bluuebunny/crossref_metadata_2025 huggingface dataset:
High‐resolution numerical‐model integrations to validate gravity‐wave‐drag parametrization schemes: A case‐study
Authors: Broad Adrian S.
Published: February 2007

A three‐dimensional non‐hydrostatic numerical model is used to simulate the lee‐wave event IOP3 (1800 GMT 14 October 1990 to 1200 GMT 15 October 1990) from the PYREX mountain experiment. Using high resolution with a horizontal gridlength of 1.5 km and a realistic terrain‐height specification the resulting model vertical‐velocity fields are compared directly with observations from aircraft flights. The overall agreement of the model‐simulated wave structure with the aircraft data is very encouraging, particularly given the high degree of nonlinearity and finite amplitude wave activity observed during this event. Given the very restrictive three‐dimensional coverage of the observations from the field experiment, the numerical‐model results are used as surrogate observational data to give exhaustive three‐dimensional data over the whole model volume. The model integrations are subsequently used to compute areally averaged surface‐pressure drags and height profiles of the vertical flux of horizontal momentum. Assuming these wave stresses to be an accurate representation of the real atmospheric stresses, they are compared, as a validation procedure, against grid‐box flux values taken from a gravity‐wave‐drag parametrization scheme in a model with resolution 2.5° latitude by 3.75° longitude. The drag scheme in question compares favourably with the model domain‐averaged stresses.
Sensitivity of a simulated tropical squall line to long‐wave radiation
Authors: Dharssi I. , Kershaw R. , Tao W. K.
Published: February 2007

A two‐dimensional cloud‐resolving model is used to simulate a tropical squall line and to examine its sensitivity to long‐wave radiation. Model results show that the magnitude of the sensitivity to long‐wave radiation depend on the organization and structure of the squall line. For one simulation, the inclusion of long‐wave radiation increased surface precipitation by nearly a third. Long‐wave radiation is also found to enhance the mid‐level rear inflow and upper‐level outflow of the squall line. Sensitivity experiments are performed to examine the role of cloud‐top cooling and cloud‐base warming, differential cooling between clear and cloudy regions, and domain‐wide cooling.
Ambiguity removal and assimilation of scatterometer data
Authors: Stoffelen By A. , Anderson D.
Published: February 2007

The ERS‐1 scatterometer has proved to be a source of high quality ocean surface wind data, but a problem remains, namely the dual directional ambiguity of the solutions. an ambiguity removal scheme, called PRESCAT, is described based on our experience (a) that information on wind direction retrieval skill is an important input to ambiguity removal, (b) that wind‐vector filtering is beneficial compared to wind‐direction filtering, and (c) that already meteorological forecast information enables us to remove correctly approximately 95% of all ambiguities. the performance of the ambiguity filter is very good compared with other operational ambiguity removal schemes. Furthermore, a statistical interpolation analysis system called ‘buddy’ check is used effectively to identify and remove the few (approx. 0.1%) wrongly selected solutions.

Assimilation of scatterometer winds has a beneficial impact on analyses and short‐range forecasts, probably mainly from improvements on the subsynoptic scales. On the wider temporal and spatial scales, scatterometer winds were also found beneficial, but only in the absence of satellite temperature soundings (SATEMs). In assimilation experiments in which the latter were included, the scatterometer did not provide a beneficial impact on the medium‐range forecast. Moreover, the conventional observations, including SATEMs are shown to have adverse effects on the surface‐wind analysis. We believe that both the redundancy and the adverse effects on the surface‐wind field are explained by the rigid formulation of the 6‐hour‐forecast error structure; the forecast error is assumed to be flow‐independent, and information on the special meteorological conditions in the atmospheric planetary boundary layer is lacking. to make observational systems more useful and complementary for numerical weather prediction the effects of the structure functions have to be investigated more precisely. In an adaptive four‐dimensional variational assimilation scheme the effect of the assumptions on forecast‐error structure will be less. We show that, in a variational framework, scatterometer backscatter measurements are difficult to assimilate directly. Instead, we derive and illustrate an alternative procedure to assimilate retrieved winds rather than backscatter measurements.
Observations of isolated wave‐turbulence interactions in the stable atmospheric boundary layer
Authors: Edwards N. R. , Mobbs S. D.
Published: February 2007

Turbulence measurements of the stable atmospheric boundary layer made at Halley station in Antarctica are presented. the interaction between small‐scale turbulence and larger solitary wave disturbances is investigated. Spectral methods are used to separate the wave and turbulence parts of the flow, and the appropriateness of this approach is discussed. Both individual wave events and an average of 18 wave events are studied. the transport of wave‐induced velocity by wave‐induced turbulence is found to be mostly upwards, whilst a strong downward turbulent heat flux accompanies each wave. Waves are often found to be associated with a burst of low‐level turbulence which occurs later than the perturbation in the mean shear. the effect of the turbulence on the wave is shown to be small, whilst the instantaneous local effect of the wave on the turbulence appears to be approximately linear in the wave amplitude. Comparisons are made between the observations and numerical predictions. These tend to confirm the need for at least second‐order closure schemes for the turbulence modelling.
An observational study of the evening transition boundary‐layer
Authors: Grant A. L. M.
Published: February 2007

Data collected during the evening transition period are described. Well‐defined flux profiles are established rapidly after the surface sensible‐heat flux changes sign. the sensible‐heat flux profile is strongly curved, so that significant cooling only occurs within the lower half of the boundary layer.

The turbulent kinetic energy flux is a significant term in the turbulent kinetic‐energy balance in the upper half of the boundary layer early in the transition period. Later the magnitude of the turbulent kinetic‐energy flux decreases to small values, so that the turbulence in the upper half of the boundary layer begins to decay. the time‐scale of this decay is of the order of u*/h, and should also be the time‐scale for the changes in the boundary‐layer depth.

Local scaling does not appear to be valid, except in a shallow layer near the surface. This is because the turbulence time‐scale is comparable to the adjustment time‐scale of the mean flow over much of the boundary layer.
Seasonal heat and freshwater budgets of the upper ocean in the North‐east Atlantic
Authors: McCulloch M. E. , Leach H.
Published: February 2007

Heat‐ and freshwater‐content changes were calculated from two repeated sections at 48°N, 18°W and 48°N, 26°W, surveyed during the Vivaldi cruise of the research ship RRS Charles Darwin in May 1991. Best estimates of the air‐sea heat‐fluxes agreed within error with the observed heat‐content changes, suggesting that diabatic forcing dominated the heat budget. This was confirmed by a numerical model which showed that Ekman and geostrophic advection and eddy diffusion together contributed less than 20 W m−2 to the heat budget.

However, the model also showed that, near the oceanic polar front under westerly winds, Ekman advection of cold, fresh water southwards over the front can cause ‐40 W m−2 and 4 mm d−1 heat‐ and freshwater‐content changes in the upper 53 m, compared to air‐sea fluxes of typically 67 W m−2 and less than 1 mm d−1. the Ekman advection contributed little to the model's density‐budget as the advective cooling and freshening had opposing effects. However, an air‐sea interaction model showed that the advective surface‐cooling may be reduced because it triggers an extra air‐sea heat flux and the remaining advective freshening then makes an important contribution to the density budget.
The impact of increased resolution on predictability studies with singular vectors
Authors: Buizza R. , Gelaro R. , Molteni F. , Palmer T. N.
Published: February 2007

The dominant singular vectors of the tangent propagator of the European Centre for Medium‐Range Weather Forecast (ECMWF) numerical weather‐prediction model are an essential component of the ECMWF Ensemble Prediction System (EPS). These singular vectors describe the principal finite‐time linear instabilities of the northern extratropical atmospheric circulation.

The impact of increasing the horizontal resolution of the tangent model from T21 to T42 on three different types of initial perturbation, which make use of these singular vectors, is considered. the increase of resolution allows the possibility of describing more accurately instabilities with an upscale cascade of energy from subsynoptic to synoptic scales. Two of the perturbations are referred to as the pseudo‐inverse and sensitivity vectors. These are both diagnostic, and involve estimating from the short‐range forecast error the component of initial error in the unstable subspace. the third type of perturbation is used to construct the set of initial states for the ensemble prediction scheme. Linear and nonlinear integrations are described using these different types of perturbation.

All the results point to the conclusion that the higher‐resolution calculations lead to more accurate results. Specifically, it is shown that significant amounts of energy in the pseudo‐inverse and sensitivity vectors reside in scales smaller than T21, and that the skill of the EPS is improved when generating the initial perturbations using T42 singular vectors. This is found to be especially true in a number of cases when the control forecast was particularly poor.

As a consequence, the predictability of synoptic‐scale disturbances in the atmosphere on time‐scales of a few days is likely to be determined by errors in the initial state on subsynoptic scales.

In addition to these conclusions, it is proved that the analysis of the relationship between the EPS and the pseudo‐inverse perturbations can be used to define the initial amplitude of the EPS perturbations.
Parametrization of momentum transport by convection. I: Theory and cloud modelling results
Authors: Kershaw R. , Gregory D.
Published: February 2007

The theory of the mass‐flux approach to parametrization of convective momentum‐transport is presented. A cloud‐resolving model is used to simulate momentum transport by ensembles of deep convective clouds in two very different regimes: a mid‐latitude cold‐air outbreak, and tropical convection forced by convergence. Idealized, unidirectional, wind‐profiles are used to simplify interpretation of the results. Diagnostics relevant to the parametrization problem are presented, and it is shown that, for both regimes, the approximations inherent in the parametrization equations are reasonable. the pressure gradients inside clouds play an important role in determining horizontal velocities in the clouds. For the cold‐air outbreak with linear shear, the results suggest that these pressure gradients are proportional to the shear and to the up/downdraught mass‐fluxes. Sensitivity studies suggest that the results are not very sensitive to resolution or parametrization of subgrid‐scale processes, giving some confidence that the results are reasonably accurate. For the tropical case with a low‐level jet, the pressure gradients change sign with the wind shear. In part II of this paper, these results will be used to develop and validate a mass‐flux parametrization of convective momentum‐transport which is tested in single column and global versions of the Meteorological Office Unified Model.
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

The 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

An 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

By 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

Non‐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

A 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

Simple 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

The 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

The 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

An 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

The 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

Simulations 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

A 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

Two 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

A 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

From 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.
Secondary frontal waves in the North Atlantic region: A dynamical perspective of current ideas
Authors: Parker D. J.
Published: February 2007

The current understanding of the dynamics of secondary cyclogenesis on frontal systems in the North Atlantic region is reviewed. These secondary cyclones are often poorly forecast, and in extreme cases can have damaging consequences, but because of their small scales and potential for rapid growth detailed observations and analyses are limited: in this paper recent work is described and compared. It is argued that, on the synoptic scale, the development of ‘primary’ baroclinic waves is well understood, but on the subscales of these systems, where the secondaries develop, there is a wider variety of mechanisms which may cause or modify cyclone growth. This variety may be reflected in a wider range of resulting systems.

Various dynamical models have been postulated: secondary frontal waves are seen as the instability of a low‐level potential‐vorticity strip or warm band at a front, with possible finite amplitude triggering by upper‐level features. Latent heating is thought to provide a suitable unstable potential‐vorticity strip but the degree to which the latent heating contributes to the instability itself is a matter of dissent. Secondary systems are often shallow, so boundary‐layer processes are important, but the influence of these is not well understood; laboratory experiments have suggested that the boundary layer will act to suppress instability. In general, frontogenetic strain flow, while acting to intensify the basic‐state front, suppresses wave development—in contrast, recent work suggests that a frontolytic strain flow may be important in enhancing the development of one class of waves.

The FASTEX experiment, an international field study of North Atlantic secondary cyclogensis, has recently attempted to observe these systems in detail. It is hoped that the data which have been obtained will help to resolve some of the outstanding issues in this field.
Effects of observation errors in linear regression and bin‐average analyses
Authors: Tolman H. L.
Published: February 2007

Effects of observation errors in linear regression and bin‐averaged (BA) validation techniques are investigated using the example of marine wind speeds. It is shown that a conventional linear regression systematically underestimates the slope of the regression line, and systematically overestimates the random model error. A BA analysis systematically understimates extreme wind speeds, incorporates spurious nonlinearity, and overestimates random model errors. Correction techniques are suggested for studies in which the observation error can be estimated. Using synthetic data the potential of the correction techniques in illustrated, and it is shown that the above errors are generally not negligible for wind speed validation studies. Practical examplies consider the random errors of anemometers and wind speed estimates from satellites. These examples highlight the importance of the error corrections, and illustrate the difficulty of estimating observation errors. Finally, it is argued that the well‐known symmetric slope regression should not be used for the validation of forecast systems. Although the present study deals with marine wind speeds, its results are expected to be valid for a wide range of validation studies.
A simple theoretical model for the intensification of tropical cyclones and polar lows
Authors: Gray Suzanne L. , Craig George C.
Published: February 2007

A simple theoretical model for the intensification of tropical cyclones and polar lows is developed using a minimal set of physical assumptions. These disturbances are assumed to be balanced systems intensifying through the WISHE (Wind‐Induced Surface Heat Exchange) intensification mechanism, driven by surface fluxes of heat and moisture into an atmosphere which is neutral to moist convection. The equation set is linearized about a resting basic state and solved as an initial‐value problem. A system is predicted to intensify with an exponential perturbation growth rate scaled by the radial gradient of an efficiency parameter which crudely represents the effects of unsaturated processes. The form of this efficiency parameter is assumed to be defined by initial conditions, dependent on the nature of a pre‐existing vortex required to precondition the atmosphere to a state in which the vortex can intensify. Evaluation of the simple model using a primitive‐equation, nonlinear numerical model provides support for the prediction of exponential perturbation growth. Good agreement is found between the simple and numerical models for the sensitivities of the measure growth rate to various parameters, including surface roughness, the rate of transfer of heat and moisture from the ocean surface, and the scale for the growing vortex.
Characteristics of heat and moisture budgets of a mesoscale convective system observed during TOGA‐COARE
Authors: Nuret Mathieu , Chong Michel
Published: February 2007

The large‐scale heat (Q1) and moisture (Q 2) budgets of a tropical convective system occurring in the warm sea surface region of the western Pacific Ocean are analysed over a 36 h period. The system was observed on 26 and 27 November 1992, during the Tropical Ocean/Global Atmosphere (TOGA) Coupled Ocean‐Atmosphere Response Experiment (COARE), and propagated from the north‐eastern part of the Outer Sounding Array (OSA) toward the Intensive Flux Array (IFA) following a south‐westward direction. It was composed of an extensive shield of high cirrus clouds over the OSA, which advected into the IFA. Rawinsonde data collected during these two days, at 6 h intervals, and also surface, satellite and operational forecast‐model data are used to investigate the budgets and the associated precipitation rates over the two contiguous regions containing different convective activity.

Globally, the vertical distributions of the heat source and moisture sink over each domain are found to be in qualitative agreement with those of other regions of the western Pacific. The heating profiles show vertical variations closely consistent with the shape of the observed mean vertical‐velocity profiles, suggesting the importance of latent heating. The mature stage provides a heating peak at the upper level in the 450–550 hPa layer, and a marked double peak drying structure at low (800–850 hPa) and middle (400–600 hPa) levels. The series of Q1 and Q2 profiles obtained every 6 hours over the two domains indicates the progressive increase of stratiform cloud which is associated with an elevation of the heating‐peak level, while the upper drying peak intensifies and the lower drying peak is decreasing. Advection contributes to these changes. Differences are observed between the profiles over both regions; these can be explained by reduced convective activity over the IFA and also by the presence of advected anvil clouds from the OSA, which lead to heating and drying over the IFA half as large as over the OSA. Similarly, the associated maxima are located at lower levels. An interesting feature is the existence of a secondary heating maximum in the high troposphere near 200–300 hPa, which can be explained as a contribution from the advected high‐level stratiform clouds.

