摘要:

AbstractIf a parametrization scheme for convective precipitation is to be used for assimilating observations of precipitation using a statistically based technique, then statistics of the errors produced by that scheme are required. These are the errors produced by the scheme's formulation itself, not counting any errors in the scheme's input. Such errors are extremely difficult to estimate, but examination of differences produced by various suitable schemes can yield qualitative descriptions of such errors. Here, hourly accumulated convective precipitation fields produced from six different versions of a short‐term forecast model are compared. The versions have identical initial and boundary conditions, but vary in the schemes used for either the convection or the planetary boundary layer, or both. The distribution of differences, or differences in logarithms of accumulations, between corresponding precipitating grid points for pairs of forecasts are examined using a simple binning technique. When the convection schemes differ, results reveal that if either a log‐normal or normal distribution is a better characterization of the distributions, it is the log‐normal one. The standard deviations of these logarithmic distributions correspond to different schemes at identical grid points producing values differing by factors of 2 or more. A large proportion of grid points that have non‐zero hourly accumulations using one model version may have no accumulation using another version. For most pairs of forecasts examined, however, grid points having larger values of accumulation for one scheme tend to have a smaller fraction of values having no accumulation in the other scheme. These results suggest that the finite probability that the model produces no precipitation when the corresponding, true atmospheric state does, should be considered in the statistical description of the model errors and that, because of the large standard deviation of model errors as well as large possible errors of hourly precipitation observations, the quantitative usefulness of assimilating such observations may be very

ISSN:0035-9009    

DOI:10.1002/qj.49712757802

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractIt is well known that classical (ana) cold fronts tend to be characterized by mesoscale circulations in which upright line convection feeds a layer of concentrated rearward slantwise ascent. Occasionally, however, as in the case‐study presented in this paper, two and sometimes more of these mesoscale circulations coexist within the same cold‐frontal zone. The two slantwise circulations described were observed to be stacked one above the other with a vertical wavelength of less than 2 km. Although it is often suspected that the circulations at ana‐cold fronts are enhanced by mesoscale processes such as conditional symmetric instability (CSI) or ΔM‐adjustment, it is notoriously difficult to discriminate between these circulations and the larger‐scale transverse circulation within which they are embedded. The occurrence in this study of multiple circulations with small vertical scale helps to distinguish them from the large‐scale circulation, and this has motivated the detailed examination of this case.Mesoscale circulations of the kind described are difficult to detect: numerical weather prediction (NWP) models, even high‐resolution models, do not usually represent them, and conventional observations do not show them clearly. This study takes advantage of observations from a high‐resolution microwave Doppler radar plus a high‐resolution analysis of ultra‐high‐frequency wind‐profiler radar data, analysed in the context of output from an operational mesoscale NWP model. The study defines the mesoscale structure of the event sufficiently carefully to provide a basis for future idealized modelling studies to investigate the possible roles of CSI and ΔM‐adjustment, both of which appear to play a part in the main

ISSN:0035-9009    

DOI:10.1002/qj.49712757803

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThis study constructs and analyses composite three‐dimensional fields of Doppler‐radar observed radial velocity and reflectivity for all precipitation events occurring in the Lago Maggiore region on the Mediterranean side of the Alps during autumn 1998 and 1999. Mean patterns for the two years are in close agreement with each other. The radar data are consistent with previous rain‐gauge studies in showing that the rain was heaviest over the lower windward slopes, and decreased toward higher terrain. The three‐dimensional reflectivity fields show that precipitation growth occurred mainly at low altitudes. The composite radar data show that the precipitation was most intense when the mean flow around the 2 km level was southerly or south‐easterly, i.e. when the mean flow was most perpendicular to the Alpine barrier.Sounding data from Milan indicated the Froude number of the flow upstream of the Lago Maggiore region. When the Froude number was high, the flow proceeded directly up and over the terrain of the lower Alpine slopes. Under these unblocked conditions, the low‐level flow (including the layer from the surface to 2 km above mean sea level) rose directly up and over the terrain, and the precipitation was greatly enhanced over the lower windward slopes and over the portions of the Po Valley just upstream of the mountains. Under unblocked conditions, the precipitation enhancement only extended a short distance (a few tens of kilometres) upstream of the Alps. When the upstream Froude number computed from the Milan sounding was low (blocked conditions), the Doppler radial velocities indicated that the low‐level flow (in the layer below 2 km above mean sea level) turned cyclonically as it approached the Alpine barrier, instead of rising over the terrain. The composite radar reflectivity data showed less precipitation enhancement directly over the windward slope, but in contrast to the unblocked case showed that the precipitation was enhanced 140 km or more upstream of the terrain. Evidently, the low‐level flow began rising far in advance of the barrier in blocked conditions.The 1998 and 1999 autumn data sample further indicates the relative roles of wind speed and stability, which are combined in the Froude number. When the wind speed upstream was strong (>8 m s−1), significant precipitation enhancement occurred on the windward slope of the Alps in the Lago Maggiore region, regardless of the static stability. However, the enhancement was far greater under unstable conditions. When the wind speed was weak (<8 m s−1), the precipitation was generally near or below average, except when the stability was low and some patchy enhancement occurred over the Po Valley just upwind of the Alps. A diurnal precipitation maximum occurred in the early morning hours (0700–1000 LST), possibly where down‐valley flow converged with synopt

