bluuebunny/crossref_metadata_2025 huggingface dataset:
The Berlin troposphere–stratosphere–mesosphere GCM: Sensitivity to physical parametrizations
Authors: Pawson Steven , Langematz Ulrike , Radek Gerd , Schlese Ulrich , Strauch Peter
Published: February 2007

The sensitivity of a troposphere–stratosphere–mesosphere general‐circulation model to changes in the radiation transfer and subgrid‐scale drag parametrizations is investigated. The large interannual variability of the middle atmospheric circulation necessitated a methodological approach. A set of 1260–day, perpetual‐January integrations was performed; this enabled significant signals to be extracted from the model variability at a reasonable cost. A Rayleigh‐friction scheme was used for the mesospheric drag. Reducing its strength led to changes in the zonal‐mean structure of the model which resembled the leading mode of variability. Increasing its depth led to significant changes throughout the stratosphere and extending into the troposphere. Including the diurnal cycle induced no significant changes in the zonal‐mean state below the stratopause. Radiation‐transfer calculations are not performed fully every time‐step; decreasing the frequency of computation from 2 to 12 hours caused marked changes in the mean structure and variability. Including a topographic gravity‐wave drag parametrization (TGWD) restored the zonal‐mean structure of the 2‐hour radiation frequency in the lower stratosphere at the expense of changes in both the temporal variability and the planetary‐wave structures. The sensitivity to the frequency of radiation calculations arose from the highly coupled nature of tropical cloud–radiation interactions: the modified structure of the upper tropospheric divergence led to changes in the Rossby‐wave source term in the extratropics. The major conclusions are: (1) the natural variability must be properly included in interpretations of the model sensitivity; (2) the adequacy of the tropospheric simulation can profoundly affect the stratosphere; (3) changes in the mesospheric drag can modify the tropospheric circulation; and (4) deficiencies in one parametrization (in this case radiation transfer) can be compensated by other changes (TGWD), but such ‘improvements’ may not apply to all aspects of the simulation.
A study of the effects of cloud processing of aerosol on the microphysics of cloud
Authors: Choularton T. W. , Bower K. N. , Beswick K. M. , Parkin M. , Kaye A.
Published: February 2007

A field experiment was conducted in which measurements were made of the microphysics of stratocumulus cloud upwind and downwind of a hill cap cloud using the UK Meteorological Office C‐130 aircraft. At the same time, measurements of the cloud microphysics and cloud chemistry were made on the ground at the hill summit. It is shown that the stratocumulus downwind of the hill has a reduced liquid‐water content due to entrainment initiated by Kelvin—Helmholtz instability above the hill top, whilst the droplet effective radius is smaller than upwind for a given value of the liquid‐water content. These latter effects are attributed to aqueous phase sulphate production caused by the oxidation of SO2 by hydrogen peroxide and ozone in the hill cap cloud, increasing the number of cloud condensation nuclei (CCN) available to the stratocumulus cloud downwind of the hill. The production of CCN by processing of aerosol in clouds with a vigorous updraught near cloud base (such as convective clouds) may be an important source of CCN in the atmosphere, further very small particles activated in the high supersaturations present in such clouds will grow rapidly into the accumulation mode.
A three‐dimensional model simulation of the impact of Mt. Pinatubo aerosol on the Antarctic ozone hole
Authors: Knight J. R. , Austin John , Grainger R. G. , Lambert A.
Published: February 2007

Seasonal integrations of a three‐dimensional fully coupled model of the radiation, dynamics and chemistry of the stratosphere and mesosphere are presented for the southern hemisphere spring. Included in the model are heterogeneous reactions which take place in sulphuric acid aerosol droplets as well as on the surface of Polar Stratospheric Clouds (PSCs). Calculations are performed for background levels of stratospheric aerosol and for conditions following the eruption of Mt. Pinatubo. For the volcanic case, surface area densities are derived from Improved Stratospheric and Mesospheric Sounder data. For background aerosol loadings there are significant increases in HOx and CIOx, as well as reductions in NOx. These effects are enhanced following the Pinatubo eruption but saturate at relatively low aerosol levels and hence can persist in the stratosphere for several years. Where PSCs are predicted to form, the sulphate aerosol chemistry does not operate in the model since the aerosols are incorporated as nuclei within the PSCs. In contrast, on the edge of the ozone hole, where temperatures are only just too high for the formation of PSCs, destruction by aerosol can result in further total ozone depletion of order 20 Dobson Units. In addition, the size and duration of the ozone hole are both increased by the presence of volcanic aerosols. The results support previous suggestions that the eruption of Mt. Pinatubo resulted in a temporary increase in ozone depletion over Antarctica and elsewhere.
Satellite measurements of global lightning
Authors: Christian Hugh J. , Latham John
Published: February 2007

The satellite‐borne NASA/MSFC Optical Transient Detector provides global distributions of lightning and lightning‐stroke radiance. Measurements made during the first year of its operation show that lightning activity is particularly pronounced over the tropics, much greater over land than over the oceans, and exhibits great seasonal variability. The values of lightning‐stroke radiance tend to be greater over the oceans, less when lightning activity is high, and greater in the northern hemisphere winter than summer.
Estimation of key analysis errors using the adjoint technique
Authors: Klinker E. , Rabier F. , Gelaro R.
Published: February 2007

An iteration procedure minimizing the short‐range forecast error leads, after some iterations, to so‐called key analysis errors. These are estimates of the part of analysis errors that is largely responsible for the short‐range forecast errors. The first step of the minimization procedure provides a scaled gradient of the two‐day forecast errors for which the ‘energy’ inner‐product provides an efficient way of identifying the analysis errors at scales that are relevant for forecast error growth. By using an ‘enstrophy’ like inner‐product as an alternative to ‘energy’ the sensitivity gradient obtains an unrealistically large scale. Performing a few more steps in the minimization provides better estimates of the analysis error in the directions spanned by the leading singular vectors of the tangent‐linear model. On a case study it is shown that three steps provide key analysis increments which, when added to the analysis, both significantly improve the fit to the available data, and substantially improve the subsequent model integration. It does not appear to be beneficial to do more steps of the minimization because of the uncertainty in the definition of the short‐range forecast error, and of approximations in the tangent‐linear model.

Key analysis errors represent an improved estimate of analysis errors compared to the scaled gradient of day‐2 forecast errors. In particular the geographical distribution shows the stability dependence of the scaled gradient. The projection of the gradient on the fastest growing errors limits maximum sensitivity to the major baroclinic zones. The close correspondence of evolved key analysis errors and forecast errors shows that key analysis errors are more realistically projecting on to full analysis errors. The close link between the stability of the flow and the gradient of the forecast errors implies an unreasonably strong seasonal variation of analysis errors estimates. In contrast, key analysis errors are nearly seasonally independent, which means that their detrimental effect on forecast errors in absolute terms in summer and winter is comparable.
Mean global surface pressure series evaluated from ECMWF reanalysis data
Authors: Hoinka Klaus P.
Published: February 2007

Time series (1979–93) of the global mean surface pressure are determined using data provided by the ECMWF ReAnalysis (ERA) project. Total pressure, water vapour pressure and dry‐air pressure are evaluated. It is shown that a trend of only 0.01 hPa per decade exists for the total surface pressure whereas that of the water vapour surface pressure is 0.05 hPa per decade. This underlines the homogeneous character of the time series of ERA data, at least for the integral quantities surface pressure and water vapour surface pressure.
An extreme cold surge over the Greek peninsula
Authors: Lagouvardos K. , Kotroni V. , Kallos G.
Published: February 2007

This paper is devoted to the detailed description of an extreme cold surge that occurred 3–13 March 1987, over Greece. This event has been rated as the worst snowfall over the last 100 years and, due to its severity and persistence, it paralysed the economic and communal life of Greece for several days. Emphasis is given to the initiation phase of this event. The structural evolution of the cold surge is analysed using both observations and model results. Model simulations, performed with the Colorado State University‐Regional Atmospheric Modelling System, provided the necessary data to diagnose the mesoscale structure of the cold outbreak.

Different mechanisms involved in the structural evolution of the cold surge are investigated. The gustiness of the observed winds and their significant departure from geostrophy are related to the role of an important isallobaric wind. The progression of this surge presented characteristics of a density current, while near the eastern slopes of the mountain barriers of continental Greece cold‐air damming occurred, leading to an accelerated flow parallel to the mountains.
Analysis of the eyes formed in simulated tropical cyclones and polar lows
Authors: Gray Suzanne L.
Published: February 2007

The structure and size of the eyes generated in numerically simulated tropical cyclones and polar lows have been studied. A primitive‐equation numerical model simulated systems in which the structures of the eyes formed were consistent with available observations. Whilst the tropical cyclone eyes generated were usually rapidly rotating, it appeared impossible for an eye formed in a system with a polar environment to develop this type of structure. The polar low eyes were found to be unable to warm through the subsidence of air with high values of potential temperature, as the environment was approximately statically neutral. Factors affecting the size of the eye were investigated through a series of controlled experiments. In mature tropical cyclone systems the size of the eye was insensitive to small changes in initial conditions, surface friction and latent and sensible heating from the ocean. In contrast, the eye size was strongly dependent on these parameters in the mature polar lows. Consistent with the findings, a mechanism is proposed in which the size of the eye in simulated polar lows is controlled by the strength of subsidence within it.
Wind‐speed limits in numerically simulated tornadoes with suction vortices
Authors: Fiedler B. H.
Published: February 2007

Three‐dimensional numerical simulations of tornado‐like vortices are presented. Of particular interest is a regime with multiple suction vortices within a larger cyclone. The simulations are designed to assess the magnitude and mechanism by which low pressure is sustained in a suction vortex. The pressure in the transient suction vortices can be nine times that supported by the buoyancy of the ‘storm’. The low pressure is sustained by transient axial flow; the pressure‐gradient force along the axis is balanced by the inertia of the axial flow, rather than by buoyancy.
Correction for attenuation of radar reflectivity using polarization data
Authors: Smyth T. J. , Illingworth A. J.
Published: February 2007

Attenuation by heavy rain is a major operational problem in radar estimation of rainfall rates, and one which is increasingly severe at wavelengths shorter than 10 cm. Gate‐by‐gate correction algorithms, including those using both reflectivity (ZH) and differential reflectivity (ZDR), are inherently unstable. In addition values of ZH and ZDR are affected by hail, which causes little attenuation. Methods dependent upon differential phase shift (KDP) do not give unique solutions but depend upon raindrop size distributions. A new technique which uses both KDP and differential attenuation between the horizontally and vertically polarized radiation (AH‐v) is proposed. The total AH‐v is measured from the negative ZDR in light precipitation behind the attenuating region. Theory and observation show that corrections based on the KDP ‐ AH‐v scheme should be unaffected by hail, stable and accurate. A simpler algorithm may be feasible at C‐band in which the total attenuation is derived from the observed total differential attenuation. It appears that the most severe cases of attenuation result from heavy rain consisting of large oblate raindrops. Accordingly, the attenuation would be appreciably reduced if operational radars used vertical polarization rather than the commonly employed horizontal polarization.
Retrieval of ageostrophic wind from a radiosounding network and a single ST radar
Authors: Le Bouar E. , Petitdidier M. , Lemaitre Y.
Published: February 2007

Previous studies have shown that ageostrophic circulations play an important role in frontogenesis phenomena. They result from the maintenance of the balanced state of the atmosphere which tends to be broken down by the large‐scale forcing of the atmosphere. Their existence explains how a frontal discontinuity can be produced in a few days. From an experimental and forecasting point of view it would be interesting to obtain these circulations in real time. Unfortunately, the ageostrophic wind is not a measurable quantity and its retrieval from experimental data is difficult, particularly at the mesoscale, since instrumental noise leads to a need for significant data correction.

In this context, this paper provides answers to some of the main problems related to the retrieval of ageostrophic winds in frontal systems. It focuses on the feasibility of retrieval while retaining, at small enough scale, details of physical interest. The framework of this study is the future implementation of regular meteorological wind‐profiler networks with a sampling time of one hour.

This paper proposes to overcome numerous practical problems using an approach based on the variational Analyse du Vent AGéostrophique (AVAG) analysis. Tests of such an approach are done on real data extracted from the Mesoscale Frontal Dynamic Project/FRONTS 87 data base for a typical two‐dimensional frontal case (the IOP 7 case). They confirm that the AVAG analysis is able to provide a non‐noisy and consistent ageostrophic circulation. Comparisons with classical methods demonstrate the robustness of AVAG and its capability to obtain physically meaningful ageostrophic wind fields. This study also estimates the best and most suitable experimental sampling for practical applications. Sensitivity tests on the quality of the retrieved wind field with respect to (i) the sampling time used, (ii) the availability of additional thermodynamic data and (iii) the geometrical pattern of the network are presented. These tests provide evidence of the importance of accounting for thermodynamical‐ and dynamical‐field along‐front variations.
Intercomparison and evaluation of atmospheric transport in a Lagrangian model (STOCHEM), and an Eulerian model (UM), using 222Rn as a short‐lived tracer
Authors: Stevenson D. S. , Collins W. J. , Johnson C. E. , Derwent R. G.
Published: February 2007

Transport of the short‐lived (half‐life 3.83 days) isotope 222Rn, which is emitted from unfrozen soils, is used to compare transport in several versions of the UK Meteorological Office Lagrangian chemistry‐transport model and in the Unified Model, the UK Meteorological Office general circulation model. The same 222Rn experiment is repeated for all the model versions, illustrating the impact on global transport of various model improvements: adding boundary‐layer schemes, including sub‐grid scale convection, increasing model spatial resolution, and increasing the temporal resolution of the meteorological fields used for driving the off‐line model. Results from all model versions are compared with a limited observational data set, and also with results from the same 222Rn simulations carried out with several global atmospheric transport models as part of the World Climate Research Program in December 1993 (Jacob et al. 1997). Versions of the Lagrangian chemistry‐transport model that include sub‐grid scale convection, transport 222Rn in a manner that is similar to the Unified Model and most general circulation models, supporting the simple and computationally inexpensive Lagrangian approach taken.
Conventional and Bayesian approach to climate‐change detection and attribution
Authors: Hasselmann K.
Published: February 2007

