bluuebunny/crossref_metadata_2025 huggingface dataset:
Troposphere to stratosphere transport at low latitudes as studies using HALOE observations of water vapour 1992–1997
Authors: Jackson D. R. , Driscoll S. J. , Highwood E. J. , Harries J. E. , Russell J. M.
Published: February 2007

In this paper we present observations of water vapour (version 18 retrievals) in the equatorial tropopause region made by the Halogen Occulatation Experiment (HALOE) instrument, which files on the Upper Atmosphere Research Satellite. These data make an important new contribution to the observationl knowledge of this region, since water vapour here has largely been observed to date using in situ instruments, and the only global longitudinally‐resolving satellite observations to be reported are those from the Stratospheric Aerosol and Gas Experiment II (SAGE II). Firstly, the HALOE water vapour climatology is presented. Comparison of HALOE and SAGE II climatologies shows considerable agreement. An exception is over Asia in June, July and August (JJA), where the HALOE fields show a distinct region of moist air (associated with convection due to the Asian summer monsoon), whereas no such feature was reported in the SAGE II observations. This suggests that HALOE is sensitive to water vapour variability near the tropopause over regions of strong convection. Secondly, the troposphere to stratosphere transport of air at low latitudes is investigated. Pressure/longitude sections near the equator show that dry air with water vapour mixing ratios of less than 2.4 parts per million by volume (p.p.m.v.) enters the stratosphere in December, January and February, then travels upwards and polewards in the subsequent seasons. A more detailed examination of pressure/longitude sections indicates episodes when dry air also enters the equatorial stratosphere in March, April and May and JJA at times of strong convenction. There is also evidence of entry of air with higher water vapour mixing ratios, of between 3.6 and 4.0 p.p.m.v, into the stratosphere in September, October and November, chiefly at 10°N and 20°N. Further examination of the region of moist air over Asia in JJA (mentioned above) shows that there is clear cross‐isentropic upward transport in the 375–410 K region of unsaturated air. The HALOE water vapour data set reported here provides an important new insight into low latitude processes near the equatorial tropopause.
Data assimilation in the presence of forecast bias
Authors: Dee Dick P. , Da Silva Arlindo M.
Published: February 2007

Statistical‐analysis methods are generally derived under the assumption that forecast errors are strictly random and zero in the mean. If the short‐term forecast, used as the background field in the statistical‐analysis equation, is in fact biased, so will the resulting analysis be biased. The only way to account properly for bias in a statistical analysis is to do so explicitly, by estimating the forecast bias and then correcting the forecast prior to analysis.

We present a rigorous method for estimating forecast bias by means of data assimilation, based on an unbiased subset of the observing system. The result is a sequential bias estimation and correction algorithm, whose implementation involves existing components of operational statistical‐analysis systems. The algorithm is designed to perform on‐line, in the context of suboptimal data‐assimilation methods which are based on approximate information about forecast‐ and observation‐error covariances. The added computational cost of incorporating online bias estimation and correction into an operational system roughly amounts to one additional solution of the statistical‐analysis equation, for a limited number of observations. Off‐line forecast‐bias estimates based on previously produced assimilated‐data sets can be produced as well, using an existing analysis system.

We show that our sequential bias estimation algorithm fits into a broader theoretical framework provided by the separate‐bias estimation approach of estimation theory. In this framework the bias parameters are defined rather generally and can be used to describe systematic model errors and observational bias as well. We illustrate the performance of the algorithm in a simulated data‐assimilation experiment with a one‐dimensional forced dissipative shallow‐water model. A climate error is introduced into the forecast model via topographic forcing. while random errors are generated by stochastic forcing. In this simple setting our algorithm is well able to estimate and correct the forecast bias caused by this systematic error, and the climate error in the assimilated‐data set is virtually eliminated as a result.
Explicit forecasting of supercooled liquid water in winter storms using the MM5 mesoscale model
Authors: Reisner J. , Rasmussen R. M. , Bruintjes R. T.
Published: February 2007

An explicit microphysical parametrization including ice physics was developed for use in the NCAR/Penn State Mesoscale Model Version 5 (MM5). This scheme includes three options of increasing complexity to represent the hydrometeor species. The scheme is evaluated by comparing model simulations with two well observed winter storms that occurred during the Winter Icing and Storms Project. The evaluation focused on the prediction of supercooled liquid water (SLW), which is of particular importance to aircraft icing. The intercomparisons showed that:

The double‐moment microphysical scheme, in which both ice mixing ratios and number concentrations were predicted, performed best, with close agreement to the observed fields.

The single‐moment schemes, in which the mixing ratio of ice species are predicted and number concentration specified, performed reasonably well if a diagnostic equation for No, s, the Y‐intercept of the assumed exponential snow distribution, is allowed to vary with snow mixing ratio.

Accurate microphysical simulations of SLW in shallow upslope clouds and cyclonic storms required accurate simulations of the kinematic and thermodynamic structure and evolution of the storms.

Though the two storms were dynamically different, the SLW formed through a balance of the condensational growth of cloud water and the depletion of cloud water by deposition and riming of snow and/or graupel for both storms. The results of this study suggest that accurate prediction of SLW over limited areas of the country may be possible using the current microphysical parametrization and high‐resolution grids (δχ <10 km).
Climatology of gravity waves in a forest
Authors: Lee Xuhui , Barr Alan G.
Published: February 2007

The objectives of this study are to establish a climatology of gravity waves in a forest and to gain insights into the dynamics of this motion type. The site is part of the BOREAS (Boreal Ecosystem—Atmosphere Study) flux monitoring network. The analysis relies mainly on 5 Hz temperature time series observed with two arrays of fine‐wire thermocouples deployed in the vertical (up to 40 m above the ground) and horizontal (separation up to 150 m) from July to November 1996. Supporting measurements include mean wind speed and direction at two heights above the forest.

Waves of various intensities are found over 40% of the night‐time observations, emphasizing the need to understand its role in transporting momentum and masses between the vegetation and the atmosphere. These waves are shear‐generated. They propagate in the direction of the mean wind at a speed that is most likely to fall between the mean wind speeds at z/h = 1.2 and 1.8, where z is the height above the ground and h is the mean tree height. The median values of wave speed, wavelength, wave vertical displacement, and wave frequency are 1.61 m s−1, 75 m, 10 m and 0.0214 Hz, respectively. The wave motion remains coherent in the horizontal over less than one wavelength but is in general persistent in time.
The tropical tropopause
Authors: Highwood E. J. , Hoskins B. J.
Published: February 2007

The physical meaning of several different tropical tropopause definitions is examined using atmospheric data from a variety of sources, and model output. The conventional lapse‐rate definition of the tropopause appears to have little physical relevance in the tropics, although it is easy to use operationally. A four year ‘climatology’ of the tropical tropopause from the European Centre for Medium‐Range Weather Forecasts (ECMWF) analyses is presented. The zonal mean picture shows the annual cycle in properties that has been related to the extratropical stratospheric wave pump. However, there are large and important zonal asymmetries. These include a relatively low pressure and temperature at the tropopause near the west Pacific heating region during December—January—February, and a striking region with low pressure on the tropopause over India during June—July—August. Results from a baroclinic model with imposed diabatic heating are used to support the hypothesis that both these features can be attributed to the direct response of the atmosphere to a large‐scale region of tropospheric diabatic heating. It is proposed that the stratospheric pump provides the general picture for the upper troposphere/lower stratosphere region, but that tropospheric convection is crucial in determining the important zonal asymmetries in this region.
Case‐studies of developing east Pacific easterly waves
Authors: Raymond Davi D. J. , López‐Carrillo Carlos , Cavazos Lucio López
Published: February 2007

This paper contains a study of developing east Pacific easterly waves observed during the summer of 1991 with aircraft in situ and radar data, and geosynchronous infrared satellite imagery. The circulation and divergence profiles, surface momentum and entropy fluxes, as well as 700 mb and 975 mb saturated and actual equivalent potential temperatures, were measured repeatedly for three disturbances that evolved into east Pacific hurricanes. As the disturbances develop, the level of maximum updraught mass flux decreases, which leads to increased convergence at low levels. This process appears to be coupled to an increase in the mid‐level relative humidity, which in turn suppresses the development of convective downdraughts. Comparison of the respective tendencies of convergence and surface friction to spin up and spin down a developing disturbance suggests that friction is unimportant in the early stages of development. However, as the surface circulation develops, the two effects become comparable in magnitude if surface friction is deposited through a fairly deep layer of the troposphere. The observations appear to rule out boundary‐layer Ekman balance in these developing systems, though more‐accurate observations of the eddy fluxes of absolute vorticity into such systems are needed to answer this question with certainty.
Air‐sea fluxes inferred from an upper ocean heat budget north‐east of the Azores
Authors: McCulloch M. E. , Leach H.
Published: February 2007

The contributions to the seasonal heat budget of the upper 500 m of the North Atlantic between 33° and 60° N and 40° and 10° W are calculated using climatology and wind‐stress data. Near the polar front, advection and diffusion are as important in the seasonal heat budget as the air‐sea heat fluxes. In contrast, along a line extending between the Azores and the British Isles the seasonal air‐sea heat fluxes are balanced primarily by heat‐content changes. Therefore, here, the air‐sea fluxes can be inferred from the heat budget with an error of only 23 W m−2 because the large errors in estimates of advection and diffusion are avoided. The inferred fluxes in this area are compared with three bulk‐formulae estimates.
Momentum fluxes due to three‐dimensional gravity‐waves: Implications for measurements and numerical modelling
Authors: Vosper S. B. , Mobbs S. D.
Published: February 2007

A three‐dimensional wave‐action equation for linear internal gravity‐waves is derived. When the basic‐state flow does not turn with height, this equation can be used to derive the three‐dimensional version of a theorem of Eliassen and Palm. This states that, for steady flows in the absence of dissipation, the vertical flux of both horizontal components of momentum is independent of height. When the basic‐state flow exhibits turning with height, analysis of the wave‐action equation indicates that, in general, the wave field cannot be regarded as steady and the three‐dimensional Eliassen and Palm theorem does not apply. It is also shown that the wave‐action equation has serious implications for the interpretation of atmospheric measurements of the momentum flux: in order to make accurate measurements of the momentum flux arising from three‐dimensional orographically forced gravitywaves, it is necessary to average ρmu'w' (where ρm is the basic‐state density and u' and w' are the horizontal and vertical wave‐induced velocity‐perturbations respectively) on horizontal planes downstream of the orography, rather than along lines in the downstream direction (as has previously been done with aircraft measurements). Using a three‐dimensional linear numerical model, we demonstrate the importance of this in even the most simple idealized cases.
Extremal principle of entropy production in the climate system
Authors: Pujol T. , Llebot J. E.
Published: February 2007

Paltridge found reasonable values for the most significant climatic variables through maximizing the material transport part of entropy production by using a simple box model. Here, we analyse Paltridge's box model to obtain the energy and the entropy balance equations separately. Derived expressions for global entropy production, which is a function of the radiation field, and even its material transport component, are shown to be different from those used by Paltridge. Plausible climatic states are found at extrema of these parameters. Feasible results are also obtained by minimizing the radiation part of entropy production, in agreement with one of Planck's results. Finally, globally averaged values of the entropy flux of radiation and material entropy production are obtained for two dynamical extreme cases: an earth with uniform temperature, and an earth in radiative equilibrium at each latitudinal point.
Distribution of precipitation and wet deposition around an Island mountain in South‐West Poland
Authors: Blaś M. , Dore A. J. , Sobik M.
Published: February 2007

A comparison has been made between measurements of precipitation intensity and chemical composition and a three‐dimensional model of airflow and orographic rainfall enhancement. the ‘seeder‐feeder’ effect is shown to be of importance in rainfall distribution, wet deposition of pollutants and the modification of the chemical composition of rainfall over an isolated island mountain. the results show increases in rainfall of between 30% and 83% over the hill, with the smallest increase corresponding to the presence of a strong inversion layer. the peak rainfall enhancement occurred between 0.5 and 2 km downstream of the summit.

The measured concentrations of major ions in the feeder‐cloud droplets were greater than those in seeder rain by factors of between two and five. the scavenging of polluted cap cloud by rain drops resulted in an increase in wet‐deposition rate by factors of between three and five, with the peak deposition occurring approximately 2 km downstream of the hill summit.
A model study of the atmospheric boundary layer in the Mars pathfinder lander conditions
Authors: Savijarvi Hannu
Published: February 2007

The diurnal cycle in the Mars Pathfinder sol 3‐6 observed temperatures is compared with results from a one‐dimensional high‐resolution atmospheric boundary‐layer model for Mars. the agreement is in general quite good, the model catching the observed cycle well with realistic regolith (soil) parameter values. the model processes driving the temperature changes, such as the surface radiation and heat budgets, and the atmospheric radiative and turbulent heating rates, are presented and discussed.

The model includes moisture, and its hydrologic cycle is described. It indicates a dryish soil and a quite dry daytime well‐mixed boundary layer up to 5 km with specific‐humidity values around 0.18 g kg−1. the relative humidity increases strongly in the evening and saturation is reached by 04 local time near the surface. the very thin radiation fog thus formed is dissolved soon after sunrise. There is weak frost formation at night and sublimation during daytime.
Maintenance of the African easterly jet
Authors: Thorncroft C. D. , Blackburn M.
Published: February 2007

The maintenance of the African easterly jet (AEJ) has been examined using a zonally symmetric general circulation model with simple parametrizations. It is shown that the AEJ is maintained in association with two diabatically forced meridional circulations: one associated with surface fluxes and dry convection in the Saharan heat‐low region and one associated with deep moist convection in the intertropical convergence zone equatorward of this. the heat‐low heating, which reaches the height of the AEJ around 700 mb, is particularly important in maintaining the AEJ and its associated meridional gradients in potential vorticity. It is concluded that the mean observed AEJ results from a combination of the diabatically forced meridional circulations which maintain it and easterly waves which weaken it.
On the numerical modelling of middle atmosphere tides
Authors: Zhu Xun , Yee Jeng‐Hwa , Strobel Darrell F. , Wang Xueliang , Greenwald Raymond A.
Published: February 2007

A linear, spectral, tidal model for middle atmosphere thermal tides has been developed. the spectral model is based on horizontal vorticity and divergence equations, and the vertical structure equation for the geopotential is solved with appropriate boundary conditions. Such an approach yields a stable temperature field and a consistent velocity field for the diurnal tide around the apparent singularity at latitudes of ±30°. Thermal forcing consists of solar near‐infrared heating by water vapour in the troposphere based on an accurate and efficient algorithm and the solar radiation heating by ozone and molecular oxygen in the stratosphere, mesosphere and lower thermosphere. the calculated diurnal and semidiurnal temperatures are in good agreement with the recent temperature measurements derived from the National Aeronautics and Space Administration Upper Atmosphere Research Satellite (UARS) and Light Detection and Ranging around the midlatitude and high‐latitude stratopause where ozone heating is a major source of tidal variability. Some discrepancies between the current model output and the wind measurements derived from the High‐Resolution Doppler Imager and Wind Imaging Interferometer on board UARS still exist. A sensitivity study shows the need for improved modelling of tropospheric heating rates and parametrization of eddy diffusion coefficients in the upper mesosphere in order to reduce these discrepancies.
Release of potential shearing instability in warm frontal zones
Authors: Chapman D. , Browning K. A.
Published: February 2007

Potential shearing instability (PSHI) exists in the atmosphere when the Richardson number in a stably stratified shear layer would be less than 1/4 if the layer were lifted to saturation. Sometimes the air entering a warm frontal zone near the position of the surface warm front is initially unsaturated, its Richardson number is greater than 1/4 but PSHI exists over a deep layer. This paper presents a study of a warm frontal zone using observations from the high‐resolution Doppler radar at Chilbolton, supported by radiosondes and output from the mesoscale version of the operational United Kingdom Meteorological Office Unified Model. Very large‐amplitude Kelvin‐Helmholtz billows were observed to form where the PSHI was realised following the onset of saturation. the billows were probably intense enough to be a hazard for air traffic and an important source of momentum transfer. Rough estimates from the radar of vertical momentum fluxes due to the billows are shown to be broadly consistent with measurements by the same radar of the change in velocity profile across the frontal zone from start to end of the billow train. the present version of the Unified Model does not parametrize vertical mixing due to shearing instability in the free atmosphere; this may be an important omission.
The representation of soil moisture freezing and its impact on the stable boundary layer
Authors: Viterbo Pedro , Beljaars Anton , Mahfouf Jean‐François , Teixeira João
Published: February 2007

The 1993 to 1996 version of the European Centre for Medium‐Range Weather Forecasts model had a pronounced near‐surface cold bias in winter over continental areas. the problem is illustrated in detail with help of tower observations. It is shown that a positive feedback exists in the land surface boundary‐layer coupling that has the potential to amplify model biases. If the surface is cooled too much the boundary layer becomes too stable, reducing the downward heat flux and making the surface even colder. This positive feedback is believed to be stronger in the model than in the real atmosphere, resulting in diurnal temperature cycles that are too large and in excessive soil cooling on a seasonal time‐scale in winter.

An important contributor to the excessive winter cooling turns out to be the lack of soil moisture freezing in the model. the importance of this process is obvious from soil temperature observations. the seasonal soil temperature evolution shows a clear ‘barrier’ at 0°C due to the thermal inertia of freezing and thawing. A more quantitative illustration is the result of a simple calculation. This shows that the amount of energy necessary to freeze/thaw 1 m3 of wet soil, would cool/warm this soil by about 50 K if the phase transition was not taken into account.

To reduce the winter cold bias in the model, three model changes have been tested and are described: (i) the introduction of the process of soil moisture freezing; (ii) revised stability functions to increase the turbulent diffusion of heat in stable situations; and (iii) an increase of the skin‐layer conductivity. the effect of these changes on the seasonal evolution of soil and 2 m temperatures is investigated with long runs that have terms that relax towards the operational analysis above the boundary layer. In this way the impact can be studied on the temperature forecasts for the winter of 1995/1996, during which the operational model showed considerable soil temperature drift over Europe. Also, short periods of data assimilation (including 10‐day forecasts) have been carried out to study the diurnal time‐scales and the impact on model performance.

The model changes eliminate to a large extent the systematic 2 m temperature biases for the winter of 1995/1996 over Europe and make the soil temperature evolution much more realistic. the soil moisture freezing, in particular, plays a crucial role by introducing thermal inertia near the freezing point, thereby reducing the annual temperature cycle in the soil. the process of soil moisture freezing leads to a considerable warming of the model's near‐surface winter climate over continental areas.
Skill and relative economic value of the ECMWF ensemble prediction system
Authors: Richardson D. S.
Published: February 2007

The economic value of the European Centre for Medium‐Range Weather Forecasts (ECMWF) operational ensemble prediction system (EPS) is assessed relative to the value of a perfect deterministic forecast. The EPS has substantial relative value throughout the medium range. Probability forecasts derived from the EPS are of greater benefit than a deterministic forecast produced by the same model. Indeed, for many users, the probability forecasts have more value than a shorter‐range deterministic forecast. Based on the measures used here, the additional information in the EPS (reflecting the uncertainty in the initial conditions) provides a benefit to users equivalent to many years' development of the forecast model and assimilation system.

The impact of ensemble size on forecast value is considered. The difference in performance between ensembles with 10 and with 50 members may appear relatively small, based on standard skill measures, yet the larger ensembles have substantial benefit to a range of users. Further increases in ensemble size may be expected to provide additional value.
On the representation of gravity waves in numerical models of the shallow‐water equations
Authors: Mohebalhojeh A. R. , Dritschel D. G.
Published: February 2007

Gravity waves, or imbalanced motions, that develop during the evolution of vortical flows in numerical models of the shallow water (SW) equations are examined in detail. The focus here is on nearly‐balanced flows, with small but non‐zero gravity‐wave activity. For properly initialized flows, it is reasonable to expect small GW activity when Froude numbers Fr < 1 and Rossby numbers Ro ≲ 1.

The guiding principle in the present study is that an accurate representation of potential vorticity (PV) is the pre‐requisite to a fair assessment of the generation of gravity waves. The contour‐advective semi‐Lagrangian (CASL) algorithm for the SW equations is applied, as it shows a remarkable improvement in the simulation of PV. However, it is shown that the standard CASL algorithm for SW leads to a noticeable numerical generation of gravity waves. The false generation of gravity waves can equivalently be thought of as the false, or numerical, breakdown of balance.

In order to understand the maintenance of balance in the SW equations, a hierarchy of CASL algorithms is introduced. The main idea behind the new hierarchy is to implement PV inversion partially, balancing algorithms directly within the SW algorithm, while still permitting imbalanced motions. The results of the first three members of the hierarchy, CA0 (standard CASL), CA1, and CA2, are described and are compared with the results of two other SW algorithms, a pseudo‐spectral and a semi‐Lagrangian one. The main body of results is obtained for a highly ageostrophic regime of flow, with|Ro|max ∼ 1 and Frmax ∼ 0.5, where sub‐index 'max' denotes maximum over the domain. Other flow regimes in the relevant parts of the Ro‐Fr parameter space are also explored. It is found that, for a given resolution and Froude number, there is an optimal CASL algorithm, i.e. one which gives rise to the least numerical generation of gravity waves.
The parametrization of drag induced by stratified flow over anisotropic orography
Authors: Scinocca J. F. , McFarlane N. A.
Published: February 2007

A new parametrization of drag arising from the flow over unresolved topography (UT) in a general‐circulation model (GCM) is presented. It is comprised of three principle components: a parametrization of the source spectrum and drag associated with freely propagating hydrostatic gravity waves in the absence of rotation, a parametrization of the drag associated with low‐level wave breaking, and a parametrization of low‐level drag associated with upstream blocking and lee‐vortex dynamics. Novel features of the scheme include: a new procedure for defining the UT in each GCM grid cell which takes account of the GCM resolution and includes only the scales represented by the parametrization scheme, a new method of representing the azimuthal distribution of vertical momentum flux by two waves whose direction and magnitude systematically vary with the flow direction and with the anisotropy of the UT in each GCM grid cell, and a new application of form drag in the lowest levels which can change the direction of the low‐level flow so that it is more parallel to unresolved two‐dimensional topographic ridges.

The new scheme is tested in the Canadian Centre for Climate Modelling and Analysis third generation atmospheric GCM at horizontal resolutions of T47 and T63. Five‐year seasonal means of present‐day climate show that the new scheme improves mean sea level pressures (or mass distribution) and improves the tropospheric circulation when compared with the gravity‐wave drag scheme used currently in the GCM. The benefits are most pronounced during northern hemisphere winter. It is also found that representing the azimuthal distribution of the momentum flux of the freely propagating gravity‐wave field with two waves rather than just one allows 30‐50% more gravity‐wave momentum flux up into the middle atmosphere, depending on the season. The additional momentum flux into the middle atmosphere is expected to have a beneficial impact on GCMs that employ a more realistic representation of the stratosphere.
On the dynamics of hurricane‐like vortices in vertical‐shear flows
Authors: Smith Roger K. , Ulrich Wolfgang , Sneddon Graeme
Published: February 2007

A simple two‐layer analogue model is used to elucidate aspects of vortex motion in a vertically sheared zonal flow. The model is based on the idea that a vortex can be considered as the sum of a pair of barotropic vortices, one whose vorticity, or potential vorticity, resides in the upper layer and the other whose vorticity resides in the lower layer. Each vortex has an associated tangential velocity distribution in the other layer, which advects the vortex in that layer. The strength of this velocity distribution is characterized by a coupling parameter, which, in the case of quasi‐geostrophic vortices, is related to the Rossby depth scale. Besides their mutual advection, the component vortices are differentially advected by vertical shear. The model leads to a set of coupled ordinary differential equations for the motion of each component vortex, which may be solved analytically in certain circumstances. The calculations indicate two types of flow behaviour according to the strength of the component vortices, the degree of vertical coupling and the strength of the shear. For weak shear and/or strong vortices and strong coupling, the vortices rotate around each other as their mean centre translates with a fraction of the mean zonal flow. For Strong shear and/or weak vortices and weak coupling, the vortices undergo a partial rotation while they are in proximity, but become progressively separated by the shear. The calculations are an aid to understanding the range of behaviour of vortices in shear in numerical calculations by other authors, and it is reasonable to presume that the processes represented by the model are fundamental processes in tropical cyclones also.

