摘要:

AbstractA three‐dimensional model of airflow over hills has been used in conjunction with a two‐dimensional model of orographic rainfall enhancement. This development has allowed the accurate production of rainfall contour maps in areas of complex terrain. A comparison has been made, using a variety of atmospheric stability profiles, between the wind fields and rainfall distributions over a bell‐shaped hill and those over a two dimensional ridge of similar height. the results show that, in conditions of either stably stratified air or with an inversion layer aloft, three‐dimensional airflow effects can lead to much reduced rainfall over a bell‐shaped hill. It is shown that in such situations the use of a two‐dimensional approximation can introduce significant errors. an example is presented of the rainfall distribution in complex hill terrain over south Kintyre on the western coast

ISSN:0035-9009    

DOI:10.1002/qj.49711850802

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractA study has been made of some aspects of frictionless stratified flow past three‐dimensional isolated mountains. the study uses a three‐dimensional non‐hydrostatic numerical model to investigate the behaviour of the flow as a function of the Froude number, and produces a picture of the dependence of the gravity‐wave drag on the Froude number for a wide range of that parameter. At the same time, the results of the numerical experiments clarify the behaviour of the flow in the transition from high to low Froude number, showing the relative importance of wave breaking and flow splitting in the transitional

ISSN:0035-9009    

DOI:10.1002/qj.49711850803

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThis paper presents the first comprehensive micrometeorological measurements to be recorded over post‐deforestation Amazonian ranchland. the ranch was managed for the production of beef cattle and had been created by felling and burning the original rainforest 12 years previously. the measurements allow derivation of the aerodynamic roughness, and a description of the response of the energy balance and surface conductance to the progressing dry season. Zero‐plane displacement and roughness length were derived from wind‐speed profiles as O.17 ± 0.03 m and 0.026 ± 0.003m respectively, while measurement of energy partition was achieved, with excellent agreement, between three independent measurement systems. During the 1990 dry season, average evaporation diminished from 3.8 to 2.1 mm d‐1as the Bowen ratio increased from 0.43 to 0.67. Values of surface conductance were derived and these compare well with expect

ISSN:0035-9009    

DOI:10.1002/qj.49711850804

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractA series of factorial experiments is used to elucidate the factors and multifactor interactions important for the performance of one complex land‐surface scheme in a tropical environment. the BATS (Biosphere‐Atmosphere Transfer Scheme) is found to be sensitive to atmospheric factors which are currently poorly predicted by global climate models (GCMs) and, more importantly, to combinations of factors rendering tuning for good performance when coupled into today's global models (a worthless practice if the tropical area is subjected to different conditions such as those induced by deforestation). Similarly, two‐factor interactions, between coded ecological parameters (here roughness length and short wave,<0.7 μm, vegetation albedo), mean even if the BATS characterization and its driving GCM perform well in today's tropical‐forest environment, altering either or both parameters, as will occur in a deforestation simulation, must greatly diminish confidence in representations of future

ISSN:0035-9009    

DOI:10.1002/qj.49711850805

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractWe have developed a system for the Simulation and Analysis of Measurements from Satellites using Operational a Nalyses (SAMSON). In this initial study, simulations of clear‐sky outgoing longwave radiation (OLR) over the oceans are compared with data from the Earth Radiation Budget Experiment (ERBE). the calculations were performed with a Malkmus narrow‐band radiation scheme, including the effects of all the radiatively important gases. Sea surface temperatures were taken from the National Meteorological Center's blended analyses and ozone data from the NIMBUS‐7 Solar Backscatter Ultra Violet experiment. Atmospheric temperatures and humidities came from operational analyses. Parallel simulations were performed with the analyses produced both by the European Centre for Medium‐range Weather Forecasts (ECMWF) and by the Meteorological Office.Simulations for the period February 1985 to October 1986 are compared with ERBE data. Differences between the humidity data in the analyses have a significant impact on the calculated clear‐sky OLR. the changes to the ECMWF model in May 1985 reduce the differences compared with ERBE. After this time, the differences are consistent with two known systematic errors; a small overestimate in the ERBE data and a tendency for the ECMWF analyses to be too dry in the ITCZ and too moist in the sub‐tropics. the simulations with the Meteorological Office's analyses generally show slightly larger differences compared with ERBE. Tests show the sensitivity of the ECMWF results to diurnal sampling of the analyses and to the time resolution employed to calculate the monthly means. These tests reveal instances of extremely low humidities in the ECMWF analyses in the upper troposphere over the tropical east Pacific.Over most of the domain, the simulations are within 5‐10 W m−2of the ERBE data. the clear‐sky OLR can thus be computed from operational analyses with an accuracy comparable with that of ERBE. This indicates the potential of such simulations and has implications for the planning of future radiation‐

ISSN:0035-9009    

DOI:10.1002/qj.49711850806

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractSolar radiative fluxes were computed for heterogeneous clouds using an extension of the Monte Carlo method of photon transport which assumes that clouds possess isotropic variability. Hence, computation of fluxes for three‐dimensional (3‐D) clouds can be achieved with only an extended, characteristic one‐dimensional (1‐D) transect of extinction coefficient, β. These are easily obtained by aircraft measurements. In order to validate the new scheme, fluxes for 3‐D, stochastic multiplicative cascade clouds were computed by the conventional approach to Monte Carlo simulation. 1‐D transects through these clouds were then strung together and used in the new scheme. Not only do both methods of calculation yield statistically identical flux estimates, but they also produce virtually identical distributions of photon optical pathlengths and number of scattering events. Furthermore, the new technique may require an order of magnitude less computation time, depending on the desired level of accuracy.Cloud microphysical data obtained by aircraft were used to represent characteristic transects of β, and corresponding fluxes were computed with the new scheme. Results suggest that internal inhomogeneity reduces cloud albedo below homogeneous values by about 5‐10% for overcast and isolated cubic clouds. Also, it is predicted that for overcast clouds of optical depth τ) ≈ 40, inhomogeneous clouds absorb less (more) radiation relative to their homogeneous counterparts. Three individual and trivial modifications of a plane‐parallel, homogeneous two‐stream solution of the radiative‐transfer equation appear to be capable of capturing the essential effects of inhomogeneity. This is promising for incorporation of inhomogeneous cloud effects into existing climate‐model solar‐radiation routines. Little evidence, however, was found to suggest that the effects of inhomogeneity alone can explain the spectral aspects of the cloud absor

ISSN:0035-9009    

DOI:10.1002/qj.49711850807

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractA new optimal nudging dynamical relaxation technique is tested in the framework of 4‐dimensional variational data assimilation, applied to an adiabatic T40 version of the National Meteorological Center (NMC) spectral model with 18 vertical layers. Several experiments are performed using the NMC operationally analysed data. the variational data assimilation algorithm is also employed in a parameter‐estimation mode to determine the vector of optimal nudging coefficients. Results of data‐assimilation experiments involving estimated nudging, optimal nudging and variational data assimilation are compared. Issues are addressed related to the dependence of the assimilation on the length of the assimilation period as well as to the ability of retrieving high‐quality model initial conditions.The study outlines the ability to obtain optimal nudging coefficients, which can vary in space, in the framework of a parameter‐estimation approach using variational data assimilation. Based on our preliminary results the optimal nudging seems to be a most promising data‐assimila

ISSN:0035-9009    

DOI:10.1002/qj.49711850808

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThe adjoint of a steady‐state linear barotropic model is presented as a tool for diagnosing regional upper‐level vorticity anomalies occurring in general circulation model (GCM) experiments. As a starting point, a simulation of a chosen GCM vorticity anomaly is made using a standard linear barotropic model forced by the corresponding anomalies in the GCM's upper‐level divergence and transient‐forcing fields. the associated adjoint model is then used to interpret this linear simulation over a chosen geographical region, in terms of the linear model's global forcing field. to do this, the adjoint model is forced by a local information source situated over the chosen region. the resulting adjoint solution consists of a global spatial weighting function which can then be used to identify regions of the forcing—local or remote—associated with the regional anomaly. the validity of barotropic linear theory can also be quantified as a by‐product.As an illustration, the model is applied to a large interdecadal anomaly over Europe, arising in a 100‐year doubled CO2integration of the Hamburg coupled ocean‐atmosphere GCM. Linear barotropic theory is found to account for over 60% of the European interdecadal anomaly amplitude in terms of anomalous divergence and the effects of transients. Although a large part of the effects of anomalous divergence are spatially concurrent with the European anomaly and just upstream of it, remote forcing regions over the east Pacific and tropical Atlantic are also implicated. Anomalous transients are found to play a secondary role. the linear results are compared with some one‐point correlation maps of the GCM's inter

ISSN:0035-9009    

DOI:10.1002/qj.49711850809

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThe internally generated variability of the global circulation on time‐scales in excess of one year is investigated using a multilevel baroclinic primitive‐equation model with moderate resolution of dynamical processes. the model includes idealized heating and friction. Maximum variability is found on time‐scales of around a decade. Empirical orthogonal‐function analysis reveals that the principal component of this variability is an alternate splitting and coalescence of the subtropical and mid‐latitude jets. Associated changes in the strength and distribution of eddy activity are found. the character of the ultra‐low‐frequency variability is completely changed for very low resolutions, but seems relatively insensitive to details of model formulation at higher resolution. the possibilities of deriving a low‐order model which includes ultra‐low‐frequency variabi

ISSN:0035-9009    

DOI:10.1002/qj.49711850810

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractDownward longwave radiation at the surface is greater in a cloudy atmosphere than in a clear one. This additional longwave radiative forcing at the surface has an average daily value in Britain of around 40 W m−2, which is small compared to observed fine‐weather shortwave radiative fluxes. However in British rainfall events, the intensity of the incoming shortwave radiation is normally around 100 W m−2or less during the day and zero at night. A numerical experiment using a multilayer model of a coniferous forest is described that suggests that during typical rainfall under British conditions the additional cloud longwave radiative forcing has a significant impact on interception loss from coniferous forest canopies. Interception loss is reduced from about 31% of rainfall, with normal cloud longwave radiative surface forcing, to about 15% with the cloud longwave forcing removed. This result is confirmed using a single‐layer model of a coniferous forest over an 80‐d

ISSN:0035-9009    

DOI:10.1002/qj.49711850811

出版商:John Wiley&Sons, Ltd    

年代:1992

数据来源: WILEY