摘要:

AbstractA series of large‐eddy simulations of the neutral‐static‐stability planetary boundary layer has been conducted. the simulations differ in the size of the computational domain and the scales represented by the subgrid‐scale turbulence parametrization. the flow eddies are always irregular but show an elongation in the direction of the local velocity shear vector. the dominant horizontal scales of the eddies increase with height in the boundary layer and become much larger than the boundary layer depth. All of this wide range of scales of motion is important in determining the gross boundary layer characteristics. the larger scales of motion have dynamics influenced by both the vertical and horizontal components of the earth's rotation. the flow statistics are compared with previous simulations and with high‐order closu

ISSN:0035-9009    

DOI:10.1002/qj.49711347602

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractWidespread gravity wave systems have been found to exist over convectively active boundary layers (CBLs) in the presence of vertical wind shear. In contrast to mountain waves, these ‘convection waves’ occur over flat terrain and are ubiquitous over fields of shallow fair weather cumuli or clear air thermals. They extend vertically to at least 9km a.m.s.l., probably to the tropopause. First discovered by glider pilots as ‘thermal waves’, they have now been systematically investigated by research aircraft during the NCAR Convection Wave Project, which also incorporates an effort in numerical simulation. the flight results are reported in this paper, while the numerical simulations are described in a companion paper.Typical wavelengths of convection waves were found to range from 5 to 15 km, (average 9 km) and typical vertical motion amplitudes from ± 1 to ± 3 m s−1. In all cases vertical wind shear exceeded 3×10−3s−1. Under these conditions cloud tops were measured to move with relative velocities of 8 to 10 m s−1against their environment, hence they present an obstacle to the surrounding airflow. the analogy to mountainous obstacles launching gravity waves into the atmosphere is suggestive and borne out by the aforementioned numerical simulations, which describe this process step by step.The interaction between the CBL and the overlying stable layers is studied by spectral and cross‐spectral analysis of aircraft data including coherence and phase relations between wave motions and cloud population underneath. Consideration is given to other potential sources of the observed gravity waves. At this stage the conclusions of our pilot study may be considered tentative. Some implications for further researc

ISSN:0035-9009    

DOI:10.1002/qj.49711347603

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractRadar observations of the differential reflectivity (ZDR) of developing convective clouds are presented. This parameter provides a measure of a mean hydrometeor shape, and when analysed in conjunction with the conventional reflectivity (Z), it gives an indication of whether the precipitation is ice or liquid water; for rain it enables an estimate of raindrop size and concentration to be made. the results suggest that some isolated echoes which subsequently develop into large cumulonimbus clouds initially have very small concentrations of large (diameter greater than 4 mm) supercooled raindrops, a very different size distribution from that normally observed in mature convective clouds. an alternative explanation in terms of an even lower population of much larger ice particles in wet growth is technically possible but does not seem physically realistic.

ISSN:0035-9009    

DOI:10.1002/qj.49711347604

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractA case study investigating the relationship of ice phase microphysical structure to differential reflectivity is presented. the distribution of ice crystal types in light precipitation is discussed and shown to agree with a theoretical model of an individual precipitation fallstreak. the particle characteristics were found to reflect sorting by size and type associated with fall through weak wind shear as well as growth in regimes of varying temperature and humidity within the fallstreak.The observed differential reflectivities were compared with thein situparticle measurements and the association of high values with planar crystal structures demonstrated. the particle size distribution was found to have a marked influence on radar returns, particularly through the obscuring effects of large particles of indefinite shape. This effect gave rise to relative variations of differential reflectivity which were found to differ from the predictions of monodisperse models. In particular large dendritic crystals of low density were found in this case to be associated with stronger signatures than denser plate‐like crystals which from theoretical studies might be expected to produce the greatest differential reflectivity. Because of these complicating factors under stratiform conditions it was concluded that differential reflectivity data must be interpreted with considerable caution in the absence ofin situmeasurements or other supporting dat

ISSN:0035-9009    

DOI:10.1002/qj.49711347605

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractA Monte Carlo model of short‐wave radiative transfer in a broken cloud field is described. Output from the model is presented and the effect of cloud shape and depth, droplet absorption and water vapour absorption on the effective fractional cloud cover,Ne, is investigated. Inhomogeneities are introduced into the cloud field in the form of a variable cloud width and a cloud optical depth which is dependent upon cloud cover. It is shown that these inhomogeneities lead to much better agreement between the modelled values ofNeand experimental values derived from data collected with the Meteorological Research Flight Hercules. the conclusion is thatNeis a nonlinear function of the fractional cloud cover,N, which may be approximated by the relationshipNe=N

ISSN:0035-9009    

DOI:10.1002/qj.49711347606

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractA comprehensive evaluation of the ECMWF operational data assimilation system has been made, based largely on statistics of its performance over a long period of time. the evaluation has identified many deficiencies, covering such aspects of the analysis as the optimum interpolation statistics, the data selection algorithms, and the quality control criteria applied within the analysis.The observation and forecast error variances were seriously in error in the stratosphere and in the tropics. A more faithful model of the forecast error spectrum is provided by the Bessel function expansion than by the Gaussian model. the vertical structure of the height error is assumed to be composed of two components: a ‘barotropic’ part related to a horizontally constant mode and a ‘baroclinic’ structure for the horizontally varying modes. the wind field errors also have a large‐scale component. the extratropical vertical correlation remained similar to the earlier correlations, but the tropical height and wind structures changed dramatically.Reasonable rejection limits were defined from the distributions of the observation departures from the first guess, as were observation errors for satellite and aircraft wind reports and Australian pseudo‐observations.A comprehensive and homogeneous data selection algorithm has been devised. However, it has been found necessary to exclude land surface wind reports from the analysis process as the wind departures from a first‐guess field commonly contradict the surface pressure departures due to difficulties in generating a reasonable 10 m first‐guess wind. the omission of surface winds alleviated the data selection problem in data‐dense regions.The representation of the vertical correlations was changed to a continuous representation in the vertical which enabled a multivariate interpolation from data on standard pressure levels directly to model coordinates. the earlier procedure used a multivariate interpolation to constant pressure surfaces in the model grid, followed by a univariate interpolation to model surfaces in the vertical.The results of these improvements, in a series of tests covering in total some 30 days of assimilations, are also described. In summary they yield an analysis and a subsequent first guess which are in closer agreement with observations. the analysis scheme yields more balanced fields in the sense that the subsequent initialization produces more modest changes. the major impact on forecasts, in particular in the short range, is also judged to be positive.These improvements have been incorporated in the ECMWF operational data assimilation system. They have also been incorporated in the reassimilation of the final FGGE level IIb

ISSN:0035-9009    

DOI:10.1002/qj.49711347607

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractThis study examines the large‐scale tropical environment of north‐east Brazil during its 1979 rainy season. Data were primarily from the First GARP Global Experiment (FGGE). Results show that in the mean NE Brazil is south of the intertropical trough zone (ITZ) and north‐west of the South Atlantic high. This pattern results in a westward low‐level flow over the region. Immediately over NE Brazil, middle‐level subsidence and low‐level moisture divergence dominate, producing a mean local environment which is unfavourable for precipitation. During wet episodes cloudiness departures show a more active ITZ over the equatorial Atlantic. Strong sea level pressure departures are found in the vicinity of the subtropical highs indicating short period fluctuations in the Hadley cell. Locally, weaker middle‐level subsidence and weaker moisture divergence during wet episodes allow for increas

ISSN:0035-9009    

DOI:10.1002/qj.49711347608

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractThe water vapour transport over Africa and the eastern Atlantic Ocean during the 1979 summer is studied. the analysis is based on wind fields from the European Centre for Medium Range Weather Forecasts and humidity fields derived from a three‐layer precipitable water data set.The evolution of the bi‐weekly mean fields of water vapour flux in the surface to 850 mb layer shows the progressive penetration of moisture over West Africa during the summer. At low levels during the maximum activity of the African monsoon, the southerly flow penetrates up to 20°N and there is a large belt of westerlies around 10°N. the fluxes between 700 and 500 mb suggest the importance of the African easterly jet (AEJ) as a major artery for moisture for West Africa.Vertically integrated fluxes across different sections have been analysed. Across the equator at the longitudes of the Gulf of Guinea the flux shows a decreasing trend due to the northward propagation of the intertropical discontinuity over west Africa. Near 22°N, the flux is mainly from the north at the beginning of the summer and becomes southerly during surges of the monsoon. the flux from West Africa toward the Atlantic Ocean is large, indicating that moisture from the Gulf is transported upward and advected westward by the AEJ. the fluxes oscillate with two main periods, around 3‐5 days and 5‐7 days.Water vapour budgets have been estimated for boxes located over the Gulf of Guinea and West Africa. the Gulf of Guinea is a source of water vapour transported northward and westward by the trade winds. Over West Africa at low levels, moisture comes from the Gulf of Guinea. In the mid‐troposphere there is a strong easterly moisture flux associated with the AEJ and originating from central Africa. This moisture seems to come initially from the Gulf of Guinea. Because it is mainly divergent, the mid‐tropospheric flux may have little influence on rainfall over West Africa. the budget study also emphasizes the importance of evapotranspiration over the belt north of the Gulf of Guinea where water vapour is recycled and transpo

ISSN:0035-9009    

DOI:10.1002/qj.49711347609

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractA new radiative transfer scheme, developed for use in a three‐dimensional dynamical model of the stratosphere and mesosphere, is used to consider the hypothetical state of the middle atmosphere in the absence of dynamical heat fluxes. Most previous work in this area has considered radiative equilibrium temperatures using a fixed solar forcing. This equilibrium state is shown to be different from a state calculated using a seasonal cycle of solar forcing; in particular, the winter pole is found to be warmer and the latitudinal temperature gradient across the edge of the polar night is found to be smaller. This difference is due to two processes. Firstly, latitudes just polewards of the edge of the polar night at solstice are in darkness for only a short period. Secondly, radiative time scales lengthen greatly as temperatures decrease, so that equilibrium cannot be reached before the sun reappears.Recent ozone climatologies are used to impose a realistic annual cycle of ozone on the model. It is found that the simulation of the lower stratosphere is in considerably better agreement with observations than previous radiation‐only calculations. the better simulation is shown to be, in most cases, due to the use of the new ozone climatology. It is also shown that a realistic ozone distribution is necessary for a good simulation of the polar stratopause at the summer solstice.A comparison of the observed and calculated annual cycle of temperature in the polar regions of the southern hemisphere lower stratosphere indicates that this region is under strong radiative control. This contrasts with an implied strong dynamical control for the same region in the northern hemisph

ISSN:0035-9009    

DOI:10.1002/qj.49711347610

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractA parametrization of Kelvin wave absorption in the equatorial stratosphere has been included in a two‐dimensional model of the atmosphere in order to improve the simulation of the semi‐annual oscillation. Additionally, a more detailed treatment of the momentum deposition associated with gravity wave breaking in the mesosphere has replaced the previous Rayleigh friction scheme. the model exhibits a semi‐annual reversal of the zonal winds at the equator in the stratosphere, with the westerly phase descending with time, as observed. the inclusion of the improved gravity wave parametrization has led to a number of significant improvements; in particular, the model also exhibits a semi‐annual oscillation in the mesosphere which is out of phase with that in the stratosphere. Meridional circulations associated with the westerly phase of the semiannual oscillation produce ‘double peak’ structures in a number of modelled tracer fields, including N2O, CH4and H2O which compare well with observations by the SAMS and LIMS satellite experiments. These structures consist of a distinct minimum at the equator and double maxima in the subtropics; they are present for several months around each equinox. Inclusion of the more realistic westerly forcing has pointed to the need for additional easterly forcing in the model around each solstice. Further supporting evidence for the importance of momentum deposition associated with planetary waves in the easterly phase of the semi‐annual oscillation comes from the difference in double peaks between the northern hemisphere spring and autumn, as suggested in an

ISSN:0035-9009    

DOI:10.1002/qj.49711347611

出版商:John Wiley&Sons, Ltd    

年代:1987

数据来源: WILEY