bluuebunny/crossref_metadata_2025_split huggingface dataset:
Microphysical and short‐wave radiative structure of wintertime stratocumulus clouds over the Southern Ocean
Authors: Boers R. , Jensen J. B. , Krummel P. B. , Gerber H. 
Published: February 2007

AbstractResults are presented from an aircraft measurement campaign carried out over the Southern Ocean near the north‐west coast of Tasmania, Australia. The microphysical and radiative characteristics of marine stratocumulus cloud sheets were sampled on four days, three of which are considered baseline days when air parcels had traversed long distances over the ocean without having been exposed to anthropogenic sources of pollution. Clouds were depleted by drizzle, with cloud liquid‐water often smaller than 50% of the expected adiabatic value. Horizontal variability in liquid‐water content associated with non‐drizzle droplets was primarily caused by variations in the droplet number concentration, which was found to be among the lowest ever recorded (10 to 40 cm −3 for clouds of up to 300 m deep). Cloud albedos integrated over the solar spectrum varied from 40 to 60%, which roughly agreed with radiative‐transfer computations. Data from one non‐baseline day were also obtained and compared with the data obtained during baseline conditions. The functional dependence of cloud optical depth on liquid‐water path was found to be much stronger on this day than the others, due to the smaller size of the cloud droplets present. Calculations show that on the days when drizzle was intense, the cloud optical depth could have been reduced by as much as 50% by shifts to larger values of the cloud droplet effective radius.
The Brewer‐Dobson Circulation In the Light of High Altitude In Situ Aircraft Observations
Authors: Tuck A. F. , Baumgardner D. , Chan K. R. , Dye J. E. , Elkins J. W. , Hovde S. J. , Kelly K. K. , Loewenstein M. , Margitan J. J. , May R. D. , Podolske J. R. , Proffitt M. H. , Rosenlof K. H. , Smith W. L. , Webster C. R. , Wilson J. C. 
Published: February 2007

AbstractFast response in situ measurements of a suite of chemical species made from the NASA ER2 high altitude aircraft, between 60°N and 70°S at potential temperatures up to 530 K from March to November 1994 at longitudes 115° W to 150°E, are considered for the view they offer of the Brewer‐Dobson circulation in the lower stratosphere and upper troposphere. In the southern hemisphere, where most of the flights occurred, comparisons are made with measurements taken in August/September 1987 at longitudes 120° W to 60° W to examine temporal and longitudinal differences. Interpretations made suggest conceptual modifications to the simple construct of advection in a two dimensional long‐term mean.
Scaling the turbulent transport of chemical compounds in the surface layer under neutral and stratified conditions
Authors: Galmarini S. , Vilà‐Guerau De Arellano J. , Duynkerke P. G. 
Published: February 2007

AbstractA model of the vertical transport of chemically reactive species is presented which is applicable to all possible stability conditions in the atmospheric surface‐layer. the non‐dimensional formulation adopted allows any chemical scheme to be included. an equation hierarchy is presented for a fully second‐order closure description of the process. the model describes comprehensively the turbulence/chemistry process by calculating the mean concentrations, the turbulent fluxes and concentration covariances of the chemical species. It explicitly takes into account the chemical transformation in first‐ and second‐order variable equations. the model is applied to the NO‐NO2‐O3 cycle and to the so‐called ‘night‐time’ chemical scheme of nitrogen oxides. an analysis of the flux budget indicates that the relative importance of the chemistry term in the flux equation varies with distance from the surface. For atmospheric neutral conditions, it is suggested that the chemistry term in the flux equation can be used to make an a priori evaluation of the possible effect of chemistry upon the flux of a chemical species. the model also shows that under stable conditions, despite the greatly reduced activity of turbulence, differences can occur between the turbulent transport of an inert tracer and that of a chemically reactive species. Under any stratification and for typical surface‐layer fluxes of nitrogen oxides, the intensity of segregation is very low and can in general be neglected in model calculations.
On the question of enhanced absorption of solar radiation by clouds
Authors: Francis P. N. , Taylor J. P. , Hignett P. , Slingo A. 
Published: February 2007

AbstractSome recent papers have claimed that clouds cause the atmosphere to absorb significantly more solar radiation than current understanding would predict. In this paper aircraft measurements of the broad‐band and narrow‐band radiation fields in and around different cloud types are presented. These measurements have been made over a number of years by the C‐130 aircraft of the UK Meteorological Office, in several different locations around the globe. Our observations show no evidence of this ‘enhanced’ cloud absorption, and they can be explained, both qualitatively and quantitatively, by currently accepted theory. the suggestion that such enhanced absorption is due to one particular mechanism, that of increased condensed‐phase water absorption in the near‐infrared, is examined. This hypothesis is rejected as being completely at variance with the observations.
Modelling isolated wave‐turbulence interactions in the stable atmospheric boundary layer
Authors: Edwards N. R. , Mobbs S. D. 
Published: February 2007

