bluuebunny/crossref_metadata_2025 huggingface dataset:
Airborne studies of the altitudinal variability of the microphysical structure of small, ice‐free, montanan cumulus clouds
Authors: Blyth Alan M. , Latham John
Published: February 2007

The University of Wyoming's instrumented King Air aeroplane made special multiple‐altitude passes through seven non‐precipitating, essentially ice‐free summertime cumulus clouds, as part of CCOPE, conducted in Montana in 1981.

The clouds were found to possess substantial microphysical variability on all detectable scales (down to 2 m) at all levels and distances from cloud boundaries. This was attributed to entrainment of environmental air, and evidence was found for a two‐stage process of turbulent mixing of cloudy and entrained air. Evaporation of the cloud drops was found to occur in a manner consistent with the ‘inhomogeneous mixing’ theory.

Generally the clouds were negatively buoyant and more turbulent in the upper regions and in near equilibrium with the environment near cloud base. the normalized liquid water content was found to be roughly independent of altitude.

Four of the clouds studied were found to contain embryonic raindrops (diameter 40 to 190 μm) in concentrations around 10 to 10001−1. Calculations and analysis showed that these particles could not have been formed in adiabatic clouds of the same base‐temperatures and depths, nor could they have been formed on ‘giant nuclei’. Their presence was shown to be explicable in terms of the ‘extreme inhomogeneous mixing’ model of accelerated growth. This mechanism also appears to offer an explanation for the small concentrations (≈0.1 m−3) of millimetric‐sized drops, shown in dual‐polarization studies often to be initiatory to substantial cloud development.
Variational inversion of simulated TOVS radiances using the adjoint technique
Authors: Thepaut Jean‐Noël , Moll Patrick
Published: February 2007

The adjoint technique allows the variational approach for assimilating various types of observations in meteorology, including raw radiances measured by satellite, to be treated exactly and at a ‘reasonable’ cost.

We have developed the tangent linear and adjoint operators of the International TOVS Processing Package radiative transfer equation. These two tools are used for treating two aspects of variational inversion of satellite sounding radiances.

First of all, we apply this technique to the computation of the error covariance matrix of the retrieval/analysis of simulated TOVS sounding radiances. Using this matrix, an original method to interpret satellite sounding radiances in terms of temperature and humidity information is then presented. We can thus evaluate the number of independent parameters which are significantly estimated from radiances, given a direct radiative transfer model, observation and background error statistics. It is found that 19 HIRS channels provide six independent pieces of information while four MSU channels provide three independent pieces of information. When grouping HIRS and MSU channels, only seven independent pieces of information are available. It is shown that in cloudy conditions MSU channels bring additional significant information to HIRS channels especially at the surface. We also study the impact of cloudiness on the quality of the retrieved profiles.

Numerical feasibility of 1‐D variational inversion using the adjoint of the International TOVS Processing Package radiative transfer model is demonstrated. Impact of the choice of the inner product on the efficiency of the method is studied. We then discuss the quality of the inversion, which turns out to be strongly related to the quality of the atmospheric information present in the sounding radiances and to the background and observation error statistics. the gain brought by MSU channels at the surface in cloudy conditions is discussed.
The temporal distribution of amazonian rainfall and its implications for forest interception
Authors: Lloyd C. R.
Published: February 2007

A model for the temporal distribution of rainfall is derived from measurements made in central Amazonian rainforest. A contrast is made with a rainfall distribution model which has been used in a stand‐alone comparison of land‐surface sub‐models of two General Circulation Models (GCMs). In terms of the rainfall interception loss, it is shown that the central Amazonian model produces an overestimate of 35%, but is within the error bars of the measured interception loss, whereas the latter model overestimates the interception loss by more than 100%. Two other time distributions of rainfall from contrasting regions of the Amazon are derived from published data in order to investigate the possible interception variation due to rainfall pattern variability. the results illustrate the need for GCM modellers to monitor their synthesized rainfall regime against real data and to be aware of the sensitivity of the forest interception loss to rainfall distribution.
Application of the semi‐Lagrangian method to a multilevel spectral primitive‐equations model
Authors: Ritchie Harold
Published: February 2007

It has previously been shown that semi‐Lagrangian schemes can be applied to spectral models of the shallow‐water equations using large time‐steps (e.g., see Ritchie 1988). The present study considers the extension of this work to a multilevel spectral primitive‐equations model.

As a first step, an Eulerian vorticity‐divergence spectral model is converted to an Eulerian model based on a vector momentum form of the equation of motion. From the latter, several semi‐Lagrangian models are prepared: one using an interpolating semi‐Lagrangian treatment of advection in the horizontal (referred to as 2DISL) while retaining Eulerian advection in the vertical, another using a 3‐dimensional interpolating semi‐Lagrangian formulation (referred to as 3DISL), and another which combines the 2DISL scheme in the horizontal and a non‐interpolating semi‐Lagrangian treatment in the vertical (referred to as NISLV). Medium‐range intercomparison experiments are performed using models that include simple physical parametrizations. It is shown that the semi‐Lagrangian semi‐implicit approach can be applied accurately and stably to produce medium‐range (5‐day) forecasts using time‐steps that are far larger than those permitted by the Courant‐Friedrichs‐Lewy (CFL) stability criterion for the corresponding Eulerian model.

The NISLV version is found to be more accurate than the 3DISL one which apparently has excessive damping in the vicinity of the tropopause, where all the model fields change abruptly in the vertical.
A three‐dimensional generalization of Eliassen's balanced vortex equations derived from Hamilton's principle
Authors: Craig George C.
Published: February 2007

A new set of equations is derived for nearly circular flow in gradient balance. For precisely axisymmetric motion, the system reduces to the well‐known balanced vortex equations of Eliassen. The derivation is based on the assumption that the radial component of velocity is small in comparison to the azimuthal component. By applying this approximation to Hamilton's principle for a continuum of fluid parcels, while preserving the time and particle‐labelling symmetries of the primitive equations, it is ensured that the resulting system possesses conservation laws for energy and potential vorticity. In potential radius coordinates, the Lagrangian equations of motion take the form of the geostrophic and hydrostatic balance conditions. The system is also presented in Eulerian form, and a practical integration scheme, based on a linear elliptic equation for geopotential tendency, is described. Finally, the set is rederived by a conventional scale analysis in order to determine constraints on the diabatic forcings which are required for consistency with the approximation.
Pressure drag and momentum fluxes due to the Alps. II: Representation in large‐scale atmospheric models
Authors: Clark Terry L. , Miller Martin J.
Published: February 2007

A three‐dimensional numerical model with interactive grid nesting is used to study the pressure drag and associated momentum fluxes due to the Alpine complex (for a specified upstream wind and static stability profile). Four basic questions are addressed which are relevant to the problems of flow description in the vicinity of mountains and the impact of orography in the context of mesoscale numerical weather prediction and general circulation models:

What is the dependence of basic quantities such as the pressure drag on the model's horizontal resolution?

Is there a need for non‐hydrostatic dynamics to account properly for the flow characteristics near steep mountains?

Is there an effective role for enhanced orography such as an envelope formulation?

Might a ‘gravity‐wave‐drag’ parametrization scheme be able to represent adequately the highest resolution fluxes and surface drag in a much coarser resolution model?

A series of experiments with both hydrostatic and non‐hydrostatic models at resolutions of 80, 40, 20, 10 and 5 km is analysed, and two of these experiments are repeated with an envelope orography.

It is concluded that hydrostatic simulations are sufficient to describe the basic dynamics even at 5 km resolution; however, there is a strong dependence of the drag and momentum fluxes on resolution, with the drag at 80 km resolution being almost half that at the highest resolution. It is also shown that the use of an envelope orography at coarser resolutions can compensate significantly for the drag under‐estimation, but is less effective in representing the vertical stress profiles or the direction of the stress vectors, which are systematically in error at the coarsest resolution. The potential role of gravity wave drag is confirmed and possible improvements considered. It is suggested that both envelope orography and gravity wave drag may make a significant contribution until resolutions are as high as 10 to 20 km.
A model of occult deposition applicable to complex terrain
Authors: Coe H. , Choularton T. W. , Carruthers D. J. , Gallagher M. W. , Bower K. N.
Published: February 2007

A model is presented of the turbulent deposition of cloud water and dissolved species to regions of complex topography where cloud forms in contact with the ground. A three‐dimensional airflow model (FLOWSTAR) which includes a treatment of atmospheric stability and boundary‐layer turbulence is used. Versions of the model are described which include either simple calculations of liquid‐water content or detailed calculations of the cloud microphysics. Both a simple equilibrium model of cloud‐water chemistry and a detailed model including the explicit calculation of the chemical processes in individual droplet size categories are used.

The deposition velocities of the cloud droplets to the terrain are calculated from recent measurements of cloud‐water deposition to forest and moorland.

It is found that good estimates of deposition may be obtained using the simplest cloud‐water and chemistry models with deposition velocities parametrized in terms of a mean droplet radius. The results indicate that the deposition is a very strong function of position within the terrain. The implications of this for the deposition of phytotoxic species to the flora and the deposition of radio‐nuclides following a nuclear accident are considered.
Mesoscale dynamics of cold fronts: Structures described by dropsoundings in FRONTS 87
Authors: Thorpe Alan J. , Clough Sid A.
Published: February 2007

The FRonts 87 project was a European experiment to make mesoscale observations to deduce the dynamics of active cold fronts approaching north‐west Europe, with a minimum of orographic influences. In this paper data from dropsoundings made by the Meteorological Office's C‐130 research aircraft are analysed. The aircraft flew a pattern with 4 or 5 runs oriented approximately at right angles to the front, but with each run displaced in the along‐front direction by about 100 km. The dropsondes gave soundings to a height of approximately 7 km with a cross‐frontal spacing of 20 km at best and 60 km on average. Between 30 and 50 soundings were made in each event, over an area of 500 × 500 km2. These data are unique in their mesoscale resolution over a synoptic‐scale region; this was made possible by the ability to track up to 5 sondes in the air at any time.

