bluuebunny/crossref_metadata_2025 huggingface dataset:
The modelling problems associated with spatial averaging
Authors: Pyle J. A. , Zavody A. M.
Published: February 2007

The problem of spatial averaging in photochemical models of the stratosphere is considered. It is shown that correlation between reacting species, or between species and temperature, can lead to a bias in the model treatment of the reaction rate. Using satellite data it is demonstrated that for reactions controlling the ozone budget the resulting error is generally not large, although it may be appreciable during times of strong dynamical activity. Finally, we speculate that the errors could be more significant for species with strong gradients in the vertical, or for variability on a smaller scale than is resolved by satellite measurements. This could afford an explanation of some current modelling problems.
Some experiments to investigate the assimilation of SEASAT altimeter wave height data into a global wave model
Authors: Francis P. E. , Stratton R. A.
Published: February 2007

A scheme for assimilating SEASAT altimeter wave heights into a global wave model, with the additional use of the collocated altimeter wind speeds, is reported. Control experiments are described in which the wave model was run with two sets of winds, one with SEASAT scatterometer wind data included in the NWP analysis and the other without, both for a period from 0000 GMT 15 September 1978 to 0000 GMT 20 September 1978. the results of forcing the wave model with these wind fields are compared with the altimeter wave heights. the differences are discussed in order to quantify errors in both the wave model and the altimetric data. Three experiments in which altimeter wave heights were assimilated are then described. the results of these are compared with the wave fields from the model runs without wave data assimilation. the assimilation experiments show that a significant improvement in model wave height is possible and that the impact of one day of assimilation is retained for at least five days.
The influence of soil wetness distribution on short‐range rainfall forecasting in the West African Sahel
Authors: Rowell D. P. , Blondin C.
Published: February 2007

Surface moisture supply is known to be a factor which can be important in triggering convection. This paper aims to investigate its impact on short‐range numerical forecasts of tropical rainfall.

The ECMWF (European Centre for Medium Range Weather Forecasts) operational forecasting model was integrated for 5 days with realistic positive and negative anomalies of the initial surface moisture distribution in the West African Sahel. the large‐scale flow is found to be relatively unaffected, but significant differences occur in the rainfall forecasts for about the first 4 days. Rainfall is greatest with a moister surface, except on days 3 and 4 in the south Sahel/Savanna region, for which a decrease of rainfall occurs. the opposite result is seen when an initially drier surface is introduced. the mechanisms involved are investigated by carrying out a detailed hydrological budget, and by studying the evolution of the atmospheric profiles of moisture and temperature.

We conclude that an improved surface moisture analysis is likely to result in some improvement of shortrange rainfall forecasts in the Sahel, and presumably also in other tropical regions. In the coming years it should be possible to achieve this using satellite‐derived soil wetness maps.
The calculation of geopotential and the pressure gradient in the ECMWF atmospheric model: Influence on the simulation of the polar atmosphere and on temperature analyses
Authors: Simmons A. J. , Jiabin Chen
Published: February 2007

The spectral atmospheric model used for prediction at the European Centre for Medium Range Weather Forecasts (ECMWF) is modified to change the spectrally‐represented thermodynamic variable from temperature to the deviation of temperature from a reference profile which depends analytically on pressure. There is only a minor change to the form of the thermodynamic equation, and the calculation of the pressure gradient is modified to eliminate some of the cancellation between the Δ and RTΔInp terms which occurs in the standard formulation. At T42 resolution the revised scheme significantly improves southern hemispheric forecasts, with differences originating over Antarctica. There is also a small improvement over the Arctic. Sensitivity is much less at T106 resolution, but a minor advantage can still be seen in high southern latitudes, and there is a reduction in noise in the vertical velocity field near the Andes at tropical latitudes. Similar results are found for a version in which surface pressure, rather than its logarithm, is chosen as a spectrally represented prognostic variable. This gives mass conservation but the stability of the semi‐implicit time integration is somewhat reduced.

The improvement seen at high latitudes is also captured by a simpler revision which retains temperature as the spectrally‐represented variable, and uses a reference temperature only in the computation of the pressure‐gradient terms in gridpoint space. Results indicate that much of the systematic difference in behaviour between T42 and T106 simulations at high latitudes can be removed by changing the pressure‐gradient calculation. In particular, the new schemes correct a systematic tendency for erroneously high pressures east of the Ross Ice Shelf over Antarctica in lower resolution simulations at medium and longer time ranges. Improvement occurs because the schemes reduce an inconsistent treatment of the Δ and RTΔInp terms in the model's semi‐implicit treatment of the vorticity equation.

The reference temperature profile has also been used in data assimilation to reduce a systematic error in the calculation of first‐guess geopotential heights at standard pressure levels. The resulting height analyses agree slightly better with radiosonde measurements, and initialization causes less of a degradation of the fit to observed data. Temperatures are systematically warmer in the upper troposphere (by almost 0.5 K in the global mean at 200 hPa) and cooler in the lower troposphere, and are closer to observed values. Results from forecasts carried out after one and two days of assimilation show small improvements in the short and early medium range, and are inconclusive at longer time ranges.
Boundary‐layer moisture regimes
Authors: Mahrt L.
Published: February 2007

Data from fifty‐two aircraft flight legs 100−150m above ground from HAPEX and FIFE are analysed to estimate characteristics of boundary‐layer moisture fluctuations in boundary layers with different bulk stability and surface energy regimes. In HAPEX, considerable effort was devoted to the quality of measurement of moisture fluctuations. The data include repeated 120km flight legs flown at 150m over the same relatively homogeneous terrain, allowing statistical examination of motion characteristics on horizontal scales up to 10km.

This study finds two prototype boundary‐layer regimes. With significant boundary‐layer instability, relatively weak surface evaporation and drier air aloft, the entrainment‐drying boundary layer may develop. This boundary layer is characterized by vertical divergence of the moisture flux and significant diffusion of dry air from the top downwards (top‐down). In this type of boundary layer, dry air occasionally reaches the lower boundary layer, leading to negative moisture skewness in spite of positive temperature and vertical velocity skewness associated with warm moist updraughts. In contrast, the moistening boundary layer associated with greater surface evaporation is characterized by positive moisture skewness near the surface.

In addition to the above turbulent moisture fluctuations, some of the above data are characterized by 10km moisture variations with horizontal gradients often concentrated in narrow zones of horizontal convergence. Since corresponding signatures of vertical velocity and temperature are weaker, these zones are referred to as ‘mesoscale’ moisture fronts. As a more general feature, moisture variations on scales of 5km and greater are negatively correlated with temperature variations associated with cool moist regions and warm dry regions. On scales of tens of kilometres and greater, such negative correlation may be due to inhomogeneity of the surface energy budget. The negative moisture‐temperature correlation leads to large mesoscale variations of relative humidity and the lifted condensation level.
Pressure drag and momentum fluxes due to the Alps. I: Comparison between numerical simulations and observations
Authors: Hoinka Klaus P. , Clark Terry L.
Published: February 2007

A three‐dimensional, non‐hydrostatic, anelastic model using interactive grid nesting is employed to simulate the airflow over and around the Alps during a strong foehn event on 8 November 1982. In the model, a single upstream sounding is used to initialize the mean flow. This single sounding was obtained from the output of a three‐dimensional objective analysis of rawinsonde data. The model results show that in low levels the airflow is forced around the Alps, in particular at its western edges, whereas above 700 hPa the flow is over the Alps. Above northern Italy a convergence line between the easterly flow and the foehn forced by the Apennine Mountains is simulated in accordance with observations. To the west of Milan vertical vortices are simulated in agreement with the complex circulation observed in this region.

The simulations show that there is significant horizontal variability in the local vectors of the pressure drag and the momentum flux. This variability complicates the comparison between model and observational data as well as restricting one's ability to extrapolate accurately local cross‐sectional observations of pressure drag and momentum flux to values representative for the entire Alpine complex. Unfortunately, there were no measurements of surface pressure drag for the entire Alps on 8 November 1982. However, measurements during ALPEX during four foehn events obtained meridional and zonal values of −4.3 and 1.8 × 1011 N. The current values of −4.7 and 1.7 × 1011 N obtained for the simulations of the 8 November 1982 case using 5 km horizontal resolution are very close to the previous observations. There were observations on a cross‐section between Vicenza and Munich on this day. The values obtained were −0.67 × 106 N m−1 for the surface pressure drag and −0.75 × 105 N m−1 for the momentum flux averaged between 5 and 10 km above mean sea level. The simulated values for the same cross‐section were −0.62 and −0.50 × 106Nm−1 for the pressure drag and momentum flux, respectively. Overall, the averaged simulated meridional momentum flux between 5 and 10 km was −2.2 × 1011 N which is about 47% of the surface pressure drag. These results and comparisons with observations suggest that while the model appears to predict reasonable values for the surface pressure drag the amplitude of the momentum flux values is too large. The simulated values are, for example, six times those observed for the Vicenza to Munich cross‐section. Simulations using even finer resolution over a reduced region suggest that poorly simulated dissipative forces by the model may be responsible for some of this discrepancy. Some other factors such as surface friction are also discussed in the text.
Size‐resolved measurements of cloud droplet deposition velocity to a forest canopy using an eddy correlation technique
Authors: Beswick K. M. , Hargreaves K. J. , Gallagher M. W. , Choularton T. W. , Fowler D.
Published: February 2007

Two case studies are presented of the direct measurement of cloud droplet fluxes to a forest canopy by the eddy correlation technique. Two instruments have been used for the liquid water measurements, and close agreement for values of liquid water content and liquid water flux have been obtained. The case studies were conducted in different weather conditions: case 1 with high values of liquid water content (typically 0.3 g m−3) and low wind speeds (2–3 m s−1), and case 2 with low liquid water content (0.1 g m−3) and higher wind speeds (typically 6 m s−1). In both cases the deposition velocity for liquid water was markedly less than for momentum. For case study 2 the cause of the small deposition velocity for liquid water was the small droplet size; the typical modal radius was 4.4 μm. The deposition velocity increased as a function of droplet size, exceeding that for momentum for droplets larger than about 6 μm. When normalized, the results agreed reasonably well with those obtained over grassland using the gradient technique. The results are compared with the predictions of the models of both Lovett and Slinn.
Validation of a regional atmospheric model over Europe: Sensitivity of wintertime and summertime simulations to selected physics parametrizations and lower boundary conditions
Authors: Giorgi Filippo , Marinucci Maria Rosaria
Published: February 2007

This paper analyses wintertime and summertime simulations over Europe using the National Center for Atmospheric Research (NCAR) regional atmospheric model (MM4). The sensitivity of the model to selected physics parametrizations (explicit moisture scheme; slower release of condensation heat; horizontal diffusion on s̀‐surfaces), and lower boundary conditions (sea surface temperature; initial soil moisture; snow cover) is examined. The simulation periods are January 1979 and June 1979; initial and lateral meteorological boundary conditions are provided by analyses of observations. The main focus of the analysis is on prediction by the model of surface air temperature and precipitation, but also sea‐level pressure and upper‐air variables are considered. In general, the model reproduces the main features of the synoptic events which prevailed during the two simulation periods. Mid‐tropospheric and upper‐tropospheric biases in the model are generally small. Surface maximum and minimum air temperature biases do not exceed a few degrees K over various regions of Europe. The largest model bias (2‐3 K) is found for summertime minimum temperature. Wintertime surface temperatures are sensitive to the precipitation parametrizations tested, as these affect cloud formation and, in turn, the surface radiative fluxes. Also, temperature biases in the driving large‐scale fields are partially transmitted to the surface air temperature calculations. Summertime temperatures are sensitive to the soil moisture content. Sea surface temperature variations influence land temperatures more strongly for wintertime than for summertime. Precipitation is sensitive to the parametrizations used. The explicit moisture scheme generally induces underprediction of precipitation. When this is not used, the biases are of the order of −10 to −20% of the observations. Summertime precipitation over the highest Alpine regions is overpredicted, except when diffusion of moisture and temperature along terrain‐following s̀‐surfaces is strongly reduced. Sea surface temperature variations substantially affect precipitation, especially over coastal areas, both in January and June. Summertime precipitation is significantly modified by the initialization of soil water content.
Baroclinic Waves Propagating From A High‐Latitude Source
Authors: Watterson I. G. , James I. N.
Published: February 2007

A time‐dependent primitive‐equation model of the southern hemisphere winter troposphere with an idealized dome‐shaped continent centred at 80°S has been used to assess the potential of Antarctica to force stationary waves and to determine the location of the mid‐latitude baroclinic storm track. Several integrations of the model for two hundred days were made with the zonal‐mean fields fixed at the climatological values. In the time mean of each run an anticyclonic wave response formed over the mountain and a planetary wave propagated into the mid latitudes with substantial amplitude. This suggests that much of the observed stationary asymmetry in the southern, mid and high latitudes may be due to Antarctica.

The response to the Antarctic forcing was different in a steady, linear version of the model. However, interaction between the steady waves appears to be of minor importance to the time‐mean pattern. Rather, E‐vector fields point to a substantial effect of the transients on the pattern. In particular, the low‐frequency transients appear to be important in the balance of the time‐mean equations near the mountain.
Enhanced Absorption of Solar Radiation By Cloud Droplets Containing Soot Particles In Their Surface
Authors: Chylek Petr , Hallett John
Published: February 2007

Laboratory studies show that when soot particles from acetylene combustion are captured by drops of size 0.1 to 1 mm they remain partially submerged in the surface for several hours. the intensity of solar radiation passing through a drop is calculated to be much greater near the surface, and therefore with such particles on the surface will give enhanced absorption compared with the situation in which the particles are uniformly distributed throughout the drop volume. This gives a possible explanation for enhanced cloud absorption when soot particles are present in amounts consistent with field measurements.
Organized Convective Systems: Archetypal Dynamical Models, Mass and Momentum Flux Theory, and Parametrization
Authors: Moncrieff Mitchell W.
Published: February 2007

A dynamical basis is established for understanding the structure and transport properties of organized convection and for expediting its parametrization in large‐scale models. A two‐dimensional model provides an exact theory of the momentum transport by mesoscale convective systems and similar phenomena. the system‐scale dynamics are supposed to dominate local processes and are modelled by a stationary triple‐branch airflow regime consisting of a double‐branch updraught and a downdraught. It is argued that a jump updraught is essential and has a fundamental effect on the momentum transport. the triple‐branch model degenerates into a form of conservative density current; another limiting case consists of a propagating positive pressure jump of maximum amplitude‐an overturning updraught but no downdraught. the functional relationship among dynamical parameters is determined by a characteristic regime equation derived from elementary nonlinear Lagrangian conservation properties and the volume integral of the horizontal‐momentum equation.

The inflow to the archetypal jump updraught is constant. an example shows that the inclusion of shear in this region alters the detailed shape of the momentum flux profile but its fundamental character, namely the negative values of momentum flux for a system travelling in the positive x‐direction, is retained. This result, together with recent numerical simulations, implies that low‐level shear directly influences the initiation and evolution of convective elements, whereas the mature‐state fluxes for which the system‐scale flow organization and tilt is paramount is a product of the distribution of heat sources/sinks and deep tropospheric shear. the universal nature of the momentum flux profiles is explained in elementary terms by appealing to dynamical theory.

The physical basis of the model and the momentum flux profiles are validated by using published results. the archetype emulates the basic character of the mass and momentum fluxes by mesoscale convective systems. For example an upper‐tropospheric flow deceleration is consistent with the observed effect of tropical cloud clusters on the mean flow but is distinct from the balanced response due to diabatic heating.

The theory is used to develop a dynamical approach to the parametrization of organized convective processes that have hitherto been neglected in global models. Mass and momentum fluxes are obtained from the archetypal model in an approach that is fundamentally different from the statistical or averaging approximations that characterize present techniques. the activation of the parametrization scheme is also studied. Mass‐flux criteria are used to define an amplitude function for the mesoscale flux divergence to incorporate the flux laws into the large‐scale equations. the work can be extended to include thermodynamic fluxes by using generalized conservation properties.
Dry Season Micrometeorology of Central Amazonian Ranchland
Authors: Wright I. R. , Gash J. H. C. , Da Rocha H. R. , Shuttleworth W. J. , Nobre C. A. , Maitelli G. T. , Zamparoni C. A. G. P. , Carvalho P. R. A.
Published: February 2007

This paper presents the first comprehensive micrometeorological measurements to be recorded over post‐deforestation Amazonian ranchland. the ranch was managed for the production of beef cattle and had been created by felling and burning the original rainforest 12 years previously. the measurements allow derivation of the aerodynamic roughness, and a description of the response of the energy balance and surface conductance to the progressing dry season. Zero‐plane displacement and roughness length were derived from wind‐speed profiles as O.17 ± 0.03 m and 0.026 ± 0.003m respectively, while measurement of energy partition was achieved, with excellent agreement, between three independent measurement systems. During the 1990 dry season, average evaporation diminished from 3.8 to 2.1 mm d‐1 as the Bowen ratio increased from 0.43 to 0.67. Values of surface conductance were derived and these compare well with expected trends.
Spatial Structure of Ultra‐Low‐Frequency Variability of the Flow In A Simple Atmospheric Circulation Model
Authors: James I. N. , James P. M.
Published: February 2007

The internally generated variability of the global circulation on time‐scales in excess of one year is investigated using a multilevel baroclinic primitive‐equation model with moderate resolution of dynamical processes. the model includes idealized heating and friction. Maximum variability is found on time‐scales of around a decade. Empirical orthogonal‐function analysis reveals that the principal component of this variability is an alternate splitting and coalescence of the subtropical and mid‐latitude jets. Associated changes in the strength and distribution of eddy activity are found. the character of the ultra‐low‐frequency variability is completely changed for very low resolutions, but seems relatively insensitive to details of model formulation at higher resolution. the possibilities of deriving a low‐order model which includes ultra‐low‐frequency variability are considered.
Structure and evolution of an isolated semi‐geostrophic cyclone
Authors: Schär Christoph , Wernli Heini
Published: February 2007

The evolution and structure of an indealized low‐pressure system is studied within the framework of the semi‐geostrophic dynamics and in the limit of uniform potential vorticiy. An isolated cyclone is grown from suitably chosen initial conditions, rather than studying the evolution of a longitudinally periodic train of baroclinic systems.

It is shown that the resulting development is able to produce a range of flow features that compare favourably with observationally based conceptual models of cyclogenesis. These features include in particular: (i) the simultaneous occurrence of both a cold and a warm front whose alignment shows some of the characteristics of occluded frontal systems and is skin to the notion of a frontal fracture, (ii) a dry descending air‐stream to the rear of the cyclone, (iii) a narrow region of maximum ascent within the warm front an its bent‐back portion, (iv) a poleward traveling air‐stream ahead of the cold front, and (v) a γ‐shaped pattern of vertical lifting comparable with the cloud patterns as commonly observed in satellite pictures.

The evolving cyclone is analysed both form Lagrangian and Eulerian viewpoints. It is demonstrated that Lagrangian criteria exist that allow for the objective definition of air‐streams and flow patterns within developing cyclones. The structures of cold and warm fronts at low levels are significantly affected by the different nature of the Lagrangian trajectories within each of these regions. In particular, the air parcels in the warm‐frontal region are transported rapidly towards the centre of the low, resulting in a low‐level warm front with an intrinsically three‐dimensional structure and an associated vorticity gradient in the along‐front direction.
Development of ice and precipitation in New Mexican summertime cumulus clouds
Authors: Blyth Alan M. , Latham John
Published: February 2007

An experiment, involving the National Center for Atmospheric Research's Kin Air aeroplane, was conducted in order to measure the microphysical properties of New Mexican summertime cumulus clouds. Since the clouds formed and developed essentially in place, over the mountains, it was possible to make multiple penetrations through a single cloud, therby observing a significant fraction of the cloud's life cycle. In this paper, the questions of primary‐ and secondary‐ice production, and the development of precipitation particles, are addressed.

Primary‐ice nucleation was found to occur when the temperature within the cloud reached a value of between −10 and −12°C irrespective of whether this was in the updraught or downdraught. Drops with diameters of about 0.5 mm were often observed in concentrations of about 10L−1 before the formation of ice. which suggests a nucleation mechanism involving large drops. The maximum concentrations of ice particles observed in these clouds (up to about 1300L−1) are much greater than typical concentrations of ice particles that can be attributed to primary‐ice nucleation. Evidence suggests that the most likely explanation is the Hallett‐Mossop process of secondary‐ice‐crystal production.

Ice particles generally were first observed in the downdraughts. The development of precipitation is often thought to occur via downdraught transport. followed by sedimentation or mixing of ice particles into fresh. liquid‐laden turrets. The multi‐thermal nature of the cloud is considered to be central to this process.
The potential for stratosphere‐troposphere exchange in cut‐off‐low systems
Authors: Price J. D. , Vaughan G.
Published: February 2007

Three mechanisms are discussed associated with cut‐off‐low (COL) systems which can transfer stratospheric air into the troposphere. These mechanisms are: (i) convective erosion of the tropopause, (ii) erosion of the tropopause by turbulence associated with a jet stream, and (iii) tropopause folding around the flank of a COL. Three case studies are presented which examine the first two of these processes and provide evidence that stratosphere‐troposphere exchange (STE) had occurred. Furthermore, results suggest that periodic reintensification of some COLs cause all these processes to recur or be prolonged. Their relative importance to STE is discussed and an estimate for that occurring by mechanism (i) is given for one of the case studies. Statistical data on annual COL numbers are used to calculate and annual mean transfer of ozone by this mechanism, which is found to be no more than 4% of the annual transfer for the northern hemisphere. This result suggests that convective STE in COLs is a minor contributor to the northern hemisphere STE.
Baroclinic‐wave life cycles, climate simulations and cross‐isentrope mass flow in a hybird isentropic coordinate GCM
Authors: Thuburn John
Published: February 2007

Results are presented from a number of simulations using a global circulation model to compare the use of a δ or hybrid δ‐p vertical coordinate with a hybrid δ‐θ‐p vertical coordinate. In idealized baroclinic instability experiments, without diabatic processes, δ coordinates and δ‐θ‐p coordinates give very similar results, with some suggestion of slightly improved Lagrangian potential‐vorticity conservation in the δ‐θ‐p coordinate case. In climate simulations it has been necessary to include extra vertical diffusion in the presence of noisy temperature profiles in order to achieve stable integrations using δ‐θ‐p coordinates (though this may by unnecessary if improved schemes for vertical advection and radiation are used.) The extra diffusion leads to a noticeably less noisy temperature profile around the tropopause. Large differences between δ‐p coordinate integrations and δ‐θ‐p coordinate integrations appear in the moisture field because the diabatic terms and vertical‐advection terms are computed in a different order, though the net moisture budget is not affected. Other diagnostics show that the two types of vertical coordinate give very similar results. Some examples are presented of a novel diagnostic: the accumulated cross‐isentrope mass flow. Global tracer conservation may be poor when hybird isentropic coordinates are used, unless care is taken to put the horizontal scale‐selective dissipation into the form of a flux divergence.
Time‐varying zonal asymmetries in stratospheric nitrous oxide and methane
Authors: Gao H. , Stanford J. L.
Published: February 2007

Previously analyses of Stratospheric And Mesopheric Sounder (SAMS) data of atmospheric constituent gases have dealt almost exclusively with zonal means (and mostly monthly means), owing perhaps to concern over data quality. The purpose of this note is to show that, with care, time‐dependent zonally‐asymmetric features may be recovered from the SAMS nitrous oxide and methane data. As an example, we demonstrate the existence of zonal wave‐1 constituent perturbations with periods of a few weeks in the middle and upper stratosphere. When the perturbations are normalized by the constituent zonal‐mean mixing ratio to compensate for the slowly varying (in both space and time) background concentration of constituents, wavepacket‐like features are found over all latitudes and seasons in the three‐year SAMS record. One specific low‐latitude case discussed had features which appear to be consistent with constituent oscillations induced by episodic equatorial Kelvin waves. Further studies are needed to better identify the nature of the plethora of observed wave‐like phenomena.
An integrated view of the 1987 Australin monsoon and its mesoscale convective systems. II: Vertical structure
Authors: Mapes Brian , Houze Robert A.
Published: February 2007

The vertical structure of monsoon thermal forcing by precipitating convection is diagnosed in terms of horizontal divergence. Airborne Doppler‐radar divergence profiles from nine diverse mesoscale convective systems (MCSs) are presented. The MCSs consisted of multicellular convective elements which in time gave rise to areas of stratiform precipitation. Each of the three basic building blocks of the MCSs—convective, intermediary, and stratiform precipitation areas—has a consistent, characteristics divergence profile. Convective areas have low‐level convergence, with its peak at 2‐4 km altitude, and divergence above 6 km. Intermediatry areas have convergence aloft, peaked near 10 km, feeding into mean ascent high in the upper troposphere. Stratiform areas have mid‐level convergence, indicating a measoscale downdraught below the melting level, and a mososcale updraught aloft.

Rawinsonde composite divergence profiles agree with the Doppler data in t least one important respect: the lower‐tropospheric convergence into the MCSs peaks 2‐4 km above the surface. Rawinsonde vorticity profiles show that monsoonal tropical cyclones spin‐up at these elevated levels first, then later descend to the surface. Rawinsonde observations on a larger, continental scale demonstrate that a large horizontal scales only the ‘gravest vertical mode’ of MCS heating is felt, while the effects of shallower components of the heating (or divergence) profiles are trapped near the heating, as predicted by geostrophic adjustment theory.
Moisture and heat budgets of a cirrus cloud from aircraft measurements during FIRE
Authors: Gültepe Ismail , Rao Gandikota V.
Published: February 2007

This study is based on the NCAR king Air aircraft and radiosonde on 31 October 1986 during the FIRE (First International Satellite Cloud Climatology Project Regional Experiment) in Wisconsin over Oshkosh. Aircraft step‐up and spiral descent flights are used to obtain kinematic and thermodynamic data. In the step‐up manoeuvre, six different penetrations were made between 1528 and 1616 UTC. Each penetration was about 30 km long separated in the vertical by about 300 m. The time difference between the two spiral soundings was about 43 minutes. The aircraft descended at a rate of 1.5 ms−1 during these spiral soundings. Kinematic, cloud physical, and radiometric observations from various instruments are used to estimate the different using the mean longitudinal wind and vertical radiative fluxes are important in forming budgets for the cirrus layers. Ice‐crystal growth is significant in the upper layers. The maintenance of cirrus can be attributed to relatively warm and moist air advection, radiative cooling at upper levels, and moisture advection in the vertical. Turbulent heat and moisture fluxes are found to be significant in the low levels of cirrus.
On the interaction of tropical‐cyclone‐scale vortices. I: Observations
Authors: Lander Mark , Holland Greg J.
Published: February 2007

A detailed analysis is made of the observed behaviour in interaction, tropical‐cyclone‐scale vortices in the western North Pacific region. It is found that all multiple‐vortex interactions can be broken down into a common model of binary interaction. The classical Fujiwhara model of converging cyclonic rotation about a centroid followed by merger is rarely followed. Capture tends to occur rapidly, within several hours, and is followed by a period of relatively stable cyclonic orbit. Cyclone merger occurs by one vortex experiencing a loss of convective organization, followed by horizontal shearing and incorporation into the outer circulation of the other vortex. However, a substantial proportion of interacting cyclones escape from the interaction, and the changeover from a stable orbiting configuration occurs rapidly. A model of binary interaction is presented.

Cases where swarms of mesoscale vortices are formed in convectively active regions without tropical cyclones are also documented. These vortices have a much narrower range of influence (a few hundred kilometres) then that observed for tropical cyclones. When groups of vortices come within this range they are observed to conform to the same interaction model as observed for tropical cyclones.
Airflow over a two‐dimensional escarpment. III: Nonhydrostatic flow
Authors: Blockley J. A. , Lyons T. J.
Published: February 2007

Forced flow over the asymmetric topography of the Darling Scarp, Western Australia is modelled, using a nonhydrostatic model, and compared with observations. Drag histories and sensitivity tests indicate that it is possible for the flow to be dominated by hydrostatic downslope windstorms even if the hydrostatic index indicates nonhydrostatic dominance. Experiments on a wide range of one‐ or two‐layered flows suggest that nonhydrostatic effects on windstorm events are very small. However, the development and location of trapped lee waves can be significantly affected if there is sufficient hydrostatic forcing in the flow. For hydrostatic forcing to dominate over trapped lee waves it is necessary for reflection, from a region of wave breaking or a critical layer, to occur at the right height.
Oscillations in the equatorial components of the atmosphere's angular momentum and torques on the earth's bulge
Authors: Bell M. J.
Published: February 2007

The angular momentum of the atmosphere is the sum of the wind term, W, due to the winds relative to the earth's surface, and the matter term, M. It is shown that if the earth were an oblate spheroid with an equipotential surface, the atmospheric torque on the earth would be ‐Ω ∧ M, Ω being the rotation rate of the solid earth. As a result, the equatorial components of M for a wave propagating at azimuthal angular velocity σ without changing shape are (Ω ‐ σ)/Ω times the equatorial components of W. Laplace's equation for tidal motions ‘on a sphere’ strictly applies to motions on such an oblate spheroid and its solutions apply a torque on the earth equal to ‐Ω ∧ M. It is shown that the resulting relationship between M and W also implies that inanyseparable wave solution of the tidal equations the surface wind is simply related to the vertical integral of the wind and the equivalent depth.

Analyses of the equatorial components of the matter term by the weather forecast systems of the European Centre for Medium‐range Weather Forecasts and the Meteorological Office are dominated by chaotic oscillations with periods of between 8 and 10 days that are well forecast out to 5 days ahead. It is argued that these are essentially free solutions of the tidal equations which exert considerable torques on the earth. The main feature of the equatorial components of the wind term is a seasonally modulated diurnal oscillation. Analyses and 2‐day forecasts of this phenomenon are in less good agreement. It is argued that it is thermally forced and depends on the compressibility of the atmosphere.
Non‐hydrostatic simulations of a cold front observed during the Fronts 87 experiment
Authors: Redelsperger J.‐L. , Lafore J.‐P.
Published: February 2007

A two‐dimensional non‐hydrostatic cloud model is used to simulate a frontal system observed on 11 November 1987 during the Fronts 87 experiment.

The cloud‐resolving model is able to reproduce a large variety of frontal bands, starting from only the meso‐α‐scale fields produced by a hydrostatic forecast model. A narrow cold‐frontal rainband is identified at the surface cold front. Two classes of wide bands are also simulated: a wide cold‐frontal rainband and warm‐sector wide rainbands. The vertical‐velocity fields exhibit similar banded features in both the simulations and the Doppler‐radar observations, with bands having the same position, intensity and width.

Several sensitivity experiments are presented. One of them shows that the initial knowledge of the ageostrophic circulation is not necessary to give a successful simulation. Other experiments show that the along‐front baroclinicity drives the wide‐band formation whereas the along‐front deformation is not important.

The heat, moisture and horizontal momentum budgets are also investigated, in order to determine the rainband effects on larger scales.
A variational modification algorithm for three‐dimensional mass flux non‐divergence
Authors: Ehrendorfer Martin , Hantel Michael , Wang Yong
Published: February 2007

The diagnosed three‐dimensional (3D) wind field should strictly satisfy the continuity equation; however, because of analysis, interpolation, and numerical errors this requirement is not met in practice. The resulting noise divergence is of the order of 10−6s−1. Its impact upon energy budgets is fatal. In order to remove the noise divergence it is suggested that a 3D minimum mean‐square modifying field is added to the diagnosed wind under the constraint that the modified wind strictly satisfies mass continuity. It is shown that the solution of the resulting variational problem, namely the modifying wind field, has the property of conserving the 3D vorticity of the diagnosed wind field.

In this paper the theory proposed for modification of the diagnosed wind field is rigorously developed for the first time for both the continuous and the discrete situation. The discrete problem—its solution being referred to as variational modification algorithm (VMA)—is approached by converting the diagnosed wind field into mass fluxes across the faces of finite boxes; the modifying mass flux components are obtained as differences of a potential function being the solution of a 3D Poisson equation which—owing to consideration of mass fluxes—is free of coordinate increments. A fast direct algorithm to solve this equation through decoupling coordinate directions is presented and compared with a fast Fourier algorithm.

The VMA is applied to the wind field over Europe, as routinely analysed by the European Centre for Medium‐range Weather Forecasts (ECMWF), for the specific case of the strong rain episode over central Europe on 1 August 1991. The wind is converted into mass flux components across the faces of boxes (100 km)2 × 100 hPa in size. The modification of the mass flux enforced by the VMA is about 7 per cent of the analysed mass flux, and 6 per cent of the initialized mass flux; this similarity suggests that the original 3D noise divergence is not in the ECMWF data (analysed or initialized), but is generated by the interpolation procedure from the ECMWF model coordinates to pressure levels. Investigation of analysed and initialized mass fluxes before and after application of the VMA reveals that the modification of the mass flux field through the initialization procedure at the ECMWF is considerably larger than the modification required to enforce non‐divergence through application of the VMA.

The impact of the modification upon the imbalance of the heat budgets in the atmosphere over Europe is investigated by a diagnostic model. It is found that the VMA reduces the root‐mean‐square values of the sensible‐heat imbalance from 518 (337) down to 66 (35) W m−2 for analysed (initialized) input data. The reduction is less dramatic for the latent‐heat imbalance (about 30 per cent).

The effectiveness of the VMA as well as its generalization to arbitrary coordinate systems and variable resolution are briefly discussed.
The northern hemisphere tropospheric response to changes in the gravity‐wave drag scheme in a perpetual January GCM
Authors: Stephenson David B.
Published: February 2007

The sensitivity of a general‐circulation‐model climatology to changes in the gravity‐wave drag scheme is investigated with particular emphasis being placed upon the northern hemisphere wintertime tropospheric circulation. It is found that by weakening the upper‐level drag, the Ferrel‐cell circulation intensifies to the north, resulting in a greater than 10 K cooling in the region of the arctic tropopause. The poleward side of the northern hemisphere sub‐tropical jet intensifies in the region of 60–70°N, and a poleward shift is observed in the eddy meridional momentum flux. A large part of the response is zonally asymmetric, with the Aleutian and Icelandic lows becoming shallower with weakened upper‐level gravity‐wave drag. By comparing the results of a gravity‐wave drag scheme having a wave‐breaking vertical profile with those of a simple uniform drag‐profile scheme it appears that the tropospheric response to weakened upper‐level drag depends very little upon the choice of vertical drag profile in the troposphere.
Transport into the troposphere in a tropopause fold
Authors: Vaughan G. , Price J. D. , Howells A.
Published: February 2007

A tropopause fold developed to the west of the British Isles on the western flank of a trough in the 300 mb flow on 6 October 1990. Radiosonde ascents over western Europe showed very dry stable layers beneath the jet stream in the potential temperature range 310‐315 K. These were evident on profiles from 12h 6 October to 00h 8 October. The European Centre for Medium‐range Weather Forecasts (ECMWF) model assimilations were examined for this period to determine how well they represented the radiosonde observations. Good agreement was found, especially for the humidity distribution. The model overestimated the vertical extent of the fold by a factor of about 2, so its representation of potential vorticity was less successful. Nevertheless, agreement was sufficiently good for the model to be used to evaluate the amount of stratosphere‐troposphere exchange in the event.

Isentropic trajectories were calculated for the air in the fold as represented by the ECMWF assimilation at 00h on the 7th. Those on the western edge of the fold split from the jet stream and transferred to the troposphere, while those on the eastern side ended up in the cut‐off low. The model fields were used to estimate a lower bound of 1.1 × 1014 kg and2 × 1032 molecules for the amount of stratospheric air and ozone, respectively, transferred into the troposphere by this fold. Owing to the model's incomplete representation of the event, the actual amount transferred may be at least twice this estimate.
The development of potential vorticity in a hurricane‐like vortex
Authors: Möller J. D. , Smith R. K.
Published: February 2007

We investigate the time evolution of the potential‐vorticity distribution in an axisymmetric hurricane‐like vortex using a numerical model based on a formulation by Schubert and Alworth. In particular, we compare the vortex response to an annular heating function with that in the analytic calculation by Schubert and Alworth in which the maximum heating rate occurs on the vortex axis. The annular heating function is intended to better represent the latent‐heat release in the eye‐wall clouds of a hurricane. We show that after about four days of integration time in the Schubert—Alworth calculation, the isentropes near the vortex centre, and hence the prescribed heating function, become concentrated near the surface in physical space. In comparison, for the annular heating distribution the descent of the isentropes is less marked and the vertical distribution of the heating in physical space remains more realistic vis‐à‐vis a hurricane. It is significant that in this case the potential‐vorticity maximum lies not on the vortex axis, as in the Schubert—Alworth calculation, but close to the radius of maximum heating. This appears to be consistent with observations in hurricanes. Finally we show that, counter to intuition, there is greater subsidence on the vortex axis in the case where the heating is a maximum on the axis. A dynamical explanation is offered for this result.
Simulation of the semi‐annual oscillation of the equatorial middle atmosphere using the Extended UGAMP General Circulation Model
Authors: Jackson D. R. , Gray L. J.
Published: February 2007

The semi‐annual oscillation (SAO) is examined in two runs of a middle atmosphere general circulation model. The first (run PGW) includes a parametrization of gravity waves with zero phase speed only, whilst the second (run AGW) also includes gravity waves with phase speeds of 20 m s−1. Run PGW successfully simulates equatorial SAOs near the stratopause and in the mid to upper mesosphere. However, the zonal winds at all heights are easterly and the amplitude of the oscillation is too weak by up to 15 m s−1 near the stratopause and too strong by about 45 m s−1 in the upper mesosphere. The corresponding SAOs in run AGW, however, have amplitudes that are within 5 m s−1 of those observed by rocketsondes, and have a pattern of easterly and westerly winds that are in reasonable agreement with the observations. Zonal mean momentum‐budget studies show that both at the stratopause and in the mid to upper mesosphere, the westerly phase of the oscillation in run AGW is stronger than in run PGW owing to an increase in total eddy forcing associated with the introduction of the 20 m s−1 phase speed gravity wave. The easterly phase of the stratopause oscillation on the summer side of the equator is driven by advection of easterlies across the equator. However, on the winter side of the equator, the net zonal forcing is westerly in May but easterly in November, since easterly eddy forcing associated with extratropical planetary waves is stronger in November than in May. The easterly phase of the mid to upper mesosphere oscillation is forced by contributions from both the residual circulation and eddy forcing. The eddy‐forcing contribution is more easterly than in other modelling studies owing to weaker Kelvin‐wave activity and to easterly forcing associated with model diurnal tides. Introduction of the 20 m s−1 gravity waves also improves the simulation of an N2O‐like tracer. In run AGW a ‘double peak’ of tracer is present in April, in reasonable agreement with observations, but this feature is absent in run PGW.