ISSN:0035-9009    

DOI:10.1002/qj.49712757804

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe development and track of a surface cyclone that produced heavy precipitation in the western Mediterranean region on 28–29 September 1994 is numerically simulated using a mesoscale model. Diagnostic calculations reveal that surface‐pressure falls‐and upward motion‐occur in response to a well‐defined pattern of upward quasi‐geostrophic forcing at all tropospheric levels. In addition, convective instability and low‐tropospheric water vapour flux convergence are significant over the western Mediterranean, and therefore the environment is highly supportive for convection development.The appreciable dynamical forcing at upper levels appears to be associated with two positive potential‐vorticity (PV) anomalies that are embedded within the large‐scale trough and rotate about each other. Motivated by the fact that the small‐scale features of the PV field are more prone to analysis or forecast error than the large‐scale components, a sensitivity study is conducted in order to analyse the dependence of the mesoscale forecast on the initial intensity and position of the two embedded PV centres. This is accomplished by first applying a piecewise PV‐inversion technique which allows the balanced flow associated with each PV centre to be calculated, and then the inverted mass and wind fields are used to modify the model initial conditions. Eight simulations are run after doubling or removing one or both anomalies (sensitivity to intensity), and eight other simulations after displacing the anomalies towards or away from the Iberian peninsula (sensitivity to position).The results exhibit a clear dependence of the track and shape of the surface cyclone and its associated pattern of low‐tropospheric warm air advection on the characteristics of the PV anomalies. Therefore, spatial details of the mesoscale forecast are shown to be highly sensitive to the precise structure of the upper‐level dynamic forcing. Nevertheless, for the full set of arbitrarily defined initial conditions, one or several low pressure systems develop in the western Mediterranean area and heavy precipitation always occurs. On the other hand, there is an overall tendency of the simulations to favour cyclone development leeward of the Atlas mountain range and local rainfall enhancements over the exposed mountains of eastern Spain. The relative roles of the orography and sea surface latent‐heat flux versus the action of the upper‐level PV centres are examined by

ISSN:0035-9009    

DOI:10.1002/qj.49712757805

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe mesoscale organization and the thermodynamic structures of cloud bands and rain bands observed on 9 January 1988 during the seventh Intensive Observation Period of the European field experiment project FRONTS 87 are investigated using both radiosounding measurements and dual Doppler‐radar data.The combined role of the conditional symmetric instability (CSI) and of the frontogenetic forcing in this banded organization is quantitatively assessed. It is shown that the observations are in agreement with the subsequent scenario, cloud bands initiated by the large‐scale forcing in the presence of a weakly CSI unstable atmosphere evolve rapidly into CSI rolls. The detailed thermodynamics within one of these rain bands is shown to be in very good agreement with the ones predicted by the CSI theory. A different dynamic behaviour between the cloud bands and the rain bands is, however, shown. Some additional role is suspected to be played by microphysics processes, in particular by snow evaporation, in the reinforcement of CSI ascending moti

ISSN:0035-9009    

DOI:10.1002/qj.49712757806

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractLiquid water path variability in unbroken marine stratocumulus clouds is examined using a simple adiabatic model and both ground‐based and airborne observations. Observations from FIRE suggest that fluctuations in liquid water path at scales smaller than around 60 km can be modelled reasonably well as a function of fluctuations in cloud‐base height. A simple adiabatic model of cloud liquid water content with a normally distributed cloud base and constant inversion height results in liquid water path distributions that compare favourably with observations. Further, it is shown how cloud‐base height fluctuations can be predicted from the sub‐cloud turbulent fluctuations of temperature, specific humidity and their covariance. The relative importance of these three contributing terms is assessed. Aircraft observations of these fluctuating variables are used to predict the widths of liquid water path distributions observed radiometrically. The observations also suggest that the standard deviation of cloud‐base height and hence liquid water path variability increases with increasing boundary‐layer depth. The model liquid water path distributions could be used to derive plane‐parallel albedo biases and offer a possible framework for future parametrizations of subgrid‐scale variability in general cir

ISSN:0035-9009    

DOI:10.1002/qj.49712757807

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractOur goal in this work is to determine the role of radiation in cirrus clouds. How it impacts the formation and evolution of cloud structure, whether or not it initiates convective instability, and what effect it has on the lifetime of these clouds. In this study, we simulate cirrus clouds using the Met Office large‐eddy simulation model with a broad‐band solar and infrared ice‐specific radiation scheme.We find that radiation can have quite an effect on cirrus clouds. Results for runs with and without radiation show many dramatic differences. In general, radiatively influenced clouds are observed to be much more dynamic and inhomogeneous. The radiation is observed to strongly enhance cellular structure within the cloud layer, and a Fourier analysis of the horizontal ice‐water path (IWP) shows that this cellular enhancement gives rise to inhomogeneity length‐scales roughly related to the thickness of the layer. The Fourier amplitudes for the radiative cases are usually two or three times larger in magnitude than the non‐radiative cases. The inhomogeneity length‐scales do appear in the non‐radiative cases but they are very weak. The radiatively driven clouds in these simulations often had horizontally averaged IWPs larger in magnitude by more than double compared to the non‐radiative cases once the radiation had taken effect, and the cloud lifetime was increased by between 30 minutes and 2 hours.To evaluate if radiation is causing convective instability, we derive and implement aradiation stability number,Rsn.For radiative instability to occur,Rsnmust satisfy the condition 0

ISSN:0035-9009    

DOI:10.1002/qj.49712757808

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractWe evaluate two candidate mechanisms for the onset of lightning in the relatively low electric fields measured inside thunderstorms. The first is conventional dielectric breakdown due to local enhancement of the electric field in the vicinity of hydrometeors. The second is runaway breakdown, due to extended acceleration of high‐energy electrons (from cosmic rays or terrestrial sources of ionizing radiation) by the in‐cloud electric field. We compare the electric fields required for lightning onset by each mechanism with those observed inside lightning‐producing clouds, and we examine the sensitivity of the computed results to input parameters and assumptions. The conclusion of our analysis is that the conventional breakdown mechanism alone cannot trigger lightning while the runaway breakdown mechanism appears a more likely candidate. We identify the parameters on which each mechanism depends and emphasize the impact of observational uncertainties on our conclu

ISSN:0035-9009    

DOI:10.1002/qj.49712757809

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractObserved wind and temperature profiles are used to initialize a model calculation of a maritime continent thunderstorm, and the numerical solution is used to explore the effect of tropospheric shear on the gravity‐wave generation. The resultant convective system is qualitatively similar to that observed. The modelled gravity waves propagate away from the cloud with wave‐fronts that are approximately circular, implying that the convective clouds do not generate waves which propagate in a preferred direction.The gravity‐wave generation is related to the oscillation of the convective updraughts about their level of neutral buoyancy. While the general features of the gravity waves are similar in most respects to a previous study which used an idealized wind profile, the frequency of the gravity waves is Doppler‐shifted by the tropospheric wind shear. The result is a much broader power spectrum in comparison to the idealize

ISSN:0035-9009    

DOI:10.1002/qj.49712757810

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe classical Prandtl model describes the basic dynamics of katabatic flows, but it fails in determining the gradients near the surface because the eddy diffusivity,K, is independent of height,z. An improvement is proposed to the Prandtl model by using the WKB method to solve the dynamic equation for any prescribed, reasonably smooth,K(z) profile.The surface fluxes are obtained by extrapolation from the katabatic jet down to the surface, thus the fluxes depend primarily on the jet which is well captured by the WKB method. The profiles of wind, temperature, momentum and heat fluxes, as well as the surface fluxes calculated using the model, are tested against observations from a meteorological field campaign on the Pasterze glacier, Austria, in 1994, and their numerical simulations. Generally there is a good agreement between observations and theory.

ISSN:0035-9009    

DOI:10.1002/qj.49712757811

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY