摘要:

AbstractMeasurements 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 thein situmeasurements. Validation of the retrieved liquid‐water paths to within 50 g m−2has been achieved. Ambiguities between cloud retrievals and water vapour and surface parameters are discussed. the observed differences between the retrieval and thein situmeasurement 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

ISSN:0035-9009    

DOI:10.1002/qj.49712152702

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractIn an effort to optimize the value of global‐scale measurements obtained with the NASA/MSFC satelliteborne Lightning Imaging System (LIS), a simple computational model of thundercloud electrification has been developed, from which it is possible to derive crude relationships between lightning frequencyf(which LIS will measure) and cloud parameters such as radar reflectivity Z, precipitation rateP, updraught speedw, cloud radiusR, ice‐crystal concentrationiand graupel‐pellet concentrationNg.Electric field‐growth is assumed to occur via the non‐inductive charging mechanism, for both Fletcher and Hallett‐Mossop types of glaciation mechanisms. A simple criterion is used to distinguish between cloud‐to‐ground and intracloud lightning discharges.fis found to be especially sensitive towin situations where, as updraught speed increases, the temperature at balance level,Tbal, of the upper boundary of the charging zone falls. In these circumstancesN1and the sizes of the ice hydrometeors are significantly increased, with a corresponding enhancement of the effectiveness of charge transfer.Over a wide range of conditions,fis found to be roughly proportional to the first power of the parametersR1NiNgandZand (in some circumstances) to at least the sixth power ofw.the relationship betweenfandPdepends critically on whether or notwandTbalare strongly linked. Hallett‐Mossop glaciation is capable of producing inverted‐polarity lightning from thunderclouds; Fletche

ISSN:0035-9009    

DOI:10.1002/qj.49712152703

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractTethered‐balloon data collected over an inhomogeneous, semi‐rural area of central England are presented. the data are compared with previous surface data to assess how representative fixed‐point flux‐measurements are of area averages. the differences between the data and ‘established’ surface‐layer results are relatively small with departures of about 10% to 20%. This supports the concept of a blending height above which the flow becomes horizontally homogeneous. Effective roughness lengths for momentum and heat transfer are derived. the ratiozeff0m/zeff0tis found to be about 80 compared toz0m/z0t≈︁10 observed for homogeneous terrain. For different wind directions the relationship between the effective roughness length and the upwind surface is investigated using recent theories on the area‐averaging of surface roughness lengths. It is found that the method of averaging transfer coefficients gives realistic values of the effective roughness lengths when compa

ISSN:0035-9009    

DOI:10.1002/qj.49712152704

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractTwo field experiments were carried out to observe the variation in wet deposition of ions caused by orography in a polluted region of the UK. the site chosen was Winter Hill which lies on the edge of the industrialised Mersey valley. Rain and cloud samples were collected daily and chemically analysed. Synoptic and meteorological data were used to classify individual rain events as frontal or convective.For the frontal events the altitude related increase in wet deposition was found to be entirely caused by the seeder‐feeder mechanism (the scavenging of cap cloud by rain from above). the concentration of marine ions in the water scavenged from the cap cloud was found to be between five and six times that in the seeder rain whereas for anthropogenically produced ions it was about twice as great. the efficiency of the seeder‐feeder mechanism is correspondingly affected.Convective events accounted for less than a quarter of the rainfall volume. However, this contained over half of the deposition of marine ions in the valley. Deposition of anthropogenically produced ions in convective events was small probably due to a lack of convection over land. A small decrease in deposition with altitude was observed for these events although this is expected to be a highly local effect.It is concluded that reliable predictions of the wet deposition of non‐marine ions can be made on the basis of the seeder‐feeder effect for northern and western regions of the UK. Elsewhere in the UK and for much of continental Europe wet deposition from convective rain should be taken into

ISSN:0035-9009    

DOI:10.1002/qj.49712152705

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe nature of unstable easterly waves growing on three different easterly jets has been examined. the ‘shearjet’ with strongest horizontal mean shear on the equatorward side of the jet is characterized by linearly unstable waves with maximum amplitude equatorward of the jet maximum and are dominated by barotropic energy conversions associated with positive horizontal momentum fluxes. the normal modes of the shear‐jet have very little amplitude at the surface.The ‘desert‐jet’ with lower mean static stability better represents the atmospheric conditions on the poleward side of the jet, and is characterized by linearly unstable waves with maximum amplitude at the surface. the modes have much larger baroclinic energy conversions than those of the shear‐jet, although barotropic energy conversions are still larger. About 25% of the value of the barotropic energy conversions is attributable to the vertical Reynolds stress term associated with vertical momentum fluxes.The ‘combined‐jet’ with both enhanced equatorward horizontal shear and low static stability combined, has also been examined. the linear normal modes associated with this jet are characterized by significant amplitudes at both the jet‐level and at the surface. the ratio of barotropic to baroclinic energy conversions is between that associated with the modes growing on the shear‐ and desert‐jets.The nonlinear behaviour of 3000 km wavelength easterly waves growing on the jets has been examined. All life‐cycles are characterized by a transition from initial barotropic energy conversions dominance to baroclinic energy conversions dominance. the nonlinear part of the shear‐jet life‐cycle is dominated by negative horizontal momentum fluxes on the poleward side of the jet and maximum amplitude at the surface in the region of the temperature gradients. In disagreement with observations only very weak nonlinear positive horizontal momentum fluxes exist on the equatorward side of the jet. the nonlinear desert‐jet life‐cycle as for the normal mode continues to be characterized by maximum amplitudes at low‐levels and becomes dominated by baroclinic energy conversions sooner. the nonlinear combined‐jet life‐cycle evolves in a similar manner to the desert‐jet life‐cycle but with more significant jet‐level amplitudes.The synoptic evolution of the relative vorticity in the life‐cycles is examined. A single surface relative vorticity maximum, associated with the meridional surface temperature gradient has been identified in all idealized integrations examined, including those with latent heating, in disagreement with observations which indicate two maxima north and south of the jet. At about 850 mb however, two positive vorticity centres do exist; a weak equatorward one associated with the developments at the jet level and a stronger poleward one associated with the low‐level temperature gradients.The shear life‐cycle is re‐examined with a simple moist parametrization included. Baroclinic energy conversions increase but barotropic energy conversions are almost unaffected by the latent heating and the jet‐level structure almost unchanged. the surface vorticity maxima is increased. the relationship between moisture availability and the jet is shown to be important in determining the nature of surface developments and the ascent pattern as well as the magnitude of the baroclinic energy conversions.It is suggested that differences between observations and the nonlinear equatorward structures identified here, may be reduced

ISSN:0035-9009    

DOI:10.1002/qj.49712152706

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractIntense vortices in the atmosphere and ocean exhibit a high degree of axisymmetry despite persistent asymmetric forcing from their environment. to further elucidate vortex axisymmetrization a variety of idealized initial‐value models for barotropic non‐divergent flow is considered. to ensure basic understanding, disturbance evolution is first examined in a rectilinear system of simple shear. Particular emphasis is placed on identifying how inviscid disturbance‐evolution depends on the zonal wave‐number and on the meridional structure of the initial conditions.Insight acquired from the rectilinear problem is then applied to a bounded Rankine vortex. Here, the dependency of disturbance evolution on the azimuthal wave‐number is of special interest. Recent development of a low‐frequency balance theory for rapidly rotating (large Rossby number) vortices has provided observational evidence that the low‐azimuthal‐wave‐number asymmetries, especially wave number one, are dominant in the nearvortex region. the results of this work provide further theoretical evidence of an inviscid wave‐number‐selection mechanism that preferentially damps the high‐wave‐number asymmetries.The radial structure and location of the initial conditions are found to be important factors in determining how rapidly a disturbance is compressed or elongated. This in turn controls the rate of disturbance growth or decay. For swirling flows, a definition of an effective shear is proposed that accounts for both the radial variations of the initial condition and the radial variation of the angular velocity. Using the reciprocal of this effective shear, time‐scales for a disturbance to decay to half its initial energy, the half‐life, are calculated for initial conditions and symmetric wind‐profiles that are found in hurricanes.Simple‐shear flow and the bounded Rankine vortex do not admit discrete modal solutions since there is no mean‐state vorticity‐gradient to support them. the unbounded Rankine vortex is examined to investigate how the presence of discrete modes (Rossby edge‐waves) associated with the radial vorticity‐gradient of the Rankine swirl modifies the con

ISSN:0035-9009    

DOI:10.1002/qj.49712152707

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractResults of the COADS data processing aimed at a classification of the structure of the large‐scale lowfrequency variability of the sea surface temperature, the sea‐level atmospheric pressure and the surface wind‐speed components are presented. Using cluster analysis, patterns of interannual variability in the temporal and spatial domains are obtained. Clusters consisting of points possessing similar temporal evolution are compared with the known dipole‐type patterns of interannual variability, and special attention is given to selection of the global ENSO‐related patterns. of particular interest are the teleconnection links between the equatorial and the extratropical regions in the Pacific Ocean. Clusters consisting of times with similar spatial patterns, as well as extended canonical correction analysis patterns, show the typical sequence of the spatial patterns during the ENSO cycle, especially for the ‘mature’ warm and cold phases. For these cases, typical features of the field anomalies, including the extratropical peculiarities in the Pacific Ocean, are considered a

ISSN:0035-9009    

DOI:10.1002/qj.49712152708

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractFor the first time, the method of four‐dimensional variational data assimilation is applied to the analysis of chemically active trace species. By combining observations with a numerical model to analyse simultaneously several species over a period of a few days, the analysis method is able to exploit information which is not available to conventional analysis techniques. Moreover, effective use can be made of asynoptic observations even for species which have strong diurnal cycles. Synoptic analyses are produced. A Lagrangian approach is adopted, allowing a separation of dynamics and chemistry which considerably reduces the computational expense of the metho

ISSN:0035-9009    

DOI:10.1002/qj.49712152709

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractCertain 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 estimateTNL≈︁ 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

ISSN:0035-9009    

DOI:10.1002/qj.49712152710

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractExamination has been made of the skill of ECMWF forecasts of the 500 hPa height field produced daily out to ten days ahead, verifying in the period from 1 December 1980 to 31 May 1994. Over this time accuracy has been improved substantially over the first half of the forecast range. the systematic (seasonal‐mean) component of the error has been greatly reduced at all forecast times, but there has been little reduction in the non‐systematic (transient) component later in the range.The simple model proposed by Lorenz for the intrinsic growth of forecast error has been applied to the evolution of differences between consecutive forecasts. the implied growth‐rates of small forecast errors have increased significantly since 1981. They do not show much variation with season, and are a little lower in the southern than in the northern hemisphere. the most recent error‐doubling times are around 1.5 days for the northern hemisphere and 1.7 days for the southern hemisphere. Error saturation levels are at present similar to or greater than those of the 1981 version of the model, having been significantly lower in intermediate years. the accuracy of recent short‐ and early medium‐range forecasts and realism of the climatology of the forecast model support the view that estimates of intrinsic error‐growth parameters from the current forecasting system are more reliable than those obtained earlier. Forecast accuracy later in the medium range may thus not have benefited fully from improvements earlier in the range because of the faster error‐growth associated with a more active, though more realistic, forecast model. Overprediction of variance may nevertheless detrimentally affect present levels of skill and estimates of predictability in all seasons other than summer.The error‐growth model currently indicates that it is possible, in principle, to make deterministic mediumrange forecasts for the extratropical 500 hPa height field of the northern hemisphere that are as accurate five days ahead as present forecasts are three days ahead, provided the one‐day forecast error can be reduced by the same factor in the future as has actually been achieved in the years since 1981. the level of error currently reached at day seven would then be reached at around day ten. the scope for improvement of forecasts for the southern hemisphere appears to be rather larger. Improvements seem to be possible throughout the spectral range studied, up to total wave‐number 40. This is found also for the rotational and divergent wind components at 850 and 200 hPa. For these components, particularly the divergent component, there is a quite pronounced error in the representation

ISSN:0035-9009    

DOI:10.1002/qj.49712152711

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY