摘要:

AbstractA self‐contained account of the semi‐geostrophic equations is given. This contains formulae not previously available in the literature. The equations are presented in terms of four alternative sets of independent variables, and the Legendre mappings between these variables are specified. The approach is facilitated by the introduction of a general theorem in vector analysis, which is proved, concerning the total or convected derivative of a Jacobian following the particle. An expression for the curvature of a front is inclu

ISSN:0035-9009    

DOI:10.1002/qj.49711750202

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe structure of the intraseasonal oscillation in a perpetual January integration of the NCAR Community Climate Model (CCM) is analysed and compared with observations. The simulated oscillation is identified as an eastward‐moving perturbation in the equatorial velocity‐potential field, with predominantly zonal wave‐number‐1 structure. Analysis of its vertical and horizontal structure, based on composites, shows good agreement with the observed oscillation from station data, and from numerical weather prediction model analyses.The simulated intraseasonal oscillation is evident in the wind fields at all longitudes but its appearance in the precipitation and outgoing long‐wave radiation is confined to the eastern hemisphere. The perturbation to the zonal wind component is characterized by an out‐of‐phase behaviour between the lower and upper troposphere which is most coherent in the convective regions of the Indian Ocean and West Pacific. The stream‐function anomalies in these regions are suggestive of a coupled Rossby‐Kelvin wave, whilst in the western hemisphere the oscillation is more characteristic of a Kelvin wave.The role of cloud‐radiation interaction was studied using a parallel integration in which the cloud long‐wave forcing in the atmosphere was removed. This was found to influence the strength of the oscillation but had no significant impact on the period.In common with other model simulations of the intraseasonal oscillation, the period was found to be too short, near 25 days. Reasons for this are discussed in the context of a similar analysis of a perpetual July integration in which a much slower (period near 54 days) and less coherent oscil

ISSN:0035-9009    

DOI:10.1002/qj.49711750203

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThis paper analyses wintertime and summertime simulations over Europe using the National Center for Atmospheric Research (NCAR) regional atmospheric model (MM4). The sensitivity of the model to selected physics parametrizations (explicit moisture scheme; slower release of condensation heat; horizontal diffusion on s̀‐surfaces), and lower boundary conditions (sea surface temperature; initial soil moisture; snow cover) is examined. The simulation periods are January 1979 and June 1979; initial and lateral meteorological boundary conditions are provided by analyses of observations. The main focus of the analysis is on prediction by the model of surface air temperature and precipitation, but also sea‐level pressure and upper‐air variables are considered. In general, the model reproduces the main features of the synoptic events which prevailed during the two simulation periods. Mid‐tropospheric and upper‐tropospheric biases in the model are generally small. Surface maximum and minimum air temperature biases do not exceed a few degrees K over various regions of Europe. The largest model bias (2‐3 K) is found for summertime minimum temperature. Wintertime surface temperatures are sensitive to the precipitation parametrizations tested, as these affect cloud formation and, in turn, the surface radiative fluxes. Also, temperature biases in the driving large‐scale fields are partially transmitted to the surface air temperature calculations. Summertime temperatures are sensitive to the soil moisture content. Sea surface temperature variations influence land temperatures more strongly for wintertime than for summertime. Precipitation is sensitive to the parametrizations used. The explicit moisture scheme generally induces underprediction of precipitation. When this is not used, the biases are of the order of −10 to −20% of the observations. Summertime precipitation over the highest Alpine regions is overpredicted, except when diffusion of moisture and temperature along terrain‐following s̀‐surfaces is strongly reduced. Sea surface temperature variations substantially affect precipitation, especially over coastal areas, both in January and June. Summertime precipitation is significantly modified by the initializatio

ISSN:0035-9009    

DOI:10.1002/qj.49711750204

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractQuasi‐periodic oscillations with a period of about 40 min are observed in visibility, temperature, water vapour, wind speed and long‐wave radiation during the fully developed stage of a shallow radiation fog. These oscillations were observed up to a height of about 60 m, whereas the fog extended up to a height of about 30 m. An interpretation of the oscillations is given in terms of gravity waves, which are rather typical under these very stable conditi

ISSN:0035-9009    

DOI:10.1002/qj.49711750205

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe aim of the paper is to demonstrate the numerical feasibility of 4‐D variational assimilation using a multilevel primitive‐equation model. The experiments consist in minimizing the distance between the model solution and the observations. The gradient of the cost function thus defined is computed by integrating the adjoint of the model.Here, assimilations are performed using model‐generated observations. In a preliminary set of experiments, assimilations were performed assuming that observations consisting of a full‐model‐state vector are available only at the end of the assimilation period. The numerical convergence of the method is proved and the results are meteorologically realistic. The use of the Machenhauer nonlinear normal mode initialization scheme and its adjoint turned out to have hastened the convergence and to have controlled to some extent the amount of gravity waves appearing in the solution. We identify a loss of conditioning of the minimization problem with an increase in the length of the assimilation period. The presence of horizontal diffusion in the model has the effect of degrading the convergence.The second set of experiments evaluates the impact of observations distributed over the whole assimilation period. Through different senarios of sets of observations, we demonstrate the efficiency of the 4‐D variational approach in extracting the information contained in the dynamics of the model, together with the information contained in the observations. In particular, observing only the small scales of the flow leads to a good reconstruction of both small scales and large scales. Observations of the mass‐field evolution lead to a good reconstruction of the vorticity field in mid latitudes but less so in the tropics.Increased resolution in the model in the experiments was found to have a negative impact on the speed of convergence of the minimizat

ISSN:0035-9009    

DOI:10.1002/qj.49711750206

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractAs faster computers emerge, resolution continues to improve in weather‐prediction and climate models. How does this affect the problems that we are dealing with in the models? In this study we have concentrated on one of these problems, namely the treatment of clouds. Among the questions addressed are: How do cloud‐related processes (condensation, precipitation) depend on grid resolution? How should we take this into account? Can we simulate orographic clouds when we have a grid resolution of hundreds of kilometres (climate models)? To tackle these questions simulations are performed with a limited‐area model that is run with horizontal grid resolutions of 150,50 and 25 km. The model has a sophisticated treatment of condensation and clouds. For verification, use is made of satellite‐derived cloud water content and cloud cover, and surface observations of precipitation.It is found that when resolution is enhanced, precipitation systematically increases while cloud cover decreases. The main reason is that much stronger vertical motions develop at high resolution, yielding dense, narrow clouds, with clear air between. The consequences of these results for cloud parametrization are discussed. It is further shown that cloud‐cover simulations over mountainous terrain can be improved by paying regard to the sub‐grid‐sc

ISSN:0035-9009    

DOI:10.1002/qj.49711750207

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractA relationship is derived between absolute vorticity in the lower stratosphere and the total‐ozone column. This relationship shows that reversible deformations to the flow in the lower stratosphere on synoptic scales or smaller (such as trough/ridge patterns) cause perturbations in total ozone which depend on the amplitude and depth of the vorticity anomaly. Examples are presented showing excellent correspondence between total ozone and vorticity charts, even on mesoscales. Grid correlations show that at least half the variance in total ozone is related to variations in the vorticity just above the tropopause. Some applications of the ozone‐vorticity relationship are discus

ISSN:0035-9009    

DOI:10.1002/qj.49711750208

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThis study, based on airborne observations obtained during the Upper Spencer Gulf experiments in South Australia, investigates the structure of turbulence in a coastal boundary layer, which represents an extreme situation of horizontal inhomogeneity. The development of the coastal boundary layer under mainly onshore wind conditions, which is characterized by the formation of a convective thermal internal boundary layer (TIBL), is illustrated using cross‐sections of potential temperature, sensible‐heat fluxes and turbulent kinetic energy. The similarity of turbulence within the TIBL and the effectiveness of local scaling for inhomogeneous conditions are examined. It is suggested that, even in advective conditions, turbulence can be considered as being in local equilibrium. By introducing a simple TIBL model, local similarity relationships can be linked to external parameters. Some turbulence properties, such as the variance of vertical velocity, are found to be determined by three external parameters: the surface heat flux over land, the onshore wind speed and the thermal stability of the approaching airf

ISSN:0035-9009    

DOI:10.1002/qj.49711750209

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractAn evaporation model based on Lagrangian turbulent diffusion principles is developed and compared with simpler single‐ and dual‐sourceK‐theory models of evaporation. The performance of the Lagrangian model is assessed against observations of total evaporation by an eddy‐correlation instrument and found to be satisfactory for a millet crop in west Africa. Three versions of existing simpler models containingK‐theory descriptions of within‐canopy turbulence are described and their results also compared. The twoK‐theory models that explicitly take into acccount the soil source perform better than the single‐source Penman‐Monteith model. A Lagrangian analysis does not seem to be necessary for this kind of crop because the low source‐density profile of the crop, associated with a low leaf‐area index, caused the near‐field effect to be very small. The overall difference between the evaporation estimates of the dual‐source and Lagrangian models is therefore small. It is concluded that, for practical purposes,K‐theory remains an adequate approximation of turbulent transport in sparse‐crop evaporation models.Significant anomalies in the surface heat fluxes, particularly those of sensible heat, accompanied the decrease in the sea ice concentration. Substantial atmospheric warming was simulated over and in the vicinity of areas in which leads were considered. In all but one experiment there were anomalous easterlies between about 40 and 60°S with westerly anomalies further to the south. The surface pressure at high latitudes appears to change in a consistent fashion with the fraction of open water, with the largest changes occurring in the Weddell and near the Ross Seas.Some of the feedbacks which may enhance the responses here, but which are not includ

ISSN:0035-9009    

DOI:10.1002/qj.49711750210

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractBy application of the stochastic coalescence equation to evolving populations of cloud droplets and raindrops, we show that in light rain there is a range of drop sizes for which, to a good approximation, raindrops grow only by cloud accretion and are neither created by autoconversion nor destroyed by breakup. In these conditions, the differential equation that describes the one‐dimensional steady‐state form of the drop‐size distribution may be solved analytically, using plausible assumptions about the cloud water content and the raindrop collection efficiency. The solution gives the drop‐size distribution as a function of distance below a reference level, from which we can calculate radar‐measurable quantities such as the reflectivity factor and the Doppler spectrum.Recent radar data from the Hawaiian Rainband Project provide the opportunity for a close comparison of the steady‐state accretion theory with observations. An example is presented of a steady situation in which, over a limited range of altitude, the observed vertical profiles of reflectivity and Doppler velocity do agree reasonably well with the theory. Other observations in equally steady‐appearing conditions, however, are not in accord with the theoretical predictions. If the parameters of the model are chosen to fit the velocity profile, the theory then tends to predict reflectivity gradients stronger than those actually observed. Similarly, if the parameters are chosen to fit the reflectivity profile, the predicted increase of Doppler velocity with distance fallen is less than what is observed. Effects of evaporation can be included in the steady‐state model, and are shown to explain the sense of the discrepancy but not its magnitude. The conclusion is that the one‐dimensional model, possibly because of its inability to account for horizontal advection, is usually inadequate to explain

ISSN:0035-9009    

DOI:10.1002/qj.49711750211

出版商:John Wiley&Sons, Ltd    

年代:1991

数据来源: WILEY