Several new aspects of cold‐front dynamics have been identified from these data; the approach here is to treat each run as an instantaneous cross‐frontal section. The discussion is centred on the hypotheses postulated before the experiment; these were intended to allow verification or refutation of aspects of models of frontal dynamics such as those embodied in semi‐geostrophic theory. A minimal and purely objective analysis has been performed on the data presented here, but the high resolution of the dropsonde observations permits evaluation of differentiated quantities, such as potential vorticity (PV), with some confidence. Issues raised here include the degree of thermal‐wind balance, the structure of conserved variables such as equivalent potential temperature and absolute momentum, the cross‐frontal circulation and the role of diabatic processes such as the evaporation, of snow, and the potential vorticity structure on the mesoscale and its interpretation. The intention is to present an integrated view of the dynamical structure of fronts in the light of theoretical rather than conceptual or airflow models.
Localized forcing of slantwise motion at fronts
Authors: Holt M. W. , Thorpe A. J.
Published: February 2007

A cold front is here modelled as a baroclinic zone in which there is a local low‐level forcing of ageostrophic motion. Such forcing may result from the outflow from line‐convection at the surface front, or from synoptic‐scale frontogenesis, and is represented by a localized fixed region in which anomalies of the inertia and/or buoyancy of the air are imposed, either continuously or impulsively.

The response to this forcing is described when the baroclinic flow is either unstable or weakly stable to symmetric instability. Such conditions are thought to be typical of cold fronts. The flow is examined using classical parcel theory, a (new) more complete parcel theory, a primitive‐equation numerical model, and the so‐called Geometric (or balanced) Model. The predicted motion involves a lens‐shaped area oriented along the sloping isentropes, with geometry dictated by the background potential vorticity and the imposed anomalies in inertia or buoyancy. The lens has zero or weakly negative potential vorticity, in the case of a background flow which is symmetrically stable.

This lens, grown in a symmetrically stable atmosphere, is offered as an alternative explanation for the observed state of (moist) slantwise neutrality at a cold front, in contrast to that explanation which assumes an irreversible adjustment to symmetric instability.
Sensitivity of the southern hemisphere circulation to leads in the Antarctic pack ice
Authors: Simmondsxs Ian , Budd W. F.
Published: February 2007

To assess the sensitivity of the southern hemisphere circulation to changes in the fraction of open water in the sea ice we have conducted four experiments with a July 21‐wave General Circulation Model (GCM) with this fraction set to 5, 50, 80 and 100%. The mean surface temperatures and the surface atmospheric temperatures over the sea ice increased as the water fraction increased and the largest changes were simulated adjacent to the coast.

Significant anomalies in the surface heat fluxes, particularly those of sensible heat, accompanied the decrease in the sea ice concentration. Substantial atmospheric warming was simulated over and in the vicinity of areas in which leads were considered. In all but one experiment there were anomalous easterlies between about 40 and 60°S with westerly anomalies further to the south. The surface pressure at high latitudes appears to change in a consistent fashion with the fraction of open water, with the largest changes occurring in the Weddell and near the Ross Seas.

Some of the feedbacks which may enhance the responses here, but which are not included in our model, are discussed.
The influence of static stability on the effective roughness lengths for momentum and heat transfer
Authors: Wood Nigel , Mason Paul
Published: February 2007

The area‐averaged properties of the planetary boundary layer over heterogeneous terrain are considered. Previous studies which dealt with the average momentum transfer properties in neutral static stability conditions are extended to include the influence of stratification and also area‐averaged properties for heat transfer. Results from numerical simulations demonstrate the utility of values of effective roughness length for both momentum and heat transfer. A heuristic model is found to show good agreement with the numerical simulations and to provide a simple method for estimating these roughness lengths for use in boundary‐layer parametrization.

Significant anomalies in the surface heat fluxes, particularly those of sensible heat, accompanied the decrease in the sea ice concentration. Substantial atmospheric warming was simulated over and in the vicinity of areas in which leads were considered. In all but one experiment there were anomalous easterlies between about 40 and 60°S with westerly anomalies further to the south. The surface pressure at high latitudes appears to change in a consistent fashion with the fraction of open water, with the largest changes occurring in the Weddell and near the Ross Seas.

Some of the feedbacks which may enhance the responses here, but which are not included in our model, are discussed.
The evaporation of frontal and other stratiform precipitation
Authors: Clough S. A. , Franks R. A. A.
Published: February 2007

The evaporation of precipitation of differing types is considered by use of one‐dimensional models. It is shown that the evaporation of snow and crystalline ice precipitation takes place in shorter times and much shallower depths than for rain, while for graupel it falls between the two. The main factors responsible for the differences in behaviour are bulk density and terminal velocity of particles, which combine to produce evaporation depths for snow less than one tenth those for rain. Further, the evaporation depth predicted for snow is shown to be rather insensitive to detailed characteristics of the snow particles within the observed range.

Some of the implications of this difference are explored by means of a model describing the interaction of evaporating precipitation with its environment when subjected to atmospheric descent. The model, originally due to Kamburova and Ludlam (1966), has been extended to study the evolution of a variable particle spectrum. It is shown that for rainfall rates of the order of 1–10 mm h−1 the evaporation of snow is sufficient to maintain the atmosphere in a near‐saturated condition despite descent rates of the order of 10–30 cms−1, typical of those observed on the mesoscale in stratiform environments like rainbands; rain exhibits no comparable behaviour.

These results suggest that the evaporation of snow plays a major role in the dynamics of fronts and other mesoscale systems with stratiform cloud. This view is supported by comparison with dropsonde observations from the FRonts 87 experiment, which show a clear indication of enhanced mesoscale descent in the region of maximum precipitation.
Cloud parametrization at different horizontal resolutions
Authors: Kristjánsson Jón Egill
Published: February 2007

As faster computers emerge, resolution continues to improve in weather‐prediction and climate models. How does this affect the problems that we are dealing with in the models? In this study we have concentrated on one of these problems, namely the treatment of clouds. Among the questions addressed are: How do cloud‐related processes (condensation, precipitation) depend on grid resolution? How should we take this into account? Can we simulate orographic clouds when we have a grid resolution of hundreds of kilometres (climate models)? To tackle these questions simulations are performed with a limited‐area model that is run with horizontal grid resolutions of 150,50 and 25 km. The model has a sophisticated treatment of condensation and clouds. For verification, use is made of satellite‐derived cloud water content and cloud cover, and surface observations of precipitation.

It is found that when resolution is enhanced, precipitation systematically increases while cloud cover decreases. The main reason is that much stronger vertical motions develop at high resolution, yielding dense, narrow clouds, with clear air between. The consequences of these results for cloud parametrization are discussed. It is further shown that cloud‐cover simulations over mountainous terrain can be improved by paying regard to the sub‐grid‐scale orography.
On the relation between total ozone and meteorology
Authors: Vaughan G. , Price J. D.
Published: February 2007

A relationship is derived between absolute vorticity in the lower stratosphere and the total‐ozone column. This relationship shows that reversible deformations to the flow in the lower stratosphere on synoptic scales or smaller (such as trough/ridge patterns) cause perturbations in total ozone which depend on the amplitude and depth of the vorticity anomaly. Examples are presented showing excellent correspondence between total ozone and vorticity charts, even on mesoscales. Grid correlations show that at least half the variance in total ozone is related to variations in the vorticity just above the tropopause. Some applications of the ozone‐vorticity relationship are discussed.
A GCM Case Study On the Maintenance of Short‐Term Subtropical Wind Maxima In the Summer Hemisphere During SOP‐1, FGGE
Authors: Hurrell James W. , Vincent Dayton G.
Published: February 2007

A recent version of the GLA fourth‐order GCM is used to investigate the short‐term relationship between tropical heating and upper tropospheric subtropical westerly maxima in the summer hemisphere. Emphasis is focused on a 15‐day forecast of the circulation corresponding to 6‐20 January 1979. During this time, observational data from the FGGE suggested that strong convection over the South Pacific was helping to maintain a nearby subtropical jet. the dominant term in the zonal momentum budget was the Coriolis force applied to the diabatically driven meridional circulation, although the mean advection of zonal momentum and transient eddy momentum and sensible‐heat transports made important contributions. A ‘full‐physics’ forecast made with the GCM is able to simulate these processes to a reasonable degree of accuracy.

To examine the impact of direct meridional overturning on the rotational flow in the South Pacific, the latent energy supplied to the model atmosphere is reduced in two ways. First, negative sea‐surface‐temperature anomalies are imposed in the area of maximum convection. Second, temperature changes resulting from precipitation processes are ignored in the model thermodynamic equation. Results from both types of experiments are consistent, and they substantiate our earlier observational findings that, at least for this case study, summertime subtropical westerly maxima are largely forced by divergent outflow from the tropics.
An Analysis of Simulated Summer Blocking Episodes
Authors: Kung Ernest C. , Min Wei , Susskind Joel , Park Chung‐Kyu
Published: February 2007

Using the high‐resolution 2 × 2.5 degree version of the Goddard Laboratory for Atmospheres general circulation model, experiments were performed for a 46‐day summer period from 1 July to 15 August 1979. During the simulation, sea‐surface‐temperature (s.s.t.) anomalies were updated daily with observed s.s.t.s to provide a realistic surface heating field. A parallel control run used climatology s.s.t.s. in lieu of observed s.s.t.s.

Both the s.s.t. updating experiment and control run were able to simulate the persistent blocking patterns in Eurasia and the eastern Pacific during July. However, only the s.s.t. updating experiment was able to generate the realistic blocking episodes in the last 10‐day period during the 46‐day simulation. Forecast skill was examined in terms of 500 mb anomaly correlation and error kinetic energy. When the blocking patterns develop or become reinforced, a high forecast skill is apparent in nearly half of the northern hemisphere including the area of blocking and its downstream region. Unlike the major winter blockings, which may be treated as the manifestation of wave number one and two, the summer blockings are localized, and have a smaller scale than that of winter blockings. However, summer blockings treated in this study are persistent, and possess a considerable predictability.
Upper Tropospheric Equatorial Waves In Ecmwf Analyses
Authors: Randel William J.
Published: February 2007

Tropical wind fields from the European Centre for Medium‐range Weather Forecasts operational analyses are analysed to search for equatorially trapped waves in the upper troposphere. Data are studied for 1980‐87. Zonal wind spectra show a predominance of power at low zonal wave numbers and low frequencies; these data do not show distinct spectral peaks corresponding to eastward‐propagating Kelvin waves or westward‐propagating equatorially‐trapped Rossby waves. the zonal wind spectra do show episodic evidence for westward‐propagating 5‐day zonal wave‐1 features with maxima over the equator; these are likely related to Rossby normal‐mode oscillations previously reported.

Meridional wind spectra show a predominance of westward‐propagating power in the 6‐ to 10‐day‐period range, primarily concentrated in zonal waves 4‐7. Enhanced meridional wind activity at the equator occurs episodically in time, with events persisting for one to several months duration. Wave packets are longitudinally localized, with preferred occurrence over the eastern Pacific Ocean. Most cases show evidence of meridional wind maxima in the tropics, suggesting a degree of equatorial trapping. the horizontal structure of these oscillations is that of mixed Rossby‐gravity waves, although they are not necessarily centred directly over the equator. These waves have largest amplitudes in the upper troposphere, showing weak coherence with the surface; they appear to be fundamentally distinct from fast 4‐ to 5‐day Rossby‐gravity waves observed in the western Pacific.

A detailed study of a group of intense Rossby‐gravity waves observed over the eastern Pacific during August‐September 1985 is presented; the reality of these features is confirmed in direct rawinsonde observations. Correlations are found between bursts of equatorial wave activity and enhanced meridional momentum fluxes from the subtropics, suggesting extratropical forcing as at least one mechanism for their generation. the waves are also strongly coupled to the upper tropospheric zonal wind field, which likely accounts for their longitudinal and temporal confinement.
A Study of Mountain Lee Waves Using Clear‐Air Radar
Authors: Ralph F. M. , Crochet M. , Venkateswaran S. V.
Published: February 2007

The tropospheric and lower stratospheric response to flow over complex terrain is investigated using observations of vertical and horizontal motions made by a clear‐air Doppler radar stationed at Toulon in southern France. Analysis of the data has disclosed that episodic enhancements in the variability of vertical and horizontal velocities are due to lee waves excited by obstacles located within 5 km of the radar. Three separate cases of lee‐wave activity are studied. Two‐dimensional linear perturbation theory and existing three‐dimensional simulation results, for appropriate parameter values, are used as guides to check against the vertical structure observed by radar. the magnitudes of vertical velocities observed during lee‐wave incidence are well within the range reported in previous observational and numerical simulation studies.

From a detailed examination of one of the cases of trapped waves, the time variation, observed by the radar, of timescales of less than an hour, has emerged as the likely result of small directional changes in the upstream winds causing a bodily rotation of the downstream three‐dimensional wave pattern centred on the wave‐generating obstacle. the observed tendency of the vertical‐velocity perturbation to change sign with time above the radar location follows as a consequence of the presence of large horizontal gradients in the motion field, which are inherent in the structure of three‐dimensional lee waves.

The temporal variability of the lee‐wave pattern has allowed the radar to sweep out a partial horizontal cross‐section, revealing vertical circulation features that are broadly consistent with gravity‐wave behaviour. However, analysis of the effects of finite antenna beam spacing on the sampled data indicates that a spurious phase‐shift might have been introduced between the vertical and horizontal velocity perturbations observed in the cross‐section. In contrast, the measured vertical velocities are not seriously affected by finite beam‐width effects.