The analogue model is evaluated in the context of quasi‐geostrophic theory, where the breakdown into the component vortices can be accomplished and where the complete problem can be solved numerically without approximation. The results of the quasi‐geostrophic model are contrasted with those of other recent studies of baroclinic vortices in the absence of vertical shear.
A general, linearized vertical structure equation for the vertical velocity: Properties, scalings and special cases
Authors: Inverarity G. W. , Shutts G. J.
Published: February 2007

A general, linear vertical structure equation for the vertical velocity component, including explicit forcing terms in the momentum, thermodynamic and continuity equations, is derived for horizontally‐homogeneous flows. The basic flow is assumed to depend on height alone and is in geostrophic and hydrostatic balance. Scale analysis is used to show that this equation incorporates a variety of familiar special cases including the lee‐wave equation, Eady's equation and the quasi‐geostrophic omega equation, the different flow regimes being identified in terms of the Rossby, Froude and Richardson numbers. Using the vertical structure equation, a wave‐stress conservation principle is derived that is valid for basic flows whose magnitude and direction vary with height. In addition to providing some unification to the many flavours of vertical velocity equation in the literature, this derivation was motivated by the need to provide a starting point for a wide class of analytical problems in the study of baroclinic instability and inertia‐gravity wave dynamics.
The dynamical consequences for tropopause folding of PV anomalies induced by surface frontal collapse
Authors: Ziemiański M. Z. , Thorpe A. J.
Published: February 2007

It has been noted previously that during frontal collapse dynamical processes lead to the formation of potential vorticity (PV) anomalies in the vicinity of the surface front. These processes can either be associated with diffusion in the presence of the tight temperature gradients or with intrusion into the atmosphere of the vanishingly thin layer adjacent to the surface. This paper explores the dynamical consequences of these PV anomalies on the parent baroclinic wave cyclone. The method used is to find the vertical motion attributable to these anomalies. The location and magnitude of this vertical motion is clearly key to the dynamical influence of the anomalies. As we are dealing with a three‐dimensional evolving cyclone with an emergent tropopause fold, the calculation of vertical motion needs to be capable of accounting for the role of the highly deformed tropopause. Hence we develop and use a nonlinear balance model approach. The result shows that the PV anomalies near the surface front induce downward vertical motion at the tropopause fold, and thus they act to amplify this feature. This role of diffusion associated with surface frontal collapse is the key finding of this paper. An analysis is made of the role of upper‐ and lower‐level PV anomalies on the overall role of the wave development.
The formation of vertical Vortices in the convective boundary layer
Authors: Kanak Katharine M. , Lilly Douglas K. , Snow John T.
Published: February 2007

A study of the dynamical formation mechanisms of atmospheric boundary‐layer vortices is being conducted. These vortices are typically manifest as dust devils although there is evidence that they exist with some frequency in the absence of visible flow tracers. For example, in 1997 MacPherson and Belts pointed out instrument observations of invisible boundary‐layer vertical vortices over the boreal forest. Several possible mechanisms for vortex formation under a variety of different dynamical regimes are described. Most observational investigators have reported that dust devils form in environments characterized by low wind speeds. The most intriguing unknown is the source of vorticity for the formation of vortices for convection in the absence (or near absence) of mean winds. Vertical vortex formation in convection without a mean shear has not often been documented in laboratory or numerical simulations.

A Large‐Eddy Simulation of the convective boundary layer is performed, without mean winds, with the purpose of examining vertical vortex‐format ion mechanisms. The current work emphasizes the conjectured larger (convective) scale vorticity‐generating mechanisms. The results indicate that vertical vortices form at some, but not all of the vertices of the simulated pattern of open convective cells. The columnar vortices may be essentially vertical or tilt with height, and may or may not extend to the surface. It is suggested that the vertical vorticity initially available to the vortices comes from the tilting of horizontal vorticity associated with local unidirectional shears due to the convective‐cell circulations, with convective‐cell asymmetries being important to the process. Subsequent evolution apparently results in the co‐location of vertical vorticity centres with local updraught maxima.
Some impact of pollutants on the development and optical properties of stratocumulus clouds
Authors: Ghosh S. , Jonas P. R. , Wood R.
Published: February 2007

Two major recent field programmes—the European Cloud Radiation Experiment (EUCREX) and the Aerosol Characterization Experiment II (ACE‐2)‐have extensively analysed the dynamical, microphysical and radiative attributes of stratocumulus clouds contaminated by continental air. Although an extensive set of dynamical and microphysical data are now available, there are no accounts of any matching theoretical modelling studies. To fully understand and numerically model the interplay between the dynamics, microphysics, radiative and chemical properties of the two clouds chosen for our case‐studies would require a full three‐dimensional large‐eddy simulation (LES) model coupled to a full‐size resolving microphysical model where the computational costs would be prohibitive. In this study we have ‘optimized’ the classic Kessler parametrization scheme so that it is effectively able to distinguish between clean and contaminated clouds. We perform LES runs with the optimized scheme to study the morphology and the dynamics of the clouds and then use a second one‐dimensional microphysical parcel model run with identical environmental conditions to study the effects of pollution on the clouds as well as the droplet spectral evolution. This procedure yields extremely good agreement with observations at modest computational expense. It is shown that nitric acid (HNO3) vapour in the parts per billion by volume (p.p.b.v.) range affects cloud formation by increasing the number of cloud droplets and decreasing the mean size compared to an acid‐free simulation. The effects of HNO3 contamination on the EUCREX case‐study is evident owing to the proximity of this cloud to sources of air pollutants. With 10 p.p.b.v. of HNO3, we are able to achieve good agreement with the observations of the droplet effective radii as well as with observations of the optical‐depth variation. For the ACE‐2 cloud which formed further away from sources of pollutants, even on a typical ‘polluted’ day when the ambient HNO3 was ∼5 parts per trillion by volume, the drop concentration was found to be insensitive to changes in the HNO3.
Deriving cloud overlap statistics from radar
Authors: Hogan Robin J. , Illingworth Anthony J.
Published: February 2007

The predictions of general‐circulation models (GCMs) are sensitive to the assumed cloud overlap within a vertical column of model grid boxes, but until now no reliable observations of the degree of cloud overlap have been available. In this note we derive the overlap characteristics of clouds from 71 days of high vertical resolution 94 GHz cloud radar data in the UK. It is found that, contrary to the assumption made in most models, vertically continuous clouds tend not to be maximally overlapped. Rather, the overlap of clouds at two levels tends to fall rapidly as their vertical separation is increased, and for levels more than 4 km apart, overlap is essentially random. A simple inverse‐exponential expression for the degree of overlap as a function of level separation is proposed that could, once results become available from a variety of other locations and seasons, be implemented in current GCMs with relatively little difficulty.
The UCD Advanced Canopy‐Atmosphere‐Soil Algorithm: Comparisons with observations from different climate and vegetation regimes
Authors: Pyles R. David , Weare Bryan C. , Pawu Kyaw Tha
Published: February 2007

The University of California, Davis (UCD), Advanced Canopy‐Atmosphere‐Soil Algorithm (ACASA) is presented and its output is compared with a comprehensive set of observations at six diverse sites. ACASA is a multi‐layer canopy‐surface‐layer model that solves the steady‐slate Reynolds‐averaged fluid flow equations to the third‐order. These equations include an explicit representation of the steady‐state, horizontally homogeneous, diabatic set of vector and scalar fluxes and flux transports. ACASA includes a fourth‐order, near‐exact technique to calculate leaf, stem, and soil surface temperatures and surface energy fluxes at various levels within the canopy. Plan! physiological response to micro‐environmental conditions is also included using Ball‐Berry/von Caemmerer‐Farquhar formulations. Observed energy fluxes and microenvironmental conditions from a grass field in the Netherlands, deciduous and coniferous forests in Canada, tropical pasture and forest in Brazil, and an ancient temperate rainforest in the USA are compared with simulated values.

Results indicate that simulated and observed estimates of monthly to annual means of all surface fluxes agree within 95% confidence thresholds for all six sites. Observed and simulated hourly estimates of net radiation are also in excellent agreement for all sites considered. Observed and simulated hourly sensible‐ and latent‐heat flux estimates are in very good statistical agreement in most cases. Differences that exist between ACASA and observed sensible‐and latent‐heat flux estimates are of the same magnitudes as observational uncertainties. Estimates of observed and simulated hourly values of canopy and ground heal storage are within 95% statistical confidence limits of agreement with observations in most cases. Simulated and measured values of daytime intra‐canopy mean wind speed, temperature, and specific humidity agree with 95% confidence within both a tropical and temperate rainforest at all levels. Results also indicate that, in general, ACASA produces flux estimates closer to observations with significantly less scatter than does the Biosphere‐Atmosphere Transfer Scheme. Sensitivity tests show that reducing the vertical resolution, linearizing surface temperature calculations, and/or simplifying the treatment of surface‐layer turbulence each altered mean sensible‐ and latent‐heat flux estimates by amounts that are statistically significant in many cases. Results show that simplifying the model alters flux predictions in manners not simply related to vegetation character, and that using ACASA at its full complexity for all vegetation regimes is warranted. Increasing the vertical resolution beyond 20 layers improved flux predictions at tropical locations but had little impact elsewhere.
The numerical stability of a parametrization of convective momentum transport
Authors: Kershaw R. , Grant A. L. M. , Derbyshire S. H. , Cusack S.
Published: February 2007

The Gregory el at. parametrization of momentum transport by convection is shown to be more prone to numerical instability than other parts of the convection scheme, because of an additional element of numerical diffusion. The limiting Courant number for linear stability is (I + c)−1, where c is the parameter which controls the dependence of the pressure gradient on environment shear. Some revised numerical treatments cure the instability problem but reduce forecast skill, apparently because the diffusive element of the original formulation is beneficial, even though it is artificial. A possible solution to the problem is to use an implicit treatment of the original formulation which is absolutely stable.
Internal wave reflection in uniform shear
Authors: Sutherland B. R.
Published: February 2007

If nonhydrostatic internal waves are of sufficiently large amplitude, they undergo significant dispersion due to interactions between the waves and wave‐induced mean flow. The effect of these interactions is investigated for internal waves propagating upward in a uniformly stratified Boussinesq flow with uniform shear. The sign of the shear is established so that the wave intrinsic frequency increases as the wave packet propagates upward; hence linear theory predicts that the waves should reflect at some level. Fully nonlinear numerical simulations of two‐dimensional wave packets are performed to study the wave‐packet evolution as a function of the initial amplitude and spatial extent of the wave packet. It is shown that if the waves are horizontally periodic, and of sufficiently large amplitude, momentum is permanently deposited to the mean flow at altitudes near, but below, the reflecting level predicted by linear theory. If the waves are horizontally compact, the waves propagate upward well above the reflection level.
The statistical structure of forecast errors and its representation in The Met. Office Global 3‐D Variational Data Assimilation Scheme
Authors: Ingleby N. Bruce
Published: February 2007

Previous studies and different methods of estimating short‐range forecast errors are summarized. Zonally and temporally averaged statistics based on differences of one‐ and two‐day forecasts valid at the same time are presented and an attempt is made to explain many of the features by reference to dynamical concepts.

Vertical correlation length‐scale tends to increase with horizontal correlation scale but to be very short in the Tropics; horizontal scale is longest in the Tropics and in the stratosphere. The variations in vertical correlation are much more pronounced for largely balanced variables such as rotational wind and temperature than they are for divergent wind or humidity. The extratropics are dominated by an equivalent barotropic mode with the level of the maximum wind amplitude (and the zero crossing of the temperature correlation) being determined by the tropopause. Surface pressure is negatively correlated with low‐level temperature as expected (except over the Antarctic plateau where it is positively correlated); it is also negatively correlated with temperatures near/above the tropopause in the extratropics.

The covariance model used in The Met. Office Global Three‐Dimensional Variational (3D‐Var) Data Assimilation system represents the variation of vertical covariances with latitude reasonably well, but the longer horizontal scales in the stratosphere are not currently reproduced. The implied covariances used operationally have been modified so that the correlation length‐scales, both horizontal and vertical, are somewhat shorter than those direct from the forecast differences. Recent changes to the representation are briefly described, with an indication of their impact on the forecasts. The impacts are significant relative to other changes tested, and the covariance model has played a major role in the successful implementation and subsequent improvement of our 3D‐Var system.
A physical basis for a maximum of thermodynamic dissipation of the climate system
Authors: Paltridge Garth W.
Published: February 2007

A mechanism is proposed by which the energy flow through a turbulent medium might be constrained to maximize its dissipation or (equivalently) its thermodynamic efficiency. The mechanism may provide a physical basis for the various findings over the years that the earth‐atmosphere system has adopted a format which maximizes its overall rate of entropy production. The qualitative picture is of a system which, because of the asymmetry of its turbulent fluctuations about the locus of possible steady states determined by energy balance, moves to a preferred steady state and therefore to a preferred turbulent transfer coefficient.
Ammonia dry deposition over grassland‐micrometeorological flux‐gradient measurements and bidirectional flux calculations using an inferential model
Authors: Spindler G. , Teichmann U. , Sutton M. A.
Published: February 2007

For a 17‐day period in late summer 1995 ammonia (NH3) fluxes over an extensive area of semi‐natural ungrazed short grassland were measured and modelled. The measurements were carried out at the Melpitz field research station near Torgau in the state of Saxony in eastern Germany (86 m above sea level, 51°31′N, 12°56′E). For the calculation of the fluxes the micrometeorological flux‐gradient technique was applied using a sensitive continuous‐flow wet annular denuder system for NH3at three heights. Both downward fluxes to the canopy and upward fluxes from the grassland to the atmosphere were observed. On the basis of the direction of the flux in relation to air concentrations, the ammonia canopy compensation point was estimated to vary within the range 0.2 to 3.0 μg m−3. Because the simple ‘canopy/surface resistance’ (Rc) model presupposes a surface concentration of zero for the observed trace gas, which cannot explain bidirectional fluxes, the results of the measurements were compared with a static ‘canopy compensation point‐cuticular resistance’ (Xc–Rw) model, which is able to quantify the stomatal compensation point (Xs) and allows bidirectional fluxes. The agreement between the model and the measured fluxes for the whole period was relatively good. Given realistic ratios of ammonium to proton concentrations (T = [NH4+]/[H+]) for the intercellular fluid of between 150 and 1000 the minimum in the difference was found for Γ ≈︁ 1000 (correlation coefficient r2= 0.23) and even bidirectional fluxes were obtained. Possible reasons for the differences between the measurements and model estimates are discussed.