cloud‐top. The terms in the turbulent kinetic‐energy budget are derived, and once again buoyant production is found to be dominant in both the cloud and the horizontal averages.The vertical turbulent fluxes of heat, moisture and liquid are analysed and found to vary in accordance with the turbulent processes driving the boundary layer. The penetration of the clouds above the main inversion shows that the clouds were effective in coupling the surface to the base of the free troposphere by transporting the heat and moisture upwards.Overall, the kinetic‐energy profiles and turbulent fluxes in the cumulus case‐study, both averaged over the layer and the clouds, show good agreement with the observations. This demonstrates the validity of the large‐eddy simulation model as a tool for studying cumulus clouds at greater vertical resolution than is possible by current observations, and provides a basis for deriving more accurate parametrization schemes of convective cloud‐capped boundary layers.
Modelling long‐range transport of CFCs to Mace Head, Ireland
Authors: Ryall D. B. , Maryon R. H. , Derwent R. G. , Simmonds P. G. 
Published: February 2007

AbstractA Lagrangian particle‐based long‐range dispersion model is used to describe the transport of inert chloro‐fluorocarbon (CFC) tracer species to Mace Head, a remote location on the Atlantic coast of Ireland, from emission sources in the European continent and from across the ocean in North America. A six‐month analysis covering the period January to June 1996 of model and observed trace‐gas concentration reveals a high level of model skill with good correlations and no significant bias in our description of long‐range transport over the 100‐2000 km scale from Europe to Mace Head. Model simulations have also supported previous suggestions that CFCs detected at Mace Head on two occasions during 1994 were of North American origin. A detailed analysis is made of the meteorological processes involved in one of these transatlantic episodes. We estimate that, on average, North American sources may account for only a few per cent of the CFC‐11 and other man‐made trace gases observed at Mace Head above the northern hemisphere baseline concentrations. During long‐range transport events, North American sources may contribute pollutant peaks at Mace Head which are about one order of magnitude smaller in concentration than those from European sources.
Detection of turbulence around jet streams using a VHF radar
Authors: Pepler S. J. , Vaughan G. , Hooper D. A. 
Published: February 2007

AbstractTwo methods for detecting turbulence near jet streams are investigated using a VHF radar‐the corrected spectral width of the vertical echo and the anisotropy of the signal returned. It is found that turbulent dissipation rates in excess of 0.2 mW kg−1 may be detected using the spectral width, but uncertainties in the derivation of this parameter preclude detection of weak turbulence. Anisotropy of the signal is capable of detecting some turbulence in regions of high static stability, but the method is not quantitative and is best employed to confirm the spectral‐width estimates when these are near the detection limit. Independent confirmation that atmospheric conditions were favourable for turbulence was provided by Richardson number profiles derived from the radar and a Raman lidar. Measurements of static stability by the lidar and spectral widths by the radar allow an estimate of the turbulent‐dissipation rate. The application of these techniques to two jet stream passages is discussed.
A new gravity‐wave‐drag scheme incorporating anisotropic orography and low‐level wave breaking: Impact upon the climate of the UK Meteorological Office Unified Model
Authors: Gregory D. , Shutts G. J. , Mitchell J. R. 
Published: February 2007

AbstractA new anisotropic gravity‐wave‐drag parametrization‐scheme which represents high‐drag states modelled on hydraulic jump, flow blocking and internal‐wave‐reflection theory, including trapped lee‐waves, is presented. The scheme is shown to represent the breaking of waves over mountains better than previous schemes by comparison with mesoscale simulations of PYREX case studies. Extended simulations of the Meteorological Office Unified Model at climate resolution are presented, showing the impact of the new scheme and a combination of the scheme with a new orographic‐roughness parametrization. Results show a changed distribution of surface‐gravity‐wave surface‐stress, mostly due to mountain anisotropy, and a greater tendency for lower‐troposphere wavc‐drag. Inclusion of the orographic‐roughness parametrization halves the gravity‐wave stress, the combined effect of these schemes for climate integrations being a slight improvement to the tropospheric flow.
Two‐day disturbances in the equatorial western Pacific
Authors: Haertel Patrick T. , Johnson Richard H. 
Published: February 2007

AbstractFrom November 1992 to February 1993, westward‐propagating disturbances with scales of 1000‐1500 km frequented the equatorial western Pacific. They propagated at about 16 m s−1 and thier passage was accompanied by a two‐day oscillation in high cloudiness. Their convection was most intense when the lower troposphere was cool and the level of free convection was relatively low. The disturbances propagated westward at about 18 m s−1 faster than the lower tropospheric flow. This rapid propagation may have been the result of a coupling between the disturbance's convection and the 20 m s−1 vertical mode. The signal of this mode was apparent in vertical profiles of temperature and wind and the most intense convection began when the mode dynamically cooled the lower troposphere, lowering the level of free convection.