摘要:

AbstractA new parametrization of drag arising from the flow over unresolved topography (UT) in a general‐circulation model (GCM) is presented. It is comprised of three principle components: a parametrization of the source spectrum and drag associated with freely propagating hydrostatic gravity waves in the absence of rotation, a parametrization of the drag associated with low‐level wave breaking, and a parametrization of low‐level drag associated with upstream blocking and lee‐vortex dynamics. Novel features of the scheme include: a new procedure for defining the UT in each GCM grid cell which takes account of the GCM resolution and includes only the scales represented by the parametrization scheme, a new method of representing the azimuthal distribution of vertical momentum flux by two waves whose direction and magnitude systematically vary with the flow direction and with the anisotropy of the UT in each GCM grid cell, and a new application of form drag in the lowest levels which can change the direction of the low‐level flow so that it is more parallel to unresolved two‐dimensional topographic ridges.The new scheme is tested in the Canadian Centre for Climate Modelling and Analysis third generation atmospheric GCM at horizontal resolutions of T47 and T63. Five‐year seasonal means of present‐day climate show that the new scheme improves mean sea level pressures (or mass distribution) and improves the tropospheric circulation when compared with the gravity‐wave drag scheme used currently in the GCM. The benefits are most pronounced during northern hemisphere winter. It is also found that representing the azimuthal distribution of the momentum flux of the freely propagating gravity‐wave field with two waves rather than just one allows 30‐50% more gravity‐wave momentum flux up into the middle atmosphere, depending on the season. The additional momentum flux into the middle atmosphere is expected to have a beneficial impact on GCMs that employ a more realistic representat

ISSN:0035-9009    

DOI:10.1002/qj.49712656802

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractA three‐dimensional, random‐walk, ballistic model is used to simulate the density and structure of an ice accretion formed by impinging supercooled droplets. The model is calibrated using experimental observations of the ice density at the stagnation line on a fixed circular cylinder. A relationship has been established between the Macklin parameter, used commonly to relate atmospheric conditions with ice accretion density, and model parameters. The model is applicable over a wide range of growth conditions, including, in the limits, growth where the droplets freeze on impact and growth where the droplets spread over and percolate into the existing ice struct

ISSN:0035-9009    

DOI:10.1002/qj.49712656803

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThe dynamical role of ice sublimation in weather systems is briefly reviewed. Observations are presented from the Fronts and Atlantic Storm‐Track Experiment (FASTEX) Intensive Observation Period 16 that show variations of static stability, humidity and mesoscale circulation corresponding to those associated theoretically with sublimation of ice precipitation. It is thus suggested that the observations display the mechanism proposed by Clough and Franks in which forward mesoscale flows are associated with moist adiabatic descent supported by the sublimation cooling. This mechanism was suggested as an important stage in the evolution of many mesoscale rain bands.A set of three model simulations of the event has been made with versions of The Mel. Office's Unified Model. Of these a mesoscale model integration with 11 km resolution and 45 levels clearly displays the symptoms, and is diagnosed to demonstrate its consistency with the Clough‐Franks mechanism. An integration omitting the cooling due to sublimation differs significantly from the full model experiment in the structure of low‐level wind fields, frontal troughs and mesoscale precipitation distribution. Il is also demonstrated that the static‐stability transition, mesoscale circulation and mid‐tropospheric potential‐vorticity perturbations are substantially weakened in this integration, thus confirming that the Clough‐Franks mechanism is also operating in the numerical weather prediction (NWP) model.We deduce from these studies that ice precipitation and its sublimation has a major role in determining mesoscale circulation and structure in mid‐latitude weather systems, affecting stratification and the formation of features such as fronts and rain bands. These are substantially affected by the fall and evaporation of ice crystals, which are both important to temperature and moisture transports and the behaviour of NWP models on time‐scales of hours to days. In our integrations dynamical feedback due to sublimation cooling coincided with extreme negative potential‐vorticity values and potentially conditional symmetric instability, hence the anticyclonic motion occurring in the cloud head or deep cloud of the moist warm sector as in this case. In moist warm sectors a substantial role for sublimation may be anticipated more generally, particularly for air trajectories receiving most ice precipitation.We suggest that the described phenomenon be referred to as sublimation enhanced descent or SED.It is concluded that in view of tins demonstrated sensitivity substantial attention should be given to refining microphysical parametrizations in NWP models, and that radar and sounding observations from the FASTEX experiment provide a suitable basis for valid

ISSN:0035-9009    

DOI:10.1002/qj.49712656804

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractWe study 20 cases in which 60‐hour Numerical Weather Prediction (NWP) of specific synoptic events have been markedly improved by observations made during the final 15 hours prior to the forecast initial time. For each case, the observing systems which played major rǒles in improving the accuracy of the forecast mean‐sea‐level pressure (m.s.1.p.) field are identified using a series of Observation System Experiments. The relative utility of each observation type is then assessed according to the frequency, over all cases, with which it delivers a major beneficial impact.The method of multiple case‐study employed has the advantage that observation value is assessed according to the benefit delivered to forecasts of events (e.g. cyclogenesis) which are of prime importance to operational meteorology. The method, therefore, provides a useful complement to the conventional Statistical approach which typically yields an ‘average’ benefit, calculated for all regimes (both active and ‘quiet’) occurring over a given period within a specific geographical region. The cases arc selected from the most significant data‐impact events detected in routine operational forecasts of m.s.1.p., for Europe and North America, during the period September 1993 to December 1995.Results indicate that, for weather systems developing over North America, radiosonde and aircraft reports contribute major forecast benefits most often, confirming their key rǒle in the North American network. These two observing systems contribute with similar frequency, suggesting that the effectiveness of the less abundant radiosonde reports is boosted significantly by their profile formal. Surface data and cloud‐track winds are the next most frequent contributors.For weather systems developing over the North Pacific and North Atlantic, aircraft winds contribute forecast benefits most frequently, and by a wide margin. Conventional surface data also play a key rǒle.Comparison of the utility of wind and temperature data suggests that, on average, the benefit of wind profiles is somewhat greater than that of temperature profiles, and that, in levels above ∼400 hPa, wind data are considerably more valuable than temperature data.The results give insights which can be used to guide the rationalization of existing networks within the northern hemisphere mid latitudes, and the following recommendations are made with a view to improving NWP over the European area.•Acquire more aircraft data over North Atlantic routes and over Europe (including reports during climb and descent). However, caution must be exercised if aircraft data are used to replace profile information from radiosondes.•Deploy more surface observations (e.g. drifting buoys) and shipborne radiosonde ascents over the North Atlantic.•Make more use of pattern‐tracking techniques for deriving ‘clear‐air’ wind data from satellite water‐vapour imagery.•Continue investigation of the effectiveness of observations ‘targeted’ on objectively defined regions where m

ISSN:0035-9009    

DOI:10.1002/qj.49712656805

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractFine‐scale three‐dimensional wind fields retrieved from airborne Doppler radar observations within an oceanic tropical mesoscale convective system (MCS), during the Tropical Ocean/Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment, are used to enhance the initial conditions of a n on‐hydrostatic mesoscale model. They are incorporated into the French CANARI‐ALADIN analysis system, based on the optimal interpolation technique. The fine‐mesh (4 km) horizontal wind components are first averaged over subdomains of 20 km × 20 km in order to provide a series soscale wind profiles. Because of the absence of dense sounding data, the synthesized vertical‐velocity profiles are transformed into humidity profiles by considering that updraughts contain nearly saturated or saturated air, while downdraughts are associated with unsaturated air.Comparison of the initial state using conventional data with the Doppler‐enhanced initial state clearly identifies the benefits of the addition of the observed mesoscale circulation features. In terms of the precipitation forecast, the control run that uses the conventional initial State dramatically fails to predict the existence of the MCS precipitation core. On the other hand, the reference run with the data‐enhanced initial state succeeds in forecasting it up to 12 hours in a way that is well consistent with the satellite imagery. In terms of the wind forecast, the mesovortex signature that could be identified from the radar observations becomes a persistent feature with the radar‐enhanced initial conditions. Sensitivity tests reveal the specific roles of wind and humidity data. As in previous studies, the initial state of humidity is fundamental for the forecast of the MCS precipitation by sustaining the convective activity, and also by forming and maintaining a vortex‐like circulation. These results suggest that moist convective processes play a major role. The inclusion of wind data is necessary for improving the system propagation

ISSN:0035-9009    

DOI:10.1002/qj.49712656806

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThe first part of this work is devoted to exploring the instability mechanism of the shallow‐water semi‐geostrophic model by investigating the evolution of disturbances under constant‐shear and cosine‐type basic flows. For the constant‐shear basic‐flow case, the wave‐packet analysis method is applied to study the evolution of potential and total energy and the meridional structure of disturbances under me assumption of a spatially slowly varying basic state. The results are found to be in good agreement with numerical calculations even when the assumption is slightly relaxed. For the cosine‐type basic‐flow case, both unstable normal modes and continuous spectra exist if the stability criterion is violated. Numerical results show that the relative importance of regular normal‐mode and continuous‐spectrum disturbances in the evolution process depends on the configuration of initial disturbances and their projections onto the normal modes and continuous spectra.In the second part of this work, in order to search for initial disturbances based on which the growth of certain norms of disturbances can be optimized, singular vectors of different norms (metrics) are computed. It is found that of all the norms, the structures of singular vectors are mainly determined by the configuration of the zonal basic flow and less affected by model parameters such as the Rossby number, rotating Froude number, and amplitude of topography. However, when the shear of the basic flow is strong, the optimal singular value can be sensitive to the model parameters. When the stability criterion is violated, the optimal singular values of all the norms have clear wave‐number preferences. The peaks of spectra of singular values of the energy‐type norms, including potential, kinetic and total‐energy norms, correspond to very weak normal‐mode instability. However, the optimal singular value of the enstrophy norm has almost the same wave‐number pref

ISSN:0035-9009    

DOI:10.1002/qj.49712656807

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractWe report on the influence of submerged bubble clouds on the re mote‐sensing properties of water. We show that the optical effect of bubbles on radiative transfer and on the estimate of the ocean colour is significant. We present a global map of the volume fraction of air in water derived from daily wind speed data. This map, together with the parametrization of the microphysical properties, shows the possible significance of bubble clouds on the albedo to incoming solar energ

ISSN:0035-9009    

DOI:10.1002/qj.49712656808

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThe necessity for treating the effects of vertically varying cloud fraction when parametrizing microphysical processes in general‐circulation models (GCMs) was recently highlighted by Jakob and Klein. In this study a parametrization to include such effects in a GCM is developed, and the new scheme is applied in the ECMWF global model. The basic idea of the new scheme is to separate the model's rain and snow fluxes into a cloudy and a clear‐sky part. The scheme is tested using the subgrid‐scale precipitation model of Jakob and Klein as a benchmark. The impact of the new scheme on the model climate is also investigated.It is shown that the new parametrization leads to a better representation of the effects of cloud and precipitation overlap, and that it alleviates most of the problems connected with their treatment in the current scheme. Due to the better treatment of cloud and precipitation overlap the new parametrization leads to a reduction in precipitation evaporation and an increase in accretion rates. When tested in seasonal model integrations the new scheme produces a drier tropical mid‐troposphere with consequences for the hydrologica

ISSN:0035-9009    

DOI:10.1002/qj.49712656809

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThe relationship between subseasonal and interannual variability of the Asian summer monsoon has been investigated through analysis of the dominant modes of variability in the 40‐year NCEP/NCAR Re‐analysis, with complementary satellite and surface‐based precipitation data. The hypothesis that the characteristics of monsoon subseasonal variability (i.e. weather regimes) are modulated on interannual time‐scales in a systematic and therefore predictable manner has been tested. The null hypothesis is that predictability of the seasonal mean monsoon behaviour requires that the effects of the slowly varying components of the climate system be correctly simulated.An interannual mode of monsoon variability has been identified which is closely related to the observed seasonal mean all‐India Rainfall (AIR). A counterpart of this mode has also been identified at subseasonal time‐scales which projects strongly on to the daily AIR, confirming that a common mode of monsoon variability exists on sub‐seasonal and interannual time‐scales. It has been shown that the temporal behaviour of this subseasonal mode, as described by the probability distribution function (PDF) of the principal component time series, does not show any evidence of bimodality, the shape of the PDF being Gaussian. Further, it has been shown that the mean of the PDF is systematically and significantly perturbed towards negative (positive) values in weak (strong) monsoon years as categorized in terms of the seasonal mean AIR. This translation in the mean of the PDF, rather than a change in shape of the PDF, suggests that anomalous monsoons are not associated with changes in weather regimes. Further analysis has confirmed that low‐frequency modulation of the basic state is primarily responsible for these shifts in the subseasonal PDFs, supporting the null hypothesis that predictability of the seasonal mean monsoon requires that the effects of the slowly varying components of the climate system be correctly simulated. Thus, model improvements to reduce systematic errors in the mean simulation and the response to low‐frequency boundary forcing may improve the prospects for dynamical seasonal prediction.However, the results indicate that only a subset of the subseasonal modes are systematically perturbed either by the El Nino Southern Oscillation or in weak vs. strong monsoon years, suggesting that predictability is likely to be limited by the chaotic, internal variability of

ISSN:0035-9009    

DOI:10.1002/qj.49712656810

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY

摘要:

AbstractThe ability of the Center for Ocean‐Land‐Atmosphere Studies Atmospheric General Circulation Model to capture the inverse snow‐monsoon relation observed in satellite‐derived data is tested. The effect of initial enhanced and reduced spring Eurasian snow mass on the evolution of the subsequent Indian summer monsoon is examined through 11‐member ensemble integrations from 1 March to 1 October. The results of the sensitivity experiment indicate that remote responses from the snow‐mass anomalies include significant changes in the sensible‐heat flux, latent‐heat flux and sea level pressure, leading to changes in summer precipitation over the land mass of the Indian subcontinent. For the pre‐monsoon season, the ensemble average of the temperature of the atmospheric column for enhanced snow mass is anomalously colder over large‐scale regions of the Eurasian continent compared with the ensemble average for the reduced snow mass. Low‐level south‐westerlies associated with monsoonal circulation are increased. The results indicate the importance of land‐surface processes in the evolution of the monsoon circulation and lend hope to the use of Eurasian spring snow mass towards improved forecast

ISSN:0035-9009    

DOI:10.1002/qj.49712656811

出版商:John Wiley&Sons, Ltd    

年代:2000

数据来源: WILEY