摘要:

AbstractAircraft observations in fields of small maritime cumulus clouds around the British Isles have been analysed to yield kinetic energy spectra and vertical turbulent fluxes, both within clouds and in the clear regions between clouds, for five cases where a cool airmass flowed over a warmer sea surface. the flights were made at various times of the year. Profiles of the kinetic energy and turbulent fluxes have been obtained from observations made at several levels. It is shown that, while the dominant source of turbulent kinetic energy may be due to evaporative cooling following entrainment at cloud top, in some cloud fields production by latent heating near cloud base dominates. the location of the dominant energy source depends on whether the clouds penetrate the inversion sufficiently for instability due to cloud‐top entrainment to occur. While the resulting flux profiles observed in some of the cases examined here resemble those observed in stratocumulus, profiles of similar shape are not always observed, and there are significant differences between cumulus layers and stratocumulus in the magnitudes of the fluxe

ISSN:0035-9009    

DOI:10.1002/qj.49712152602

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA high‐resolution large‐eddy simulation model has been used to investigate the effect of scale in surface inhomogeneities on the turbulence structure of the convective boundary layer (CBL). Surface heat fluxes were varied sinusoidally in two dimensions with wavelengths of 250, 500 and 1000 m, corresponding to about 0.27zi, 0.55ziand 1.1zi(ziis the boundary layer depth). the geostrophic wind speed was 1 ms−1. Horizontally‐averaged statistics of the CBL above surface heat‐flux inhomogeneities of different two‐dimensional scales were compared. the phase‐average method was used to study the horizontal variation of turbulence and the mean circulation in the CBL.Results from horizontally‐averaged statistics show that surface inhomogeneities of small horizontal extent (x

ISSN:0035-9009    

DOI:10.1002/qj.49712152603

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractObservations from 58 dropwindsondes released in a mesoscale array during the FRONTS92 experiment are interpreted in the context of satellite imagery to derive the mesoscale structure and evolution of parts of a frontal cyclone developing over the eastern North Atlantic. A conceptual model involving the intertwining of ‘dry intrusion’ and ‘cloud head’ flows is corroborated and is used to provide the framework for interpreting the detailed mesoscale behaviour. In the cold air, two distinct dry‐intrusions were responsible for two cold fronts, trailing south‐westwards from the tip of the cloud head. Both were surface features at the beginning of our study but the leading one evolved into an upper front with mid‐level convection as the dry intrusion responsible for it overran the warm sector. Areas of both high and low potential vorticity were indicated within the dry intrusions. Upon encountering a critical level of zero system‐relative velocity at the top of the moist boundary layer, the dry‐intrusions' arrival was associated with the development of multiple dry and moist laminae near the top of the boundary layer. the vertical wavelength of the laminae was about 1 km and they extended over 200 km in the front‐normal direction, with a slope of typically 1 in 60. Although most parts of the laminae were subsaturated, their circulations combined with the double structure of the dry intrusions to produce multiple shallow cloud‐lines within the boundary layer. These formed as an extension of the south‐western tip of the cloud head associated with the developing cyclone. Possible mechanisms for generating the observed s

ISSN:0035-9009    

DOI:10.1002/qj.49712152604

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThis paper investigates the occurrence of conditional symmetric instability during MFDP/Fronts87IOP2. A saturated negative equivalent‐potential‐vorticity area is detected by the mesoscale analysis in the upper troposphere, and this is confirmed by dropsounding data. Two‐dimensional hydrostatic simulations are performed with the Météo‐France PERIDOT model which give rise to the onset of symmetric instability. A complete life cycle is obtained as well as the growth rate of the perturbation (about 2.5 × 10−5s−1≈ (11 hours)−1). Particular emphasis is put on the meanings and results of the kinetic‐energy conversions. Most of the perturbation energy comes from the basic state through vertical momentum fluxes, which is consistent with previous theoretical or numerical works. the decay of the perturbation is also shown to be due to its own dynamics, the symmetric rolls advecting positive equivalent potential vorticity into the unstable region. Deformations of equivalent potential temperature (θe) surfaces and m‐surfaces give an insight into the parcel trajectories: the ascending parcels follow θe‐surfaces. leading to a buckling of the m‐surfaces, and the descending parcels follow θ‐surfaces. the small value of the growth rate is stressed as well as of the maximum intensity (w ≈ 5 cm s−1). Thus, interactions with the frontogenetical forcing are expected owing to the small growth rate obtained. This is t

ISSN:0035-9009    

DOI:10.1002/qj.49712152605

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThis paper deals with the mesoscale circulations that may appear in a frontogenetically forced flow. Two mechanisms are studied: conditional symmetric instability (CSI) and transient adjustment of a finite momentum anomaly. the study is focused on two situations selected from frontal events that were documented during the MFDP/FRONTS87 field experiment.In Part I of this paper, a successful numerical simulation of CSI during the second intensive observing period (IOP2) was presented. However, the growth rate of the perturbation was very small (≈ f/4) and 42 hours were needed before vertical velocities reached their maximum value ≈5 cm s−1. Therefore, interactions between CSI and the large‐scale frontogenetic forcing are expected; these are investigated in this Part. CSI is released (without significant increase of the growth rates) for small forcing only since confluence is found to shrink the area fulfilling the pure CSI criterion rapidly. It is thus not likely to occur in active fronts.The IOP7 is then investigated. This very anisotropic front was embedded in a strong large‐scale confluence. the creation and maintenance of the observed ‘Wide Cold‐Frontal Rainband’ (WCFR) (some 100 km wide) above the sloping frontal surface and behind the surface cold front are investigated. Using a hierarchy of threedimensional (3D) and two‐dimensional (2D) versions of the model, this band is shown to be produced by the inertial, transient adjustment of low pseudo‐angular‐momentum air that was advected into the mid‐troposphere by the frontal convective jet. the 3D, as well as the 2D, forced simulationsproducea negative equivalent potential vorticity chimney‐like area which matches well with the observational data. This area evolves on the same time‐and space‐scales as the roll‐like circulation associated with the WCFR. This mechanism bears similarities with mature CSI and their distinction in diagnostic studies may be difficult. However, the presence of a momentum source (the frontal convection) is found her

ISSN:0035-9009    

DOI:10.1002/qj.49712152606

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractRecent observations and analyses of the atmospheric flow in the vicinity of the Pyrénées made during PYREX are compared with numerical experiments carried out with the T213L31 European Centre for Medium‐range Weather Forecasts (ECMWF) global model. This study shows that the model simulates a certain proportion of the measured pressure drag, but always underestimates it. In the current model, this deficit is not adequately made up by any of the parametrized subgrid‐scale processes. When the flow is along the range, the pressure‐drag value depends on the synoptic background flow structure. It is close to zero when the flow impinging on the range is baroclinic. It is not zero when the atmosphere is more barotropic (e.g. in the absence of any fronts) and the model misses important lift processes. When the flow is normal to the ridge, the model response to the mountain forcing is essentially ageostrophic. In this case comparison between the vertical profiles of the model stresses and the measured profile suggests that further drag should be applied in the model at low level. This is supported by comparisons of isentropic diagnostics, from the ECMWF model and the French mesoscale model, Peridot. These diagnostics show that the ECMWF model underestimates irreversible low‐level processes, such as the flow separation on the mountain flanks and the associated downstream wake, which can be related to the drag. They also show that low‐level parametrized drags have a realistic impact on the model simulation of the mountain wake. This analysis supports the current developments at the ECMWF of a new subgrid‐scale orographic drag scheme which parametrizes the low‐level impact of meso

ISSN:0035-9009    

DOI:10.1002/qj.49712152607

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractAn 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 downwardFsin the cyclone warm sector is anticyclogenetic during the phase of most rapid deepening. the sensitivity indicates thatFscan be cyclogenetic in one location and anticyclogenetic at the same time in another location, so thatFseffects on cyclone intensity are partially self‐cancelling. Surface momentum stress is anticyclogenetic, with sensitivity highly localized in the cyclone warm

ISSN:0035-9009    

DOI:10.1002/qj.49712152608

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractLong‐range transport models for acidic sulphur species are being used to develop emission‐control strategies to combat environmental acidification in the remote, sensitive regions of Europe. the incorporation of seeder‐feeder enhancement into a simple trajectory model for sulphur has markedly improved the representation of wet deposition across Great Britain, as shown by a detailed comparison of the model results with the observations from monitoring networks. an uncertainty analysis reveals that those input parameters which contribute most to uncertainty in the total sulphur deposition are the sulphur‐dioxide emissions, wind speeds and wet‐removal coefficients. the total Great Britain dry and wet depositions of sulphur are estimated to be 216 and 241 thousand tonnes per year, respectively, making a total sulphur deposition per year of 457 ± 110 thousand tonnes, in excellent agreement with the observed quantity of 433 thousand tonnes. the model was used to give an indication of the origin of the deposited sulphur across Great Britain. European land‐based sulphur sources contribute 34% of the deposition to Great Britain, with significant dry deposition in south‐east England and wet deposition in upland western Britain. UK power stations and oil refineries contribute 46.7% of total deposition, with shipping and natural sulphur sources from algal blooms, a mere 4.6%. Differences in source attribution between this UK‐scale study and those previously published for the long‐range transport scale are explored and it is concluded that national‐scale models have an important role to play i

ISSN:0035-9009    

DOI:10.1002/qj.49712152609

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractPresent‐day climate simulations for Europe are presented, based on a 50 km regional‐climate model (RCM) driven by output from a global general‐circulation model (GCM) using a one‐way nesting approach. Both models are components of the Meteorological Office Unified Forecast/Climate Model and use the same subgrid‐scale physics. the relationship between the RCM circulation and that of the driving GCM was assessed in seasonal RCM integrations using domains of different sizes. In the larger domains, both the mean flow and the day‐to‐day variability in the RCM diverge from that of the GCM on the synoptic scale, rendering the RCM solution physically inconsistent with the GCM solution external to the RCM domain. At the grid‐point scale the RCM freely generates its own features, even in the smaller domains—only at points adjacent to the boundary buffer zone is there evidence of significant distortion by the lateral boundary forcing from the GCM.Using one of the smaller domains, a 10‐year RCM simulation was carried out, driven by a coupled atmosphere/mixed‐layer‐ocean version of the GCM. Over the region of interest the general circulation and daily synoptic variability is realistically simulated by the GCM and, therefore, also by the RCM (see above). Stronger vertical motions in the RCM lead to a general increase in dynamical precipitation relative to the GCM, and thus a drier and warmer troposphere and reduced convective cloud and precipitation. Layer‐cloud cover is also reduced in the RCM, due to a time‐step dependence in the treatment of the dissipation of ice cloud. Significant changes occur in the surface heat balance. the spatial patterns of surface air temperature and precipitation over Europe are well simulated by both the GCM and the RCM on scales resolved by the former. At finer scales the RCM contains a strong signal which is related to orographic height. Validation against a detailed observed climatology for Great Britain demonstrates that this signal

ISSN:0035-9009    

DOI:10.1002/qj.49712152610

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractAs the spatial scales simulated by general circulation models are refined and the parametrizations they employ are improved, greater accuracy can be achieved in their simulations of regional climate and events on short time‐scales, and greater confidence attached to their predictions of these for perturbed climates. In the present paper, we examine the daily (interdiurnal) variability of surface temperature and precipitation in Europe simulated by a version of the UK Meteorological Office (Hadley Centre) general circulation model, having 11 levels in the vertical and a horizontal resolution of 2.5°latitude × 3.75° longitude, coupled to a slab ocean, in the current climate and the equilibrium climate for a doubled atmospheric concentration of carbon dioxide. We evaluate the simulation of current conditions by comparison with climatologies and time‐series of observed data. In the modified climate, we find reductions in temperature variability in winter, related to a lessened land‐sea temperature contrast, and increases in summer, caused by changes in the surface heat balance. Precipitation in all seasons shows significant tendencies both to more dry days and to heavier events, with less frequent moderate daily amounts; this is connected with an increase in convective activity. We conclude that simulation of daily variability is strongly dependent on particular local physical processes and hence on the parametrizations used to represent them. While acknowledging deficiencies in the model's performance, we consider that our results are indicative of possible changes for a future

ISSN:0035-9009    

DOI:10.1002/qj.49712152611

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY