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1.
Summary Mechanisms associated with Alpine lee cyclogenesis during the early phase of their generation are investigated using a variational quasigeostrophic filter technique. It was possible to extract the quasigeostrophic signal from the available analyzed real data set.The results presented here are for the 11–12 March 1982, an example of so-called orographically induced lee cyclogenesis. Non-quasigeostrophic fields, calculated as a difference between observations and the quasigeostrophic fields, show significant magnitudes indicating the possible importance of non-quasigeostrophic processes. A dipole structure in the residual geopotential field was observed, similar to the results of numerical model experiments. Also, a strong upper-level non-quasigeostrophic divergence was found in the Alpine region 24 hours prior to lee cyclogenesis, lasting for 6–12 hours. On the other hand, quasigeostrophic results indicate only a local effect of mountain slopes, suggesting possibly a dominant role of the low-level blocking. A hypothetical scenario of Alpine lee cyclogenesis is proposed, based on results obtained here.With 14 Figures  相似文献   

2.
ALPEX-Simulation     
Summary In a project ALPEX-Simulation, sponsored by the Österreichischer Fond zur Förderung der wissenschaftlichen Forschung (FWF), all eight cases of ALPEX-SOP cyclones were numerically simulated with a fine mesh isentropic model of the atmosphere. These numerical simulations in six-hourly intervals allow a deeper insight into the synoptics and dynamics of the cyclogeneses in the Western Mediterranean, especially into the genesis of the two basic types of cyclones: the so-called Überströmungs-type and Vorderseiten-type. In the first phase of cyclogenesis of the Überströmungs-type, the blocking and flow splitting of the cold air due to the Alps and the canalization between the Alps and the Massif Central are important. Cold air flows cyclonically around the western part of the Alps, creating a vorticity maximum at the south western edge of the Alpine, bow and leads also to an enhanced PV. In connection with warm air in the Mediterranean, a strong baroclinic zone is generated. The interaction between the arriving PV maximum in the upper troposphere and the enhanced PV at the bottom leads to cyclogenesis in the Western Mediterranean. In the case of the Vorderseiten-type warm air advection dominates with the exception of a shallow layer of cold air in the inner Po-Valley, which is shielded by the Alpine ridge. A well-pronounced PV maximum builds up and couples with the PV maximum arriving at upper levels, even before the cold air, coming from the north-west, has surrounded the Alps. The cold air only intensifies the development by raising the baroclinity. Therefore, the Vorderseiten-cyclogenesis is an orographically modified cyclogenesis, in the course of which the cyclonic development is triggered by the Alps, whereas the Überströmungs-cyclogenesis is an orographically induced cyclogenesis i.e. a true lee cyclogenesis.With 14 FiguresDied in a tragic traffic accident on June 6, 1993.  相似文献   

3.
Summary An analysis of the report of the (U.S.) National Academy of Sciences (NAS) on atmospheric effects of a nuclear exchange leads to conclusions that differ from those of the NAS and of the earlier TTAPS and AMBIO studies. Any cooling of the earth's surface is likely to beshort-lived because of rapid removal of the smoke clouds originating from nuclear burst-initiated fires, andminor because of appreciable green-house effects due to several distinct physical causes. (One of these, neglected in prior analyses, is the infrared absorption from cirrus clouds produced directly by the nuclear bursts.) Taken together, these effects may even induce slight surface warming (nuclear summer) instead of cooling (nuclear winter). The consequences to atmospheric ozone are similarly ambiguous; depending on the detailed nuclear scenario, the net ozone content may increase-rather than decrease as argued by TTAPS. Experiments could settle some uncertainties.With 2 Figures  相似文献   

4.
Summary Several cases of lee cyclogenesis that occurred during ALPEX-SOP have been analyzed, with the aim of separating the large scale structures from the subsynoptic/meso-a scale features that are characteristic of this meteorological phenomenon. The results presented here are mainly based on composites of the analyzed cases. We assume that the deformation caused by the orography can be, at least to a reasonable extent, isolated from the undisturbed state using scale separation. The analysis technique we employ provides the scale separation as built-in in the interpolation algorithm. The scale separation error due to the large inhomogeneities of the data density distribution is partly corrected using a method described in the text. The orographic disturbance appears in different mesoscale fields as a quasiantisymmetric dipolar structure. For example, high/low pressure, cold/warm temperature and anticyclonic/cyclonic couplets characterize the mesoscale fields near the Alps. A qualitative agreement is found with the structure of the orographically induced perturbations predicted by the normal mode theory of lee cyclogenesis.With 7 Figures  相似文献   

5.
Summary During an expedition to the high Andes of Southern Peru in June–July 1977, measurements of direct solar radiation in four spectral bands (0.270–0.530–0.630–0.695–2.900 ) were conducted at six sites in elevations ranging from sea level to 5645 m. These measurements were evaluated in Langley plots to determine total optical depths () and irradiances at the top of the atmosphere. In addition, water vapor optical depths (wv) were calculated from the mean radiosounding over Lima during the expedition, and Rayleigh (ray) and ozone (oz) optical depths were obtained from published tabulations. Subtracting ray, oz, and wv from yielded estimates of aerosol optical depth aer. The components ray and oz decrease from the shorter towards the longer wavelength bands and from the lower towards the higher elevation sites; aer also decreases towards the higher elevations. Particularly pronounced is the decrease of aer and from the lowlands of the Pacific coast to the highlands of the interior, reflecting the effect of a persistent lower-tropospheric inversion and the contrast from the marine boundary layer to the clear atmosphere of the high Andes.With 4 Figures  相似文献   

6.
Summary The performance of evaporation schemes with and approach and their combination within resistance representation of evaporation from bare soil surface is discussed. For this purpose nine schemes, based on different functions of or , on the ratio of the volumetric soil moisture content and its saturated value are used.The quality of the chosen schemes has been evaluated using the results of time integration by the coupled soil moisture and surface temperature prediction model, BARESOIL, using in situ data. A sensitivity analysis was made using two sets of data derived from the volumetric soil moisture content of the top soil layer. One with values below the wilting point (0.17 m3m–3) and the second with values above 0.20m3m–3. Data sets were obtained at the experimental site Rimski anevi, Yugoslavia, from the bare surface of a chernozem soil.With 4 Figures  相似文献   

7.
Six locations across mainland Portugal were selected for exposing Parmelia sulcata, for a one-year period (8 months for one site), with simultaneous measurement of total (dry + wet) deposition (one-month periods). The exposed lichens and the total (dry + wet) deposition were analysed for cobalt contents by INAA (instrumental neutron activation analysis) and ICP-MS (inductively coupled plasma mass spectroscopy), respectively. The designated wet deposition was evaluated through the collected water volume; the designated dry deposition was assessed after the (dried) residual mass of the wet deposition. An excellent agreement between Co contents in exposed lichens and the cumulative (1) Co contents in the dry deposition, (2) dry deposition, and (3) wet deposition has been found for the locations with alternate drought and precipitation months, high dry deposition, and high Co contents in the latter. Continuous rainfall was found to hinder the Co accumulation in the lichen due to its release from the lichen and/or lower Co contents in the dry deposition. At three locations, P. sulcata Co contents, after subtraction of the background (before exposure), equalled or exceeded the Co contents in the cumulative dry deposition at the end of the exposure time. The optimal exposure period for this species likely depends on the exposure conditions.  相似文献   

8.
Wind and temperature profiles in the stable boundary layer were analyzed in the context of MoninObukhov similarity. The measurements were made on a 60-m tower in Kansas during October 1999 (CASES-99). Fluxprofile relationships, obtained from these measurements in their integral forms, were established for wind speed and temperature. Use of the integral forms eliminates the uncertainty and accuracy issues resulting from gradient computations. The corresponding stability functions, which were nearly the same for momentum and virtual sensible heat, were found to exhibit different features under weakly stable conditions compared to those under strongly stable conditions. The gradient stability functions were found to be linear, namely m = 1+ 5.8 and h = 1 + 5.4 up to a limit of the MoninObukhov stability parameter = 0.8; this is consistent with earlier findings. However, for stronger stabilities beyond a transition range, both functions were observed gradually to approach a constant, with a value of approximately 7. To link these two distinct regimes, a general but pliable functional form with only two parameters is proposed for the stability functions, covering the entire stability range from neutral to very stable conditions.  相似文献   

9.
Summary ¶In order to better understand land-atmosphere interactions and increase the predictability of climate models, it is important to investigate the role of forest representation in climate modeling. Corresponding to the big-leaf model commonly employed in land surface schemes to represent the effects of a forest, a so called big-tree model, which uses multi-layer vegetation to represent the vertical canopy heterogeneity, was introduced and incorporated into the National Center for Atmospheric Research (NCAR) regional climate model RegCM2, to make the vegetation model more physically based. Using this augmented RegCM2 and station data for China during 1991 Meiyu season, we performed 10 experiments to investigate the effects of the application of the big-tree model on the summer monsoon climate.With the big-tree model incorporated into the regional climate model, some climate characteristics, e.g. the 3-month-mean surface temperature, circulation, and precipitation, are significantly and systematically changed over the model domain, and the change of the characteristics differs depending on the area. Due to the better representation of the shading effect in the big-tree model, the temperature of the lower layer atmosphere above the plant canopy is increased, which further influences the 850hPa temperature. In addition, there are significant decreases in the mean latent heat fluxes (within 20–30W/m2) in the three areas of the model domain.The application of the big-tree model influences not only the simulated climate of the forested area, but also that of the whole model domain, and its impact is greater on the lower atmosphere than on the upper atmosphere. The simulated rainfall and surface temperature deviate from the originally simulated result and are (or seem to be) closer to the observations, which implies that an appropriate representation of the big-tree model may improve the simulation of the summer monsoon climate.We also find that the simulated climate is sensitive to some big-tree parameter values and schemes, such as the shape, height, zero-plane displacement height and mixing-length scheme. The simulated local/grid differences may be very large although the simulated areal-average differences may be much lower. The area-average differences in the monthly-mean surface temperature and heat fluxes can amount to 0.5°C and 4W/m2, respectively, which correspond to maximum local/grid differences of 3.0°C and 40W/m2 respectively. It seems that the simulated climate is most sensitive to the parameter of the zero-plane displacement among the parameters studied.  相似文献   

10.
A numerical study of stably stratified flow over a three-dimensional hill is presented. Large-eddy simulation is used here to examine in detail the laboratory experimental flows described in the landmark work of Hunt and Snyder about stratified flow over a hill. The flow is linearly stratified and U/Nh is varied from 0.2 to 1.0. Here N and U are the buoyancy frequency and freestream velocity respectively, and h is the height of the hill. The Reynolds number based on the hill height is varied from 365 to 2968. The characteristic flow patterns at various values of U/Nh have been obtained and they are in good agreement with earlier theoretical and experimental results. It is shown that the flow field cannot be predicted by Drazin's theory when recirculation exists at the leeside of the hill even at UNh 1. The wake structure agrees well with a two-dimensional wake assumption when U/Nh 1 but lee waves start to influence the wake structure as U/Nh increases. The dividing-streamline heights obtained in the simulation are in accordance with experimental results and Sheppard's formula. The energy loss along the dividing streamline due to friction/turbulence approximately offsets the energy gained from pressure field. When lee waves are present, linear theory always underestimates the amplitude and overestimates the wavelength of three-dimensional lee waves. The simulated variations of drag coefficients with the parameterK (=ND/ U) are qualitatively consistent with experimental data and linear theory. Here D is the depth of the tank.  相似文献   

11.
Parameterizing turbulent diffusion through the joint probability density   总被引：3，自引：3，他引：0  
The convective mass flux parameterization often used in meteorological modeling expresses the vertical flux of a transported scalar as proportional to the product of the difference in mean values of the scalar in updrafts and downdrafts and their characteristic velocity. The proportionality factor is a constant to be specified. We show that this proportionality factor also appears in the relaxed eddy accumulation technique of Businger and Oncley. That associates the surface-layer flux of a scalar with the product of the standard deviation of vertical velocity and the mean concentration difference between updrafts and downdrafts.We show that this constant (b) is determined uniquely by the joint probability density (jpd) of vertical velocity and the scalar. Using large-eddy simulation, we generate this jpd for a conservative scalar diffusing through a convective boundary layer. It has quite different forms in top-down and bottom-up diffusion geometries. The bottom-up jpd is fairly well represented by a jointly Gaussian form and implies b ~ 0.6, in good agreement with the surface-layer value reported by Businger and Oncley. The top-down jpd is strikingly non-Gaussian and gives b ~ 0.47. Updrafts carry the bulk of the scalar flux - 70% in the bottom-up case, 60% in the top-down case.The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

12.
Surface-Layer Fluxes in Stable Conditions   总被引：2，自引：2，他引：0  
Micrometeorological tower data from the Microfronts experiment are analyzed. Scale-dependencies of the flux and flux sampling error are combined to automatically determine Reynolds turbulence cut-off time scales for computing fluxes from time series. The computed downward heat flux at the 3 m height averaged over nine nights with 7.3 hours each night is 20% greater than the downward heat flux computed at the 10 m height. In contrast, there is only a 1.2% difference between 3 m and 10 m heat fluxes averaged over daytime periods, and there is less than a 2% difference between 3 m and 10 m momentum fluxes whether averaged over nighttime or daytime periods.Stability functions, M(z/L) and H(z/L) are extended to z/L up to 10, where z is the observational height and L is the Obukhov length. For 0.01 < z/L < 1 the estimated functions generally agree with Businger-Dyer formulations, though the H estimates include more scatter compared to the M estimates. For 1 < z/L < 10, the flux intermittency increases, the flux Richardson number exceeds 0.2, and the number of flux samples decreases. Nonetheless the estimates of the stability function M based on 3-m fluxes are closer to the formula proposed by Beljaars and Holtslag in 1991 while the M functions based on 10-m fluxes appears to be closer to the formula proposed by Businger et al. in 1971. The stability function H levels off at z/L = 0.5.  相似文献   

13.
This study details the observed effects ofatmospheric stability on characteristics of thesurface layer in a low wind speed (U = 1.5 m s-1)regime of tropical West Africa. Theaerodynamic roughness length, z0, anddisplacement height, d, obtained from profilewind-speed data at our bush land site (height 2 m)have values of 0.24 ± 0.10 m and 1.54 ± 0.04 mrespectively. In the unstable range (-2.5 < Ri < -0.1; Riis gradient Richardson number), thestandard deviation in wind speed fluctuations, u, increased from 0.57 ± 0.19 m s-1 toa maximum of 0.7 ± 0.2 m s-1 in near-neutralconditions, and in the stable range, the parameterdecreased rapidly to 0.41 ± 0.15 m s-1 at Ri 0.2.In the same stability range, the horizontal winddispersion, , decreased withincreasing stability from 19 ± 8 deg. to 13 ± 5 deg.The surface-layer integral quantity, u/u*, when plottedas a function of stability, is in agreement with theempirical results. The ratio ofsensible heat flux (estimated) to the net radiationranged between 0.1 and 0.2 at nighttime,increasing to about 0.5 during the daytime, and showeda strong dependency on season.  相似文献   

14.
Periodicity of annual precipitation in different climate regions of Croatia   总被引：1，自引：1，他引：0  
Summary The periodicity of a 100-year series of annual precipitation over Croatia has been studied by means of power spectrum analysis at 3 stations representing the different climatic regions of Croatia. The annual precipitation variance spectra in the continental lowland (Osijek) and at the north East Adriatic coast (Crikvenica) can be fitted by Markov white noise continuum, but in the transitional region between the Dinaric Alps and the Pannonian lowland (Zagreb-Gri) a non-white noise continuum is necessary. Quasi-periodic oscillations appear in two spectra ranges: short (2.2 and 4.7 years) and medium (25.0 and 33.3 years). These results are compared with those of other authors for other parts of the Europe.With 2 Figures  相似文献   

15.
Flux densities of carbon dioxide were measured over an arid, vegetation-free surface by eddy covariance techniques and by a heat budget-profile method, in which CO2 concentration gradients were specified in terms of mixing ratios. This method showed negligible fluxes of CO2, consistent with the bareness of the experimental site, whereas the eddy covariance measurements indicated large downward fluxes of CO2. These apparently conflicting observations are in quantitative agreement with the results of a recent theory which predicts that whenever there are vertical fluxes of sensible or latent heat, a mean vertical velocity is developed. This velocity causes a mean vertical convective mass flux (= cw for CO2, in standard notation). The eddy covariance technique neglects this mean convective flux and measures only the turbulent flux c w. Thus, when the net flux of CO2 is zero, the eddy covariance method indicates an apparent flux which is equal and opposite to the mean convective flux, i.e., c w = – c w. Corrections for the mean convective flux are particularly significant for CO2 because cw and c w are often of similar magnitude. The correct measurement of the net CO2 flux by eddy covariance techniques requires that the fluxes of sensible and latent heat be measured as well.  相似文献   

16.
Turbulence characteristics in a near neutrally stratified urban atmosphere   总被引：4，自引：1，他引：4  
Turbulence measurements from the city of Uppsala, Sweden, are analysed. Measurements were taken at two sites: one in the central area, ca. 6 m above roof level, the average building height being ca. 15 m; the other at ca. 8 and 50 m above the ground on a tower situated 100 m downwind of a sharp discontinuity between the densely built-up urban area and flat grass-covered land. The average stability was close to neutral, the range being -0.2 < z/L < 0.2. The main emphasis of the study is on the non-dimensional standard deviations of the velocity components i /u *t and on the corresponding non-dimensional energy spectra, u *t being a local velocity scale defined as i /( l is the local momentum flux). Comparison with results obtained from surface-layer measurements at ideal sites (with u *, being the ordinary friction velocity) shows good general agreement. The most complete agreement is found for the tower 50 m measurements, a result which is notable as this measurement point is found to be within a distinctly transitional zone between the urban and post-urban boundary layers. The results from the central city measurement point are also fairly close to the ideal results, the deviations found being small in view of the fact that the site is probably inside the layer in which the roughness elements (the buildings) have direct influence. The measurements at the tower 8 m level show certain distinct deviations from ideal results: all three i /u *l , are higher by ca. 10%, the excessive energy being found at the low frequency end of the spectrum. Arguments are presented for this feature to be due to a spectral lag effect.  相似文献   

17.
This paper describes the similarity between atmospheric fluctuations of carbon dioxide, water vapor and temperature using data which cover a wide range of instability (0.02 < < 10). The is the Monin-Obukhov stability parameter including the humidity effect.The spectral analysis shows that the coherency between fluctuations of carbon dioxide and water vapor or temperature is very close to unity, and the phase difference is basically out of phase for whole frequency ranges analyzed. The stability dependence of the normalized standard deviation of carbon dioxide is very similar to those of water vapor and temperature. The normalized standard deviation is about 2.5 under near neutral conditions, and it decreases with increasing instability following the -1/3; power law as (-)-1/3. The skewness factors of carbon dioxide, water vapor and temperature show a systematic departure with increasing instabilities for 0.02 < s- < 1, and level off at high instabilities for 1 < -\s < 10. The stability dependence of the flatness factors is not so clear as that noted in the standrard deviation and skewness factors. Dissipation rates of carbon dioxide, water vapor and temperature variance are well related to the spectral peak wavelength. This seems to be real since the local production and local dissipation rates are the main terms, almost balancing one another in the variance budget equations for scalar entities.  相似文献   

18.
Plume dispersion in the convective boundary layer (CBL) is investigated experimentally in a laboratory convection tank. The focusis on highly-buoyant plumes that loft near or become trapped in the CBL capping inversion and resistdownward mixing. Such plumes are defined by dimensionless buoyancy fluxes F* 0.1, where F* = Fb/(U w* 2 zi), Fb is the stack buoyancy flux,U is the mean wind speed, w* is the convective velocity scale, and zi is the CBL depth. The aim is to obtain statistically-reliable mean (C) and root-mean-square (rms, c) concentration fields as a function of F* and the dimensionless distance X = w*x/(U zi), where x is the distance downstream of the source.The experiments reveal the following mainresults: (1) For 3 X 4and F* 0.1, the crosswind-integrated concentration (CWIC) fields exhibit distinctly uniform profiles below zi with a CWIC maximum aloft, in contrast to the nonuniform profiles obtained earlier by Willis and Deardorff. (2) The lateral dispersion (y) variation with X is consistent with Taylor's theory for * 0.1 and a buoyancy-enhanced dispersion, y/zi F* 1/3X2/3, forF* = 0.2 and 0.4. (3) The entrapment, the plume fraction above zi, has a mean (E) that follows a systematic variationwith X and F*, and a variability (e/E) that is broad ( 0.3 to 2) near the source but subsides to 0.25 far downstream. (4) Vertical profiles of the concentration fluctuation intensity (c/C) are uniform for z < zi and X > 1.5, but exhibit significant increases: (a) at the surface and close to the source (X 1.5), and(b) in the entrainment zone. (5) The cumulative distribution functions (CDFs) of the scaled concentration fluctuations (c/c) separate into mixed-layer and entrainment-layer CDFs for X 2, with the mixed-layer group collapsing to a single distribution independent of z.These are the first experiments to obtain all components of the lateral and vertical dispersion parameters (rms meander, relative dispersion, total dispersion) for continuous buoyant releases in a convection tank. They also are the first tank experiments to demonstrate agreement with field observations of: (1) the scaled ground-level concentration along the plume centreline, and (2) the dimensionless lateral dispersion _y/z_i of buoyant plumes.  相似文献   

19.
Equilibrium evaporation beneath a growing convective boundary layer   总被引：1，自引：1，他引：0  
Expressions for the equilibrium surface Bowen ratio ( s ) and equilibrium evaporation are derived for a growing convective boundary layer (CBL) in terms of the Bowen ratio at the top of the mixed layer i and the entrainment parameter A R . If AR is put equal to zero, the solution for s becomes-that previously obtained for the zero entrainment or closed box model. The Priestley-Taylor parameter is also calculated and plotted in terms ofA R and i . Realistic combinations of the atmospheric parameters give values of in the range 1.1 to 1.4.  相似文献   

20.
The reactions of alkoxy radicals determine to a large extent the products formed during the atmospheric degradations of emitted organic compounds. Experimental data concerning the decompositions, 1,5-H shift isomerizations and reactions with O2 of several classes of alkoxy radicals are inconsistent with literature estimations of their absolute or relative rate constants. An alternative, although empirical, method for assessing the relative importance under atmospheric conditions of the reactions of alkoxy radicals with O2 versus decomposition was derived. This estimation method utilizes the differences in the heats of reaction, (H)=(Hdecomposition–HO 2 reaction), between these two reactions pathways. For (H)[22–0.5(HO 2 reaction)], alkoxy radical decomposition dominates over the reaction with O2 at room temperature and atmospheric pressure of air, while for (H)[25-0.5(HO 2 reaction)], the O2 reaction dominates over decomposition (where the units of H are in kcal mol–1). The utility and shortcomings of this approach are discussed. It is concluded that further studies concerning the reactions of alkoxy radicals are needed.  相似文献   

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