bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison between the land surface response of the ECMWF model and the FIFE‐1987 data
Authors: Betts Alan K. , Ball John H. , Beljaars Anton C. M.
Published: February 2007

An averaged time series for the surface data for the 15 × 15 km FIFE site was prepared for the summer of 1987. Comparisons with 48‐Hour forecasts from the ECMWF model for extended periods in July, August and October 1987 identified model errors in the incoming short‐wave radiation in clear skies, the ground heat flux, the formulation of surface evaporation, the soil‐moisture model, and entrainment at boundary‐layer top.

The model clear‐sky short‐wave flux is too high at the surface by 5–10%. The ground heat flux is too large by a factor of 2 to 3 because of the large thermal capacity of the first soil layer (which is 7 cm thick), and a time truncation error. The surface evaporation was near zero in October 1987, rather than of order 70 W m−2 at noon. The surface evaporation falls too rapidly after rainfall, with a time‐scale of a few days rather than the 7‐10 days (or more) of the observation. On time‐scales of more than a few days the specified ‘climate layer’ soil moisture, rather than the storage of precipitation, has a large control on the evapotranspiration. The order 2g Kg‐1 in forecasts from an experimental analysis with nearly realistic surface fluxes; this because there is insufficient downward mixing of dry air.
Anomaly in the Alpine diurnal pressure signal: Observations and theory
Authors: Frei Ch. , Davies H. C.
Published: February 2007

An examination is undertaken of the diurnal surface‐pressure pattern across the main east‐west ridge of the European Alps. A case study for clear‐sky, quiescent synoptic conditions reveals a pronounced across‐ridge asymmetry—to the north the primary feature is an approximate 0.5 hPa semi‐diurnal oscillation whilst on the southern side there is an approximate 2.5 hPa diurnal oscillation. A one‐year statistical climatology and a two‐month synoptic climatology confirm the existence, and indicate the prevalence of this asymmetry. The across‐ridge variation in the amplitude of the solar‐diurnal (i.e S1) component is about 1 hPa and it has a lateral scale of around 200 km.

From a theoretical standpoint physical arguments and dynamical considerations are advanced to suggest that the S1 anomaly is not readily attributable to purely in situ effects. In addition a heuristic model is deployed to examine the possible nature of the Alpine modification of the planetary diurnall tide. The model, comprising a cylindrical island embedded in ‘reduced‐gravity’ shallow water, reveals a terrain‐induced modification in the form of a mesoscale, terrain‐trapped Kelvin wave that interferes destructively (constructively) with the global S1 signal to the north (south) of the terrain. The pattern's amplitude, phase and spatial structure are consistent with the main features of the observed signal.
Importance of diabatic processes on ageostrophic circulations observed during the FRONTS 87 experiment
Authors: Lagouvardos K. , Lemaitre Y. , Scialom G.
Published: February 2007

The impact of condensation and evaporation on real cross‐frontal circulations is studied, using data from the FRONTS 87 field experiment. The Sawyer‐Eliassen equation is solved numerically in its primitive equation form, for one intensive observing period of FRONTS 87. Three different schemes of parametrization of condensation heating are used. Comparison between diagnosed and observed ageostrophic circulations shows that latent heat release plays an important role in enhancing the ascending branch of the ageostrophic circulation ahead of the frontal zone. In particular, when a parametrization scheme for the low‐level moisture convergence is applied, a cell of strong ascending motion is formed ahead of the frontal zone, in good agreement with the observations. Further, a simple parametrization of evaporative cooling below the frontal discontinuity leads to a reinforcement of downdraughts in the cold sector of the front, as observed in regions where precipitation originating from the strong line convection evaporates. In comparison with their adiabatic counterparts, the current solutions show strong similarities with the observed circulation and indicate the predominance of diabatic effects in the frontal zone.
Nonlinear balance on an equatorial beta plane
Authors: Raymond David J.
Published: February 2007

Extension of the nonlinear balance model to include the equator is examined here from a theoretical perspective. It is found that this model is ill‐posed, in that zonally symmetric equatorial Kelvin waves of arbitrary amplitude can occur. Two possible solutions are outlined. The zonally‐averaged zonal momentum equation provides auxiliary information that can be imposed on the flow. This equation can be solved simultaneously with the other parts of the problem. Alternatively, when small, isolated disturbances can be considered to have a negligible effect on the mean flow; one can simply assume that zonal means remain unchanged.
Thermal compression waves. II: Mass adjustment and vertical transfer of total energy
Authors: Nicholls Mel Ville E. , Pielke Roger A.
Published: February 2007

A fully compressible model is used to simulate the mass adjustment that occurs in response to a prescribed heat source. Results illustrate the role that thermal compression waves have in this process. The vertical mass transport associated with compression waves decreases rapidly with height. Most of the mass transport occurs in the horizontal, with the vertical structure of the disturbance similar to that of a Lamb wave. The vertical transfer of total energy in a thermally driven mixed layer is also examined. It is shown that the upward transport of total energy is accomplished by a compression effect rather than by the exchange of warm and cold air by buoyant thermals. Model results are analysed to determine budgets of total energy, mass and entropy. It is demonstrated that buoyant thermals are predominantly responsible for a transfer of entropy, rather than total energy. In the light of these results the notion of ‘heat transport’ in a fluid is discussed.
Passage of a frontal zone over a two‐dimensional ridge
Authors: Egger Joseph , Hatt Hans
Published: February 2007

