bluuebunny/crossref_metadata_2025_split huggingface dataset:
Secondary frontal waves in the North Atlantic region: A dynamical perspective of current ideas
Authors: Parker D. J. 
Published: February 2007

AbstractThe current understanding of the dynamics of secondary cyclogenesis on frontal systems in the North Atlantic region is reviewed. These secondary cyclones are often poorly forecast, and in extreme cases can have damaging consequences, but because of their small scales and potential for rapid growth detailed observations and analyses are limited: in this paper recent work is described and compared. It is argued that, on the synoptic scale, the development of ‘primary’ baroclinic waves is well understood, but on the subscales of these systems, where the secondaries develop, there is a wider variety of mechanisms which may cause or modify cyclone growth. This variety may be reflected in a wider range of resulting systems.Various dynamical models have been postulated: secondary frontal waves are seen as the instability of a low‐level potential‐vorticity strip or warm band at a front, with possible finite amplitude triggering by upper‐level features. Latent heating is thought to provide a suitable unstable potential‐vorticity strip but the degree to which the latent heating contributes to the instability itself is a matter of dissent. Secondary systems are often shallow, so boundary‐layer processes are important, but the influence of these is not well understood; laboratory experiments have suggested that the boundary layer will act to suppress instability. In general, frontogenetic strain flow, while acting to intensify the basic‐state front, suppresses wave development—in contrast, recent work suggests that a frontolytic strain flow may be important in enhancing the development of one class of waves.The FASTEX experiment, an international field study of North Atlantic secondary cyclogensis, has recently attempted to observe these systems in detail. It is hoped that the data which have been obtained will help to resolve some of the outstanding issues in this field.
Effects of observation errors in linear regression and bin‐average analyses
Authors: Tolman H. L. 
Published: February 2007

AbstractEffects of observation errors in linear regression and bin‐averaged (BA) validation techniques are investigated using the example of marine wind speeds. It is shown that a conventional linear regression systematically underestimates the slope of the regression line, and systematically overestimates the random model error. A BA analysis systematically understimates extreme wind speeds, incorporates spurious nonlinearity, and overestimates random model errors. Correction techniques are suggested for studies in which the observation error can be estimated. Using synthetic data the potential of the correction techniques in illustrated, and it is shown that the above errors are generally not negligible for wind speed validation studies. Practical examplies consider the random errors of anemometers and wind speed estimates from satellites. These examples highlight the importance of the error corrections, and illustrate the difficulty of estimating observation errors. Finally, it is argued that the well‐known symmetric slope regression should not be used for the validation of forecast systems. Although the present study deals with marine wind speeds, its results are expected to be valid for a wide range of validation studies.
A simple theoretical model for the intensification of tropical cyclones and polar lows
Authors: Gray Suzanne L. , Craig George C. 
Published: February 2007

AbstractA simple theoretical model for the intensification of tropical cyclones and polar lows is developed using a minimal set of physical assumptions. These disturbances are assumed to be balanced systems intensifying through the WISHE (Wind‐Induced Surface Heat Exchange) intensification mechanism, driven by surface fluxes of heat and moisture into an atmosphere which is neutral to moist convection. The equation set is linearized about a resting basic state and solved as an initial‐value problem. A system is predicted to intensify with an exponential perturbation growth rate scaled by the radial gradient of an efficiency parameter which crudely represents the effects of unsaturated processes. The form of this efficiency parameter is assumed to be defined by initial conditions, dependent on the nature of a pre‐existing vortex required to precondition the atmosphere to a state in which the vortex can intensify. Evaluation of the simple model using a primitive‐equation, nonlinear numerical model provides support for the prediction of exponential perturbation growth. Good agreement is found between the simple and numerical models for the sensitivities of the measure growth rate to various parameters, including surface roughness, the rate of transfer of heat and moisture from the ocean surface, and the scale for the growing vortex.
Characteristics of heat and moisture budgets of a mesoscale convective system observed during TOGA‐COARE
Authors: Nuret Mathieu , Chong Michel 
Published: February 2007

AbstractThe large‐scale heat (Q1) and moisture (Q 2) budgets of a tropical convective system occurring in the warm sea surface region of the western Pacific Ocean are analysed over a 36 h period. The system was observed on 26 and 27 November 1992, during the Tropical Ocean/Global Atmosphere (TOGA) Coupled Ocean‐Atmosphere Response Experiment (COARE), and propagated from the north‐eastern part of the Outer Sounding Array (OSA) toward the Intensive Flux Array (IFA) following a south‐westward direction. It was composed of an extensive shield of high cirrus clouds over the OSA, which advected into the IFA. Rawinsonde data collected during these two days, at 6 h intervals, and also surface, satellite and operational forecast‐model data are used to investigate the budgets and the associated precipitation rates over the two contiguous regions containing different convective activity.Globally, the vertical distributions of the heat source and moisture sink over each domain are found to be in qualitative agreement with those of other regions of the western Pacific. The heating profiles show vertical variations closely consistent with the shape of the observed mean vertical‐velocity profiles, suggesting the importance of latent heating. The mature stage provides a heating peak at the upper level in the 450–550 hPa layer, and a marked double peak drying structure at low (800–850 hPa) and middle (400–600 hPa) levels. The series of Q1 and Q2 profiles obtained every 6 hours over the two domains indicates the progressive increase of stratiform cloud which is associated with an elevation of the heating‐peak level, while the upper drying peak intensifies and the lower drying peak is decreasing. Advection contributes to these changes. Differences are observed between the profiles over both regions; these can be explained by reduced convective activity over the IFA and also by the presence of advected anvil clouds from the OSA, which lead to heating and drying over the IFA half as large as over the OSA. Similarly, the associated maxima are located at lower levels. An interesting feature is the existence of a secondary heating maximum in the high troposphere near 200–300 hPa, which can be explained as a contribution from the advected high‐level stratiform clouds.The budget‐derived rainfall rates compare well with those deduced from surface and satellite data, but to a lesser degree with model‐forecast results. This comparison highlights the importance of data density in the behaviour of the large‐scale model in representing convection in equatorial regions, as already identified for the present TOGA‐COARE. Although vertical advection of moisture globally balances the quantity of water that condenses and precipitates, the contribution of horizontal moisture advection can be an important component of the moisture budget, to counterbalance the storage of moisture in a column.
The response of a moist zonally symmetric atmosphere to subtropical surface temperature perturbation
Authors: Zheng Xinyu 
Published: February 2007

AbstractA moist zonally symmetric model has been developed in order to inveestigate the response of a moist atmosphere to subtropical surface temperature perturbations. This is an extension of the study by Plumb and Hou (1992) in which they found that the response of a dry atmosphere to a localized subtropical thermal forcing exhibits threshold behaviour. An inviscid steady‐state theory for zonally symmetric moist atmospheres by Emanuel (1995) is presented first. The model calculations confirm the theory. Then the model is used to examine how moisture‐dynamics feedback influences the threshold behaviour, for a given subtropical surface temperature perturbation. The threshold behaviour is not changed by the inclusion of moisture‐dynamics feedback. Finally, calculations are performed for cases with time‐varying surface temperature perturbations, in order to check the extent to which steady‐state theory can be applied. It is found that the theory is applicable to cases with seasonally varying surface temperature perturbations. Finally some implications of this study are discussed.
The response of bora‐type flow to sea surface temperature
Authors: Enger Leif , Grisogono Branko 
Published: February 2007

AbstractA non‐linear, two‐dimensional, hydrostatic, incompressible numerical model with a higher‐order turbulence closure scheme is used to study the effect of sea surface temperature on the severe downslope wind called bora at the Adriatic coast. A non‐linear large‐amplitude mountain wave is generated and is broken beneath and within its critical layer, due to resonant tuning between the initially single‐layer atmosphere and the terrain. The tuning is governed by the Froude number. A qualitative and sometimes quantitative analogy exists between the wave‐breaking (unsteady, stratified) flow and the hydraulic jump (steady, two‐layer flow). It is also known that the strongest Adriatic bora appears during the winter season, when the sea surface temperature is typically larger than the ground surface temperature.Firstly, a relatively higher (lower) sea surface temperature means an additional distortion (moderation) of the mountain wave and consequently a larger (smaller) area with bora wind maxima. For a relatively higher sea surface temperature a propagating hydraulic jump occurs. Typically bora maxima are about three to four times larger than the related geostrophic wind (8 m s−1). Secondly, the presence and importance of the inertial oscillation are indicated. Since the wave‐breaking is the vital component of the strongest bora cases, there is a relatively large, elevated area–i.e. the critical layer–with substantial flow decelerations and generally low wind speeds. The wave‐breaking area has a Rossby number O(1). Hence, the earth's rotation appears to be an important part of bora evolution. The simulations presented consider generalized bora cases which may pertain to other similar orographic flows.
The use of TOVS clear radiances for numerical weather prediction using an updated forward model
Authors: Rizzi Rolando , Matricardi Marco 
Published: February 2007

AbstractThe impact of TOVS clear radiances, processed using a new transmittance database, on the ECMWF forecast system, is evaluated. This involved the production of a line‐by‐line transmittance database, of sets of coefficients for the operational fast radiative‐transfer scheme, and the use of the operational bias‐correction procedure to reduce residual long‐term systematic differences. Two case‐studies are used to examine data from TOVS instruments on three NOAA satellites, and the differences in radiances measured by the various platforms are discussed. Measured and simulated data are compared before and after the operational bias‐removal scheme, and a final impact assessment is made using an assimilation and forecast experiment.
The Berlin troposphere–stratosphere–mesosphere GCM: Sensitivity to physical parametrizations
Authors: Pawson Steven , Langematz Ulrike , Radek Gerd , Schlese Ulrich , Strauch Peter 
Published: February 2007

AbstractThe 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

AbstractA 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

AbstractSeasonal 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

AbstractThe 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

AbstractAn 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

AbstractTime 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

AbstractThis 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

AbstractThe 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

AbstractThree‐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.