The lee waves examined in this study are quite moderate, nevertheless, this circumstance has provided a favourable opportunity to conduct a critical evaluation of the capabilities and limitations of this radar technique for observing lee‐wave phenomena in general. We have been able to show clearly how the incidence and vertical structure of the waves respond to the synoptic‐scale situation. In particular we have been able to distinguish between trapped and vertically propagating waves and to relate a transition between these wave types to synoptic‐scale changes in the static stability structure.
On the Shape and Fall Velocities of Raindrops
Authors: Brazier‐Smith P. R. , Brazier‐Smith P. R.
Published: February 2007

A numerical model for the calculation of the shapes and fall velocities of raindrops is presented. Central to the operation of the model is the calculation of the flow over the front portion of the drop. This is done on the basis that the flow in that region is potential and that the surface streamline separates at the waist. In its final form the model predicts fall velocities within 4% of the experimental values for drops of 1 mm radius and greater, and shapes that correspond very closely to observed shapes. the incorporation of electrical forces and their effects are briefly explored.
On the spectral behaviour of atmospheric boundary‐layer parameters at Thumba, India
Authors: Winston Prakash J. Jeeva , Ramchandran Radhika , Nair K. Narayanan , Gupta K. Sen , Kunhikrishnan P. K.
Published: February 2007

The features of the spectra of meteorological fields of wind velocity and temperature observed at Thumba, a site on the western coast of India, are discussed. Data utilized for the study are mostly from an instrumented tower, sodar and long‐period temperature data available for the site. The influence of local circulations like the sea breeze is manifested also in the general spectral characteristics of horizontal wind, as with the spectral gap in the mesoscale region between the synoptic‐scale region and the turbulent‐scale region of the spectra. The width of the spectral gap observed between the wave‐active region and the microscale region in the wind velocity spectra is seen to be dependent on the stability of the medium, and the gap is found to shift to the high‐frequency side with increasing stability. The composite spectrum of temperature also reveals a prominent gap at about an hour. It is seen that with the advection of a sea breeze over the land, a shallow thermal internal boundary layer is formed and regions above one tenth of this depth obey mixed‐layer similarity.
Surface‐layer description under free‐convection conditions
Authors: Sykes R. I. , Henn D. S. , Lewellen W. S.
Published: February 2007

Large‐eddy simulation is used to study the structure of the boundary layer close to the surface under free‐convection conditions. The magnitude of the surface friction velocity induced by the large convective eddies is determined as a function of the convective velocity scale, w*, and the surface, roughness length, z0. A simple analytic scaling form is obtained from a balance of forces in the wall layer induced by the convective eddies, giving the effective thickness of the layer and the effective surface stress and temperature dependence. The depth of the wall layer varies significantly with surface roughness and is extremely thin for small roughness, e.g. 0.5% of the mixed layer depth for a roughness length of 10−4zi. The analytic forms are consistent with the numerical predictions over the range of roughness length considered. The probability distribution of the instantaneous surface friction velocity (magnitude only) is examined and found to be reasonably close to Gaussian.
A GCM simulation of the impact of Amazonian deforestation on climate using an improved canopy representation
Authors: Lean J. , Rowntree P. R.
Published: February 2007

To obtain a estimate of the impcat of Amazonian deforestation on local climate it is critical that the representation of the forest canopy within general circulation models (GCMs) is as realistic as possible. Recent measurements from the Amazonian forest have highlighted major weaknesses in the Meteorological Office GCM simulation of the interception of rainfall from the forest canopy. Here, we present results for Amazonia from a new 3‐year control experiment which incorporates an improved representation of micrometeorological processes within the forest. A detailed assessment of the control simulation reveals that the adjusted GCM provides a realistic description of the climate of Amazonia. In determining the impact of Amazonian deforestation on climate we present a comprehensive analysis of the simulated climate following the replacement of Amazonian forest by pasture%.

A comparison of the new results with those from the earlier deforestation experiment carried out by Lean and Warrilow (1989) suggests that the reductions in local rainfall (14%) and evaporation (24%) are smaller than those obtained with the previous formulation of interception. It was concluded by Lean and Warrilow that with a wet canopy, decreases in roughness in the deforested case reduce evaporation. With the introduction of the new interception formulation the canopy is less often wet, and so the effect of deforestation is reduced.
The density and structure of ice accretion predicted by a random‐walk model
Authors: Szilder Krzysztof
Published: February 2007

A random‐walk, ballistic‐type model is used to simulate the ice accretion process due to impinging supercooled droplets. the model is applicable over a wide range of growth conditions including, in the limit, growth where the droplets freeze virtually as spheres (dry growth), and growth where the droplets spread into and across the ice surface losing their identity (wet growth). The structure and density of the ice are determined by the probabilities governing droplet motion and freezing during their random walk along the ice deposit. A relationship between the Macklin parameter, commonly used to relate atmospheric conditions to ice density, and the probabilities of droplet motion has been established. The ice density and accretion structure perdicted by the random‐walk model agree qualitatively with experimental observations at the stagnation line on a fixed circular cylinder. The model also predicts finger‐like ice structures for small values of droplet momentum, also in keeping with observations.
Turbulent transfer relationships over an urban surface. II: Integral statistics
Authors: Roth M.
Published: February 2007

A comprehensive study of atmospheric turbulence over an urban surface has been carried out. Part II presents the normalized standard deviations of all important atmospheric variables from unstable conditions Measured at z'/z0=37 and analysed within the Monin‐Obukhov similarity framework. They differ from the rural reference data in the following ways.

The normalized standard deviation of vertical velocity is systematically smaller.

The efficiency of urban transfer of heat and especially momentum are larger.

The humidity statistics cannot be described within the similarity framework.

It is suggested that the roughness sublayer effects and large‐scale space and time inhomogeneities of the motions in the suburban atmosphere are responsible for the observed dissimilarities.
Microphysical and radiative properties of small cumulus clouds over the sea
Authors: Raga G. B. , Jonas P. R.
Published: February 2007

Microphysical and radiative data obtained in fields of cumulus clouds over the sea around the United Kingdom are presented. The Meteorological Research Flight C‐130 aircraft was used to make radiation and cloud microphysical measurementsin situon four different occasions. The sub‐cloud aerosol concentrations ranged from 100 to 5000 cm−3, from fairly maritime to heavily polluted. Vertical profiles of microphysical variables indicate that small cumuli over the sea have some properties that are more similar to marine stratocumuli than to continental cumuli. Nonetheless, variables, such as liquid‐water content and droplet‐number concentration, exhibit a larger variability than in stratocumuli, owing to more vigorous entrainment in cumuli.

Mie theory was used to compute extinction cross‐sections, single scattering albedo, back‐scattering cross‐sections, and asymmetry parameters for all in‐cloud samples. The average vertical profiles are in agreement with values computed by previous authors for trade‐wind cumuli. Our results show that these properties depend upon the background aerosol concentrations%.

A simple two‐stream radiative‐transfer model, using the back‐scattering cross‐section, is presented and shows good agreement with run‐averaged radiometric observations.
A multiple sounding technique for the study of gravity waves
Authors: Shutts G. J. , Healey P. , Mobbs S. D.
Published: February 2007

Radiosondes released in rapid succession, with a time separation of a few minutes, were launched during a field experiment in central Wales, U.K. to study orographic gravity waves. Rate of ascent fluctuations were used to infer the existence of vertical‐velocity fluctuations associated with orographic gravity waves, and differences between consecutive ascents provided information on the steadiness and phase‐line slope of the waves.

The potential of the multiple sounding technique is examined here and an analysis strategy proposed. Three triple‐sounding cases from the field experiment are used to illustrate the strengths and weaknesses of the technique. In one case it was possible to estimate a phase‐line tilt with height and this, combined with the inferred vertical‐velocity amplitude, allowed the estimation of the vertical momentum flux.

It is suggested that the near‐simultaneous release of between three and five sondes upstream of mountainous terrain could provide a useful method for calculating the vertical flux of momentum due to gravity waves.
Studies of the radiative properties of ice and mixed‐phase clouds
Authors: Sun Zhian , Shine Keith P.
Published: February 2007

Radiative parametrizations for both ice and water clouds are developed in terms of liquid/ice water content, based on Mie scattering theory. For ice crystals the application of Mie theory is guided by the hexagonal‐crystal/equivalent‐spheres comparison of Takano and Liou. These parametrizations are extensively tested against measurements from aircraft and are shown to perform satisfactorily, although corrections for unobserved small crystals and the effect of crystal shape are large and not currently well defined. The parametrizations are then used to investigate the effect of mixed‐phase clouds on radiative transfer. It is found that, because the radiative properties of ice crystals and liquid droplets are significantly different, the radiative properties of mixed‐phase clouds cannot be simulated successfully if the ice in clouds is converted into liquid water. Both the albedo and the rate of change of albedo with ice fraction are significantly dependent on the method by which the phases are mixed; these factors may be of especial importance in climate‐sensitivity experiments that incorporate mixed‐phase clouds. The presence of ice in clouds below the cirrus level is often ignored in climate‐model and radiation‐budget studies. The calculations presented here indicate that this neglect may lead to a serious bias in cloud albedo for a given path of condensed water.
On the reflectance of cellular cloud layers
Authors: Jonas Peter R.
Published: February 2007

A Monte Carlo model is used to investigate the effects on the reflectance of low cloud layers of organized cellular structure, which is often observed in such layers. It is assumed that the cloud is composed of a series of cells each of which comprises a core region surrounded by a wall region of reduced water content. It is shown that the sensitivity of reflectance to the structure is largest for wavelengths that are not absorbed by liquid water. The reflectance of the cloud layer is reduced when the water content in the regions of low water content is less than about 30% of the water content in the remainder of the layer. The effect on the cloud‐layer reflectance of organized structure in the water content is large compared with the effect of changes in the effective radius at cloud top. It is further shown that, for narrow (∼100 m) regions of low water content, estimations of the layer average reflectance based on the area weighted average of the reflectance of layer clouds with high and low water contents are significantly in error. For small solar zenith angles the regions of low water content tend to permit incident radiation to pass through the cloud, reducing the reflectance, while for large solar zenith angles the reflectance is increased by the effects of cloud shadowing.
Dynamics of a low‐level jet observed during the Fronts 87 experiment
Authors: Kotroni V. , Lemaitre Y. , Petitdidier M.
Published: February 2007

A study of the internal dynamics of a low‐level jet, its role on the frontal discontinuity evolution, and its relevance to the organization of convection at mesoscale and small scale is presented. The low‐level jet studied is associated with an active cold front that was observed during the French—British co‐operative field experiment Fronts 87. The originality of the present low‐level‐jet description lies in the fact that it is observed not only by means of soundings but also by means of a new tool for mesoscale meteorology observations, a wind profiler (stratospheric—tropospheric UHF radar). The high resolution of the wind‐profiler data in both time and height allows one to assess, in precipitating as well as in clear‐sky areas, a detailed image of the low‐level jet and the associated mesoscale and convective‐scale circulations. The comparison between observations and predictions of various theories provides information on the physical mechanisms responsible for the initiation and maintenance of the observed mesoscale features. The flow resulting from the presence of the low‐level jet produces inertial acceleration and baroclinic symmetric instability which lead to a narrow mesoscale line of sloping motions. Moreover, the interaction of the cold surface outflow with the low‐level vertical shear below the jet maximum is found to be crucial in the intensification and maintenance of the observed narrow cold‐frontal rainband.
Sensitivity of optimal unstable structures
Authors: Buizza Roberto
Published: February 2007

Optimal unstable perturbations, i.e. structures that grow fastest over a finite time interval, can be used in numerical weather prediction to construct the initial conditions of an ensemble of integrations. Optimal perturbations superimposed on a basic state provide a representation of the uncertainty of the initial state of the atmospheric flow. The computation of the fastest‐growing perturbations over a finite time interval can be achieved in the linear approximation by using the forward and adjoint tangent version of a full nonlinear model.

Previous results, obtained using a version of a primitive‐equation model with a reasonable horizontal and vertical resolution, showed that the lack of parametrization of turbulent processes can lead to fast‐growing perturbations characterized by ‘non‐meteorological’ structures. The first part of this paper focuses on this problem. Numerical experiments have been performed to study the impact of a simple surface‐drag and vertical‐diffusion scheme on the most unstable perturbations. It is shown how the very simple parametrization of the turbulent processes implemented inhibits the growth of non‐meteorological structures close to the surface.

A second very important problem to face when constructing the initial conditions of an ensemble of forecasts, using optimal perturbations, is the definition of the optimization time interval over which the growth of these unstable structures is maximized. This problem will be investigated in the second part of this work, where the impact of the optimization time interval on the definition of the unstable sub‐space is studied for time periods up to three days. Results from different cases seem to indicate that unstable sub‐spaces computed with an optimization time interval longer than 36 hours are very similar.
A three‐dimensional model study of nitrogen oxides in the stratosphere
Authors: Lary D. J. , Pyle J. A. , Carver G.
Published: February 2007

A new three‐dimensional model for stratospheric chemistry studies is described. The model, a spectral general circulation model, is developed from the model used at the European Centre for Medium‐range Weather Forecasts. Schemes to describe atmospheric chemistry and photochemistry as well as advection of trace gases have been implemented. In this study a perturbed period in January 1987 is studied. The model performs well. An interesting feature, similar to the Noxon cliff, is reproduced in which the NO2 column is reduced significantly in high latitudes. In this calculation the cliff is produced by gas phase conversion of NO2 to N2O5.
Comments on ‘Spatial structure of ultra‐low frequency variability of the flow in a simple atmospheric circulation model’ by I. N. James and P. M. James (October 1992, 118, 1211–1233.)
Authors: Feldstein Steven B. , Robinson Walter A.
Published: February 2007

James and James (1992) examine internally generated interannual variability in a multi‐level primitive‐equation model. The authors claim that the variability in the global mean relative angular velocity is associated with fluctuations in the mid‐latitude zonally‐averaged zonal winds. We question this relationship, and present results from an aqua‐planet GCM which indicate that, instead, the global mean relative angular velocity is primarily related to changes in the tropical zonally‐averaged zonal winds. Furthermore, we also question whether the ‘ultra‐low‐frequency variability’ examined by James and James is not simply just ‘climate noise’.