The passage of a frontal zone over a two‐dimensional triangular ridge is investigated using a semigeostrophic model. The model atmosphere consists of three layers of uniform density and potential vorticity. The densest (coldest) layer represents the airmass behind the cold front, the next layer the frontal zone itself, and the least dense (warmest) layer represents the flow ahead of and above the frontal zone. At ground level, the frontal zone is contained between two surface fronts. A prescribed pressure field imposes translation at a speed U, and confluence with a deformation rate α. A one‐layer version of the model has also been considered. The main conclusions are that (a) the mountain‐induced shift of the position of a front does not depend on U or α; it is mainly negative on the windward slope and the upper part of the leeward slope, but is positive in the downstream plain and on the lower part of the leeward slope (there are no effects far‐afield), and (b) frontal collapse must occur for α > 0. The collapse is delayed over most of the mountain, compared to the no‐mountain case, but occurs earlier downstream from the mountain. Prior to the collapse, there is up‐slope frontogenesis when part of the frontal zone is in the plain. Frontolysis prevails if all of the frontal zone is on the up‐slope and if α is not too large. Frontogenesis occurs on the down‐slope and in the plain.
The shapes of snow crystals—Fitness for purpose?
Authors: Mason B. J.
Published: February 2007

Snow crystals exhibit six quite sharp changes of habit between 0 and −25°C as between hexagonal plates, columns/needles and stellar dendrites. The questions arise as to why, and do these changes, unique in crystal physics, serve a useful purpose?.

Calculations of the growth rates of the various forms, and also of ice spheres, as they fall through atmospheres with specified profiles of temperature and supersaturation, reveal that the transitions from plates to stellar dendrites that grow only between −12 and −16°C, and from plates to columns below −25°C, permit the more effective release of precipitation from layer clouds. The interruption of the plate regime by the growth of columns and needles between −8 and −3°C leads to the formation of larger precipitation elements that better survive evaporation below cloud base than would plate crystals, by virtue of having much higher collection efficiencies for supercooled cloud droplets.
Physical retrieval of liquid water contents in a North Atlantic cyclone using SSM/I data
Authors: Prigent C. , Sand A. , Klapisz C. , Lemaitre Y.
Published: February 2007

In the framework of the Mesoscale Frontal Dynamics Project/FRONTS 87, a cyclone over the North Atlantic Ocean is studied with the help of the Special Sensor Microwave/Imager (SSM/I) on board the satellite of the Defense Meteorological Satellite Program. After a brief recall of the relevant studies on the frontal system of 11 November 1987 in the context of the second intensive observing period of Fronts 87, the SSM/I images are analysed to show the potential of microwave observations for diagnosing the mesoscale structure of the front. Then, considering the poor accuracy of the SSM/I regression‐type algorithms in estimating the integrated cloud‐liquid‐water contents, we propose to retrieve this quantity with the help of a radiative‐transfer model, coupled with a realistic first guess of the atmospheric characteristics in the meteorological area of concern. A physically consistent estimation is deduced from a nonlinear iterative inversion procedure, which aims at reducing the difference between simulated and measured brightness temperatures. The mesoscale structures of the retrieved fields of water vapour, cloud liquid water and rain rate appear consistent with the Fronts 87 meteorological analysis of this situation, and they complement the understanding of the processes involved in the dynamics of the front.
Horizontal structure of height‐forecast errors over the Southern part of South America
Authors: Vera Carolina
Published: February 2007

A study of the height‐forecast error autocovariances was made for a simple intermittent data‐assimilation system (SADI) applied over the southern part of South America. The main objective of this study was to determine from the data the height‐forecast error autocorrelation, and to provide new estimations of the forecast‐ and observation‐error standard deviations. In performing this derivation several significant differences were found between the estimates obtained with a simple forecast model in an area which is data sparse, and those derived for data‐rich regions and advanced forecast models. The forecast‐error correlations were fitted to the Fourier—Bessel series and also to a negative square exponential (NSE) function at the six model levels. The fitted Fourier—Bessel series was able to represent more accurately the wavelength range resolved by the analysis scheme than the fitted NSE function. New estimations of forecast and observation errors have been obtained from the Fourier—Bessel expansion. As the representative errors are much larger because of the coarse resolution of the network and the low‐order truncation of the fit, the observation errors are greater than those used for the northern hemisphere analysis systems. Results also show that the forecast errors are dominated by the synoptic‐scale components in all the levels considered. An evaluation of the impact of using the new set of horizontal covariances was done and it mainly showed that the analysis done with the fitted Fourier‐Bessel series better represents the data than both the original SADI system and an analysis system using a fitted NSE function.
A pole problem in the reduced Gaussian grid
Authors: Courtier P. , Naughton M.
Published: February 2007

It is shown that a pole problem may arise from the use of a reduced Gaussian grid in the spectral transform method on the sphere. The problem is related to an asymptotic property of the associated Legendre functions and can be solved by slightly increasing the number of points close to the pole.

It is also shown that the reduced grid controls aliasing arising from quadratic terms only as an asymptotic property. Nevertheless, a small increase in the number of points (everywhere) is enough to reduce the aliasing to a negligible level.
The effect of the passage of cold fronts on the vertical distribution of ozone at the CHMI observatory at Prague‐Libuš
Authors: Halenka T. , LašTovička J. , Radicella S.
Published: February 2007

The response of the total ozone content to meteorological phenomena of a synoptic scale has been studied for many years and is relatively well known now. Much less is known about the response of the ozone profile to such meteorological phenomena. In this paper the response of the shape of the ozone profile to the passage of intense cold fronts and to strong advection from a significantly northern direction is investigated using the results of ozone soundings carried out by the observatory of the Czech Hydrometeorological Institute (CHMI) in Prague‐Libuš during the period January to April for the years 1981 to 1990. Some data from Uccle (Belgium) have also been used.

The synoptic patterns studied here were those that were followed by the occurrence of secondary peaks in ozone profiles between the tropopause height (often significantly decreased during such events) and the usual height of the main ozone maximum. Such secondary peaks can be sometimes even larger than the usual ozone maximum. The observed changes in ozone profiles could be explained by the transport of air of polar origin having a greater ozone content in the lower stratosphere behind the upper‐level frontal zone.
Large‐eddy simulation of stable atmospheric boundary layers with a revised stochastic subgrid model
Authors: Brown A. R. , Derbyshire S. H. , Mason P. J.
Published: February 2007

Simulations of stable atmospheric boundary layers have been conducted at various stabilities with a revised, stochastic backscatter model based on Mason and Thomson's theoretical and numerical results. Compared to the previous results of Mason and Derbyshire, there is more turbulence and a deeper boundary layer, as was expected from the tendency of the parametrization change to increase the surface stress. Scatterplots of various local non‐dimensional quantities against the gradient Richardson number are presented which indicate a high degree of collapse to a single local scaling curve. Agreement of the non‐dimensional gradients with the surface‐layer observations is greatly improved by the inclusion of backscatter.
Diagnosis of hydrometeor profiles from area‐mean vertical‐velocity data
Authors: Braun Scott A. , Houze Robert A.
Published: February 2007

A simple one‐dimensional microphysical retrieval model is developed for estimating vertical profiles of liquid and frozen hydrometeor mixing ratios from observed vertical profiles of area‐mean vertical velocity in regions of convective and/or stratiform precipitation. The mean vertical‐velocity profiles can be obtained from Doppler radar (single and dual) or other means. The one‐dimensional results are shown to be in good agreement with two‐dimensional microphysical fields from a previous study. Sensitivity tests are performed.
The parametrization of surface fluxes in large‐scale models under free convection
Authors: Beljaars Anton C. M.
Published: February 2007

This paper investigates the idea of formulating free convection in large‐scale models as a special case of forced convection. In free convection, the wind speed in the surface transfer law is related to the velocity of the large eddies in the mixed layer and is proportional to the convection velocity scale. The empirical coefficient is estimated with the help of large eddy simulation data by Sykes et al. (1993) and the resulting formulation is compared with field data by Stull (1994). This concept is shown to be applicable to smooth ocean surfaces as well as rough land surfaces. It is argued that within this framework, free convection is a natural extension of forced convection and only needs a minor modification of traditional transfer laws.
Rotating convective disturbances in the trades
Authors: Raymond David J. , Lewis Sharon A.
Published: February 2007

Observations of rotating convective disturbances in the trade‐wind region just east of Hawaii are presented. Rotation results from stretching of vertical ambient vorticity in the convective regions. Surface friction is shown to play a significant dissipative role in these disturbances. Ekman pumping associated with the rotation is probably only of secondary importance. Convection is found to be associated with weak local maxima in the boundary‐layer equivalent potential temperature. This suggests that convection occurs where such anomalies make the buoyancy of surface parcels larger than average. Convective available potential energies are found to be small, which means that such equivalent‐potential‐temperature anomalies are important in determining parcel buoyancy. It is hypothesized that mesoscale circulations induced by the ascending motion of convection reinforce the anomalies, resulting in the intensification of the circulations. The equivalent potential temperature of the boundary layer results from a balance between surface fluxes and entrainment from above. Ascending motion apparently reduces entrainmcnt, thus allowing the equivalent potential temperature to rise, whereas subsidence has the opposite effect.
Mesoscale aspects of a dry intrusion within a vigorous cyclone
Authors: Browning K. A. , Golding B. W.
Published: February 2007

The results of a diagnostic study using radar and satellite imagery, together with surface reports and output from regional and mesoscale models, are presented for an occasion of a rapidly deepening cyclone that crossed the British Isles. The study shows how air descended from near‐tropopause level in the form of mesoscale dry intrusions which appeared to overrun parts of the warm conveyor belt ahead of a surface cold front. One of the dry intrusions is analysed in detail, because it is thought to have led to the large changes that were observed in the character of the wide frontal rain band: in some places the precipitation was entirely suppressed, whilst in other places the precipitation became convective, with a tornadic squall line developing. Although the effect of the dry intrusion was seen most clearly in the precipitation distribution as determined by radar, the approach of the dry intrusion was also clearly detected in the satellite water‐vapour imagery, and foreshadowed in the model forecasts.

The mesoscale model, despite some limitations, reproduced many of the observed mesoscale features. It also provided insight into the detailed behaviour of the dry intrusion associated with the tornadic squall line. It showed that the dry intrusion was characterized by a mesoscale filament of high potential vorticity (PV) which was extruded from the lower part of a large region of high PV near a low tropopause. The extruded PV anomaly plunged to within a kilometre of the surface where it overran part of the warm conveyor belt. The dry intrusion was also characterized by relatively low wet‐bulb potential temperature, θw, and it led to potential instability where it overran the warm air. The warm conveyor belt itself was characterized not only by a high θw but also by a separate strip of high PV generated locally, mainly by condensation. The quality of the mesoscale‐model simulation gives grounds for optimism that such models are capable in principle of predicting severe mesoscale weather events when, as in this case, they are orchestrated by resolved larger‐scale dynamics.
Global processing of satellite sounding radiances in a numerical weather prediction system
Authors: Gadd A. J. , Barwell B. R. , Cox S. J. , Renshaw R. J.
Published: February 2007

This paper gives an account of the global soundings system (GLOSS), which is the new method introduced at the Meteorological Office at Bracknell for the processing of global TOVS radiance data for assimilation into numerical weather prediction (NWP) models. The assimilating NWP models themselves provide the prior information necessary to infer temperature and humidity information from radiances. After a brief summary of the historical background, the GLOSS processing is described, noting in particular the differences from similar work elsewhere. Results are then presented from NWP impact studies of the assimilation of temperature profiles derived from satellite soundings. An advantage is demonstrated for the GLOSS temperature retrievals relative to the retrievals distributed by NESDIS and produced from the same radiance data. The advantage of GLOSS is clear and consistent in the extratropical regions of the northern hemisphere and, especially, of the southern hemisphere. In the tropics the results are more mixed. The paper concludes with a note of additional work required before operational implementation of GLOSS, and with an outline of other expected future developments.
Data sparsity and the tropical‐cyclone analysis and prediction problem: Some simulation experiments with a barotropic numerical model
Authors: Weber Harry C. , Smith Roger K.
Published: February 2007

Some simulation experiments relevant to the problem of determining the minimum data requirements for the accurate reconstruction of tropical‐cyclone‐scale vortices from given sparse data distributions are described. Also, the accuracy with which the vortex track can be predicted using these data is investigated. The experiments are based on a 96‐hour numerical integration of the barotropic vorticity equation on a beta‐plane, starting with a symmetric tropical‐cyclone‐scale vortex embedded in zonal shear flow. After 48 hours, when asymmetries in the vortex circulation have developed, the model fields in this ‘control’ calculation are used as a ‘perfect’ data set to assess the analysis strategy described. The 20 km horizontal resolution of these new initial data is progressively degraded to test the ability of the analysis procedure to reconstruct the perfect analysis. The analyses of the degraded data sets are used as an initial condition for a further 48‐hour integration of the model, and the resulting vortex tracks are compared with that in the second half of the control run. The influence of the analysed vortex asymmetry on the subsequent vortex track is also studied.

As the resolution of the simulated initial data is degraded, so is the information that these data provide on the symmetric circulation and the vortex‐induced asymmetries, and ultimately it becomes necessary to introduce ‘synthetic’ data in the analysis. The calculations indicate some of the problems that need to be overcome to introduce such data properly.

The analysis method provides a general way to partition the flow into four components: the large‐scale environment, the symmetric vortex, the vortex asymmetry, and the small‐scale environment. The large‐scale environment is characterized by the lowest few wave numbers in a two‐dimensional Fourier analysis. The residual field is subjected to an azimuthal Fourier analysis about the cyclone centre, and the last three components of the partition are defined by the symmetric component, the wave‐number‐one component and the sum of all higher wave‐number components of this analysis, respectively. An attractive feature of such a partition is that the total flow across the centre of the symmetric vortex, which governs the vortex motion in a barotropic model, is contained in only two components: the large‐scale environment and the vortex asymmetry.
Simulated sensitivity of the earth's radiation budget to 'changes in cloud properties
Authors: Sinha A. , Shine K. P.
Published: February 2007

Simulations are made of the long‐term mean clear‐sky and total‐scene components of the earth's radiation budget (ERB), using detailed solar and thermal infrared radiative transfer schemes. These employ as input a wide range of multi‐year observations of atmospheric, surface and cloud parameters. For clear skies, differences between the simulations and results from the Earth Radiation Budget Experiment (ERBE) are generally within the expected accuracy of the observations and calculations, with the most notable disagreement at high latitudes. For cloudy skies, the agreement is less good, but still generally within the systematic error bounds. the absorbed shortwave radiation (ASR) is simulated least well over oceanic regions, with the pattern of disagreement resembling the pattern of response when low cloud cover or liquid water path is perturbed; this suggests that the representation of low cloud cover or liquid water path dominates the ASR discrepancy between the simulation and ERBE. the simulated outgoing longwave radiation (OLR), which is sensitive to the high‐cloud emissivity, differs most from the ERBE OLR in deep convective regions; whilst assuming the high cloud to be black lessens agreement between the simulation and observations in some areas, agreement is substantially improved in others. This indicates that better knowledge of high‐cloud emissivity remains an important issue. the spatial and temporal variations of the sensitivity of the ERB to perturbations to low, mid and high cloud properties are investigated. Principal results include (a) the dominant importance of cloud amount, as compared to cloud temperature, in determining the sensitivity of the ERB to high‐cloud properties, and (b) the greater sensitivity of the net ERB to low‐cloud properties than to those of mid and high cloud. A major uncertainty in the calculations is the degree of cloud overlap; the inclusion of overlapping clouds is found to be capable of reversing the net high‐cloud sensitivities in certain locations. Global mean sensitivity to low cloud perturbations are compared with previous results obtained using a GCM. the simulated sensitivity of the ERB to low cloud cover is substantially larger than in the previous results, although the effects of changes in liquid water path are similar.
A realistic three‐component planetary wave model, with a wave‐breaking parametrization
Authors: Kinnersley J. S. , Kinnersley Jonathan S.
Published: February 2007

A fairly simple stratospheric model of the three longest planetary waves is constructed, which includes interference terms between the three wave components, and a diffusive parametrization of wave breaking in regions where the local meridional potential vorticity gradient is negative. In order to allow comparison with the observed behaviour of planetary waves the model is forced near the tropopause with observed wave amplitudes and the model waves propagate over the observed zonal‐mean state. the model is able to mimic the behaviour of the observed waves fairly accurately, with the thermal damping, interference and wave breaking terms all being important to its success. Good results are obtained with a value of local wave‐breaking diffusion coefficient between 106 and 107m2s−1. It is shown that the deficiencies in Matsuno's (1970) model are probably due mainly to the neglect of wave‐wave interactions and an over‐simplified zonal‐mean temperature structure.

It is shown that Garcia's (1991) idea that wave breaking completely absorbs any build‐up of zonal‐mean wave activity is probably too severe, but that better results are obtained if the local nature of the wave‐breaking is taken into account.

The zonal‐mean diffusion coefficient sometimes has large values within the vortex, as well as in the midlatitude surf zone, with the two regions of diffusion being separated by the vortex edge.

The difficulty in estimating the diffusion coefficient required for a tracer is discussed, as well as the behaviour of the zonal‐mean potential vorticity flux during a wave‐breaking episode. the assumption of turbulent diffusion in reversed‐gradient regions implies a positive diffusive contribution to the potential vorticity flux.

It was also found that changes in the equatorial zonal wind result in a modulation of the extra‐tropical potential vorticity flux similar to that found by Dunkerton and Baldwin (1991), but that the modulation did not depend strongly on a movement of the tropical zero‐wind line.
The role of ozone‐induced diabatic heating anomalies in the quasi‐biennial oscillation
Authors: Li Dingmin , Shine K. P. , Gray L. J.
Published: February 2007

The diabatic effects of the quasi‐biennial oscillation (QBO) in ozone on the oscillation of temperature and zonal mean winds are investigated using a 2‐dimensional model with full interaction between dynamics, radiation and photochemistry. It is found that the temperature anomaly caused by the ozone QBO makes a significant (25%) contribution to the total temperature anomaly of the QBO although the corresponding change in wind is smaller (about 10%). It is shown that the diabatic effects of the ozone QBO should not be neglected in attempts to simulate the QBO in other parameters.
Evaluation of physical processes in an idealized extratropical cyclone using adjoint sensitivity
Authors: Langland Rolf H. , Elsberry Russell L. , Errico Ronald M.
Published: February 2007

An adjoint model is used to examine the sensitivity of an idealized dry extratropical cyclogenesis simulation to perturbations of predictive variables and parameters during the cyclone life cycle. the adjoint sensitivity indicates how small perturbations of model variables or parameters anywhere in the model domain can influence cyclone central pressure. Largest sensitivity for both temperature and wind perturbations is located between 600 and 900 hPa in the baroclinic zone above the developing cyclone. Perturbations of a given size have more influence on cyclone intensity when located in high‐sensitivity regions (the middle and lower troposphere in this simulation). the effects of physical processes can be interpreted with adjoint sensitivity by considering perturbations that are proportional to temperature and wind tendencies in the basic state (nonlinear forecast). In the early phase of the cyclone life cycle, temperature advection near the steering level in the lower troposphere (about 800 hPa) is strongly cyclogenetic and resembles a Charney mode of baroclinic instability. During the phase of most rapid deepening, temperature advection in the lower troposphere remains important, while interpretation of sensitivity to wind perturbations suggests that increased vorticity in the middle and upper troposphere above the surface low‐pressure centre may also be significant for cyclone intensification.

Adjoint techniques can provide insight into spatial and temporal sensitivity not easily obtained from other methods. Higher sea surface temperature (SST) has a cyclogenetic effect mainly in a localized region corresponding to the cyclone warm sector. Outside the areas of high sensitivity, small perturbations of SST have very little effect on central pressure of the forecast cyclone. When strong upward sensible‐heat flux, Fs, exists, it can have a cyclogenetic (preconditioning) influence early in the cyclone life cycle, although downward Fs in the cyclone warm sector is anticyclogenetic during the phase of most rapid deepening. the sensitivity indicates that Fs can be cyclogenetic in one location and anticyclogenetic at the same time in another location, so that Fs effects on cyclone intensity are partially self‐cancelling. Surface momentum stress is anticyclogenetic, with sensitivity highly localized in the cyclone warm sector.
Airborne radiometric observations of cloud liquid‐water emission at 89 and 157 GHz: Application to retrieval of liquid‐water path
Authors: English S. J.
Published: February 2007

Measurements of the microwave brightness temperature of stratocumulus cloud at 89 and 157 GHz using the Microwave Airborne Radiometer Scanning System on the UK Meteorological Research Flight's C‐130 aircraft have been analysed. Comparisons of observed and calculated brightness temperature using models available in the literature have given good agreement for sea‐surface emission and atmospheric attenuation in clear and cloudy skies. A nonlinear retrieval scheme has been applied to the observations to retrieve cloud liquid‐water paths for comparison with the in situ measurements. Validation of the retrieved liquid‐water paths to within 50 g m−2 has been achieved. Ambiguities between cloud retrievals and water vapour and surface parameters are discussed. the observed differences between the retrieval and the in situ measurement are not found to correlate strongly with cloud temperature, but a higher than expected correlation is found with the drop‐size distribution. It is demonstrated that the scheme is applicable to satellite soundings of cloud, and that a similar level of accuracy should be achieved.
Optimal perturbation time evolution and sensitivity of ensemble prediction to perturbation amplitude
Authors: Buizza Roberto
Published: February 2007

Certain characteristics of the perturbations which grow most rapidly over a finite time interval in a primitiveequation atmospheric model are discussed. They are the singular vectors of a linear approximation of the European Centre for Medium‐Range Weather Forecasts primitive‐equation model. They are computed using the adjoint technique at horizontal spectral truncation T21 with 19 vertical levels. Linear combinations of singular vectors, named optimal perturbations, can be used in ensemble prediction to generate the initial conditions of perturbed integrations.

Firstly, having specified the initial amplitude to be comparable with the amplitude of analysis‐error estimates, the nonlinear time evolution of optimal perturbations when added to the control initial conditions are studied. In particular, estimates are made of the time limit, TNL, after which nonlinear processes cannot be neglected. Considering optimal perturbations generated using singular vectors with maximum growth over a 36‐hour time interval, and characterized by amplitudes comparable with analysis‐error estimates, two different methods estimate TNL ≈︁ 2‐2.5 days.

Secondly, the sensitivity of ensemble predictions to the optimal perturbation amplitude is analysed. This sensitivity study suggests that an increase of the root‐mean‐square amplitude of the initial perturbation can give a more realistic ensemble spread.

Lastly, an estimate of the possible impact of the reduction of the amplitude of analysis errors on the skill of numerical weather prediction is deduced from the comparison of ensemble experiments run with T21 initial perturbations characterized by different amplitudes. Results indicate that a reduction of the root‐mean‐square amplitude of the analysis error by a factor √2 may lead to an improvement of medium‐range predictability up to 1 day, and that a reduction by a factor 2√2 may reduce the errors of the 7‐day forecast to values shown, at present, at forecast‐day 5.
Winds in the strait of gibraltar
Authors: Dorman Clive E. , Beardsley Robert C. , Limeburner Richard
Published: February 2007

Automated meteorological observations were made from October 1985 to September 1986 in the narrows of the Strait of Gibraltar. Additional surface and upper‐air measurements were made in the Strait from May to July 1986. the main wind events in summer were mostly mesoscale levanters (winds from the east through the Strait), and a lesser number of synoptic‐scale levanters and westerlies. During mesoscale levanters, the wind speed increased and the pressure decreased from Gibraltar downstream through the narrows as far as Tangier. A mesoscale low and the highest winds were found over the axis of the Strait, north of Tangier. High‐velocity (20 m s−1) winds occurred only under 1 ‐km height. the air‐temperature profile became progressively warmer moving from Gibraltar towards Tangier. A weak air‐temperature inversion decreased in height to the west of the narrows.

Venturi flow as an explanation for the downstream increase of the low‐level winds during a levanter is inconsistent with observations. A new model is proposed to explain the wind field associated with levanter winds in the Strait that includes some orographic blocking and low‐level convergence into the narrows, some west‐side divergence due to drag over water, and leeside heating that generates a leeside, mesoscale low pressure. the surface low accelerates the surface winds to their maximum strength well downstream of the narrows. A similar flow regime has been observed recently in the sea‐level channel between Hawaii and Maui.
Inertial instability on an asymmetric low‐latitude flow
Authors: Clark P. D. , Haynes P. H.
Published: February 2007

Both satellite data and numerical model simulations have suggested a link between propagation of planetary waves into the low‐latitude middle atmosphere and the occurrence of dynamical structures with small vertical scale similar to those predicted by the theory of symmetric inertial instability. It is here argued that existing theories that consider the growth of disturbances to a basic flow that is independent of longitude are unlikely to be directly relevant to such observations. A theory is presented that takes account of longitudinal variations in the structure of the basic flow, using WKB methods and the notion of absolute instability to find localized unstable modes. In the limit of large vertical wavenumber it is demonstrated analytically that the flow is absolutely unstable. For finite vertical wavenumber the calculation is based on numerical derivation of the dispersion relation. The growth rates and the spatial structure of the unstable modes relative to the variation of the basic flow are predicted. The results are compared to those from numerical solution of the linear stability problem and with full numerical simulation in a three‐dimensional primitive‐equation model.

It is found that under some circumstances the longitudinal variation causes the maximum growth rate to occur at finite vertical wavenumber, rather than at infinite vertical wavenumber as is the case when the basic flow is independent of longitude.
The rapid glaciation of slightly supercooled cumulus clouds
Authors: Mason B. J.
Published: February 2007

It is demonstrated that, contrary to current opinion, the rapid glaciation of shallow supercooled cumulus clouds with tops no colder than −8°C can be accounted for by the growth of a very low concentration (<10 m−3) of graupel pellets in a cloud of modest liquid‐water content of ∼1 g m−3 and a modest updraught of ∼3 m s−1. In an illustrative calculation, a graupel pellet, in ascending from the −3°C to the −7°C level and returning to the −5°C level, attains a radius of 3.6 mm and sheds > 104 ice splinters by the Hallett—Mossop mechanism in 750 s, one‐third of them in only 150 s. Ice splinters shed in the early stages of graupel growth develop into small graupel pellets of diameter ∼1 mm and account for these being observed in concentrations of ∼1 per litre.

The whole process, from activation of the ice nuclei at the −8°C level to the final stage of explosive glaciation, takes about one hour and therefore occurs only in aged cumulus with weak updraughts and not in rapidly ascending towers.
A two‐dimensional Lagrangian stochastic dispersion model for daytime conditions
Authors: Rotach Mathias W. , Gryning Sven‐Erik , Tassone Caterina
Published: February 2007

A two‐dimensional (x, z) Lagrangian stochastic dispersion model is presented that is correct (i.e. fulfils the well‐mixed condition) for neutral to convective conditions. The probability density function (pdf) of the particle velocities is constructed as a weighted sum of a neutral pdf (u and w jointly Gaussian) and a convective pdf (w skewed, u and w uncorrelated). The transition function ℱ varies continuously with stability and therefore ensures that the model results are not confined to a finite number of stability classes.

The model is described in full detail and some sensitivity tests are presented. In particular, the role of Co, the universal constant in the Lagrangian structure function for the inertial subrange, in determining average plume characteristics is discussed. Furthermore, the evolution of average plume‐height and ‐width is investigated for different boundary‐layer stabilities ranging from ideally neutral to fully convective. Finally, the model is applied to the situation of a tracer experiment in Copenhagen, and it is shown that the measured surface‐concentrations can be well simulated.
Some aspects of the variation with time of katabatic flow over a simple slope
Authors: Helmis C. G. , Papadopoulos K. H.
Published: February 2007

Features of the variations in katabatic flow along a simple slope in Athens, Greece are examined both observationally and from a simplified theoretical basis. When undisturbed periods of established flows are selected it is observed that the velocity and temperature fluctuations reveal characteristics which are in agreement with the theory proposed by Fleagle. Interesting implications of the theory are discussed, such as the effects of prescribing a decreasing cooling rate throughout the night or of taking into account the vertical structure of the katabatic layer.
Studies with a flexible new radiation code. I: Choosing a configuration for a large‐scale model
Authors: Edwards J. M. , Slingo A.
Published: February 2007

A comprehensive new radiation code based on the two‐stream equations in both the long‐wave and short‐wave spectral regions is described. The spectral resolution of the code is variable, enabling it to be used in a wide range of applications. Because of its flexibility, the code is well‐suited to the investigation of the sensitivity of radiative calculations to changes in the way in which physical processes are parametrized. The gaseous transmission data are derived from a line‐by‐line model. Particular attention is directed towards the treatment of the water vapour continuum, the overlap between gases, and the sensitivity to changing the carbon dioxide concentrations.

The performance of the code is examined both at high spectral resolution and in a lower‐resolution configuration designed for the UK Meteorological Office Unified Forecast/Climate Model (UM). Particularly for use in the UM, the code must be shown to perform satisfactorily across the whole range of atmospheric conditions. Comparisons are therefore made with reference calculations in both the long‐wave and the short‐wave, in clear and cloudy skies, and the accuracy with which various processes may be represented is studied.

For the cloudy calculations in the short‐wave, a new method is presented for deriving the single‐scattering properties in broad bands, based on the analytic expression for the reflectivity of an optically thick cloud. This minimizes the errors in calculating the short‐wave radiative properties of water clouds when the spectral resolution is reduced to that designed for the UM. In contrast, for ice clouds the errors are minimized by deriving the single‐scattering properties using linear averaging, as appropriate for optically thin clouds. In the long‐wave, the vertical distribution of the radiative heating in cirrus clouds is examined at high spectral resolution. The effect of scattering of long‐wave radiation, usually ignored in large‐scale models, is examined in some detail and is explained using a simple model. Taking all these studies into account, it is concluded that the configuration designed for the UM retains the generality of the code, without significantly compromising the overall accuracy.
The response of a variable resolution semi‐Lagrangian NWP model to changes in horizontal interpolation
Authors: Tolstykh Mikhail A.
Published: February 2007

The effect of changing the horizontal interpolation for evaluating the quantities at departure points in the global spectral semi‐Lagrangian variable resolution NWP model ARPEGE/IFS is described. The two interpolations involved are conventional cubic Lagrangian and Hermite with estimates of derivatives by fifth‐order compact upwind differencing (CUD‐5). Their properties are compared with examples of different one‐dimensional and two‐dimensional linear‐advection tests. With each kind of horizontal interpolation, twelve 5‐day forecasts are performed, beginning on the fifth day of each month from February 1994 to January 1995, using the same vertical non‐interpolating scheme.

On average, the difference between forecasts with these interpolations in geopotential and mean sea‐level pressure fields is negligible within the first three days, but noticeable on the fifth day, especially in the mean sea‐level pressure field. However, these improvements are present beyond the range for which the variable resolution strategy is designed.

The root‐mean‐square errors for the relative‐humidity fields at 1000 hPa and 850 hPa demonstrate some improvement as a result of the change of interpolation, from the second day of the forecasts onwards. Hermite CUD‐5 interpolation also results in a reduction of the amount of negative moisture.
A new long‐wave formula for estimating downward clear‐sky radiation at the surface
Authors: Prata A. J.
Published: February 2007

A new formula is proposed for estimating the long‐wave radiation from clear skies. The formula depends on screen‐level air temperature and water vapour pressure. The formula has been extensively tested using long‐wave measurements covering a large range of environmental temperatures (–40 to +40°C), and by using radiosonde profiles and an accurate radiative‐transfer model (Lowtran–7). It is shown that the new formula out‐performs five other widely used formulas.
Stratospheric inertia–gravity waves generated in a numerical model of frontogenesis. I: Model solutions
Authors: Griffiths Morwenna , Reeder Michael J.
Published: February 2007

The generation of stratospheric inertia–gravity waves is investigated with the aid of a dry two‐dimensional anelastic numerical model. The model calculations follow the development of upper‐level and low‐level fronts together with their attendant jets. Long‐wavelength low‐frequency inertia–gravity waves are generated in the model runs. The key constituents of the model basic‐state are: (1) a time‐dependent shear flow normal to the front in thermal‐wind balance with the along‐front potential temperature; and (2) a time‐ and height‐independent confluent‐deformation wind‐field with its axis of dilation parallel to the front.

When the cross‐front geostrophic wind shear A is negative, and consequently the upper‐level jet advects cold air along the upper front, prominent vertically propagating inertia–gravity waves are evident in the lower stratosphere. Like those observed in the atmosphere, the modelled waves have frequencies close to the inertial frequency, horizontal wavelengths of order 400 – 1200km, and vertical wavelengths in the range 2 – 10km. Numerical experiments in which Λ ⩾ 0 showed little gravity‐wave activity.
A simple semi‐Lagrangian scheme for advection equations
Authors: Sun Wen‐Yih , Yeh Kao‐San , Sun Rueen‐Yuh
Published: February 2007

A semi‐Lagrangian scheme is proposed by using forward trajectories and a splitting approach to the interpolation procedure. Several polynomial interpolation schemes are tested, including cubic spline and Lagrange polynomials of degree 3, 5 and 7. A simple filter is also proposed to eliminate spurious short waves and to achieve positive‐definiteness. Uniform, rotational and Smolarkiewicz's deformational flows are tested, a solution to the inviscid Burger's equation is also provided. This new algorithm employing cubic‐spline interpolation and the new filter yields efficient and accurate short‐term simulations. The advantage in efficiency of adopting split trajectories for interpolation is accompanied by a restriction which is slightly more stringent than the stability criterion for conventional semi‐Lagrangian advection schemes. It is also found that the simple filter can eliminate spurious short waves very effectively without degrading the solutions.