bluuebunny/crossref_metadata_2025_split huggingface dataset:
Flux‐gradient relationships for momentum and heat over a rough natural surface
Authors: Fazu Chen , Schwerdtfeger Peter 
Published: February 2007

AbstractThe flux‐gradient relationships for momentum and heat have been investigated over extensive flat bushland (h = 2.3 m, zo = 0.4 m) with observations being made at several levels extending up to 10 times the canopy height. It has been found that: (1) Eddy diffusivities for momentum and heat, KH and KM, are enhanced by factors of 1.65 and 2.5 respectively under near‐neutral conditions and within the layer close to the canopy. (2) These enhancements distinctly depend on instability, and their variations with height are not simply monotonic, as expected in earlier investigations. (3) Even a counter‐gradient flux of heat could occur under some circumstances. (4) The level of enhancement remains substantially unchanged over different surfaces for heat transfer but decreases as the canopy density increases in the case of momentum transfer. Possible mechanisms are discussed. (5) The length scale of atmospheric turbulence should be taken into account in studies of turbulent transfer over very rough surfaces, where the large‐scale eddies may have a key role.
The structure of radiatively driven convection in stratocumulus
Authors: Nicholls S. 
Published: February 2007

AbstractData from a series of five research flights in marine stratocumulus are used to investigate the structure of radiatively driven, free convective layers. Conditional sampling methods are used to determine the properties of the primary convective elements: negatively buoyant downdraughts. The turbulent velocity data are also subjected to spectral analysis and the turbulent kinetic energy (TKE) balance is evaluated for each case. Results from each case are found to be very similar if scaled using appropriate mixed layer quantities.Downdraughts are selected using a criterion based on vertical velocity. The distributions of their intersected widths at cloud top, and the velocity spectra, are consistent with downdraughts occupying narrow regions (∼0.1‐0.15 h wide, h being the mean mixed layer depth) around the periphery of larger, regular (∼0.5 h‐0.75 h diameter) updraughts as suggested by cellular patterns observed in the cloud tops. Downdraughts are found to carry over half of the total turbulent fluxes of heat and water substance within cloud, and also transport variance down the mean gradients. Their thermodynamic properties are consistent with their containing a small fraction of air from above cloud top although their negative buoyancy is almost entirely due to the incorporation of radiatively cooled air. The properties of the downdraughts near cloud top suggest they are formed primarily as a result of the local horizontal convergence of upwelling motions constrained by the inversion. While being cooled radiatively, these quasi‐horizontal motions scour the base of the inversion, incorporating some drier air before being forced downwards in relatively narrow zones. Comparisons with other convective layers over land or sea suggest that convection in stratocumulus proceeds with a greater mass flux and correspondingly reduced differences between convective elements and the environment, possibly reflecting the improved ventilation possible at an inversion interface compared with much rougher surfaces.The TKE balance divides into three main regions and is consistent with the interpretations given above. The pressure transport is implied to be the largest source term close to cloud top. Comparisons with model results reveal some important differences.
Numerical modelling of the offshore extent of sea breezes
Authors: Arritt Raymond W. 
Published: February 2007

AbstractTwo‐ and three‐dimensional versions of a nonlinear atmospheric model were used to investigate environmental controls on the offshore influence of sea‐breeze circulations. The temperature of the water surface relative to the atmospheric temperature had a small effect when the water was colder than the overlying air, and a greater effect when the water temperature was high enough to create convective mixing in the planetary boundary layer over the water. Ambient thermal stratification, including elevated inversions with thermal stratification similar to trade wind inversions, had a small influence. The predicted offshore extent of the sea breeze was substantially affected by latitude. The effects of both ambient stratification and latitude were generally consistent with predictions of linear theory. Prevailing synoptic winds were also found to have a significant effect on the sea breeze offshore, with offshore influence greatly suppressed by onshore large‐scale flow. Curvature of the coastline produces slightly stronger winds offshore for a concave coast than for a convex coast. The subsidence offshore also extends further over the water for a concave coast than for a convex coast.The model predictions were compared with previous investigations which used linear theory. It was found that the present results agreed with the linear theory of R. Rotunno with regard to the effects of latitude and ambient thermal stability. Linear theory is less suitable for explaining the effects of water temperature, owing to the simplified treatment of turbulent exchange necessary for a tractable linear model.
Monthly‐mean diabatic circulations in the stratosphere
Authors: Pawson S. , Harwood R. S. 
Published: February 2007

AbstractThree‐dimensional fields of net radiative heating rate have been calculated from temperature, ozone and water vapour data observed by the LIMS instrument. Monthly‐mean distributions of net heating rates are presented for the months November 1978–May 1979; particular attention is paid to the distribution of net heating rates at two isentropic levels in the northern hemisphere, one near 30 km in the mid‐stratosphere, the other near the stratopause. The seasonal evolution of the heating rate is discussed in relation to the vortex position. We also consider standard deviations of the daily radiative heating distributions from the monthly‐mean fields; large variability is found in the dynamically active months of the winter. An interhemispheric comparison of the net heating rate in November in the northern hemisphere with May in the southern hemisphere reveals similarities in the heating distribution at the lower level but differences at the higher level, where the southern hemispheric distribution is far more zonal than that of the northern hemisphere.The continuity equation may be used to determine the divergent horizontal flux of mass at isentropic levels. Over long periods the transient component is found to be dominated by the diabatic component, so that the net radiative heating distributions can be used to obtain this divergence. Near the stratopause we find an anti‐correlation between the northward components of divergent and geostrophic winds.The zonal‐mean diabatic circulation has also been calculated and found to agree with the rather broad range of results of several other recent studies. Knowledge of the three‐dimensional heating rates and tracer distributions has been applied to the problem of tracer transport by mean and eddy motions across isentropic surfaces. The mean advection is found to be dominant, a consequence of the eddy motions being almost adiabatic. The implication of this result is that it is the zonal‐mean diabatic circulation which is responsible for any systematic cross‐isentropic tracer transport, the eddies playing only a minor role.
A general method for the linear stability analysis of stratified shear flows
Authors: Mobbs S. D. , Darby M. S. 
Published: February 2007

AbstractA general method for testing the stability of a stratified, parallel shear flow is presented. The technique allows the routine determination of unstable modes from any high resolution radiosonde data and is a development of the procedure described by Lalas and Einaudi. Stability analyses of several idealized velocity profiles have been performed in order to validate the method. A particularly important feature is the ability to predict marginally unstable modes with more than one critical level; several examples are presented. An example using real radiosonde data is given.
The formation of internal bores in the atmosphere: A laboratory model
Authors: Rottman James W. , Simpson John E. 
Published: February 2007

AbstractRecently several atmospheric observations have been interpreted as internal undular bores or internal solitary waves evolving from internal bores that propagate along low‐level temperature inversions. It has been speculated that such disturbances are generated by some type of gravity current (such as cold fronts, sea breeze fronts and thunderstorm outflows) interacting with an existing temperature inversion. In this paper we describe a systematic laboratory study in a water channel, of internal bores and their generation by the movement of gravity currents through a two‐layer model of the atmosphere. We compare the results of our laboratory experiments with previous theories and numerical simulations and with several detailed atmospheric observations of internal bores at different stages of development.
A simple tropical moist model applied to the ‘40‐day’ wave
Authors: Davey Michael K. 
Published: February 2007

AbstractThe ‘40‐day’ wave is a baroclinic eastward‐propagating disturbance commonly observed in the tropical atmosphere, with a period that actually lies in the band 30 to 60 days. Its structure resembles an equatorially‐trapped Kelvin wave, predominantly with zonal wavenumber 1. In this paper a simple model with marginally stable moist static stability is used to investigate one possible mechanism for its appearance: namely, as the response to variable thermal forcing amplified by the effect of interactive latent heating.The model has just one vertical baroclinic mode, forced by prescribed heating and by interactively‐determined latent heating. Latent heating only occurs when and where there is low‐level convergence and moisture is tending to increase beyond some saturation level: in such regions the static stability takes its moist value, which is considerably less than the dry value acting in regions without precipitation.Analytic solutions are first provided for cases with static stability everywhere constant. Waves propagate more slowly when the moist value is used, so the peak linear response to wave‐like thermal forcing (essentially a resonant response) occurs at longer periods than for dry static stability. For dry conditions the period of a free Kelvin wave with zonal wavenumber one is about 10 days, but for wet conditions the period increases toward the ‘40‐day’ range.When static stability varies the model must be solved numerically. Solutions show that the increase in period predicted analytically is indeed realized, even when precipitation is patchy and dry static stability is acting almost everywhere. Effectively the amplified waves travel at a speed consistent with the maintenance of their latent‐heating energy source. Eastward waves are prominent near the equator. By analysing particular moisture‐dependent combinations of variables, these eastward waves can be dynamically identified as Kelvin waves.Saturation moisture content is closely linked to sea surface temperature. The warm western Pacific and Indian Oceans are regions of high atmospheric moisture content, and hence of very low moist static stability. This ocean temperature effect is studied using the simple model, showing that intraseasonal waves are largest in such regions, as observed.
Modelling wind‐generated surface gravity waves using similarity in a coupled discrete wave model
Authors: Perrie W. , Günther H. , Rosenthal W. , Toulany B. 
Published: February 2007

AbstractIn deep water, there is a growing observational consensus that supports an ƒ4 variation for the Kitaigorodskii subrange of the high‐frequency energy spectra. We therefore re‐examine and extend the TMA (Texel Marsen Arsloe) similarity and concepts for shallow water waves, which were originally derived for an ƒ5 Phillips subrange. The similarity for an ƒ4 a subrange is found to closely resemble that of the ƒ5 subrange. The former exists when nonlinear transfer due to wave‐wave interactions dominates over other source terms, such as wind input, in the energy balance.Starting with a standard second generation deep water coupled discrete spectral wave model, we implement the TMA similarity. To evaluate our modifications, we perform the real and synthetic shallow water tests of the recent SWIM study. Thus we intercompare our model with HYPAS, an analogously modified deep water coupled hybrid model, and with other wave models which were not based on the TMA similarity. Our model and HYPAS are found to behave quite similarly in the sloping and flat‐bottom SWIM tests, compared to the other wave models of the SWIM intercomparison. The more difficult and complex North Sea SWIM hindcast extends this property, in some regards.
Assessment of the practical utility of extended range ensemble forecasts
Authors: Murphy J. M. 
Published: February 2007

AbstractLagged‐average forecast experiments, produced with the Meteorological Office 11‐level general circulation model, are used to assess the benefits arising from the use of ensemble forecasts in practical monthly prediction.As measured by anomaly correlation, forecast skill is consistently positive at days 6‐15, but very small on average beyond 20 days. For the monthly mean forecast, removal of an independent estimate of the model systematic error improves the average score significantly. Compared to the mean score amongst individual members, a modest increase in skill is achieved by ensemble averaging. Theoretically, however, there remains scope for a much greater impact in the future, through the use of models capable of more accurate forecasts.Local forecast skill is correlated with a measure of the agreement between ensemble members, although the overall relationship is weak beyond the medium range. Nevertheless, for areas of high agreement, the probability of obtaining useful skill is considerably higher than average at days 6‐15 and 11‐20.The experiments are also assessed as probability forecasts, using the Ranked Probability Score to measure skill. the ensemble probability forecast (EPF) is created by averaging the probability forecasts of the individual members. Relative to the average score for the latter, the EPF gives improved skill. For the medium‐range period of the forecast (days 1‐10), the use of an optimized EPF, calculated by weighting each ensemble member according to its age, leads to a small increase in skill compared to the individual forecast from the most recent analysis. This effect is not observed in the corresponding anomaly correlation results. the above results for local skill prediction are confirmed, and somewhat enhanced, in the probabilistic formulation, using the ensemble variance of forecast categories as the predictor. A simple method of issuing confidence‐based adjustments to the forecast probabilities is discussed.
An analytical model of the growth of a frontal discontinuity
Authors: Holt M. W. , Shutts G. J. 
Published: February 2007

AbstractAn analytic semi‐geostrophic model of frontogcnesis is described which exhibits the growth of a frontal discontinuity surface. the initial distribution of temperature on a plane horizontal boundary consists of a warm two‐dimensional strip embedded in an isothermal background; the potential vorticity in the atmosphere above is uniform and positive. A barotropic deformation field with axis of dilation along the warm strip forces a discontinuity in the velocity field as the warm air is occluded and becomes detached from the lower boundary.The solution demonstrates some important general properties of the inviscid semi‐geostrophic equations when discontinuities are present. In particular, the vertical velocity on the lower boundary becomes non‐zero at the front. the maximum value of the vertical velocity occurs here when the front first forms, but then lifts off the boundary. the front itself constitutes a line of infinite potential vorticity, so that the global integral of mass‐weighted potential vorticity is conserved only if the boundary of the domain is redefined to exclude the front. the implications of these properties for numerical modelling are discussed.For typical atmospheric rates of deformation forcing, the accelerations of air parcels extremely close to the front are large enough to violate the conditions of validity of semi‐geostrophic theory. the conditions of hydrostatic balance are, for all practical purposes, never violated.
Estimates of cyclone track predictability. II: Fractal analysis of mid‐latitude cyclones
Authors: Fraedrich Klaus , Grotjahn Richard , Leslie Lance M. 
Published: February 2007

AbstractA nonlinear systems analysis is applied to cyclone tracks over the North Pacific area (based on surface pressure analyses and twelve‐hourly sampling time) and over the Australian region (based on 500mb heights and daily sampling) for the respective winter seasons. First estimates are obtained of the degree of their chaotic or irregular behaviour in phase space and of the self‐affinity of the planar tracks. This information is deduced from distance distributions (correlation integrals) between independent pairs of cyclone trajectories and from the structure function of individual cyclone paths.The correlation integrals reveal average structures of the substitute phase space (such as the dimension of the attractor, a measure of predictability) after stratifying the North Pacific tracks into a western domain of growing and an eastern domain of large amplitude cyclones. A further subdivision into large‐scale flow anomalies (the positive and negative Pacific/North America pattern in the eastern North Pacific) provides flowdependent estimates which indicate two subsystems differing in dimension and predictability. the time scaling analysis of the planar cyclone paths characterizes systems of no scale preference: the associated variance spectra decrease by a 2 to 3 power law with increasing frequency.
Frontal instability generated by tropospheric potential vorticity anomalies
Authors: Joly Alain , Thorpe Alan J. 
Published: February 2007

AbstractThe development of secondary disturbances on a two‐dimensional front is described using a linear stability analysis. A two‐stage process is envisaged in which, first, strong frontogenesis produces condensation at the front. This leads to the generation of a lower tropospheric zone of high potential vorticity (PV) lying along the front. As frontogenesis weakens, the second stage is entered in which an essentially steady front is susceptible to the growth of linear normal modes along the front owing to the existence of the PV anomaly or, equivalently, the frontal rainband. Thus for a typical band of width 160 km a frontal wave of some 800 km is produced with a growth rate of about 1 day−1. the dynamics of these modes is examined by considering both the realistic frontal flow as well as idealized strips of PV anomalies. the idealized case is potentially applicable to a variety of meteorological situations such as upper jet streaks. Useful insight is gained from an analysis of the energetics of the modes. This shows that for narrow frontal PV anomalies the waves main energy source is the basic kinetic energy and further they have a negative vertical heat flux. Recent observations suggest that frontal waves are indeed characterized by such a heat flux. As most frontal zones involve significant condensation, and therefore are likely to exhibit a lower tropospheric PV maximum, it is stressed that studies of frontal waves need to include this aspect of the frontal structure.
Buoyancy reversal and cloud‐top entrainment instability
Authors: Siems Steven T. , Bretherton Christopher S. , Baker Marcia B. , Shy Shenqyang , Breidenthal Robert E. 
Published: February 2007

AbstractIn 1980, Randall suggested that cloud top entrainment instability, a runaway positive feedback between mixing, evaporative cooling and entrainment, may cause the transition from the subtropical stratocumulus layers, frequently observed over the cold water of the eastern oceans, to tradewind cumulus. Predicting how the mean position of this transition, and the associated large change in albedo, would vary with, and feed back on, a change in climate is important for any credible model of the climate system.We have used numerical and laboratory simulations, as well as field observations, to study entrainment in buoyancy‐reversing systems. We consider a system in which an upper fluid stably overlies a second lower fluid; but some mixtures are less buoyant than either fluid. the laboratory results show that buoyancy reversal does not cause runaway ‘explosive entrainment’ unless D (the ratio of the density difference between the most cooled mixed parcel and the lower fluid to the density difference across the unmixed layers, a measure of CEI potential) exceeds 1.3, which is far greater than the range of D observed in stratocumulus. For D below this threshold, buoyancy reversal leads to weak circulations rather reminiscent of Rayleigh‐Bénard convection in the lower layer, but only slightly distorts the interface. For D > 1.3 an entrainment tongue of upper fluid penetrates a sizable fraction of the depth of the lower layer and entrainment is explosively rapid. Numerical experiments suggest that even when 0 < D < 1.3, entrainment can break up idealized subtropical marine stratocumulus within about an hour if radiation and surface fluxes are removed, but comparison of numerical simulations with the laboratory experiments suggests that the numerically simulated entrainment rate may be substantially too large in this regime owing to the inability to accurately produce and resolve the entraining eddies. Our observations of stratocumulus from FIRE typically had a strong stable layer above the cloud top inversions, leading to some ambiguity in defining D, but clearly show that stable stratocumulus layers often have 0 < D < 0.2. We suggest that sudden breakup of stratocumulus due to runaway entrainment is unlikely, but evaporatively enhanced entrainment may aid the transition from stratocumulus to tradewind cumulus in conjunction with some other mechanism, such as short‐wave absorption by liquid water, that can help dynamically decouple the cloud layer from the surface layer.
A perturbation analysis of turbulent flow through a porous barrier
Authors: Wilson J. D. , Swaters G. E. , Ustina F. 
Published: February 2007

AbstractEarlier attempts to calculate the mean velocity field in flow about a thin, finite porous barrier (wall‐mounted or otherwise) are reviewed. By simplifying the governing mean vorticity equation and expanding the flow variables in powers of the pressure‐loss coefficient of the barrier (k,) we derive an analytical solution for the mean velocity field in unbounded and homogeneous turbulent flow through a finite barrier. This solution is compared with the earlier solution of Kaiser (1959) and with numerical solutions for windbreak flow (bounded, inhomogeneous turbulence), and it is shown that all these solutions demonstrate a rate of recovery of the mean velocity field which is slower than is observed. It is suggested that the weakness of all the solutions lies in the treatment of the Reynolds stresses. A further point of interest is that a useful approximation to the pressure field may be obtained by dropping the Reynolds stress terms and solving a simplified equation expressing a balance between streamwise advection, pressure gradient, and localized momentum removal at the barrier; this might prove useful for the solution of more general porous barrier flows.