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1.
We have studied the response of the Atlantic meridional overturning circulation to surface freshwater forcing using an ocean GCM coupled to an energy-moisture-balance atmosphere model. The overturning collapses rapidly when a slowly increasing forcing applied to the North Atlantic passes a positive threshold, and spins up equally quickly when the forcing falls below a negative threshold. This well-known behaviour is referred to as hysteresis because the thresholds in forcing are different for the transitions in opposite directions. However, we argue that the behaviour of the Atlantic salinity is more fundamental than the forcing. Hysteresis as a function of freshwater forcing occurs because the states with North Atlantic overturning on and off each tend to reinforce their associated salinity distributions and inhibit the transition to the other state. During the collapse, the Atlantic becomes less saline because of the import of 80 Sv year of freshwater by ocean transports across 30°S; during the spin-up this freshwater is exported again. We show that qualitatively similar hysteresis behaviour can be produced by perturbing the system without any net freshwater forcing. The salinity flip-flop is associated with the appearance and disappearance of a shallow reverse overturning circulation south of the Equator, which is present while the northern overturning is absent, and may provide the mechanism for the ocean freshwater influx during collapse.  相似文献   

2.
The effect of changes in zonal and meridional atmospheric moisture transports on Atlantic overturning is investigated. Zonal transports are considered in terms of net moisture export from the Atlantic sector. Meridional transports are related to the vigour of the global hydrological cycle. The equilibrium thermohaline circulation (THC) simulated with an efficient climate model is strongly dependent on two key parameters that control these transports: an anomaly in the specified Atlantic–Pacific moisture flux (Fa) and atmospheric moisture diffusivity (Kq). In a large ensemble of spinup experiments, the values of Fa and Kq are varied by small increments across wide ranges, to identify sharp transitions of equilibrium THC strength in a 2-parameter space (between Conveyor On and Off states). Final states from this ensemble of simulations are then used as the initial states for further such ensembles. Large differences in THC strength between ensembles, for identical combinations of Fa and Kq, reveal the co-existence of two stable THC states (Conveyor On and Off)—i.e. a bistable regime. In further sensitivity experiments, the model is forced with small, temporary freshwater perturbations to the mid-latitude North Atlantic, to establish the minimum perturbation necessary for irreversible THC collapse in this bistable regime. A threshold is identified in terms of the forcing duration required. The model THC, in a Conveyor On state, irreversibly collapses to a Conveyor Off state under additional freshwater forcing of just 0.1 Sv applied for around 100 years. The irreversible collapse is primarily due to a positive feedback associated with suppressed convection and reduced surface heat loss in the sinking region. Increased atmosphere-to-ocean freshwater flux, under a collapsed Conveyor, plays a secondary role.  相似文献   

3.
General circulation model experiments with surface albedo changes   总被引：1，自引：0，他引：1  
K. Laval 《Climatic change》1986,9(1-2):91-102
In 1975, Charney proposed a biogeophysical feedback mechanism to partly explain the droughts that occur in desert border areas. He showed that a perturbation of albedo (due to a natural or anthropogenic decrease of vegetation) can be unstable and lead to a variation of precipitation in the region where albedo is changed.Several numerical experiments have been achieved with general circulation models to study the sensitivity of climate to surface albedo. We compare the GLAS and LMD model results for the Sahel. For all models, rainfall decreases when albedo increases and net radiative heating of soil decreases. We show the variations of circulation simulated by the LMD model that we obtain when albedo is increased. These changes are compared to the weakening of Easterly Jet at 200 mb observed during dry years.  相似文献   

4.
A numerical case study with a second-order turbulence closure model is proposed to study the role of urban canopy layer (UCL) for the formation of the nocturnal urban boundary layer (UBL). The turbulent diffusion coefficient was determined from an algebraic stress model. The concept of urban building surface area density is proposed to represent the UCL. Calculated results were also compared with field observation data. The height of the elevated inversion above an urban center was simulated and found to be approximately twice the average building height. The turbulent kinetic energy k, energy dissipation rate , and turbulence intensities u 2 and w 2 increase rapidly at the upwind edge of the urban area. The Reynolds stress uw displayed a nearly uniform profile inside the UBL, and the vertical sensible heat flux w had a negative value at the inversion base height. This indicates that the downward transport of sensible heat from the inversion base may play an important role in the formation of the nocturnal UBL.  相似文献   

5.
Studies have suggested that sea-ice cover east and west of Greenland fluctuates out-of phase as a part of the Atlantic decadal climate variability, and greater changes are possible under global warming conditions. In this study, the response of the Atlantic meridional overturning circulation (MOC) to the distribution of surface fresh-water flux is explored using a global isopycnal ocean model. An Arctic ice related fresh-water flux of 0.1 Sv entering the Nordic Seas is shown to reduce the maximum overturning by 1 to 2 Sv (106 m3 s–1). A further decrease of 3 to 5 Sv in the MOC is observed when the fresh-water flux is shifted from the Fram Strait to the southern Baffin Bay area. Surprisingly, the salinity in much of the upper Nordic Seas actually increases when the Arctic fresh-water source is the strongest there, as a result of enhanced global overturning. It reflects the great influence of Labrador Sea convection on this models MOC. By applying a weaker surface fresh-water transport perturbation (0.02 Sv) on the Baffin Bay area and therefore perturbing the Labrador Sea Water (LSW) formation, we have also investigated the interaction between the overflows across the Greenland–Scotland Ridge and the LSW and find that, with the same surface forcing conditions in the Nordic Seas, volume transport of the overflows weakens when the LSW formation intensifies.  相似文献   

6.
A simple nonlinear three-box ocean model of the North Atlantic Ocean including the rudiments of eddy mixing, vertical stratification and thermohaline circulation is first presented. It is subject to uniform latitudinal differential heating, q, and net evaporation m e , and includes a linear equation of state. Two quite different limiting steady-state solutions exist. The first has a warm saline surface water and a cold, low-salinity deep ocean; deep water is primarily formed in higher latitudes by the prevalence of differential heating. A second limiting solution consists of a warm saline deep ocean underlying a cool, low-salinity surface ocean; deep water is formed primarily in lower latitudes as a consequence of large differential evaporation. A coupled ocean-atmosphere model, in which the oceanic surface heat fluxes are determined internally but with differential evaporation at the ocean surface m e remaining an external parameter, is next presented. The atmosphere component is a simple energy balance model that emphasizes the vertical fluxes of radiative, sensible and latent heat fluxes but does not include temperature-albedo feedback. Model response depends on the external parameters m e and , controlling the magnitude of the thermohaline-driven circulation, and on the magnitudes of the eddy mixing coefficients and the solar constant. For small m e , a steady-state solution corresponding to a cold fresh deep ocean is found, qualitatively similar to the modern ocean. For large m e , a steady-state solution with a warm saline deep ocean occurs; this solution resembles conceptual models that have been proposed for the warm saline Cretaceous ocean. There exists an intermediate region of values of m e for which the solutions are more complex. On the lower end of this region, both the cold fresh deep-ocean and warm saline deep-ocean circulations coexist as stable equilibria. On the upper end, the cold-deep ocean becomes unstable, manifesting oscillations with growing amplitude, and ultimately reaches the warm saline deep-ocean solution. In the neighborhood of a cusp on the , m e plane, that is, for relatively small , more complex behaviour occurs, which has not yet been fully analyzed. The model response in the region of complexity is not sensitive to changes in the solar constant but is sensitive to the eddy mixing coefficients.  相似文献   

7.
An ocean general circulation model of global domain, full continental geometry and bottom topography, is used to study the influence of the Bering Strait on the general circulation by comparing equilibrium solutions obtained with and without a land-bridge between Siberia and Alaska. The model is integrated with restoring boundary conditions (BC) on temperature and salinity, and later, with mixed BC in which a restoring BC on temperature is maintained but a specified flux condition on salinity is imposed. In both cases, the effect of the Bering Strait is to allow a flow of about 1.25–1.5 Sv from the North Pacific to the Arctic Ocean and, ultimately, back to the North Pacific along the western boundary current regions of the Atlantic and Indian Oceans. When a restoring BC on salinity is used, the overturning associated with North Atlantic Deep Water and Antarctic Intermediate Water formation are increased if the Bering Strait is present in the model geometry. The result of switching to a specified flux BC on salinity is to cause a transition in the THC in which the overturning associated with North Atlantic Deep Water formation increases from about 12 Sv to about 22 Sv. This transition occurs in an essentially smooth fashion with no significant variability and is about 12% smaller in magnitude if the Bering Strait is present in the model geometry. Because the Bering Strait appears to exert some influence on the general circulation and the formation of deep water masses, it is recommended that this Strait be included in the geometry of similar resolution models designed to study the deep ocean and potential changes in climate. Correspondence to: CJC Reason  相似文献   

8.
A review of flux-profile relationships   总被引：33，自引：5，他引：33  
Flux-profile relationships in the constant flux layer are reviewed. The preferred relationships are found to be those of Dyer and Hicks (1970), namely, H = W =(1–16(z/L))–1/2, M =(1–16(z/L))–1/4 for the unstable region, and H = W = M = 1+5(z/L) for the stable region.The carefully determined results of Businger et al. (1971) remain a difficulty which calls for considerable clarification.  相似文献   

9.
The rainwater composition in the vicinity of Mainz, FRG, has been investigated with special emphasis on insoluble constituents. The number size distribution was determined in the range from 0.1 m up to 100 m radius. For particles with r>0.5 m radius the shape of the size distribution of insoluble particles in rain follows the shape of the average urban and rural aerosol. In this particular size range no major size selective removal processes could be seen. For r<0.5 m the number size distribution tends to flatten compared to the average aerosol. This might be the indication of a size selective removal process (Greenfield Gap).  相似文献   

10.
The results from an integration of a global ocean circulation model have been condensed into an analysis of the volume, heat, and salt transports among the major ocean basins. Transports are also broken down between the model's Ekman, thermocline, and deep layers. Overall, the model does well. Horizontal exchanges of mass, heat, and salt between ocean basins have reasonable values; and the volume of North Atlantic Deep Water (NADW) transport is in general agreement with what limited observations exist. On a global basis the zonally integrated meridional heat transport is poleward at all latitudes except for the latitude band 30°S to 45°S. This anomalous transport is most likely a signature of the model's inability to form Antarctic Intermediate (AAIW) and Antarctic bottom water (AABW) properly. Eddy heat transport is strong at the equator where its convergence heats the equatorial Pacific about twice as much as it heats the equatorial Atlantic. The greater heating in the Pacific suggests that mesoscale eddies may be a vital mechanism for warming and maintaining an upwelling portion of the global conveyor-belt circulation. The model's fresh water transport compares well with observations. However, in the Atlantic there is an excessive southward transport of fresh water due to the absence of the Mediterranean outflow and weak northward flow of AAIW. Eddies in the mid-latitudes act to redistribute heat and salt down the mean gradients. Residual fluxes calculated from a sum of the computed advective (including eddies), forced, and stored fluxes of heat and salt represent transport mostly due to vertical sub-grid scale mixing processes. Perhaps the model's greatest weakness is the lack of strong AAIW and AABW circulation cells. Accurate thermohaline forcing in the North Atlantic (based on numerous hydrographic observations) helps the model adequately produce NADW. In contrast, the southern ocean is an area of sparse observation. Better thermohaline observations in this area may be needed if models such as this are to produce the deep convection that will achieve more accurate simulations of the global 3-dimensional circulation.  相似文献   

11.
Summary The influence of agricultural management on the CO2 budget of a typical subalpine grassland was investigated at the Swiss CARBOMONT site at Rigi-Seebodenalp (1025m a.s.l.) in Central Switzerland. Eddy covariance flux measurements obtained during the first growing season from the mid of spring until the first snow fall (17 Mai to 25 September 2002) are reported. With respect to the 10-year average 1992–2001, we found that this growing season had started 10 days earlier than normal, but was close to average temperature with above-normal precipitation (100–255% depending on month). Using a footprint model we found that a simple approach using wind direction sectors was adequate to classify our CO2 fluxes as being controlled by either meadow or pasture. Two significantly different light response curves could be determined: one for periods with external interventions (grass cutting, cattle grazing) and the other for periods without external interventions. Other than this, meadow and pasture were similar, with a net carbon gain of –128±17g Cm–2 on the undisturbed meadow, and a net carbon loss of 79±17g Cm–2 on the managed meadow, and 270±24g Cm–2 on the pasture during 131 days of the growing season, respectively. The grass cut in June reduced the gross CO2 uptake of the meadow by 50±2% until regrowth of the vegetation. Cattle grazing reduced gross uptake over the whole vegetation period (37±2%), but left respiration at a similar level as observed in the meadow.  相似文献   

12.
E- turbulence model predictions of the neutralatmospheric boundary layer (NABL) are reinvestigated to determine thecause for turbulence overpredictions found in previous applications. Analytical solutions to the coupled E and equations for the case of steady balance between transport and dissipation terms, the dominant balance just below the NABL top, are derived. It is found that analytical turbulence profiles laminarizeat a finite height only for values of closure parameter ratio c 2 /e equal toor slightly greater than one, with laminarization as z for greater . The point = 2 is additionally foundthat where analytical turbulent length scale (l) profilesmade a transition from ones ofdecreasing ( < 2) to increasing ( > 2)values with height. Numerically predicted profiles near the NABL topare consistent with analytical findings. The height-increasingvalues of l predicted throughout the NABL with standard values ofclosure parameters thus appear a consequence of 2.5(> 2), implied by these values (c 2 = 1.92, = 1.3, e = 1). Comparison of numericalpredictions with DNS data shows that turbulence overpredictions obtained with standard-valued parameters are rectifiedby resetting and e to 1.1 and 1.6, respectively, giving, with c 2 = 1.92, 1.3, and laminarization of the NABL's cappingtransport-dissipation region at a finite height.  相似文献   

13.
 This work concerns an analysis of inter-basin and inter-layer exchanges in the component ocean part of the coupled ECHAM4/OPYC3 general circulation model, aimed at documenting the simulation of North Atlantic Deep Water (NADW) and related thermohaline circulations in the Indian and Pacific Oceans. The modeled NADW is formed mainly in the Greenland– Iceland–Norwegian Seas through a composite effect of deep convection and downward cross-isopycnal transport. The modeled deep-layer outflow of NADW can reach 16 Sv near 30 °S in the South Atlantic, with the corresponding upper-layer return flow mainly coming from the “cold water path” through Drake Passage. Less than 4 Sv of the Agulhas “leakage” water contributes to the replacement of NADW along the “warm water path”. In the South Atlantic Ocean, the model shows that some intermediate isopycnal layers with potential densities ranging between 27.0 and 27.5 are the major water source that compensate the NADW return flow and enhance the Circumpolar Deep Water (CDW) flowing from the Atlantic into Indian Ocean. The modeled thermohaline circulations in the Indian and Pacific Oceans indicate that the Indian Ocean may play the major role in converting deep water into intermediate water. About 16 Sv of the CDW-originating deep water enters the Indian Ocean northward of 31 °S, of which more than 13 Sv “upwell” mainly near the continental boundaries of Africa, South Asia and Australia through inter-layer exchanges and return to the Antarctic Circumpolar Current (ACC) as intermediate-layer water. As a contrast, only 4 Sv of Pacific intermediate water is connected to “upwelling” flow southward across 31 °S while the magnitude of northward deep flow across 31 °S in the Pacific Ocean is significantly greater than that in the Indian Ocean. The model suggests that a large portion of the deep waters entering the Pacific Ocean (about 14 Sv) “upwells” continually into some upper layers through the thermocline, and becomes the source of the Indonesian throughflow. Uncertainties in these results may be related to the incomplete adjustment of the model’s isopycnal layers and the sensitivity of the Indonesian throughflow to the model’s geography and topography. Received: 12 August 1997/Accepted: 12 March 1998  相似文献   

14.
A link between the Antarctic sea-ice extent and low-frequency atmospheric variations, particularly ENSO, has been suggested by recent modeling and empirical studies. This question is examined here using a high-resolution (by week, by region) data base of Antarctic sea-ice extent for the 1973–1982 period. Although of relatively short duration by Northern Hemisphere standards, such a data base offers an opportunity rare in Southern Hemisphere climate studies. The seaice variations are examined in the context of longer-term indices of the large-scale atmospheric circulation. These are a Southern Oscillation Index (SOI) and an index of sea-level pressure (SLP) wavenumber one in the Southern Hemisphere extratropics. The indices are updated through 1982, and their associations with regional-scale pressure indices in the Australia-New Zealand sector are also examined. The 1973–1982 period is anomalous when compared with the period 1951–1972. Correlation analysis of the monthly sea ice and circulation index values reveals that much of the apparent link between the ice and the SOI suggested in previous studies arises from autocorrelations present in both data sets and the strong annual cycle of sea-ice extent. Removing these effects from the data and re-running the correlations reveals that most of the resulting significant associations between the ice and one or other of the circulation indices can probably be explained on the basis of chance. In order to reconcile these findings with previous studies that show some strong ice-circulation interactions on regional scales, only those months in which significant correlations occur between both largescale circulation indices and the sea ice are examined further. These occur preferentially in the Ross and Weddell sectors, which constitute the regions contributing most to the variability of Antarctic sea ice. The analysis suggests that the sea-ice-extent changes lag the SOI by several months but may precede changes in extratropical SLP wavenumber one. Confirmation of these tentative regional ice extent-circulation teleconnections necessarily awaits the forward extension of the high-resolution sea-ice data base beyond the 10 years available here.This paper is based on material presented at the Conference on Mechanisms of Interannual and Longer-Term Climatic Variations held at the University of Melbourne, Australia: December 8–12, 1986.  相似文献   

15.
Analyses indicate that the Atlantic Ocean seasurface temperature (SST) was considerably colder at the beginning than in the middle of the century. In parallel, a systematic change in the North Atlantic sea-level pressure (SLP) pattern was observed. To find out whether the SST and SLP changes analyzed are consistent, which would indicate that the SST change was real and not an instrumental artifact, a response experiment with a low-resolution (T21) atmospheric GCM was performed. Two perpetual January simulations were conducted, which differ solely in the Atlantic Ocean (40° S-60° N) SST: the cold simulation utilizes the SSTs for the period 1904–1913; the warm simulation uses the SSTs for the period 1951–1960. Also, a control run with the model's standard SST somewhat between the cold and warm SST was made. For the response analysis, a rigorous statistical approach was taken. First, the null hypothesis of identical horizontal distributions was subjected to a multivariate significance test. Second, the level of recurrence was estimated. The multivariate statistical approaches are based on hierarchies of test models. We examined three different hierarchies: a scale-dependent hierarchy based on spherical harmonics (S), and two physically motivated ones, one based on the barotropic normal modes of the mean 300 hPa flow (B) and one based on the eigenmodes of the advection diffusion operator at 1000 hPa (A). The intercomparison of the cold and warm experiments indicates a signal in the geostrophic stream function that in the S-hierarchy is significantly nonzero and highly recurrent. In the A-hierarchy, the low level temperature field is identified as being significantly and recurrently affected by the altered SST distribution. The SLP signal is reasonably similar to the SLP change observed. Unexpectedly, the upper level stream-function signal does not appear to be significantly nonzero in the B-hierarchy. If, however, the pairs of experiments warm versus control and cold versus control are examined in the B-hierarchy, a highly significant and recurrent signal emerges. We conclude that the cold versus warm response is not a small disturbance that would allow the signal to be described by eigenmodes of the linear system. An analysis of the three-dimensional structure of the signal leads to the hypothesis that two different mechanisms are acting to modify the model's mean state. At low levels, local heating and advection are dominant, but at upper levels the extratropical signal is a remote responce to modifications of the tropical convection.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dilmenil.AWI Publication no. 254  相似文献   

16.
Effect of finite sampling on atmospheric spectra   总被引：2，自引：0，他引：2  
The effect of a finite averaging time on variances is well known, but its effect on power spectra is less clearly understood. We present numerical solutions for the spectral distortion arising from sampling over a finite time interval T and show that the commonly used filter function (1 – sinc2f T), valid for variances, is a reasonable approximation for power spectra only when T 10 m , where f is the cyclic frequency, and m is the dominant time scale of the process. Our results exhibit an increasingly steeper low-frequency roll-off as T decreases relative to m , indicating that the measured spectrum is subject to a greater suppression of the lower frequencies (f > 1/T) than predicted by (1 – sinc2f T). This suppression is, in a sense, compensated by an overestimation of spectral estimates in the frequency range f 1/T.  相似文献   