The budget‐derived rainfall rates compare well with those deduced from surface and satellite data, but to a lesser degree with model‐forecast results. This comparison highlights the importance of data density in the behaviour of the large‐scale model in representing convection in equatorial regions, as already identified for the present TOGA‐COARE. Although vertical advection of moisture globally balances the quantity of water that condenses and precipitates, the contribution of horizontal moisture advection can be an important component of the moisture budget, to counterbalance the storage of moisture in a column.
The response of a moist zonally symmetric atmosphere to subtropical surface temperature perturbation
Authors: Zheng Xinyu
Published: February 2007

A moist zonally symmetric model has been developed in order to inveestigate the response of a moist atmosphere to subtropical surface temperature perturbations. This is an extension of the study by Plumb and Hou (1992) in which they found that the response of a dry atmosphere to a localized subtropical thermal forcing exhibits threshold behaviour. An inviscid steady‐state theory for zonally symmetric moist atmospheres by Emanuel (1995) is presented first. The model calculations confirm the theory. Then the model is used to examine how moisture‐dynamics feedback influences the threshold behaviour, for a given subtropical surface temperature perturbation. The threshold behaviour is not changed by the inclusion of moisture‐dynamics feedback. Finally, calculations are performed for cases with time‐varying surface temperature perturbations, in order to check the extent to which steady‐state theory can be applied. It is found that the theory is applicable to cases with seasonally varying surface temperature perturbations. Finally some implications of this study are discussed.
The response of bora‐type flow to sea surface temperature
Authors: Enger Leif , Grisogono Branko
Published: February 2007

A non‐linear, two‐dimensional, hydrostatic, incompressible numerical model with a higher‐order turbulence closure scheme is used to study the effect of sea surface temperature on the severe downslope wind called bora at the Adriatic coast. A non‐linear large‐amplitude mountain wave is generated and is broken beneath and within its critical layer, due to resonant tuning between the initially single‐layer atmosphere and the terrain. The tuning is governed by the Froude number. A qualitative and sometimes quantitative analogy exists between the wave‐breaking (unsteady, stratified) flow and the hydraulic jump (steady, two‐layer flow). It is also known that the strongest Adriatic bora appears during the winter season, when the sea surface temperature is typically larger than the ground surface temperature.

Firstly, a relatively higher (lower) sea surface temperature means an additional distortion (moderation) of the mountain wave and consequently a larger (smaller) area with bora wind maxima. For a relatively higher sea surface temperature a propagating hydraulic jump occurs. Typically bora maxima are about three to four times larger than the related geostrophic wind (8 m s−1). Secondly, the presence and importance of the inertial oscillation are indicated. Since the wave‐breaking is the vital component of the strongest bora cases, there is a relatively large, elevated area–i.e. the critical layer–with substantial flow decelerations and generally low wind speeds. The wave‐breaking area has a Rossby number O(1). Hence, the earth's rotation appears to be an important part of bora evolution. The simulations presented consider generalized bora cases which may pertain to other similar orographic flows.
The use of TOVS clear radiances for numerical weather prediction using an updated forward model
Authors: Rizzi Rolando , Matricardi Marco
Published: February 2007

The impact of TOVS clear radiances, processed using a new transmittance database, on the ECMWF forecast system, is evaluated. This involved the production of a line‐by‐line transmittance database, of sets of coefficients for the operational fast radiative‐transfer scheme, and the use of the operational bias‐correction procedure to reduce residual long‐term systematic differences. Two case‐studies are used to examine data from TOVS instruments on three NOAA satellites, and the differences in radiances measured by the various platforms are discussed. Measured and simulated data are compared before and after the operational bias‐removal scheme, and a final impact assessment is made using an assimilation and forecast experiment.