摘要:

AbstractFinite‐amplitude solutions for infinitely deep stratified flow over orography are obtained for upstream conditions with uniform wind and a two‐layer static‐stability structure. the flow within each layer is governed by Long's equation. the governing equations are solved using a semi‐analytic model with an open (radiation) boundary condition in the upper layer. A simplified, computationally efficient form of the finite‐amplitude radiation upper‐boundary condition is presented and utilized in the two‐layer calculations.When the higher stability is in the lower layer, finite‐amplitude effects may produce large‐amplitude waves in circumstances where linear theory predicts a weak response. the relationship between the continuously stratified system and shallow‐water hydraulic theory is explored, and an expression is derived for the effective Froude number in unbounded continuously stratified flow. In those cases producing the strongest finite‐amplitude response, the flow undergoes a transition from subcritical to supercritical just to the lee of the mountain crest.When the higher stability is in the upper layer, the finite‐amplitude response is also sensitive to the location of the stability interface. the dependence of the solution on the height of the interface is, once again, different from that predicted by linear theory. the interface height that produces the strongest response increases as the mountain height increases. These results support the idea, originally suggested through the analysis of linear multi‐layer models, that the vertical phase shift between the ground and the tropopause has an important influence on the strength of downslope windstorms. the optimal phase shift for finite‐amplitude waves cannot, however, be reliably de

ISSN:0035-9009    

DOI:10.1002/qj.49711850502

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractHigh‐spatial‐resolution data were collected for nearby convective storms by using a volume scanning weather radar with 200m resolution, and for stratiform events by using a high‐resolution (20m) vertically pointing radar. Errors in the estimation of rainfall by the scanning radar, due to the radar beam intersecting or overshooting features such as the bright band, were simulated by using these high‐resolution images. It was found that these effects define a maximum useful range for the radar that is a strong function of the prevailing meteorological conditions (e.g. bright‐band height, intense cell diameter and height).Sudden changes in bright‐band height over short times and the large scatter observed in its thickness limit the accuracy with which corrections for the vertical profile of reflectivity may be precalculated in stratiform rain events. A scheme for using a vertically pointing radar to produce these corrections in real time is proposed. the convective events show enough significant storm‐to‐storm variability that caution is required in extending the useful range of th

ISSN:0035-9009    

DOI:10.1002/qj.49711850503

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractExperiments with a one‐dimensional version of the Meteorological Office's 11‐layer GCM are used to derive a simple calibrated subgrid parametrization of rainfall interception by the Amazonian rain forest. Two interception parametrizations, both incorporating subgrid variability of precipitation within a GCM grid box are presented and tested, and compared with observations made at a single location. the 1‐D model is a fully interactive atmospheric column and incorporates all the GCM physics but is forced at its boundaries by climatic data. the interception losses appear to be sensitive to what fraction of area in the GCM grid box is covered by the rain and to the precise formulation of the interception process. the implications for the parametrization of the wet area in a GCM grid box are discussed and suggestions are made as to how to parametrize the fraction of wetted area in a GCM gri

ISSN:0035-9009    

DOI:10.1002/qj.49711850504

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThe sensitivity of modelled northern hemisphere climate to modification of the snow‐covered surface albedo is investigated using the United Kingdom Meteorological Office (UKMO) general circulation model (GCM). the UKMO GCM is a global, primitive‐equation model with 11 layers in the atmosphere. Surface processes in the model are highly parametrized, with bulk aerodynamic formulation of the surface fluxes and a ‘bucket’ soil‐moisture accounting method. the experiment represents in a highly simplified fashion the role of the boreal forests in reducing the surface albedo under snow‐covered conditions. A parametrization of snow‐covered land was developed which allows the maximum albedo attainable with a snow cover to be prescribed as a function of vegetation type. In the standard version of the model the maximum snow‐covered surface albedo attainable is 0.60, which exceeds observed values for the forested areas of the northern hemisphere. the model was integrated twice, with different albedos representing forested and deforested conditions. the sensitivity of the heat and hydrologic budgets for the northern hemisphere and deforested areas is discussed. A detailed analysis of the deforested regions reveals systematic reductions in temperature of up to 2.8 K. Precipitation shows a systematic decrease in the affected regions. the largest decreases occur generally in the months with largest evaporation changes. For the case of no masking by forest vegetation (equivalent to boreal deforestation) the model produces a significant change in the pattern of snow‐melt. the removal of forest affects both the magnitude and the timing of spring snow‐melt, and consequently also the runoff. There are delays in snow‐melt‐induced runoff peaks by a month and increases in the magnitudes by on average 32%. the role of large‐scale advection is investigated by comparison with the results from a single‐column‐model experiment. A realistic representation of the snow‐covered surface albedo is evidently a requirement for simulations of

ISSN:0035-9009    

DOI:10.1002/qj.49711850505

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractAltitude and latitude distributions of the populations of the bending and symmetric stretching levels of CO2and of the cooling rates induced by the bands originating from them in the middle atmosphere (40‐120km), have been calculated for the CIRA 1986 reference atmosphere using a non‐LTE radiative transfer model. A sensitivity study of the more uncertain parameters that strongly affect these quantities has been undertaken. These factors are the carbon dioxide and atomic oxygen volume mixing ratios, and the rate coefficient for the deactivation of the bending mode of CO2by atomic oxygen. the model makes use of the most recent values of these parameters: the abundances of CO2in the upper mesosphere and lower thermosphere as measured by, and derived from, rocket and satellite instruments; the global distribution of atomic oxygen density calculated by Rees and Fuller‐Rowell (1988) and the measurements of Thomas (1990); and the fast rate for the deactivation of CO2(0, v2, 0) by O(3P), recently derived by Sharma and Wintersteiner (1990). Among the most important results are the following: (i) the CO215 μm fundamental band is close to LTE (Tv‐ Tk≤ 6K) up to about 100km altitude; (ii) the first and second hot bands exhibit high vibrational temperatures, especially in the daytime summer mesopause region; (iii) the cooling rates by the CO215μm bands in the lower thermosphere can be very large, depending on the CO2abundance. Finally, our results have been compared with other recent non‐LTE models in order to quantify the effects of different radiative transfer schemes on the calculations of vibrational temperatures and

ISSN:0035-9009    

DOI:10.1002/qj.49711850506

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractA low‐order model of global barotropic flow with intrinsic long‐term variability is presented. the model is based on the barotropic vorticity equation with Newtonian forcing of the zonal flow and realistic orography. Subgrid eddy motion is represented by point vortices which move within the low‐order flow of the model and affect it through their own circulation. the vortices are inserted at locations of maximum flow speed and are removed after a prescribed lifetime has elapsed. In the absence of Newtonian forcing, mid‐latitude westerlies and strong polar easterlies are induced if there are only cyclonic vortices. Anticyclonic vortices create equatorial easterlies.If ‘realistic’ Newtonian forcing is added, two stationary orographic troughs dominate the model's mean flow in the northern hemisphere. Cyclones are generated downstream of the troughs, and pronounced storm tracks extend eastward from these genesis areas. the model atmosphere exhibits slow variability of a realistic order of magnitude with maxima over the ‘oceans’. This long‐term variability is due to the interaction of long‐lived circulation patterns, like blocks, with the point vortices. Moreover, cyclone generation is important in inducing slow motions. In the southern hemisphere there is little variation of flow statistics with longitude. All in all the low‐order model is surprisingly successful in simulating the observed patterns of atmospheric transience. This suggests that a low‐order model with point vortices might be a useful t

ISSN:0035-9009    

DOI:10.1002/qj.49711850507

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractA probabilistic approach accounting for the variability of error growth in the atmosphere is developed and applied to an idealized low‐order atmospheric circulation model. Error growth in a single‐realization, ensemble‐averaged error versus time and the time‐dependent as well as the asymptotic probability distribution of errors are determined. A wide dispersion around the mean is found, showing clearly the inadequacy of a description that is limited to averaged properties only. A self‐consistent one‐variable, two‐parameter model of error dynamics is constructed in the form of a stochastically driven logistic equation, and the results are shown to be in good quantitative agreement with the simulation of the full equations. A generalization to more realistic atmospheric models is

ISSN:0035-9009    

DOI:10.1002/qj.49711850508

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractAn earlier review paper of analysis methods for numerical weather prediction showed how many methods could be derived from the same basic analysis equation, expressing the Bayesian probability that any state is the true state. Prior probabilities depend on the error covariance of the background prior estimate, and these covariances figure in most solutions. This paper extends the analysis of iterative solution methods (one of which is known as the ‘Successive Correction’ method). In a simple example the methods are compared with a direct solution: the ‘Optimal Interpolation’ method. It is shown that the iterative methods can be equally ‘optimal’. If the number of iterations is limited, or the background weighting is omitted, then the methods normalizing the increments in observation space are shown to be more reliable than those normalizing in grid‐point space. the traditional successive‐correction method has a grid‐point space normalization.The covariances in these methods perform a filtering function. It is possible to replace them by a filter acting on the analysis increments. Iterative methods using such a filter are derived, and are shown to correspond exactly to the iterative methods using covariance functions. A simple and efficient recursive filter is described and applied to the same example. the analysis using a two‐pass filter is almost identical to that using a ‘second‐order auto‐regressive’ covariance function. A filter with many passes corresponds to a Gaussian‐shaped covariance function.Approximate filters can be devised to model the effect of observational‐error correlations, with an accuracy adequate in view of the lack of knowledge of the real correlations. With this filter, the iterative methods can be extended to deal effectively with data from remote‐sensing instruments.Published iterative methods (many originally empirically derived) are reviewed, and fitted into the optimal theory. Pra

ISSN:0035-9009    

DOI:10.1002/qj.49711850509

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

ISSN:0035-9009    

DOI:10.1002/qj.49711850511

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

ISSN:0035-9009    

DOI:10.1002/qj.49711850512

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY