bluuebunny/crossref_metadata_2025 huggingface dataset:
Interannual displacements of convection and surface circulation over the equatorial Indian Ocean
Authors: Reverdin Gilles , Cadet Daniel L. , Gutzler David
Published: February 2007

The interannual variability of surface observations in the equatorial Indian Ocean is investigated from 23 years of ship reports (1954‐1976). In October and November, during certain years, the monthly analyses show strong wind anomalies in the eastern and central equatorial Indian Ocean. Simultaneously, cloud cover anomalies of opposite signs are observed in the eastern Indian Ocean and in the central Indian Ocean. A linear analysis of heat‐induced circulation suggests that the surface wind anomalies are forced by anomalies of rainfall. This circulation is probably dissipative with time scales in the range two to five days.

The reasons for the maintenance of anomalous circulations are not well known. It is noted that during most El Niño events in the Pacific Ocean, anomalies are observed in the Indian Ocean. However, there are other years with the same anomalous pattern, 1961 for example. During the years with anomalously little cloudiness in the eastern Indian Ocean, sea surface temperatures lower than normal are found in the eastern Indian Ocean. This suggests that coupled air‐sea dynamics must be taken into account to understand the interannual variability.
Numerical experiments on the sensitivity of the monsoon circulation to differential heating
Authors: Chen Shou‐Jun , Dell'Osso Lorenzo
Published: February 2007

Some numerical simulations from real data were carried out to examine the response of the monsoon circulation to moist processes. Two cases were selected: one for the monsoon onset and the other for the active (fully established) period. The results show that latent heat is the main feed‐back process in the monsoon circulation. The south‐west monsoon current from the Arabian Sea to the South China Sea in the lower troposphere and the easterly jet stream over southern Asia in the upper troposphere are greatly enhanced by this process both in the onset and active periods. The surface latent heat flux (mostly over the ocean) is important in the maintenance of the circulation during the active period but is less important for the onset.
Field studies of the optical and microphysical characteristics of clouds enveloping Great Dun Fell
Authors: Choularton T. W. , Consterdine I. E. , Gardiner B. A. , Gay M. J. , Hill M. K. , Latham J. , Stromberg I. M.
Published: February 2007

In an effort to study aspects of their evolution, simultaneous measurements have been made, in two separate studies, of the microphysical properties of cap clouds over Great Dun Fell, in Cumbria, at two vertically displaced sites along the line of the wind. Dynamical, optical (10.6 γm) and acoustic sounder measurements were also made.

Influences upon the cloud properties of humidity fluctuations near to cloud base, turbulent deposition of droplets to ground, additional activation caused by orographically induced accelerations within the clouds, entrainment of environmental air and long‐wave radiation to space were identified and examined.

The clouds studied exhibited structure in their microphysical and optical properties on all scales from a few metres to several hundred metres.

The cloud model of Carruthers and Choularton was found to predict the optical and microphysical characteristics of clouds not significantly influenced by entrainment, to a reasonable degree of accuracy.
Systematic errors of the ECMWF operational forecasting model in mid‐latitudes
Authors: Arpe K. , Klinker E.
Published: February 2007

The systematic errors of the ECMWF spectral operational model (April 1983 to April 1985) in midlatitudes are described. The main errors are:

* a zonalization of the time‐mean flow with considerably reduced diffluent flow at the jet exits;

* a wavenumber‐2 pattern in the mean height error field with forecast heights too low over Europe and the Bering Strait, and too high over Canada and Siberia;

* an upward shift of the subtropical jet and the tropopause;

* a cooling of the stratosphere;

* a cooling of the middle troposphere which decreases the static stability in the lower troposphere;

* an increase of the horizontal and vertical tilts of baroclinic waves;

* changes in the geographical distributions of baroclinic waves and cyclones, with enhanced activity near jet exits and reduced activity near areas of cyclogenesis;

* a drop of eddy kinetic energy during the 10‐day forecasts, first in the baroclinic waves and later in the long waves.

The errors in both hemispheres have similar characteristics.

The relations between the different errors and possible causes are discussed and comparisons with the errors of the former ECMWF operational grid point model are made. The introduction of the spectral model has led to significant improvements in the speed of baroclinic waves, which are now predicted more accurately up to day 3.
Super‐rotation and diffusion of axial angular momentum: I. ‘Speed limits’ for axisymmetric flow in a rotating cylindrical fluid annulus
Authors: Read P. L.
Published: February 2007

‘Super‐rotation’ can be defined with respect to the limitations on the magnitude of specific angular momentum, m, in an inviscid fluid. In a viscous fluid, a steady, super‐rotating axisymmetric flow is shown to require diffusion of m against its gradient ∇m in the meridional plane. The conditions under which m can be diffused in an incompressible fluid by molecular viscosity against ∇m in a cylindrical system are shown to be consistent with the normal properties of isotropic Newtonian viscosity (i.e. appropriate for the laminar flow of a viscous liquid in the laboratory) when cylindrical curvature is fully represented. A series of numerical simulations of thermally‐driven axisymmetric circulations in a cylindrical fluid annulus, subject to various mechanical boundary conditions, is then presented. The role of diffusion in the angular momentum budget of the simulations is examined by diagnosing m, its diffusive flux F (due to molecular viscosity) and divergence ∇·F from the final equilibrium (steady) flow. Up‐gradient diffusion (with respect to ∇m) is found to be particularly important for flows in a system with stress‐free top and side boundaries and a non‐slip base. Similar up‐gradient diffusion can also result in angular momentum expulsion effects in a system confined entirely by stress‐free boundaries. The characteristic dynamics of super‐rotation in a viscous fluid, and the associated limitations upon its magnitude and dependence upon the external conditions and fluid properties, are then explored in a scale analysis for the cylindrical annulus with a non‐slip base (based on a scheme due to Hignett, Ibbetson and Killworth). The most rapid super‐rotation (with zonal Rossby number > 1) is found to be favoured at moderate rotation rates, with strong cylindrical curvature, and a large meridional aspect ratio and Prandtl number. The results of the scale analysis are verified by means of further numerical experiments.
Conditional symmetric instability and mesoscale rainbands
Authors: Xu Qin
Published: February 2007

The linear theory of conditional symmetric instability (CSI) is re‐examined in a rigorous framework. In comparison with symmetric instability a new feature of CSI is that the moist updraught tends to be narrow, as with conditional buoyancy instability (CBI). As the width of the moist updraught varies from its tolerance maximum to infinitesimal, the inviscid growth rate increases from zero to its maximum and the slope of the moist updraught increases from the absolute momentum surface to the moist most unstable surface. The fact that CSI circulations absorb energy from the basic shear and moist thermal field but lose energy to the dry basic thermal field is responsible for the narrow and slant feature of the moist updraught. When a bulk viscosity is accounted for, the most rapidly growing CSI modes bear a qualitative resemblance to some observed rainbands. The stability criterion of viscous CSI also shows a better comparison with observational data than inviscid CSI. The linear CSI theory here predicts that the isolated mode is preferred to other non‐isolated (periodic or irregular spacing) modes. The preference of non‐isolated modes is speculated to occur in the nonlinear stage.
Diurnal variation and cloud in a general circulation model
Authors: Wilson C. A. , Mitchell J. F. B.
Published: February 2007

It is shown that changing the resolution of the diurnal cycle of cloud and radiative fluxes in an atmospheric general circulation model can affect the simulated climate. The effect on radiative forcing of using a temporal resolution which is unable to represent accurately the phase of diurnal variation in cloud is illustrated using a radiative‐convective version of the model. Similar results are found in the full model simulation for July, especially over land in the tropics. A detailed study of the changes over North Africa shows that the reduction in solar heating at the surface leads to a reduction in low‐level convergence and rainfall, and increased low cloud. The results indicate that cloud amounts need to be updated more frequently than four times a day if the model simulation is not to be degraded when the diurnal cycle is represented.
Momentum exchange at the sea surface by wind stress and understress
Authors: Bye John A. T.
Published: February 2007

Momentum exchange at the interface between two fluids is considered using general reasoning on energy dissipation in the coupled planetary boundary layers.

It is shown that the surface shearing stress can be expressed as a function of the relative surface geostrophic velocity between the two fluids, and that the surface velocity is a linear combination of the two surface geostrophic velocities. This latter result is in agreement with a result previously derived by E. B. Kraus directly from the momentum equations.

The analysis enables the ratio of the dissipation rates between the two fluids in the interfacial boundary layer to be determined. For the coupled ocean‐atmosphere, we find this ratio to be approximately (p1/p2)12 : where p1 and p2 are the densities of air and water, and we have made use of observations of the surface drift velocity and a dimensional argument which both suggest that the drag coefficients in the two fluids are approximately equal.

From the form of the expression for the surface shearing stress, it is clear that both fluids lose energy in the coupled boundary layer, and also that they either transfer energy to each other or extract energy from each other by interaction at the interface.

These energy fluxes are carried by two elemental stresses due to the general circulations of the atmosphere and the ocean respectively and which we call ‘wind stress’ and ‘understress’.

The mutual loss of energy implies that the interface exerts a drag on both the atmosphere and the ocean, and the interfacial energy exchanges show that the ocean does work on the atmosphere analogous to the work done by the atmosphere on the ocean.
Comparative energetics of the observed and simulated global circulation during the special observing periods of FGGE
Authors: Kung E. C. , Baker W. E.
Published: February 2007

Energetics of the observed and simulated global circulation are evaluated in the zonal spectral domain for the special observing periods of FGGE. The study utilizes GLA analyses of FGGE observational data and parallel simulation experiments. There are noticeable differences in energy transformations between the observation and simulation during SOP‐1. These include the baroclinic conversion C(n) by the zonal mean motion and short‐wave disturbances, and the nonlinear wave‐wave interaction L(n) at the long and short waves. The energy transformations of the short‐wave disturbances are much more intense in the simulated circulation than in the observation. However, good agreement is noted in the conversion and dissipation of kinetic energy in the large‐ and cyclone‐wave range n = 1–10. Spectral distributions of global energy transformations at the long‐ and cyclone‐wave range indicate that the SOP‐2 simulation agrees more closely with the observed fields than the SOP‐1 simulation. Other pertinent points of energetics diagnosis are also included in the discussion.
A study of wintertime circulation anomalies during past El Niño events using a high resolution general circulation model. I: Influence of model climatology
Authors: Palmer T. N. , Mansfield D. A.
Published: February 2007

In this paper the response of an 11‐layer general circulation model (GCM) to an enhanced composite El Niño sea surface temperature (s.s.t.) anomaly is studied. The integrations were run for 540 days in a perpetual January mode, and had either mean or envelope orography.

A basic objective of this paper is to study the influence of model climatology on the response to a tropical s.s.t. anomaly. This was done both by comparing our results with those from a lower resolution GCM (the NCAR Community Climate Model) with an identical s.s.t. anomaly, and by performing control and anomaly integrations with both mean and envelope orography. The specification of orography strongly influences the strength of the mean zonal flow, and stationary and transient wave activity in our model. Whereas the runs with envelope orography produced an anomalous Pacific/North American (PNA) teleconnection pattern in the northern hemisphere, the runs with mean orography did not. In the extratropical southern hemisphere. our model produced a statistically significant response which had a substantial zonally symmetric component. By contrast, the NCAR model, with weaker zonal flow, produced a weaker and mainly statistically insignificant response in the southern hemisphere. Finally, in the tropics, over the Central and Eastern Pacific, the anomalous rainfall and vorticity fields correlated strongly with the corresponding fields in the control integration. Overall, therefore, we conclude that a GCM's response is strongly sensitive to the basic model climatology.

In trying to understand dynamically the GCM response it was concluded that the PNA pattern may not simply be the result of Rossby wave dispersion from a tropical source. The PNA pattern may also be associated with a combination of two separate influences, the interaction with the anomalous mean flow of enhanced storm track activity over the Pacific, and with orography on the North American continent.

The implications of the model's tropical response for simple linear theory were also discussed. In these composite El Niño anomaly experiments enhanced rainfall and upper divergence occurred well to the west of the upper tropical anticyclone pairs, implying that a Sverdrup type balance in the vorticity equation cannot hold. It is suggested that the strength of the anomalous vortex stretching term could be a small residual compared with the horizontal advection terms, and that a simple linear relationship between anomalous latent heating and the position and strength of the anomalous anticyclone pairs is not always valid. An alternative qualitative explanation for the relative position of these anticyclone pairs is given in terms of a reduction in the zonally asymmetric component of upper tropospheric vorticity across the tropical Pacific, whilst the position of the rainfall maximum is located close to the s.s.t. maximum.
A new convective adjustment scheme. Part II: Single column tests using GATE wave, BOMEX, ATEX and arctic air‐mass data sets
Authors: Betts A. K. , Miller M. J.
Published: February 2007

The schemes proposed in part I are tested using single‐column data sets from tropical field experiments (GATE, BOMEX, ATEX) and an arctic air‐mass transformation. Both the deep and shallow schemes perform well. The sensitivity of the schemes to adjustable parameters is also studied. Preliminary global forecasts show significant improvements in the global surface fluxes and mean tropical temperature tendency over the operational Kuo convection scheme.
Trace constituents in the Austral stratosphere
Authors: Galbally I. E. , Roy C. R. , O'Brien R. S. , Ridley B. A. , Hastie D. R. , Evans W. F. J. , McElroy C. T. , Kerr J. B. , Plumb R. A. , Hyson P. , Laby J. E.
Published: February 2007

Measurements of NO, NO2, HNO3, O3, N2O, CCl3F, H2O, CH4 and aerosol were made during balloon flights in the stratosphere over Australia at 23°S and 34°S from March to October 1979. These data and ground‐based column density measurements of O3 and NO2 are presented and compared with previous measurements of the same species in the southern hemisphere stratosphere.

Diurnal variations of NO and NO2 are presented. These measurements show that there is a temporary reservoir which, during the daytime, releases perhaps one third of the total NOx present at sunset.

Vertical and latitudinal variations of NOy (NO + NO2 + HNO3) and the partitioning between the three species are presented. There is a substantial increase in NOy towards high latitudes. HNO3 is a larger fraction of NOy in the mid‐latitude westerlies compared with the subtropical easterlies as observed in a mixing event involving air from higher latitudes.

Unusual profiles of aerosol, CCl3F and NO were observed that can be explained by an intrusion of air from lower altitude into the 27 to 32 km height range in the equatorial and temperate latitudes of the southern hemisphere during March to May 1979.
Multiple state stochastic models for the long‐range transport and removal of atmospheric tracers
Authors: Egbert Gary D. , Baker Marcia B.
Published: February 2007

We discuss a general mathematical framework for modelling long‐term average concentration and deposition patterns for atmospheric tracers which are subject to removal and transformation. For a generalized ‘particle’ which can exist in a finite number of states, the joint statistics of random particle transport and interstate transformation can be used to calculate long‐term mean fields for deposition and concentration. We show that a number of models presented in the recent literature can be considered as simple cases of this formulation. These models generally assume a weak statistical relationship between vertical transport, horizontal transport and removal processes. We consider a general approach to linking these processes, based on calculating approximate transport statistics conditional on particle states. We demonstrate this approach with a rather schematic, but almost analytically solvable, stochastic model which allows for the linkage of transport and removal processes. We use these ideas to construct a two‐layer model of the transport and removal of sulphur in a region of convective precipitation, demonstrating how the correlation of vertical transport and precipitation can change the precipitation statistics in the particles' Lagrangian frame and hence affect deposition patterns.
The effect of rimer surface temperature on ice splinter production by the Hallett‐Mossop process
Authors: Griggs D. J. , Choularton T. W.
Published: February 2007

The effects of rimer surface temperature and environment temperature on spike production as supercooled water droplets are accreted by an ice surface immersed in a liquid bath have been investigated.

When the rimer surface temperature and the environment temperature were the same, spikes were produced only in the temperature range −3° to −9 °C. However, when they were varied independently it was found that the upper temperature limit for spike production of −3 °C is controlled more by the rimer surface temperature, provided that the environmental temperature is at or below −3 °C. By contrast, the low temperature cut‐off for spike production at −9 °C is almost independent of the rimer surface temperature and is controlled more by the environmental temperature.

If applicable, this could suggest that the Hallett‐Mossop process will not be displaced to lower temperatures when rapid riming of hailstones occurs in clouds with high liquid water contents.
Studies of internal gravity waves at Halley Base, Antarctica, using wind observations
Authors: Rees J. M. , Mobbs S. D.
Published: February 2007

At Halley Base, which is situated on the Brunt Ice Shelf in Antarctica, internal gravity waves are frequently observed in the stably stratified atmospheric boundary layer. In this paper, time series of wind speed and direction at a height of 8 m, observed from three meteorological masts, are used to determine some of the physical properties of the waves, such as their wavelengths, phase speeds and directions of propagation. The relationship of the results to various theories of the generation of internal gravity waves is considered. It is shown that the waves do not usually propagate in the direction of the surface wind; typically the direction of propagation is rotated about 45° clockwise from the wind vector. The waves most frequently propagate from a direction of about 135°, which is perpendicular both to the Hinge Line (line where the ice shelf meets the land) and to the ridges in the ice shelf, suggesting a topographic influence. It is shown that the waves obey a dispersion relation similar to that for neutral, trapped internal gravity waves.
On the nitrogen budget for the United Kingdom and north‐west Europe
Authors: Derwent R. G. , Dollard G. J. , Metcalfe S. E.
Published: February 2007

The application of a simple trajectory model approach to the emission, transformation and deposition of a coupled series of atmospheric pollutants including SO2, NOx and ammonia is described. A detailed comparison of the model results with observations is provided using United Kingdom monitoring data. Whilst reasonable agreement can justifiably be claimed for some species, whether in the gas phase, aerosol or precipitation, it is clear that for the others there are either severe disagreements or a lack of suitable data for comparison purposes. Background sources of these pollutants are assessed by applying wind sector analysis to precipitation measurements at remote sites. The budgets for the United Kingdom and north‐west Europe have a similar balance between wet and dry deposition to that given in the literature for the global nitrogen budget. This is in conflict with some literature budgets for Europe as a whole which have apparently overestimated the dry deposition terms.
A dual‐satellite algorithm for deriving sea surface temperature
Authors: Dudhia Anu
Published: February 2007

This paper describes an extension of conventional multichannel s.s.t. algorithms for NOAA‐7 AVHRR/2 data to include data from the Meteosat‐2 IR1 (11 μm) channel, generally viewing through a different zenith angle. This provides a combination of multichannel and dual‐angle techniques for determining the atmospheric correction. These algorithms, derived statistically from the results of transmission calculations on a number of temperature/humidity profiles, were tested under ‘pseudo‐operational’ conditions against ship measurements of s.s.t., and the results compared with the AVHRR‐only algorithms of other workers. The conclusion is that the technique is feasible but, due to the uncertainties in the Meteosat radiances caused by residual cloud contamination, the additional data are given only a low weight in the algorithms and the influence on retrieved s.s.t. is small. There were also opportunities to test the Rutherford‐Appleton Laboratory transmission model used in deriving the algorithms, and the results suggest that the model underestimates the absorption by the water vapour continuum in the 11 μm window.
A global numerical weather prediction model with variable resolution: Application to the shallow‐water equations
Authors: Courtier Philippe , Geleyn Jean‐Francois
Published: February 2007

We follow the approach suggested by F. Schmidt to implement a spectral global shallow‐water model with variable resolution. A conformal mapping is built between the earth and a computational sphere and the equations are discretized on the latter using the standard spectral technique associated with a collocation (Gaussian) grid. We prove that the only non‐trivial conformal mapping which exists between the two spheres is based on the transformation introduced by Schmidt, but the pole of the collocation grid has no longer to coincide with the pole of dilatation. We implement the technique in an explicit model, where only minor modifications to a uniform resolution model are needed. The semi‐implicit scheme and the nonlinear normal mode initialization are proved to work satisfactorily. 24‐hour forecasts show that the method is successful in dealing with the shallow‐water equations and allow us to discuss the potential of the approach.
An analysis of the vertical structure equation for arbitrary thermal profiles
Authors: Cohn Stephen E. , Dee Dick P.
Published: February 2007

The vertical structure equation is a singular Sturm‐Liouville problem whose eigenfunctions describe the vertical dependence of the normal modes of the primitive equations linearized about a given thermal profile. The eigenvalues give the equivalent depths of the modes. We study, for arbitrary thermal profiles, the spectrum of the vertical structure equation and the appropriateness of various upper boundary conditions.

Our results depend critically upon whether or not the thermal profile is such that the basic state atmosphere is bounded. This is not surprising since, as we point out, the vertical structure equation is not meaningful at large heights because of the traditional shallowness approximations which are used to derive the primitive equations. The nature of the spectrum of a singular Sturm‐Liouville problem depends only on the behaviour of the coefficients of the differential equation near the singular boundary. Spectral results therefore have no physical significance for unbounded atmospheres.

For all bounded atmospheres we show that the spectrum is totally discrete, regardless of details of the thermal profile. For the barotropic equivalent depth, which corresponds to the lowest eigenvalue, we obtain upper and lower bounds which depend only on the surface temperature and the atmosphere height. All eigenfunctions are bounded, but always have first derivatives which become unbounded near the top. We prove that the commonly invoked upper boundary condition that vertical velocity must vanish as pressure tends to zero, as well as a number of alternative conditions, are well posed.

For unbounded atmospheres, on the other hand, we show that typically there is a continuous spectrum, that the boundary condition of vanishing vertical velocity is not well posed, and that the eigenfunctions. if any. are unbounded.
Kinetic equations for 2‐D turbulent flow
Authors: Tapp M. C.
Published: February 2007

A formal BBGKY kinetic theory for inhomogeneous 2‐D fluid turbulence is developed from the Liouville equation for Fourier mode amplitudes. The analysis indicates that all members of the hierarchy are present except in the special case of unforced barotropic flow where only the two‐coefficient distributions are explicitly absent. A graphical representation of the BBGKY hierarchy is presented, providing a vivid picture of the interaction processes involved. Equations representing the turbulence dynamics of correlations are derived for binary‐ and triad‐wave groups and exhibit a highly nonlinear structure. It is shown that, in the inhomogeneous case, binary‐wave interaction via the mean flow plays an important role, in contrast with accepted homogeneous theories. A phenomenological approximation belonging to the eddy‐damped class of turbulence models is developed and yields a set of kinetic equations for which conservation laws and an H‐theorem can be established. The kinetic equations can be recast into a familiar Fokker‐Planck form written in intrinsic coordinates and demonstrate the approach to absolute equilibrium for the case of forced barotropic inhomogeneous flow. A certain information measure is shown to be a maximum at t = 0 for the exact dynamics and suggests that forcing gives rise to an increased rate at which information is lost.
Tidal fluctuations as seen in ECMWF data
Authors: Hsu Huang‐Hsiung , Hoskins Brian J.
Published: February 2007

Diurnal and semi‐diurnal fluctuations in the troposphere and lower stratosphere during the 1986 summer and 1986/87 winter, deduced from data of the European Centre for Medium Range Weather Forecasts, are documented. In general, the tidal fluctuations are consistent with the results of previous investigators based on station data. It thus suggests the adequacy of the data generated by the analysis‐forecast system for presenting the global structure of tidal fluctuations in the troposphere.

The semi‐diurnal fluctuations in ECMWF data are consistent with the predictions of tidal theory. The diurnal fluctuations, which are more sensitive to local factors such as the sea‐land contrast and topography, exhibit a complicated geographical dependence. It appears that the current tidal theory, by neglecting the lower boundary effects and zonally varying factors, is incomplete for explaining the diurnal tidal motions in the troposphere.
A comparison of satellite‐retrieved parameters with mesoscale model analyses
Authors: Saunders R. W.
Published: February 2007

A scheme to retrieve quantitative products from radiances recorded by the AVHRR (Advanced Very High Resolution Radiometer) on NOAA satellites over the same area as the Meteorological Office mesoscale model is outlined. Satellite products and model fields are both averaged over fifth of a degree latitude/longitude grid squares. Results for one satellite overpass over the British Isles on 21 June 1986 are described in detail. Fields of surface temperature over land and sea, fractional cloud cover and cloud top temperature retrieved from the AVHRR data are compared with the corresponding model‐analysed fields nearest in time to the satellite overpass. The AVHRR and model sea surface temperatures are in reasonable agreement (∼0.7K), although the satellite data provide the more detailed field. The satellite land surface brightness temperatures could not be easily compared with the model ground temperatures as the former are radiative skin temperatures whereas the latter are related to screen temperatures and near‐surface lapse rates. Differences of up to 10 K are noted in clear sky situations. The satellite and model fractional cloud cover fields are in reasonable agreement though the satellite fields did highlight one or two areas where the model analysis is in error. This is mainly in areas devoid of conventional observations. Fields of satellite cloud top temperatures show more detail and are more realistic than the corresponding model fields. This is not surprising as cloud top temperatures cannot be easily inferred from the surface observations analysed by the model. Satellite products, such as the ones described here, could in the future be used operationally to improve mesoscale or regional model analyses.
A study of air particle motions during a stratospheric warming and their influence on photochemistry
Authors: Austin John , Butchart Neal
Published: February 2007

Middle stratospheric ten‐day trajectories are calculated using satellite data for air particles initially situated on the edge of the circumpolar vortex during the minor warming of January 1979. The trajectory behaviour is consistent with the observed evolution of the isentropic distribution of potential vorticity. Further, from an analysis of a large number of trajectories calculated from closely spaced initial positions it is argued that the trajectories are realistic indicators of transport. In particular, the trajectories divide naturally into a limited number of distinct categories representing either vortex erosion or the continued advection of air around the distorted vortex edge. None of the categories indicate horizontal transport of air into the vortex.

A comprehensive photochemical model is integrated along the trajectories and the results are compared with similar integrations along trajectories calculated for a suitably chosen axially symmetric reference atmosphere. In this way it is established that, for the late January warming, the distortion of the vortex from zonal symmetry led to substantial decreases in the destruction of ozone and in the production of nitric acid on the edge of the vortex. Once the particles are stripped from the vortex by the wave breaking, they experience only small changes in photochemistry if the particles remain in middle latitudes, or much larger changes if they are transported as far as the subtropics. Results from the photochemical model are also compared with co‐located satellite observations of the constituents ozone, water vapour and nitric acid for the different categories of trajectories. In many cases there are statistically significant discrepancies between observed and modelled concentrations. It is suggested that these discrepancies are due to the presence of other particles transported along trajectories from different locations, and therefore with different chemical properties, contaminating the coarse resolution co‐located observations. The findings are entirely consistent with the view that outside the vortex the wave breaking will mix or generate small unresolvable scales in the tracer fields. Some implications of the results for the photochemical modelling of the stratosphere are discussed.
The equatorial Pacific Ocean prior to and during El Niño of 1982/83—a normal mode model view
Authors: Bigg Grant R. , Blundell Jeffrey R.
Published: February 2007

An equatorial ocean model with multiple vertical and horizontal modes is developed for the Pacific Ocean, and run for the five years 1979 to 1983. In the year prior to the warm event, or El Niño, of 1982/83 the model shows an unusually strong and delayed seasonal Kelvin wave generated in the western Pacific. By analogy with the ocean‐atmosphere coupling mechanism postulated by several recent authors for explaining the El Niño/Southern Oscillation cycle, it is suggested that this wave could ultimately have been instrumental in causing the subsequent El Niño to be so strong. The course of events during El Niño is described in terms of equatorial normal modes and related to the evolving wind field. The strength and importance of the second vertical mode during the event is emphasized. Evidence for a coupled system decay mechanism is also found. The model is compared with several types of observations, but principally the expendable bathythermograph (XBT) data of the Pacific Ship of Opportunity Programme. These data are analysed from a normal mode perspective which is related to that of the model, supporting the main conclusions derived from it. Such data have not been analysed previously in this manner, converting observed variables directly to theoretical equatorial wave amplitudes. The adequacy of the data for such analysis is discussed.
Response to small‐scale forcing on two staggered grids used in finite‐difference models of the atmosphere
Authors: Janjić Zaviša I. , Mesinger Fedor
Published: February 2007

‘Source‐sink’ shallow water integrations are performed, using two limited area models with the Coriolis terms included. One of the models was defined on the staggered C grid, and the other on the semi‐staggered E/B grid. For each grid, experiments were performed with grid sizes of 250, 125 and 62.5 km. A source and a sink, of constant intensity, were placed symmetrically in the central part of the domain. They covered, for the three resolutions considered, areas of one, four and sixteen grid points, respectively.