The conventional multi‐variate, multi‐fingerprint theory of climate‐change detection and attribution, expressed in terms of existing frequency distributions, is reviewed and generalized to a Bayesian approach based on subjective probabilities. Bayesian statistics enable a quantitative determination of the impact of climate‐change detection tests on prior subjective assessments of the probability of an externally forced climate change. The Bayesian method also provides a potentially powerful tool for enhancing statistical detection and attribution tests by combining a number of different climate‐change indicators that are not amenable to standard signal‐to‐noise analyses because of inadequate information on the associated natural‐variability statistics. The relation between the conventional and Bayesian approach is illustrated by examples taken from recent conventional analyses of climate‐change detection and attribution for three cases of climate‐change forcing by increasing greenhouse‐gas concentrations, increasing greenhouse‐gas and aerosol concentrations, and variations in solar insolation. The enhancement of detection and attribution levels through a joint Bayesian analysis of a number of different climate‐change indices is demonstrated in a further example. However, this advantage of the Bayesian approach can be achieved only within the framework of a subjective rather than objective analysis. The conventional and Bayesian approach both exhibit specific advantages and shortcomings, so that a parallel application of both methods is probably the most promising detection and attribution strategy.
Comparison of several numerical schemes applied to advection equations
Authors: Sokol Zbyněk
Published: February 2007

Eight numerical‐solution methods of advection equations are compared using two tests: the rotational flowfield test and the deformational flow‐field test. Beside the leap‐frog difference scheme used as a reference method, all the other methods are positive definite or monotonicity preserving. the positive definiteness is either the property of the advection scheme or it is obtained by applying the flux transport limiter. the following schemes are studied: the leap‐frog scheme completed by a flux transport limiter, Bott's scheme, the piecewise parabolic method, one version of the total variation diminishing (TVD) scheme and three modifications of essentially non‐oscillatory (ENO) schemes. the aim is to compare the schemes with respect to the application of TVD and ENO schemes in atmospheric modelling with high resolution. the test results confirm that at least one of the ENO schemes tested may be applied, but apparently the best results are obtained by Bott's scheme.
Effects of resolution on the simulation of stratocumulus entrainment
Authors: Stevens D. E. , Bretherton C. S.
Published: February 2007

Three‐dimensional large‐eddy simulations (LESS) of a smoke cloud beneath a strong inversion are used to investigate how grid resolution affects simulated entrainment‐rate. These simulations are generalizations of a 1995 GEWEX Cloud System Studies intercomparison of models of smoke clouds. Radiative cooling at the top of the smoke cloud induces convection in the smoke cloud in a manner closely analogous to that in marine stratocumulus‐capped boundary‐layers.

The simulations are compared to a laboratory analogue for a range of inversion strengths and radiative forcings. the inverse Richardson‐number scaling of non‐dimensional entrainment‐rate found in the laboratory experiments is largely captured by the numerical model, but the modelled entrainment‐rates for strong inversions are two large by a factor of two. Better agreement is obtained when the numerical model resolves the undulations of the inversion driven by the underlying convection. By this criterion, most LES studies of subtropical maritime stratocumulus made so far are under‐resolved.

For determining the bulk entrainment‐rate, a nested‐grid model which has fine resolution only in the entrainment zone is as effective as a model with uniformly fine resolution. If horizontal resolution is much coarser than the vertical resolution, modelled entrainment‐rates are less than those obtained with simulations which have comparable vertical and horizontal resolutions.
The sensitivity of large‐eddy simulations of shallow cumulus convection to resolution and subgrid model
Authors: Brown A. R.
Published: February 2007

Results are presented from a series of large‐eddy simulations of shallow cumulus convection, including some at much higher resolution than previously performed. Many of the ensemble‐averaged turbulence statistics which are relevant for mass‐flux parametrization schemes are shown to be reasonably insensitive to increases in resolution, but the cloud fields change markedly. the implications for the best interpretation of large‐eddy simulation results are discussed.
Linear evolution of baroclinic waves in saturated air
Authors: Fantini Maurizio
Published: February 2007

A primitive‐equation model which includes a moisture parametrization that assumes that ascending air is always saturated, has been used to study the characteristics of moist baroclinic waves, exploring their dependence on the Richardson number, besides zonal and meridional wavelength. Growth rates are shown to increase, in consequence of latent‐heat release, in a larger measure for the meridionally structured waves than for the two‐dimensional ones. Approximate analytic relationships for the normal‐modes growth rates, based on the concept of a weighted average of dry and saturated parameters, compare favourably with the numerical results. Asymmetric horizontal structures, previously obtained in quasi‐geostrophic approximation, are shown to be consistent with potential‐vorticity generation by diabatic heating in the updraught.
Frontogenesis and the development of secondary wave cyclones in FASTEX
Authors: Chaboureau Jean‐Pierre , Thorpe Alan J.
Published: February 2007

Two major mechanisms of frontogenesis‐deformation and shear‐are important in frontal wave cyclone development. Horizontal deformation can suppress the nonlinear wave development. Using an analytic model, Bishop and Thorpe have shown that large strain rates inhibit any wave‐slope amplification. For real cases, this ambient strain can be measured using the vorticity‐divergence attribution method developed by Bishop. This technique permits us to confirm the crucial role of such strain on the evolution of cases of wave development during the Fronts and Atlantic Storm Track Experiment (FASTEX).

Horizontal shear in the presence of an along‐front thermal gradient is also an important mechanism of frontogenesis. Using an Eady model, Joly and Thorpe have shown that, in cases of large along‐front thermal gradient, frontal waves have growth rates smaller than the front itself, and thus would not develop. the domain‐independent attribution method developed by Bishop is here extended to a geopotential‐field partition. This leads, via a nonlinear balance condition, to the estimation of the ambient along‐front potential‐temperature gradient. the role of such an along‐front potential‐temperature gradient is discussed. as well as the relative contributions of the two frontogenesis mechanisms for the FASTEX cases.
A model study of corona emission from hydrometeors
Authors: Schroeder Vicki , Baker M. B. , Latham John
Published: February 2007

The maximum measured electric fields in thunderclouds are an order of magnitude less than the fields required for electric breakdown of the air. Several explanations for lightning initiation in these low‐intensity fields exist. One explanation is that electric breakdown first occurs at the surfaces of hydrometeors where the ambient field is enhanced very locally due to the drop geometry. Laboratory experiments have indicated that colliding raindrops which coalesce to form elongated water filaments, can produce positive corona in ambient fields close to those measured in thunderclouds. We calculate the electric field distribution around a simulated coalesced drop, and use a numerical model to study the positive‐corona mechanisms in detail. Our results give good agreement with the laboratory observations. At the altitudes and reduced pressures at which lightning initiation is observed, our results show that positive corona can occur at observed in‐cloud electric fields strengths.
Do orographic gravity waves break in flows with uniform wind direction turning with height?
Authors: Broad Adrian S.
Published: February 2007

The properties and behaviour of orographically generated gravity waves which have a vertical component of propagation are examined by analysing the structure of stationary, hydrostatic waves embedded in a basic state wind field, U, whose direction and magnitude may vary linearly with height, z. From a ray‐tracing analysis it is evident that the wave energy density decays to zero approaching a three‐dimensional (3‐D) critical level in such an ambient flow field where U. k = 0 for the particular horizontal wave vector k = (k, l, 0). This decay of the wave energy density would appear to imply that wave breaking does not naturally occur in 3‐D flows where the wind turns uniformly with height (excepting that wave vectors which do not experience a 3‐D critical level can break due to the decrease of density with height). However, as a linear theory analysis of convective overturning demonstrates, the criterion for wave breaking relaxes as a 3‐D critical level is approached. the gravity wave amplitude is only required to exceed the inverse of the vertical wave number (which is proportional to U. k and decays to zero at the critical level) for overturning to occur. Consequently a linear estimate of whether convective instability will occur depends on the asymptotic limit of the ratio of the wave energy density to the square of the amplitude. It can be shown from the ray‐tracing analysis that for 3‐D flow fields with a mean wind which turns uniformly with height this ratio diverges, implying that the criterion for convective breaking of gravity waves will always be exceeded on approaching such a 3‐D critical level.
Critical levels and mixing layers induced by convectively generated gravity waves during CEPEX
Authors: Teitelbaum H. , Moustaou M. , Sadourny R. , Lott F.
Published: February 2007

We study data from the Central Equatorial Pacific Experiment, in particular the soundings providing temperature, pressure, wind and ozone profiles, and we link them to the large‐scale conditions deduced from the analyses of the European Centre for Medium‐Range Weather Forecasts. to the west of the date line, the mean wind shows a strong shear at the tropopause between the easterlies of the upper troposphere and the westerlies of the lower stratosphere. These are conditions propitious to the formation of critical levels for quasi‐stationary gravity waves generated by deep convection. the measured wind profiles show the main characteristics of the encounter of a wave with its critical level and, in the ozone and potential‐temperature profiles, the existence of a conspicuous mixing layer in the vicinity of this critical level. Our interpretation that the mixing layer is a consequence of the critical level is supported by numerical simulations with a nonlinear time‐dependent two‐dimensional model. the occurrence of a mixing layer in the tropopause structure seems to be characteristic of large‐scale equatorial convective areas during westerly phases of the quasi‐biennial oscillation.
Passage of a tracer through frontal zones: A model for the formation of forward‐sloping cold fronts
Authors: Parker D. J.
Published: February 2007

It is shown that a forward‐sloping cold front may form in a simple two‐dimensional model of an atmospheric frontal zone, through differential advection of dry‐ and wet‐bulb potential temperature. As such, two frontal zones may coexist, with opposing slopes. This behaviour is manifest in the neutral Eady edge wave, which exhibits kata‐cold front behaviour, while the unstable Eady wave, with an ana‐cold front, does not generate a forward‐sloping cold front.

These results come about as a consequence of the wave‐like behaviour of the fronts in the model, that is, the ability of a front to propagate through the air locally. As such, they highlight the fact that a frontal zone need not be a barrier to airflow and that any air properties, from water vapour to atmospheric pollution, may pass through a front.
The dependence of clear‐sky outgoing long‐wave radiation on surface temperature and relative humidity
Authors: Allan R. P. , Shine K. P. , Slingo A. , Pamment J. A.
Published: February 2007

A simulation of the earth's clear‐sky long‐wave radiation budget is used to examine the dependence of clear‐sky outgoing long‐wave radiation (OLR) on surface temperature and relative humidity. the simulation uses the European Centre for Medium‐Range Weather Forecasts global reanalysed fields to calculate clear‐sky OLR over the period from January 1979 to December 1993, thus allowing the seasonal and interannual time‐scales to be resolved. the clear‐sky OLR is shown to be primarily dependent on temperature changes at high latitudes and on changes in relative humidity at lower latitudes. Regions exhibiting a ‘super‐greenhouse’ effect are identified and are explained by considering the changes in the convective regime associated with the Hadley circulation over the seasonal cycle, and with the Walker circulation over the interannual time‐scale. the sensitivity of clear‐sky OLR to changes in relative humidity diminishes with increasing relative humidity. This is explained by the increasing saturation of the water‐vapour absorption bands with increased moisture. By allowing the relative humidity to vary in specified vertical slabs of the troposphere over an interannual time‐scale it is shown that changes in humidity in the mid troposphere (400 to 700 hPa) are of most importance in explaining clear‐sky OLR variations. Relative humidity variations do not appear to affect the positive thermodynamic water‐vapour feedback significantly in response to surface temperature changes.
Radiative‐dynamic effects of the antarctic ozone hole and chemical feedback
Authors: Mackenzie I. A. , Harwood R. S. , Stott P. A. , Watson G. C.
Published: February 2007

A southern hemisphere winter/spring is simulated with the Coupled Stratosphere‐Mesosphere Interactive Chemistry model, a mechanistic model of the middle atmosphere with an interactive chemistry scheme. Simulations are initialized and forced at the lower boundary with dynamic fields taken from UK Meteorological Office assimilations for 1992. Radiative and dynamic effects of ozone depletion are investigated by comparing runs that include heterogeneous chemistry and hence develop an ozone hole, with otherwise identical control runs with no heterogeneous chemistry and no ozone hole. Temperatures in the ozone hole and control runs start to diverge from around mid‐September; by late October the lower stratosphere is up to ∼12 K cooler when an ozone hole is present. In the mid and upper stratosphere, a strengthened general circulation and an enhanced radiative heating rate in the ozone‐depleted runs lead to slightly higher temperatures at the high southern latitudes than in the control runs. the initially smooth evolution of the temperature difference at the upper levels is interrupted in mid‐October by more transient events associated with planetary‐wave activity.

The feedback onto the chemistry of the physical changes wrought by using the depleted ozone in the radiative calculations is investigated by comparing runs of a chemical transport model forced by winds and temperatures calculated with and without an ozone hole. the ozone‐hole induced cooling does not change the spatial extent of the chlorine and bromine activation, and only slightly extends its duration. Consequently, the depth and size of the ozone hole are unchanged by chemical feedback. Local deficits in the ozone column of up to ∼30 Dobson units which arise in the cooler run in October are due to differences in horizontal transport. the increased diabatic descent resulting from using the depleted ozone for the radiation calculations does not significantly alter the vertical transport of polar ozone, the rate of which is low compared with the rate of chemical ozone destruction.
Predictive skill of the UKMO unified model in the lower stratosphere
Authors: Lahoz W. A.
Published: February 2007

This paper studies the short‐term (up to 10 days) predictive skill of the UK Meteorological Office Unified Model (UM) in the lower stratosphere by applying conventional verification statistics used in numerical weather prediction studies. Simulations from four versions of the UM, each of them having a different representation of the stratosphere, are used to assess the dependence of the model's skill on stratospheric vertical resolution, location of the highest model full‐level and location of the highest model half‐level. Results are shown for two winter periods: February 1994 (northern winter with a weak wave‐number two warming) and October 1994 (southern winter with a typical top‐down break‐up of the stratospheric polar vortex). the results of this paper indicate that the model's skill in the lower stratosphere depends on a number of factors, including flow structure, stratospheric vertical resolution and the location of the highest model full‐level. By contrast, the model forecast skill in the mid troposphere is insensitive to the stratospheric representation on the 10‐day time‐scale. the highest skill (10 days and above for temperature and geopotential height at 50 hPa) is associated with steady and deep stratospheric flows which span the upper troposphere. the poorest skill is associated with rapidly varying flows; flows with shallow vertical structure also tend to have relatively poor skill. In all cases the model's skill is a significant improvement over persistence. the model has higher forecast skill in the lower stratosphere than in the mid troposphere; it also has higher skill in northern winter than in southern winter: these differences in skill can be ascribed to the flow regime in the lower stratosphere being dominated by lower wave numbers than in the mid troposphere, and to larger initialization errors in the southern hemisphere. It is shown that higher stratospheric vertical resolution does not necessarily give higher skill in the lower stratosphere; this is particularly true in northern winter. the model results show evidence of links between tropospheric and stratospheric errors.
Equilibrium atmospheres of a two‐column radiative‐convective model
Authors: Nilsson J. , Emanuel K. A.
Published: February 2007

Interaction between steady, large‐scale atmospheric circulations and a radiative‐convective environment is considered. As a model tool, we use a two‐column radiative‐convective model with an explicit hydrological cycle that uses clear‐sky conditions in the radiation calculation. A flow field is calculated by the linearized, hydrostatic equations of motion in a non‐rotating frame of reference. Mechanical damping is represented by vertical diffusion of momentum and surface drag. the flow advects heat and moisture, and thereby modifies the local radiative‐convective equilibrium. A dynamically passive ocean mixed layer is situated below the model atmosphere.

All externally specified parameters are identical in the two columns, implying that local radiative‐convective equilibrium is a steady solution. For weak mechanical damping (or small column length), the local equilibrium is generally unstable due to a positive feedback between large‐scale subsidence and infrared cooling, which operates via advective drying. A circulating equilibrium, in which the air ascends in one column and descends in the other, is attained. Due to a reduced content of clear‐sky water vapour, which is the major infrared absorber in the model, the circulating equilibrium can emit the absorbed solar radiation at a significantly lower surface temperature than the corresponding local equilibrium.

In the limit of a nearly inviscid atmosphere, the intensity of the large‐scale circulation is controlled chiefly by the mid‐tropospheric radiative cooling in the downdraught column. In this regime, we find two distinct equilibria with circulation that are distinguished by the features of the downdraught column: one branch with deep convection but where the integrated convective heating vanishes due to evaporation of precipitation; and one branch with shallow (or no) convection where the surface boundary layer is disconnected from the free atmosphere.
3D‐Var Hessian singular vectors and their potential use in the ECMWF ensemble prediction system
Authors: Barkmeijer J. , Buizza R. , Palmer T. N.
Published: February 2007

Singular vectors are computed which are consistent with 3D‐Var (three‐dimensional variational) estimates of analysis error statistics. This is achieved by defining the norm at initial time in terms of the full Hessian of the 3D‐Var cost function. At final time the total energy norm is used. the properties of these Hessian singular vectors (HSVs) differ considerably from total energy singular vectors (TESVs) in such aspects as energy spectrum and growth rate. Despite these differences, the leading 25 TESVs and HSVs explain nearly the same part of the 2‐day forecast error.

Two experimental ensemble configurations are studied. One configuration uses perturbations based on HSVs in the computation of initial perturbation, the other uses TESVs and 2‐day linearly evolved singular vectors (ESVs) of two days before. the latter approach provides a way to include more stable and large‐scale structures in the perturbations. Ten pairs of ensembles are compared to the operational European Centre for Medium‐Range Weather Forecasts Ensemble Prediction System. the ensembles using ESVs perform slightly better. the ensembles based on HSVs show a slightly worse performance and are lacking some spread in the medium range. Possible directions to improve the computation of HSVs are discussed.
Variational analysis of effective soil moisture from screen‐level atmospheric parameters: Application to a short‐range weather forecast model
Authors: Rhodin A. , Kucharski F. , Callies U. , Eppel D. P. , Wergen W.
Published: February 2007

A variational analysis scheme for soil moisture that assimilates analysed fields of screen‐level atmospheric temperatures and relative humidities in the domain of the operational regional weather forecast model of the Deutscher Wetterdienst, is investigated over a five‐day period in March 1994.

To reduce the computational load implied by the variational analysis scheme, a stand‐alone system comprising the soil module plus a simplified atmospheric boundary‐layer description has been isolated from the full forecast model. the coupling between the two systems is mediated via boundary conditions and source terms.

The retrieved soil moisture values are consistently much lower than those in the operational reference run. They can be regarded as tuned effective parameters providing optimum lower‐boundary conditions for the atmosphere. the improved quality of the resulting sensible‐ and latent‐heat fluxes can be substantiated by better atmospheric forecasts.

Numerical experiments give estimates of the influence on analysed soil moistures of the misspecification of radiative forcing. Soil moisture values derived from observations of atmospheric relative humidity alone are more stable with respect to radiation errors than those derived from atmospheric temperature observations.
Conceptual models of upper‐level frontogenesis in south‐westerly and north‐westerly flow
Authors: Schultz David M. , Doswell Charles A.
Published: February 2007

The conceptual model proposed by Shapiro as it is applied to the evolution of an upper‐level frontal zone within a baroclinic wave is reviewed and its limitations are investigated through previous literature and two casestudies presented in this paper. the early stages in the evolutions of these two cases are used to examine specific limitations of this conceptual model: (1) upper‐level frontogenesis in south‐westerly flow that evolves from a state of equivalent barotropy to a state of cold advection along the front, and (2) upper‐level frontogenesis in northwesterly flow with along‐front variation in the sign of the thermal advection, such that warm advection occurs upstream of cold advection in the thermal trough.

Vector‐frontogenesis diagnostics for the Lagrangian rate of change of the magnitude and direction of the horizontal potential‐temperature gradient, including tilting due to vertical motion, are derived. These diagnostics are applied to the two cases to examine the maintenance of the potential‐temperature gradient and the development of cold advection along each upper‐level front. the upper‐level front in south‐westerly (north‐westerly) flow was maintained primarily by deformation (tilting) frontogenesis, in agreement with previous research. the increasing cold advection along the upper‐level front in both cases was related to an upstream vorticity maximum. For the case in south‐westerly flow, the pre‐existing vorticity maximum approached a downstream equivalent‐barotropic upper‐level front in a manner similar to an instant occlusion, resulting in cold advection along the length of the upper‐level front. For the case in north‐westerly flow, an intensifying vorticity maximum concentrated the cold advection in the base of the thermal trough, as warm advection developed upstream.

These two cases are compared to upper‐level fronts in previous literature and a climatology of upper‐level fronts associated with landfalling cyclones over the eastern North Pacific Ocean. the results indicate that these two cases are typical of early evolutions of upper‐level fronts that can occur in south‐westerly and north‐westerly flow. Therefore, a revised version of the Shapiro conceptual model is presented that represents more accurately the early evolutions exhibited in the present and previous studies.
Frontogenesis and frontal motion due to confluent deformation with a translating dilatation axis
Authors: Cunningham P. , Keyser D.
Published: February 2007

A two‐dimensional model of primitive‐equation frontogenesis is formulated in which the forcing is due to a basic‐state field of confluent deformation with a dilatation axis that translates in the cross‐front direction. Although the translation speed and direction of motion of the basic‐state dilatation axis have no effect on frontal structure when the total deformation is spatially uniform, they can influence considerably the motion of the front. At the surface, this motion may be separated into two components: (i) internal motion, directed towards the warm air, which is associated with the secondary ageostrophic circulation of the front itself; and (ii) external motion, either towards cold or warm air, which is associated with the initial location and translation speed of the basicstate dilatation axis. the latter component ultimately dominates, and thus the motion of a mature front may be thought of as being controlled by the large‐scale flow. However, the former may dominate early on in the frontal evolution, resulting in significantly different frontal trajectories depending on the initial location of the basic‐state dilatation axis and on whether this axis is moving towards the cold or the warm air. the relative locations of various conventional frontal measures are examined in the special case of a uniformly translating dilatation axis. of these measures the axes of maximum baroclinicity and vorticity become collocated at the surface as the front intensifies. However, the basic‐state dilatation axis and the confluence axis, the latter being defined as the location of zero total wind in the cross‐front direction, may be displaced considerably from this merger point, with this displacement depending on the initial location and subsequent motion of the basic‐state dilatation axis. Potential extensions of the model are discussed with regard to improved understanding of the difference in structure and motion between cold and warm fronts.
The January 1992 stratospheric sudden warming: A role for tropical inertial instability?
Authors: Rosier Suzanne M. , Lawrence Bryan N.
Published: February 2007

Twelve simulations of the stratosphere and mesosphere have been performed to investigate the near‐major stratospheric warming of January 1992. All were initialized close to the observed warming. the resulting simulations fall into two classes: those that accurately reproduce the timing and anticyclonic merger mechanism of the observed warming and those that fail. the model used was a mechanistic stratosphere‐mesosphere model. with a prescribed troposphere; thus differences among the simulations must only be due to differences among the initial conditions, which were constructed using different combinations of observed and climatological data. the main differences between the initial conditions of those simulations that succeed and those that fail are in low latitudes. A detailed analysis of two contrasting simulations reveals that the one which fails does not accurately reproduce the observed conveyor belt feeding tropical low‐potential‐vorticity (PV) air into the Aleutian high. Several factors appear to have combined to make the difference between the successful and the unsuccessful simulation of this sudden warming. the amount of low PV already present in higher latitudes in the initial condition appears to have an influence, as does the strength of the subtropical PV barrier. Also important is the Rossby wave structure which, by determining the degree of zonal asymmetry of the PV barrier, influences the injection of low PV into higher latitudes. In addition, however, we propose that the inertial stability of the tropical upper stratosphere might have played a significant role. In the unsuccessful simulation the tropical upper stratosphere is more inertially unstable than in the successful simulation; in this region more vigorous small‐scale roll structures develop which, in stabilizing the flow, might have inhibited the northward transport of tropical air that appears to be a key part of the development of the observed sudden warming.