bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimation of skill and uncertainty in regional numerical models
Authors: Anthes Richard A. , Kuo Ying‐Hwa , Hsie Eirh‐Yu , Low‐Nam Simon , Bettge Thomas W.
Published: February 2007

The results from 72‐hour simulations and forecasts from the Penn State/NCAR limited‐area model, in which a number of numerical and physical factors are varied, are analysed to understand the contribution to model error or uncertainty introduced by these factors. The factors include the initial conditions, horizontal resolution (80 km and 160 km) and domain size, lateral boundary conditions, and physical parametrizations. The results are compared with those of a global forecast model (the NCAR Community Climate Model). The simulations and forecasts are verified both for 12 individual cases and for the ensemble average of the 12 cases, using several objective measures of skill. The differences in these skill scores between different models are tested for their statistical significance.

The use of observed lateral boundary conditions (LBC) exerts a strong control on the growth of errors over a domain size 3600 × 4800 km2. Objective measures of error show little growth beyond about 36 hours, so that 72‐hour errors are nearly as low as the 36‐hour errors. On these time and space scales, the quality of the LBC is more important than any other factor tested in the temporal evolution of the model errors. These results show that the large‐scale atmospheric motions have a major effect on the evolution of small‐scale features in the model.

Small variations in initial conditions have little effect on the model skill beyond 12–24 hours. Of all the factors examined, small uncertainties or errors in the initial conditions have the least effect on model skill beyond 12 hours.

Latent heating effects associated with condensation and precipitation, and sensible and latent heat fluxes from the surface have a statistically significant effect on model skill. However, relatively simple parametrizations of these effects produce nearly the same skill as do more complex schemes for the cases studied here.

There is a large variation in model skill from case to case; some cases are easier to forecast than others. Also, there is much greater variation in different model performances for a single case than when averaged over all 12 cases.

The concept of climatological use and verification of regional models is introduced. The climatological skill of the control model version which includes all physical processes appears to be quite good. The skill scores of the ensemble average simulation are considerably better than the average scores for individual cases, the model has small bias errors, and the horizontal structure of the model‐simulated atmosphere is similar to the observed structure for scales of motion resolved by the upper‐air observational network over the United States.
A numerical study of the parametrization of deep tropical convection
Authors: Gregory D. , Miller M. J.
Published: February 2007

A two‐dimensional mesoscale cloud model is used to simulate an ensemble of deep tropical convective clouds using GATE data. Random heating perturbations near the sea surface initiate the development of a population of clouds which grow under the influence of specified large‐scale, radiative and surface forcing. Budgets of convective heating and moistening are presented and the model found to be a reasonable representation of the atmosphere.

The model is used to investigate the parametrization of cumulus convection in large‐scale models. Equations governing the effect of convection upon the large‐scale atmosphere are derived in full and compared with approximate forms used by previous workers. These forms are the basis of many parametrization schemes. Simple representations of cumulus heating (Q1), cumulus moistening (Q2), and dynamical transports for use in parametrization schemes in large‐scale atmospheric models are studied. The importance of shallow convection in determining the vertical structure of Q2 is recognized. The dependence of convective rainfall upon atmospheric structure and large‐scale processes (the closure problem) is investigated.

The importance of downdraughts is emphasized and difficulties in the calculation of Q2 identified. Study of parametric forms for cumulus momentum transports indicates that the ‘cumulus friction’ hypothesis is inadequate. It is suggested that present closure schemes are too restrictive.
The role of diabatic heating in maintaining the upper‐tropospheric baroclinic zone in the South Pacific
Authors: Robertson Franklin R. , Vincent Dayton G. , Kann Deirdre M.
Published: February 2007

A modified set of ECMWF FGGE level III‐b analyses is used to examine the four‐dimensional structure of the region containing the South Pacific convergence zone (SPCZ) during a period when two cyclones form in the tropics and propagate south‐eastward along the SPCZ into middle latitudes. During this period. 10–18 January 1979, the atmosphere is characterized by a highly baroclinic state which, together with the existence of an upper tropospheric subtropical jet, supports the growth and development of the observed cyclone activity. In a previous study, it was found that convective activity was prominent during the life cycle of each cyclone.

The goal of the present study is to examine the significance of kinematic and thermodynamic processes in maintaining the upper baroclinic region coincident with the subtropical jet. This baroclinic zone lies just poleward of the warm core associated with the convectively‐active SPCZ. A partitioned form of the fronto‐genetical function is used to diagnose the adiabatic and diabatic contributions to the maintenance of baroclinicity in the vicinity of the SPCZ. A major balance is found between the frontogenetical contribution by differential diabatic heating and the opposing diabatic tilting processes. In contrast to the results of most mid‐latitude cyclone studies, the adiabatic contributions from the deformation and tilting terms appear to be of lesser importance. Also, the contribution from the deformation associated with the horizontal branch of the diabatically forced circulation is found to be relatively unimportant. The latter finding is in disagreement with some previously reported numerical results concerned with middle latitude moist frontogenesis. Possible reasons for these differences are discussed.
Reflection and attenuation of equatorial waves in the stratosphere and mesosphere
Authors: Reddy C. A. , Vijayan Lekshmi
Published: February 2007

Quantitative theoretical estimates are made of the reflection and radiative damping of the Kelvin and Rossby‐gravity (RG) waves propagating upward in the 20–90 km altitude region. An atmosphere with realistic temperature, wind and cooling rate coefficient values is approximated by a model of vertically stacked, thin, homogeneous layers. The amplitude reflection coefficients are shown for different wave periods (T) and zonal wavenumbers (kx). Combining the attenuation due to radiative cooling with the growth due to density decrease, the net changes of wave amplitudes with height in the 20–90 km region are shown for various wave modes.

For Kelvin waves the reflected wave energy is generally less than 15% of the incident wave energy, but the reflected wave amplitude can be 20%‐30% of the incident wave amplitude for many modes, at 20 km height. In contrast, many RG wave modes suffer strong or total reflection at various heights if they are not severely damped at lower heights. Calculations for January and July wind conditions reveal a good deal of sensitivity of RG wave reflection to wind conditions.

Radiative damping is found to be severe in the mesosphere for Kelvin waves with T ⩾ 6–8 days, depending on wind conditions; it is severe even in the lower stratosphere for RG waves with T > 4 days. Considerable changes in the damping rates and wave amplitudes at various heights take place from January to July owing to wind changes. Kelvin waves in the 2–8‐day period range and RG waves in the 1.5‐3‐day range can propagate into the upper mesosphere/lower thermosphere without large attenuation, depending upon wind conditions.

It is shown that our theoretical results can explain several observed characteristics of equatorial waves in the stratosphere and mesosphere, including the recent results on wave disturbances in satellite‐measured temperatures. Further modelling studies based on our methodology can lead to a good physical insight and quantitative interpretations of the SAO‐ and the QBO‐generation processes.
Precipitation development in a New Mexico thunderstorm
Authors: Raymond D. J. , Blyth A. M.
Published: February 2007

Observations were made of a thunderstorm over Langmuir Laboratory for Atmospheric Research on 2 August 1984. Four Doppler radars were closely spaced, allowing observations to be taken at high elevation angles. As a result, the vertical component of the precipitation particle velocity was accurately measured. In conjunction with the vertical air velocity, obtained by integration of the continuity equation, the particle terminal velocity field was computed.

Backward integration of particle trajectories showed that most precipitation embryos originated near the tops of thunderstorm precursor clouds when these cloud tops were 7–8 km above sea level. Early and mature phase precipitation grew by accretion at levels below 7 km, and resulted in relatively high density graupel or hail. Late phase precipitation underwent growth mainly above 8 km, and was sufficiently small in diameter and low in density to undergo rapid melting at the freezing level.
The effects of oppositely sloping boundaries with Ekman dissipation in a nonlinear baroclinic system
Authors: Weng H.‐Y.
Published: February 2007

A modified Eady channel model with oppositely sloping top and bottom Ekman layers is used to examine analytically and numerically the effect of the slope, Δ, with dissipation, δ, on baroclinic flows in linear and nonlinear systems. the spectral wave solution is truncated up to six components.

In the linear system, when δ = 0, waves are dispersive and move westward owing to Δ. the maximum value of Δ for which unstable waves may exist, ΔCmax, is equal to the slope of the isentropes of the basic state. Δ exhibits only a weak stabilizing influence on short waves, while exhibiting a strong stabilizing influence on long waves when it exceeds a critical value. When δ ≠ = 0, a small δ has a destabilizing effect on short waves by shifting the short‐wave cut‐off in the shorter wave direction and extending Δcmax to a larger value. However, the combined effect of Δ and δ strongly stabilizes long waves. the most unstable wavenumber may be shifted to a higher one by increasing Δ, in both inviscid and viscous cases. the relation between the most unstable wavenumber and Δ is similar to some annulus experiments with oppositely sloping boundaries. It is also similar to the relation between the most unstable wavenumber and β in β‐plane models.

In a nonlinear system without wave‐wave interaction, Δ stabilizes the flow even for small δ and reduces the domain of vacillation and aperiodic flow regimes, while enlarging the domain of single‐wave steady state. It also qualitatively agrees with the results from β‐plane dissipative systems. Within the single‐wave steady state regime, the preferred nonlinear wavenumber may also be shifted to a higher one by increasing Δ. When wave‐wave interaction is allowed, a variety of flow regimes is observed in four truncated systems. There are single‐wave steady states, multi‐wave steady states where wave dispersion or structural vacillation may occur, multi‐wave vacillation, and aperiodic flows. In contrast to the system without wave‐wave interaction, the domain of vacillation is enlarged. A ‘frequency‐locking’ mechanism is found in multi‐wave steady states, where all three conditions required for a resonant triad are strictly satisfied.
Adjusted NOAA outgoing long‐wave and net solar irradiances
Authors: Weare Bryan C. , Soong Albert
Published: February 2007

The outgoing long‐wave (OLR) and net solar (NSR) radiation at the top of the atmosphere, derived from the NOAA series satellites, are compared with those measured by the wide‐field‐of‐view Earth Radiation Budget (ERB) radiometers aboard Nimbus 7. For selected months between 1979 and 1983 important regional differences are found for both the OLR and NSR between the ERB observations and the NOAA, which have been spatially smoothed to approximate the resolution of the former. Knowledge of the systematic differences is utilized to derive linear regression equations, which adjust the smoothed NOAA data to match as well as possible the ERB measurements. These adjustment equations reduce the mean squared difference between the NOAA and ERB data for independent samples, not used in the statistical model development, by approximately 50% for both OLR and NSR. the adjustment equations are applied to the 2.5° NOAA data for June 1974 through May 1988. Long‐term zonal averages of the annual and monthly radiation budget at the top of the atmosphere are displayed and discussed. the adjusted data budgets are much more realistic than those derived from unadjusted data and suggest physical features not clearly evident in the previously published analyses based upon lower resolution data.
Baroclinic adjustment in a zonally varying flow
Authors: Mole Nils , James Ian N.
Published: February 2007

In agreement with previous published results, based on a 2‐layer model measure of baroclinicity, the extratropical northern hemisphere zonal mean atmosphere is found to be close to marginal stability in both winter and summer of the FGGE year. In winter, however, considerable zonal variations are present. Results of some numerical model runs, with both zonally symmetric and varying thermal forcing, are presented, and it is found that for sufficiently short forcing time scales a state close to marginal stability, and a storm‐tracklike region, can be produced. Large steady eddies do not seem to result so readily from the thermal forcing. Time‐lagged correlations provide evidence of forcing of the high frequency (periods less than 10 days) eddy heat flux by the baroclinicity, and it is suggested that the low frequency eddy heat flux may be a result of a nonlinear cascade from the baroclinic scales.
Some measurements of turbulence over an antarctic ice shelf
Authors: King J. C.
Published: February 2007

Profiles of wind speed, temperature and turbulent fluxes of heat and momentum in the lowest 32 m have been measured at a station on an antarctic ice shelf. During the antarctic winter the surface layer often shows strong static stability, with temperature gradients as large as 1 Km−1 in the lowest few metres. the surface inversion is destroyed during periods of high wind speed but the wind profile shows significant deviation from the expected logarithmic form under such conditions. Measurements of stress at 5 m indicate that the roughness length of the snow surface is about lO−4 m.

At 5 m height, the variations of the dimensionless wind shear, ϕm, and potential temperature gradient, ϕT, agree with previously determined forms of the Monin‐Obukhov similarity functions. Above 5 m, the behaviour of ϕm and ϕT is only qualitatively similar and surface‐layer similarity theory does not provide a good description of the profiles.

Turbulence length scales have been deduced from vertical velocity power spectra. Under near‐neutral conditions, the ratio of turbulence length scale to measurement height is observed to decrease with increasing height of measurement. This observation is consistent with the variation of turbulence length scale with height implied by the measurements of ϕm. It is suggested that the stability of the overlying atmosphere restricts the depth of the turbulent boundary layer and hence the length scales of turbulence within this layer. Increasing stability causes a decrease of turbulence length scales at all levels.

The ratios of turbulence kinetic energy to stress and temperature variance to heat flux are examined. Measurements are somewhat scattered, but the distribution of values varies little with height or stability. the form of the distribution suggests that large‐scale motions, possibly internal gravity waves, may be playing an important role in boundary‐layer processes.
Frontogenesis at the boundary between air‐masses of different potential vorticity
Authors: Thorpe Alan J.
Published: February 2007

Theoretical models of frontogenesis using the semi‐geostrophic approximation have imagined the process to occur between air‐masses whose only difference is their respective surface potential temperature. This is equivalent to assuming the Ertel potential vorticity (PV) to be constant in the troposphere and that there is a horizontal temperature gradient at the surface. For many cases this is a good approximation and furthermore the constraint on the dynamics of such uniform PV is a fundamental one. In some locations and synoptic situations, however, the air‐masses not only have a different surface potential temperature but also have significantly different static stability. In this case the PV, as well as the boundary potential temperature, has a horizontal variation. Here we take the variation in PV to be entirely due to the static stability, holding f constant, although in reality there is a contribution due to the variation of f with latitude. Examples of this type of frontogenesis are coastal fronts, where the air over the ocean has a different stability to that over the land, some mesoscale polar vortices which form at the ice margin, and cold fronts with polar air moving southwards over a warmer ocean giving enhanced convection and hence lower stability.

Results are presented from a two‐dimensional semi‐geostrophic model of frontogenesis between two rigid horizontal surfaces to illustrate the effects of variations in PV. In many cases the conclusions can be deduced from the geometrical aspects of the problem. If the cold air‐mass has the larger stability the frontogenesis at the lower boundary proceeds at an increased rate compared to that at the upper boundary. Indeed it is possible for a particular stability difference between the air‐masses to eliminate the upper frontogenesis completely, producing a purely lower tropospheric phenomenon. the geometrically converse conclusions apply if the cold air‐mass has the smaller stability. the results also allow the role of the mean value of the PV or stability of the two air‐masses to be described.
Investigation of systematic errors by relaxation experiments
Authors: Klinker E.
Published: February 2007

The systematic errors of the ECMWF forecast model have been investigated by relaxing forecasts towards analyses in selected regions. This method proved to be a useful tool to study the initial error growth in a certain region and the subsequent error propagation.

The forecast fields in the tropics were relaxed towards the analysis by adding a contribution proportional to the current forecast error to the tendencies of temperature and momentum. From the comparison of control forecasts with tropically relaxed forecasts it was possible to separate large errors generated in the extra‐tropics from comparatively small errors penetrating from the tropics into the extra‐tropics.

Relaxation in the major mountain regions showed different results for the Rocky Mountains and the Himalayas. the errors originating from the Rocky Mountains are substantial contributors to errors of both transient eddies and stationary eddies. the baroclinic waves in the Atlantic storm track are weakened and the stationary eddies develop a wave‐train‐like error pattern which is typical for many operational forecasts in the winter season.

The errors in the Pacific are much less dependent on errors from the mountainous region. the error structure of the transient and stationary eddies, as revealed from experiments in which the Asian mountains are excluded or included from the relaxation, shows little similarity to operational forecast errors.
Extended‐range predictions with ecmwf models: Interannual variability in operational model integrations
Authors: Palmer T. N. , Branković Č. , Molteni F. , Tibaldi S.
Published: February 2007

A set of 30‐day integrations made with the operational ECMWF model over a period of three and a half years (from 1985 to 1988) is studied. the impact of model reformulations during this period on the climate drift of the model is assessed, and the level of extended‐range forecast skill achieved by the operational model is studied.

Diagnostics of zonal average 30‐day mean wind and temperature error show a systematic reduction in the extratropics over three years. It is argued that these improvements are consistent with the changes in model formulation. Eddy fluxes of heat and momentum, and levels of eddy kinetic energy are similarly improved. We also show that the ability to simulate blocking activity in the extended range has improved substantially over the three winters.

From maps showing the ratio of systematic to total error it is concluded that, by the final winter, the middle‐latitude extended‐range error is almost entirely associated with random errors. This is supported by the growth of the dispersion, or spread, between forecasts initialized 24 hours apart. In particular, for the extended winter 1987/88, it is shown that the asymptotic level of (internal) spread is comparable with the asymptotic level of (external) skill. By contrast, in the tropics, the total error is dominated by the systematic error for all years.

Despite improvements to the model, extended‐range forecast skill is modest. Skill has been measured in a spatially‐filtered three‐dimensional phase space, spanned by rotated EOF (empirical orthogonal function) coefficients of 500 mb height, which contain the principal weather regimes in the atmosphere. On average the skill scores are consistent with those of other major centres studying dynamical extended‐range prediction. In particular there is evidence of skill to day 15 and possibly to day 20 in middle latitudes. Beyond day 20 skill is marginal. We demonstrate that in the winter sample of operational‐model forecasts, skill is correlated with the observed value of the first rotated EOF, which can be described as a global version of the Pacific/North American mode.

By contrast, the ability to predict monthly mean fields in the tropics depends primarily on a knowledge of the boundary forcing, including sea surface temperature. It is shown that some aspects of the interannual variations in the summer monsoon circulation can be predicted, in particular the tropical easterly jet anomalies, African rainfall, and, to a much lesser extent, Indian monsoon rainfall.
Travelling waves in Nimbus‐7 SBUV ozone measurements: Observations and theory
Authors: Prata A. J.
Published: February 2007

Observations of travelling waves in the ozone measurements made by the Solar Backscatter Ultraviolet (SBUV) instrument on board the Nimbus‐7 satellite are presented. In one month's data for January 1979 evidence for a westward travelling 5‐day wave and an equatorially trapped eastward travelling Kelvin wave of 7‐day period is found. These modes arise through the coupling of ozone with the temperature perturbation field: their structure depends on the mean zonal wind, the mean zonal ozone and the photochemistry of ozone. It is shown that the structure of these waves can be described reasonably well by a simple theory involving only the latitudinal variation of the important physical processes. the waves are shown to reach highest amplitudes in the transition region between about 3 mb and 1 mb and it is speculated that they may contribute to the transport of ozone by wave interaction in the stratosphere. Some data are also presented showing wave‐induced effects on the temperature dependence of ozone in the stratosphere.
The maximum entropy production principle in climate models: Application to the faint young sun paradox
Authors: Gerard Jean‐Claude , Delcourt Dominique , Francois Louis M.
Published: February 2007

Current theories of stellar evolution imply that the solar luminosity was 25‐30% less than at present during early phases of the Earth's history. the response of the global annual climate to changes of the solar input is examined using the maximum entropy production (MEP) principle and a zonal energy balance model including ice‐albedo feedback. It is shown that this formulation reproduces satisfactorily the present meridional heat flux and latitudinal temperature distributions. It is found that the model predicts that the efficiency of the meridional heat transport decreases for low values of the solar constant. Consequently, if the solar irradiance is lowered, a progressive development of the polar caps and a decrease of the global surface temperature are obtained without major irreversibility in the system. A comparison with a similar model including diffusive heat transport formalism shows that the value of the critical solar luminosity causing a global glaciation is decreased when the meridional heat fluxes are determined by the MEP principle.
On the generation and propagation of the southern African coastal low
Authors: Reason C. J. C. , Jury M. R.
Published: February 2007

The time‐height variability of the lower marine atmosphere during the eastwards propagation of coastal lows around the coast of southern Africa is presented. These coastal lows are shallow, mesoscale disturbances that are trapped vertically by a strong subsidence inversion and horizontally, through Coriolis effects, against the steep escarpment that fringes the subcontinent. Prior to the event, warm offshore flow descending off this escarpment at about the inversion level provides an input of buoyancy which, together with the cyclonic vorticity acquired during the descent, acts to generate a coastal low rather than a high.

Analysis of the data shows that coastal lows are characterized by a marked lowering of the inversion level, a switch in the direction of the winds below the inversion and a drop in the near‐surface temperature, which are all consistent with the hypothesis that the coastal low is an internal Kelvin wave propagating in the marine layer. Good agreement between the observed propagation speed of the coastal low and the theoretical phase speed of such a wave reinforces this hypothesis.

A model of the vertical buoyancy advection into the marine atmosphere that results from the warm offshore flow is presented. It is illustrated that an internal Kelvin wave, in the form of a coastal low, results from this buoyancy advection.

Comparisons are made between coastal‐low dynamics and those characteristics of other mesoscale, coastally‐trapped disturbances which propagate in the marine layers of California and south‐eastern Australia.
Dynamical aspects of the october storm, 1987: A study of a successful fine‐mesh simulation
Authors: Shutts G. J.
Published: February 2007

A diagnostic study of a successful numerical simulation of the October storm 1987 is described. This good, non‐operational forecast was obtained using the Meteorological Office's fine‐mesh model run from a revised analysis at 00 GMT on 15 October 1987. the Meteorological Office's mesoscale model was also run using initial conditions at 12 GMT 15 October obtained by interpolating the above fine‐mesh forecast fields to examine the benefits of very high horizontal resolution (15km grid length).

Diagnostic maps and cross‐sections derived are used to address two scientific issues:

the role of weak moist slantwise stability (or, alternatively, positive slantwise convective potential energy)

the dynamical significance of the striking cloud structure (the cloud head) seen in the satellite imagery early in the storm's life.

It appears that the baroclinic zone, in which the storm develops, is highly receptive to forcing by an upper trough and the warm boundary‐layer air just ahead of the main surface front is characterized by substantial slantwise convective available energy. the cloud head is identified with a region of intense slantwise ascent in the warm air just above the main frontal zone. Its great meridional extent is associated with the upper tropospheric outflow and horizontal spreading of air which has recently undergone rapid slantwise ascent. the low‐level temperature field exhibits, throughout the development period, a remarkable shape‐preserving thermal ridge of considerable intensity. the sensitivity of the forecast to the removal of surface energy fluxes and latent heat of condensation are investigated separately. It is found that on the timescale of the simulation, surface energy fluxes have negligible impact on the explosive development. On the other hand, the contribution of latent‐heat release to the development appears to dominate the dry baroclinic instability process.
On the parametric representation of the tropospheric thermodynamic structure for mid‐latitude convective situations
Authors: Binder Peter
Published: February 2007

The thermodynamic structure related to mid‐latitude continental summer convection is investigated from a set of upper‐air soundings from a station in western Europe. In particular an examination is undertaken of the applicability to mid‐latitude situations of the convective adjustment scheme proposed by Betts (1986), which was based primarily on tropical data. It is shown that, given a proper set for the adjustable parameters derived from the observations, the deep‐convection reference profile of Betts characterizes well the convectively adjusted atmosphere of the extra‐tropical continental environment. In comparison with the reference profile determined by Betts and Miller (1986), the temperature structure derived from the present data‐set is found to be even more unstable below melting level. In addition the humidity profile exhibits some distinctive physical characteristics.
Three‐dimensional baroclinic instability and summertime frontogenesis in the Australian region
Authors: Reeder Michael J. , Keyser Daniel , Schmidt Brian D.
Published: February 2007

The structure of a mature three‐dimensional baroclinic wave and its attendant cold front is examined using an idealized numerical model with a simple parametrization of dry buoyant convection resulting from diurnal heating. Relative‐flow isentropic analyses of the numerical solution and of an observed Australian summertime frontal system are compared. It is shown that the model well represents the typical synoptic environment accompanying frontogenesis in the Australian region. A diagnosis of the three‐dimensional vertical circulation is obtained using a technique involving a vector streamfunction. The circulation in a cross‐sectional plane parallel with the low‐level temperature gradient is found to be highly two‐dimensional, with geostrophic confluence constituting the principal frontogenetical forcing at low levels, while at upper levels the frontogentical forcing is dominated by geostrophic horizontal shear. In a second cross‐section, taken zonally, the frontogenetic forcing is found to consist almost solely of geostrophic horizontal shear throughout the entire troposphere. However, in this case the circulation in the plane of cross‐section is found to be only qualitatively two‐dimensional, as a significant proportion of the low‐level postfrontal subsidence is associated with circulation normal to the cross‐section. It is shown that the frontal structure analysed is sensitive to the location of the chosen cross‐sectional plane, and that this in turn provides an explanation for the observed relative shallowness of Australian summertime fronts. The effects of diurnal heating are important but confined to the surface‐based mixed layer. With the addition of diurnal heating, the prefrontal updraught amplifies almost threefold and the frontal temperature contrast over the heated region approximately doubles. The diabatic forcing term in the well‐known Sawyer‐Eliassen equation is shown to be negligible. This suggests that the increased low‐level convergence is a reflection of the model's enhanced response to frontogenetic forcing in the presence of low static stability in the surface‐based well‐mixed layer. Finally, the numerical model solution is compared with a simpler two‐dimensional model specifically developed to study Australian summertime fronts.
A numerical study of tropical cyclone motion using a barotropic model. II: Motion in spatially‐varying large‐scale flows
Authors: Ulrich Wolfgang , Smith Roger K.
Published: February 2007

The motion of an initially symmetric vortex in a spatially‐varying large‐scale flow on a beta plane is investigated using a nondivergent, barotropic numerical model. The calculations extend those carried out for the case of zero basic flow in Part I. The large‐scale flows are provided by meridionally‐varying zonal flows, or single‐mode, stationary, finite‐amplitude planetary waves in a channel. Interest is focused on the evolution of vortex asymmetries and their role in determining vortex motion relative to the basic large‐scale flow.

As in Part I, the calculations are used to assess averaging procedures for computing the environmental wind field of a tropical cyclone from observed wind data. It is shown that averaging over an annular region centred on the vortex is, in principle, more accurate when there is a basic flow, in comparison with the case of zero basic flow.
The Papal Front of 3 May 1987: A remarkable example of frontogenesis near the Alps
Authors: Volkert Hans , Weickmann Ludwig , Tafferner Arnold
Published: February 2007

All available, routinely collected surface data are used to document the development of the ‘Papal Front’, which crossed southern Germany on 3 May 1987 causing casualities and damage. Objective analyses on isentropic surfaces, initialized with the European aerological data, indicate that the development of the violent mesoscale front, which exhibited squall‐line characteristics, began when a distinct mid‐troposopheric anomaly of potential vorticity reached the north‐western rim of the Alpine arc. The forced uplift ahead of the associated shortwave trough combined with ageostrophic motions due to a jet streak at higher levels apparently provided a trigger for deep convection as was observed over the Alpine foreland. Numerical simulations with a dry hydrostatic model indicate the relative importance of the synoptic‐scale forcing and the Alpine orography for the generation of the ‘Papal Front’. A westerly ‘orographic jet’ over the foreland below crest height only develops in the simulation with full Alps and is corroborated by the few mountain wind observations that are available.

A comparison with studies of frontal propagation along other mountain ranges reveals that the routine network in the Alpine region and its surroundings is at least of similar density, but so far hardly used for frontal case‐studies. It is concluded that a series of ‘routine‐data case‐studies’ could help to determine the variability of frontal progression along the Alps and, thus, provide guidance when assessing the significance of single events, particularly those sampled during special observation programmes.
Growth of droplets in cloud edge downdraughts
Authors: Jonas Peter
Published: February 2007

Thermodynamic analysis of aircraft observations of the downdraughts at the edges of small maritime cumulus clouds is used to obtain estimates of the rate of entrainment of cloudy air into the downdraughts. The entrainment rates are around 10−4 to 10−2s−1. A simple entraining parcel model is used to investigate the fate of droplets entrained at this rate into such downdraughts. It is shown that the majority of small droplets evaporate totally, resulting in a subsaturation in the downdraught of less than 1%, except close to cloud top or cloud base. However, a few droplets which contain large soluble nuclei continue to grow, or undergo only a small amount of evaporation, during their descent. The limited evaporation ensures that some droplets undergoing a cycle of ascent and descent will grow even without net ascent. The largest droplets at some levels are therefore to be found in the downdraughts although the droplet concentration and liquid water content are much lower than in the cloud at the same level. The calculated growth and the thermodynamic properties of the downdraughts are in good agreement with observations of small maritime cumulus.
The initial value problem for tropical perturbations to a baroclinic atmosphere
Authors: Hoskins Brian J. , Jin Fei‐Fei
Published: February 2007

Large‐scale wave propagation ideas have proved extremely valuable in interpreting real and model data. These ideas have been developed in the context of barotropic models, and here the extension to baroclinic atmospheres is investigated. The dispersion of waves from an initial, large‐scale, tropical perturbation to a baroclinic basic flow is studied using a direct approach. A 15‐level, spectral, primitive equation model is integrated forward in time. Even in baroclinically unstable flows it is found that for a period, normally of order 12 days, the model yields what can be considered to be the direct response to the tropical perturbations. After this time baroclinic instability dominates. The basic flows are a resting atmosphere, a climatological westerly, an equatorial longitudinal modification to this, and a 3‐D climatological December‐February state. For the perturbations used here, equatorial Kelvin waves play only a very small role. The large‐scale equatorial Rossby wave shows only small dispersion and tends to move with a Doppler‐shifted phase speed which allows it to ‘accumulate’ to the east of an equatorial maximum of 10 ms−1 or more. Rossby wavetrains propagate into the middle and high latitude westerly regions. With the climatological flow, perturbations in the Indian Ocean‐west Pacific sector trigger a modal structure in the North Pacific and wavetrain across North America very much as barotropic theory would suggest and similar to observed Pacific‐North America (PNA) patterns. In such a flow the middle latitude waves tend to extend to the equator in the westerly regions of the east Pacific and Atlantic, in agreement with observed behaviour.
On the dependence of travelling wave stability on basic state wave phase speed
Authors: Manney Gloria L.
Published: February 2007

The dependence of stability on basic state wave phase speed is examined in a nondivergent barotropic model on a sphere for planetary scale waves with phase speeds typical of waves observed in the atmosphere. Triad interactions are examined analytically and compared to numerical results from a full spectral model. A number of triads may result in growth of the perturbation components for each basic state wave; for each triad there is a basic state wave phase speed where it is resonant, identified as the point where the critical amplitude for instability to that perturbation becomes zero. Critical amplitudes for instability obtained using the full spectral model generally agree well with triad results. Since the basic state wave phase speed determines which triad will grow, spatial structure and critical amplitudes for instability for the growing disturbances depend strongly on basic state wave phase speed. The results of this idealized study suggest that phase speed may be an important factor in determining the stability of planetary scale waves in the atmosphere.
Data sampling strategies for general circulation models
Authors: Thuburn John
Published: February 2007

For climate study purposes, complex general circulation models are often run for extended periods; the model state is sampled regularly and various diagnostics calculated. A convenient and often used sampling period is 1 day, but in this case diurnal and semi‐diurnal variations, for example those associated with the atmospheric tides, will alias into the time mean.

Using the UK Universities Global Atmospheric Modelling Project model, adapted from the ECMWF forecast model, an attempt is made to quantify this aliasing in various diagnostics. A variety of data sampling/processing strategies are investigated, for example time‐averaging the model fields before writing them out, in an attempt to eliminate the aliasing problem. It is found that sampling the model state every 3/4 day or every 5/4 day is a simple and effective solution.
Role of detailed wind‐topography interaction in orographic rainfall
Authors: Alpert P. , Shafir H.
Published: February 2007

A meso γ‐scale (Δx = 1 km) model for orographic rainfall was used to investigate the dependence of orographic enhancement on wind speed and direction and on detailed topography. Applying the model to cases reported in south Wales, high sensitivity to wind direction and the corresponding upwind topographical cross‐sections—factors neglected in some previous studies—was found. This suggests that studies of high‐resolution orographical rainfall characteristics (such as orographic enhancement as function of wind speed) or model verification should account for detailed interaction of the wind vector with the high‐resolution topography. Comparison with microphysical models illustrates that the effect of detailed wind‐topography interaction is at least of similar magnitude to that due to microphysical processes.
Variations common to the interplanetary magnetic field, the zonal atmospheric circulation and the earth's rotation
Authors: Djurovic D. , Páquet P.
Published: February 2007

A study of the cyclic fluctuations in the earth's rotation, the geomagnetic field, the global atmospheric circulation and solar activity reveals the existence of 50‐day and 120‐day oscillations common to all. Our results, as well as the results obtained by radiometric measurements from the satellites Nimbus‐7 and the Solar Maximum Mission (Wilson 1982; Pap 1985), suggest that their origin lies in the physical processes in the sun. The mechanism remains unexplained but from this work it follows that the role of the interplanetary magnetic field could very well be important.
Lamination in ozone profiles in the lower stratosphere
Authors: Reid S. J. , Vaughan G.
Published: February 2007

A survey of the distribution and scale of layers of enhanced and depleted ozone in the lower stratosphere was conducted over the altitude range 9.5–21.5 km using data recorded by balloon‐borne sondes released at various sites around the world.

The results show clearly that lamination is primarily a winter‐spring phenomenon from subtropical to polar regions in both hemispheres, and seems to be linked to the presence of the polar vortex. A peak altitude of occurrence was found near 14 km. The interannual fluctuations in lamina abundance were found to be extremely large, varying at high latitudes by as much as a factor of three.

Both the mean vertical thickness of the laminae and the associated rise or fall in ozone partial pressure within them vary progressively with latitude, season and altitude.
Dynamic initialization by repeated insertion of data
Authors: Macpherson B.
Published: February 2007

In this study we isolate and assess the impact of dynamic initialization by repeated insertion of data (RI), also known as ‘nudging’. A novel control experiment compares this iterative technique with an alternative intermittent analysis‐forecast cycle (AF) framework within the same assimilation system. Both schemes are versions of the operational global data assimilation system for numerical weather prediction at the Meteorological Office. A secondary issue is the impact of divergence damping as an extra initialization measure.

The effectiveness of RI for control of high‐frequency gravity waves is demonstrated, together with its beneficial impact on the short‐period forecast of surface pressure. Changes to analysed fields due to RI are only a small fraction of the changes due to model evolution in six hours. RI is found to enhance upper‐level divergence in the equatorial west Pacific and the problem of spin‐up of the global mean precipitation is virtually absent, except when divergence damping is imposed during assimilation and removed during the forecast. The main benefit from divergence damping is a more accurate tropical wind field.

RI is found to degrade the short‐period forecast of the wind field in the basic system studied, and the extra errors are largest in strong upper winds at mid latitudes. This fact is discussed in the context of an earlier idealized study of RI. Analysis constraints are a primary source of difference between various ‘nudging’ schemes used in numerical weather prediction. The sensitivity of the impact from RI to these is evaluated by relaxing constraints in the basic system. The result emerges that as the analysis step of a nudging scheme becomes better balanced, the scope for improvement by using RI for initialization becomes smaller. Also, if the better‐balanced analysis step entails stronger enforcement of non‐divergence on the wind increments, then the damaging effect of RI on the wind field through advection becomes larger and can outweigh the benefit through initialization. A scheme combining RI for mass data with an iterative AF method for wind data retains much of the benefit and avoids most of the problems in the scheme with RI of all data. This suggests a method of improving the contribution of an RI structure to the overall performance of an assimilation scheme based on the nudging approach.
The summertime variation of heat and freshwater contents between the Azores and Greenland
Authors: Bauer J. , Leach H. , Woods J. D.
Published: February 2007

The Kiel Sea Rover System was used to make a series of hydrographic surveys north of the Azores, which provided the first detailed description of the spatial structure of the summertime heat and freshwater contents. Our results confirm the conclusion based on earlier studies that advection plays a dominant role in the seasonal change in the freshwater content. Using the new data we have assessed the reliability of estimates of the various contributions to the summertime changes of fresh water content based on climatological estimates of the surface fluxes, on Ekman and geostrophic transport and on horizontal eddy diffusivity. We show that they contain substantial inconsistencies and consider how these might have arisen. Joint regression of the heat and freshwater variations recorded in our synoptic data suggest a balance of contributions that is significantly different from that derived from climatology, but, in view of the complexity of the region, we do not claim that they do more than reveal the difficulty of the problem.
A concise derivation of the semi‐geostrophic equations
Authors: Chynoweth S. , Sewell M. J.
Published: February 2007

A self‐contained account of the semi‐geostrophic equations is given. This contains formulae not previously available in the literature. The equations are presented in terms of four alternative sets of independent variables, and the Legendre mappings between these variables are specified. The approach is facilitated by the introduction of a general theorem in vector analysis, which is proved, concerning the total or convected derivative of a Jacobian following the particle. An expression for the curvature of a front is included.