At the sink ‘point’, after 24 hours, substantial differences in the depth of the water existed between the lowest resolution experiments on the two grids. The differences were very much reduced in the two 125 km experiments, and were virtually absent in the highest resolution, 62.5 km, experiments. With increasing resolution, the depths at the sink on the two grids converged with about equal rapidity towards a value in between the two lowest resolution values. As to be expected on the basis of the geostrophic adjustment theory when applied to the spatially discretized systems, the depth on the low resolution C grid solution was found to be an underestimate, and that on the E grid an overestimate, of the true value. It is demonstrated that the rate of convergence on the E grid can be improved by increasing the weight of the modification introduced to cope with the grid separation problem.

An implication of the results obtained in the simple experiments reported here is that forcing schemes should be considered which would avoid forcing at single grid points. Where appropriate, simultaneous forcing at several neighbouring points may be an attractive alternative approach.
An extended version of a nonhydrostatic, pressure coordinate model
Authors: White A. A.
Published: February 2007

The acoustically‐filtered, nonhydrostatic flow model formulated by M. J. Miller and R. P. Pearce in 1974 may be modified so as to remove dependence on a certain thermodynamic reference state. The modified model seems appropriate for mesoscale integrations. It implies an elliptic diagnostic equation for the height field, a satisfactory energy equation and an analogue of Ertel's potential vorticity theorem. These properties are also possessed by Miller and Pearce's original model, but in different forms (which have been obtained previously by various authors).
A scheme for predicting layer clouds and their water content in a general circulation model
Authors: Smith R. N. B.
Published: February 2007

A cloud water variable has been included in the Meteorological Office atmospheric general circulation model. the cloud amount and water content are generated by a scheme which assumes a distribution of thermodynamic and water content variables about their grid‐box‐mean values. A parametrization of the rate of depletion of cloud water by precipitation is included. the model's boundary layer mixing is reformulated in terms of variables conserved during changes of state of water, allowing a more realistic calculation of atmospheric stability in cloudy regions.

The scheme produces realistic cloud distributions and the predicted cloud water content verifies well against microwave radiometer data in all regions except the mid‐latitudes of the summer hemisphere. the parametrization of cloud optical properties in terms of the cloud water content will be an important future development of the model.
The theoretical relationship between foliage temperature and canopy resistance in sparse crops
Authors: Shuttleworth W. James , Gurney Robert J.
Published: February 2007

One‐dimensional, sparse‐crop interaction theory is reformulated to allow calculation of the canopy resistance from measurements of foliage temperature. A submodel is introduced to describe eddy diffusion within the canopy which provides a simple, empirical simulation of the reported behaviour obtained from a second‐order closure model. the sensitivity of the calculated canopy resistance to the parameters and formulae assumed in the model is investigated. the calculation is shown to exhibit a significant but acceptable sensitivity to extreme changes in canopy aerodynamics, and to changes in the surface resistance of the substrate beneath the canopy at high and intermediate values of leaf area index. In very sparse crops changes in the surface resistance of the substrate are shown to contaminate the calculated canopy resistance, tending to amplify the apparent response to changes in water availability. the theory is developed to allow the use of a measurement of substrate temperature as an option to mitigate this contamination.
Numerical prediction of thunderstorms—has its time come?
Authors: Lilly Douglas K.
Published: February 2007

An approach to developing numerical prediction of convective storms and storm environments over the next decade is outlined. the principal scientific challenges are described, along with some early progress made in meeting them.
Eastward travelling waves in the southern hemisphere stratosphere during the spring of 1983
Authors: Shiotani M. , Kuroi K. , Hirota I.
Published: February 2007

This paper presents a case study of planetary waves in the southern hemisphere stratosphere during the spring of 1983, using the global temperature and geopotential height data provided by the United States National Meteorological Center (NMC). the eastward travelling wave of zonal wavenumber 2 with a period of 9.2 days persisted from mid September to early November. It had a phase structure which showed only slight tilt with height and latitude, while the quasi‐stationary wave 1 tilted westwards with increasing height and towards the equator. During the vigorous period of the eastward travelling wave 2, the amplitude of the quasi‐stationary wave 1 varied with a period of about 10 days. This variation was due to a coincidence between the phases of the stationary wave 1 and the eastward travelling wave 2; when the ridges of waves 1 and 2 overlap, wave‐1 amplitude reaches its maximum. the active period of the travelling wave 2 is closely related to the seasonal change in the mean zonal winds. In 1983 the eastward travelling wave 2 became vigorous after the zonal‐mean westerly jet moved rapidly poleward and downward. Compared with the other 5 years for which we have data, the wind structure for the month of September 1983 was peculiar because of the stronger meridional curvature of the westerly jet.
Computational aspects of moisture transport in global models of the atmosphere
Authors: Rasch Philip J. , Williamson David L.
Published: February 2007

Computational aspects of methods used to simulate the transport of water vapour in a global atmospheric general circulation model are examined. A set of properties useful in characterizing numerical methods for modelling atmospheric transport are identified. Spectral and semi‐Lagrangian methods, which are very different in terms of these desired properties are compared.

The extent to which the schemes do not satisfy certain properties of the continuous equations provides a measure of one component of the error of the solution. For the spectral scheme, negative specific humidities q indicate such an error component. Conventional semi‐Lagrangian schemes are also susceptible to generating negative values. In addition, they are not inherently conservative. Shape‐preserving semi‐Lagrangian methods do not generate negative values, but still are non‐conservative. the degree to which the advection process does not conserve mass provides a measure of another error associated with the numerical solution. the negative error is shown to be large for the spectral transport scheme, measured either locally or globally. Measured globally, the semi‐Lagrangian transport schemes' conservation errors are equally large. Locally, the correction of this error can be made very much smaller, relative to physical processes in the model.

The study highlights the computational problems which still exist within the better numerical methods used to simulate the transport of water vapour, and demonstrates the care with which one must apply computational constraints to the solution. the spectral and semi‐Lagrangian transport schemes produce very different climatologies in model simulations. A comparison of these climatologies will appear elsewhere.
A study of aerosol properties and solar radiation during a strawburning episode using aircraft and satellite measurements
Authors: Kilsby C. G.
Published: February 2007

Aircraft measurements of radiative fluxes over the North Sea during a heavy straw‐burning pollution episode are presented and compared with results from a radiative transfer model.

The model uses aerosol properties derived from aircraft and satellite measurements, and acceptable agreement with the measured fluxes is obtained.

High levels of aerosol concentration are described, with correspondingly large radiative effects. Boundary layer heating rates of 3K day−1 are measured, and rates of 5‐10K day−1 in regions of higher aerosol loading are inferred.
Parametrization of tropical ocean heat flux
Authors: Wells N. C. , King‐Hele S.
Published: February 2007

A heat‐flux scheme, based on the assumption of a constant relative humidity and air‐sea temperature difference, is tested for the tropical oceans. It is compared with heat fluxes over the Pacific Ocean, determined by empirical formulae and maritime observations, both for long‐term monthly‐mean fluxes and for interannual variations in heat fluxes over the equatorial Pacific Ocean.

It is shown that the differences in latent heat flux between values obtained by the scheme and those calculated from maritime observations are generally no greater than the uncertainty in the bulk aerodynamic method. the method is also shown to be successful in reproducing the interannual variation in surface heat loss in the eastern Pacific Ocean, though the variability is less than that obtained from maritime observations in the coastal upwelling zone adjacent to South America.

Finally, the sensitivity of the heat‐flux scheme to sea surface temperature, for wind speeds typical over the tropical Pacific Ocean, is compared to other schemes. It is shown that the sensitivity coefficient is a function of both wind speed and sea surface temperature, and has values in the range 2‐16 W m−2 K−1 for the tropical ocean.
North atlantic blocking during January 1979: Linear theory
Authors: Frederiksen J. S. , Bell R. C.
Published: February 2007

The second half of January 1979 was a time of persistent and large‐scale blocking in the North Atlantic region. In this study we examine the instability properties of the flow during this period and relate the growing modes to the development that occurs. We have analysed the growth rates, phase frequencies and structures of the ten fastest‐growing modes, within a five‐level quasi‐geostrophic model incorporating spherical geometry, on each day for the period between 10 and 30 January 1979.

Large‐scale equivalent barotropic dipole or multipole instability modes are prevalent in the North Atlantic during the second half of January. These modes are stationary or slowly propagating and grow rapidly with efolding times as short as 1.7 days in some cases. We find that there is a close connection between these modes and the dynamical developments observed subsequently a few days later. Associated with the early stages of block formation, there are large‐scale eastward propagating westward tilting onset‐of‐blocking wave‐train modes. the onset‐of‐blocking modes also appear to be associated with rapid cyclogenesis off the east coast of North America.

The formation of the North Atlantic blocking system and diffluent flow also produces corresponding changes in the storm tracks. During the early parts of this period the fastest‐growing monopole cyciogenesis instability modes have regions of preferential development in the climatological storm tracks over the Pacific, Atlantic and Siberian regions. Subsequently, however, the Atlantic cyclogenesis modes split into wave trains poleward and equatorward of the block.

In the developments during this period, we also examine the roles of the adjoint eigenmodes and of optimal perturbations which produce the largest initial tendency of the L2 norm squared of the streamfunction.