AbstractThe interaction between a large‐scale isolated wave and turbulence is considered in the context of the stable atmospheric boundary layer. A two‐dimensional model with a second‐moment turbulence closure is used to simulate the interaction numerically. Results obtained using other closures are discussed. the initial form of the wave‐like disturbance is suggested by observations, which are described in a companion paper. the turbulence is shown to exert only a small influence on the wave, while the turbulence itself remains close to production‐dissipation equilibrium. Wave‐like components of the turbulence fluxes vary roughly linearly with wave amplitude, but overall the wave is strongly nonlinear and depends heavily on the background flow. Only one term in the equations for the quadratic turbulence quantities varies in a simple way with wave amplitude; this term represents a net transfer of energy from the wave to the turbulence.
A numerical model of the cloud‐topped planetary boundary‐layer: Impact of aerosol particles on the radiative forcing of stratiform clouds
Authors: Bott Andreas 
Published: February 2007

AbstractIn a numerical sensitivity study, the microphysical model of stratus MISTRA, is used to investigate the impact of aerosol particles on the evolution of stratiform clouds. Four model runs are presented, each for a different type of background aerosol. Two include aerosol particle size‐distributions which are typical of marine and rural continental air masses; a third represents a mixture of marine and rural continental aerosol particles, and the fourth rural continental aerosol particles with a reduced solubility in water.The results show that the microphysical structure of layer clouds was strongly affected by the physico‐chemical properties of the aerosol particles from which the cloud droplets grew. the relatively low concentration of the typical marine aerosol resulted in clouds with a low concentration of large droplets. By contrast, the rural continental and mixed aerosols yielded higher concentrations of relatively small cloud‐droplets. Because of their low solubility in water, relatively few of the aerosol particles simulated in the fourth run of the model became activated, so that the microstructure of the cloud was again similar to that with marine aerosol. the different size‐distributions of the cloud droplets had a direct influence on the radiative forcing of the clouds. When there were many small droplets, the reflectivity of the clouds was distinctly higher than when there were fewer, but larger, cloud‐droplets. In all simulations, the effective radii of the cloud droplets showed a strong diurnal variation.
The singular vectors of a coupled ocean‐atmosphere model of Enso. I: Thermodynamics, energetics and error growth
Authors: Moore A. M. , Kleeman R. 
Published: February 2007

AbstractThe singular vectors of a dynamical system are the most rapidly growing perturbations that can exist in the system before nonlinearity becomes important. We have studied the singular vectors of an intermediate coupled ocean‐atmosphere model of El Niño/Southern Oscillation (ENSO) in an effort to understand the dynamics responsible for the growth of small perturbations in the tropics. In particular, we have examined how the singular vectors of the observed seasonal cycle are influenced by various thermodynamic processes which operate in the upper‐ocean mixed‐layer, and which affect sea‐surface temperature (SST). These processes include vertical movements of the main thermocline, zonal advection and vertical upwelling. the main findings are:
the singular vector spectrum is dominated by the fastest growing member, regardless of which thermodynamic processes are active;
the growth factors of the singular vectors exhibit a strong seasonal dependence;
the western and central parts of the Pacific Ocean are the areas most often favoured for growth by the singular vectors, and the general criteria that must be met for singular‐vector growth have been determined;
at a given time of year, the growth factors of the singular vectors are sensitive to different combinations of thermodynamic processes, although the configuration of the dominant singular‐vector wind field after optimal growth has been achieved is very similar in all cases. This suggests that in the tropics, the coupled system has a preferred response that the ocean thermodynamics controlling SST conspire to produce, regardless of which processes are operating in the mixed layer;
the observed ENSO cycle is likely to influence singular‐vector growth significantly.
These results and ideas have important ramifications for the growth of errors and uncertainties in models during ENSO forecasts, and for the way this influences the predictability of ENSO. They also have important ramifications for the way perturbations grow in the real coupled ocean‐atmosphere system.
Using forecast sensitivity patterns to improve future forecast skill
Authors: Pu Zhao‐Xia , Kalnay Eugenia , Derber John C. , Sela Joseph G. 
Published: February 2007

AbstractA simple, relatively inexpensive technique has been developed for using past forecast errors to improve the future forecast skill. the method uses the forecast model and its adjoint and can be considered as a simplified 4‐dimensional variational (4‐D VAR) system. One‐or two‐day forecast errors are used to calculate a small perturbation (sensitivity perturbation) to the analyses that minimizes the forecast error. the longer forecasts started from the corrected initial conditions, although better than the original forecasts, are still significantly worse than the shorter forecasts started from the latest analysis, even though they both had access to information covering the same period.As a much less expensive alternative to 4‐D VAR, the adjusted initial conditions from one or two days ago are used as a starting point for a second iteration of the regular NCEP analysis and forecast cycle until the present time (t = O) analysis is reached. Forecast experiments indicate that the new analyses result in improvements to medium‐range forecast skill, and suggest that the technique can be used in operations, since it increases the cost of the regular analysis cycle by a maximum factor of about 4 to 8, depending on the length of the analysis cycle that is repeated. Several possible operational configurations are also tested.The model used in these experiments is the NCEP's operational global spectral model with 62 waves triangular truncation and 28 ő‐vertical levels. an adiabatic version of the adjoint was modified to make it more consistent with the complete forecast model, including only a few simple physical parametrizations (horizontal diffusion and vertical mixing). This adjoint model was used to compute the gradient of the forecast error with respect to initial conditions.
A numerical study of the effects of land proximity and changes in sea surface temperature on hurricane tracks
Authors: Dengler Klaus 
Published: February 2007

AbstractA three‐layer shallow‐water model with a convection parametrization is used to study the track deflections of slow‐moving hurricanes on a β‐plane as they approach straight coastlines of different orientations with respect to north, or as they cross to a cooler or warmer body of water. In the landfall experiments, the land surface is level and is characterized by an abrupt increase of the drag coefficient and zero evaporation. the tracks of a 72 h integration on a β‐plane are compared with an ocean control experiment.It is found that the divergent flow in the boundary layer in regions of offshore winds is responsible for the observed track deflections. In this region middle‐layer air descends into the boundary layer and increases the positive potential‐vorticity anomaly in the middle layer. In the case of land conditions to the west of the vortex, the maximum potential‐vorticity anomaly extends less to the north of the vortex centre and results in a more southerly track than in the ocean control case. If land conditions are specified in the north, the positive potential‐vorticityasymmetry maximum extends more to the north‐west and results in a northward deflection of the track. On the other hand, land conditions to the north‐west of the initial vortex centre do not significantly change the location of the asymmetry, and therefore the track deflection is small. the magnitude of the track deflections, as well as the drift speed, increases when the roughness of the land is increased. It is significant that track deflections commence about 20 h before landfall, reaching a value of about 150 km at the time of landfall.Compared with landfalling hurricanes, track deflections of model hurricanes which drift into a region of different sea surface temperature (SST) are smaller and are a result of intensity changes of the vortex circulation. the maximum wind speed at the time of landfall increases if a region of higher SST is located adjacent to the coastline.
The structure of katabatic flows down a simple slope
Authors: Papadopoulos K. H. , Helmis C. G. , Soilemes A. T. , Kalogiros J. , Papageorgas P. G. , Asimakopoulos D. N. 
Published: February 2007

AbstractAn analysis of the katabatic flow structure is presented, based on data collected along a simple slope over a two‐month period. Four meteorological stations were deployed along the nearly two‐dimensional east slope of Mt. Hymettos in Greece. The study focuses on the momentum and energy budgets of the flows during carefully selected stationary periods with negligible ambient winds. In order to achieve a proper selection, the mean features of the observed flows are presented along with their response to external winds. The observed profiles are successfully compared with the theoretical Prandtl profiles. Bulk quantities of the katabatic layer are compared with proposed experimental values in order to check the representativity of the observed flows. These comparisons offer a further test of theoretical considerations and assist in discussions of processes affecting mean‐flow features.The flows were ∼ 20 m deep, with velocities 1‐2 m s−1. During steady‐state periods the katabatic acceleration was mainly balanced by the surface friction, the contribution of advective terms being small. The estimated interfacial drag was less important than the surface drag, which proved large compared with usually suggested values. Within the experimental uncertainties, values of the drag coefficient of the order of 10−2 seem appropriate. The steady‐state energy balance showed that the downslope advection balanced the turbulent heat flux to the ground surface.
Particle trajectories above sinusoidal terrain
Authors: Stout J. E. , Janowitz G. S. 
Published: February 2007

AbstractAs heavy particles fall towards windswept topography, their motion is governed partly by gravitational forces and partly by fluid forces resulting from the relative motion of the particles through the flow field. Topographically induced perturbations of the flow field distort particle paths and ultimately modify deposition patterns at the surface. Here, we calculate trajectories of particles falling toward a series of low‐amplitude hills. Particle motion is obtained by simplifying the dynamical equations of particle motion to kinematic form and then applying perturbation techniques. This simplification is possible when the timescale of fluid motions is much longer than the characteristic response‐time of the particles. It is shown that, under the right atmospheric conditions, alternating regions of convergent and divergent particle‐paths will occur. an extension of the trajectory analysis yields expressions that predict the point of surface impact as functions of the initial release point. This leads to a method for predicting the point along the surface where deposition is either a maximum or a minimum.
Mode selection, wave breaking and parametric sensitivity in the quasi‐biennial oscillation
Authors: Li X. , Read P. L. , Andrews D. G. 
Published: February 2007

AbstractAn analytical model is developed for the propagation of equatorially trapped waves on an equatorial β‐plane in a uniform zonal flow in the presence of Rayleigh friction and Newtonian cooling. By comparing solutions obtained from this model under conditions appropriate to the troposphere and stratosphere, it is shown that wave modes which are most likely to contribute to the excitation of zonal flow oscillations occur in a ‘spectral window’, for which modes propagate freely in the vertical in the troposphere but are strongly damped in the stratosphere. This spectral window leads to a mode‐selection criterion which accounts for the dominant modes observed in association with the main stratospheric quasi‐biennial oscillation (QBO), and other aspects of equatorial‐wave propagation in the tropical stratosphere and mesosphere.Solutions from this model are also used to verify a new set of numerical models of equatorial‐wave propagation (using the ‘transformed Eulerian‐mean’ formulation) in the presence of zonal flows with general lateral and vertical shear and arbitrarily specified stratification. the latter numerical models are shown to simulate a realistic stratospheric QBO successfully, under conditions similar to those observed. the models are then used to investigate the probable role of large‐scale wave‐breaking processes in modifying the form of the QBO, via a convective parametrization scheme, which is shown to act in certain respects like Rayleigh friction. Finally, a sensitivity study is carried out to investigate the effect of varying parameters‐such as the vertical diffusion coefficient, buoyancy frequency and forcing by tropospheric wave‐on the form, strength and period of the QBO.
Long‐term fluctuations in the statistical properties of low‐frequency variability: Dynamical origin and predictability
Authors: Molteni Franco , Corti Susanna 
Published: February 2007

AbstractThe stability and predictability of the statistical properties of low‐frequency variability are investigated by studying the dynamical origin of long‐term changes arising, respectively, from the nonlinear response to variations in atmospheric forcing and from chaotic fluctuations in the energy partition between different variability patterns. Results from long integrations of a realistic quasi‐geostrophic (QG) model are analysed, and interpreted on the basis of the behaviour of simple nonlinear dynamical systems. It is shown that chaotic energy fluctuations not only modify the variance explained by different variability patterns, but also may either obscure or enhance the multimodality in the distribution of model states in periods as long as several decades. On the other hand, it is argued that significant variations in atmospheric statistics may be induced by anomalies in the atmospheric forcing originated from the lower boundary. QG simulations show that a strengthening of the planetary‐wave forcing due to the asymmetry in the divergent circulation between the western and eastern Pacific increases low‐frequency variability in the extratropical Pacific, and may turn the distribution of model states from unimodal to bimodal. Anomalies in low‐frequency variance related to changes in atmospheric forcing seem to be larger than those originated by internal dynamics, at least in certain periods and regions, suggesting a potential predictability of these statistics by coupled ocean‐atmosphere models.
Observations of a cut‐off low over southern Australia
Authors: Griffiths Morwenna , Reeder Michael J. , Low David J. , Vincent Robert A. 
Published: February 2007

AbstractThe structure and evolution of an intense cut‐off low is documented in detail unprecedented for the Australian region, by combining high time‐resolution radiosonde profiles with co‐located wind‐profiler observations and routine Australian Bureau of Meteorology analyses. The cut‐off low develops over the ocean south of Australia and subsequently interacts with a subtropical frontal system over the central part of the continent. Throughout the period of observation, the strongest temperature gradients lay through the subtropics. Moreover, the subtropical temperature gradients strengthened while the midlatitude temperature gradients associated with the cut‐off low weakened. The Australian Bureau of Meteorology's 150 km Regional Assimilation and Prognosis System analyses capture the broad‐scale structure and evolution of the system reasonably well. The system documented here is similar in structure to the instant occlusion investigated by Browning and Hill (1985), the major difference being that our cut‐off low develops poleward of a subtropical front. The cut‐off low was cold‐cored and produced a very deep tropopause fold. Over Adelaide, the tropopause descended to an altitude of about 5 km, and the temperature at 500 hPa fell to −30°C. The western edge of the low‐level cold dome was marked by a strong secondary warm front, which passed over Adelaide about 8 hours after the tropopause height minimum. The warm front was characterized at the surface by an absolute minimum in the pressure, and a temperature rise of about 7°C. Significantly, this increase in temperature took place in the early hours of the morning, against the diurnal trend. The cut‐off low produced very high rainfalls, resulting in flash flooding and much damage in the Adelaide Hills area. The most severe convection took place in the region behind the lowered tropopause, around the passage of the warm front.