17.
The relation between the turbulence Reynolds numberR and a Reynolds numberz* based on the friction velocity and height from the ground is established using direct measurements of the r.m.s. longitudinal velocity and turbulent energy dissipation in the atmospheric surface layer. Measurements of the relative magnitude of components of the turbulent kinetic energy budget in the stability range 0 >z/L 0.4 indicate that local balance between production and dissipation is maintained. Approximate expressions, in terms of readily measured micrometeorological quantities, are proposed for the Taylor microscale and the Kolmogorov length scale .  相似文献   

18.
Summary A zonally averaged global energy balance model with feedback mechanisms was constructed to simulate (i) the poleward limits of ITCZ over the continent and over the ocean and (ii) a simple monsoon system as a result of differential heating between the continent and the ocean. Three numerical experiments were performed with lower boundary as (1) global continent, (2) global ocean and (3) continent-ocean, with freezing latitudes near the poles. Over the continent, midlatitude deserts were found and the ITCZ migrates 25° north and south with seasons. Over a global swamp ocean results do not show migration of ITCZ with time but once the ocean currents are introduced the ITCZ migrates 5° north and south with seasons. It was found that the seasonal migration of ITCZ strongly depends on the meridional distribution of the surface temperature. It was also found that continent influences the location of the oceanic ITCZ. In the tropics northward progression of quasi-periodic oscillations called events are found during the pre- and post-monsoon periods with a period of 8 to 15 days. This result is consistent with the observed quasi-periodic oscillations in the tropical region. Northward propagation of the surface temperature perturbation appears to cause changes in the sensible heat flux which in turn causes perturbations in vertical velocity and latent heat flux fields.List of Symbols vertical average - 0 zonal average - vertical mean of the zonal average - 0s zonal average at the surface - 0a zonal average at 500 mb level - latitude We now define the various symbols used in the model rate of atmospheric heating due to convective cloud formation (K/sec) - dp/dt (N/m2/sec) - density - potential temperature (K) - rate of rotation of the earth (rad/sec) - empirical constant - humidity mixing ratio - * saturated humidity mixing ratio - opacity of the atmosphere - 1,2 factors for downward and upward effective black body long wave radiation from the atmosphere - Stefan-Boltzmann constant - emissivity of the surface - D subsurface temperature (K) - a specific volume - 0xs ,0ys eastward and northward components of surface frictional stress - * vertical velocity at the top of the boundary layer (N/m2/sec) - P Thickness of the boundary layer (mb) - nondimensional function of pressure - P pressure - P a pressure of the model atmosphere (N/m2) - P s pressure at the surface (N/m2) - t time (sec) - U eastward wind speed (m/sec) - V northward wind speed (m/sec) - surface water availability - T absolute temperature (K) - heat addition due to water phase changes - g acceleration due to gravity (m2/sec) - a radius of the earth (m) - R gas constant for dry air (J/Kg/K) - C p specific heat of air at constant pressure (J/Kg/K) - k R/C p - L latent heat of condensation (J/Kg) - f coriolis parameter (rad/sec) - H s H 0s (1) +H 0s (2) +H 0s (3) +H 0s (4) +H 0s (5) (J/m2/Sec)=sum of the rates of vertical heat fluxes per unit surface area, directed toward the surface - H a H 0a (1) +H 0a (2) +H 0a (3) +H 0a (4) (J/m2/Sec)=sum of the rates of heat additions to the atmospheric column per unit horizontal area by all processes - H 0s (1) ,H 0a (1) heat flux due to short wave radiation - H 0s (2) ,H 0a (2) heat flux due to long wave radiation - H 0s (3) ,H 0a (3) heat flux due to small scale convection - H 0s (4) heat flux due to evaporation - H 0a (4) heat flux due to condensation - H 0s (5) heat flux due to subsurface conduction and convection - e * saturation vapor pressure - R solar constant (W/m2) - r a albedo of the atmosphere - r s albedo of the surface - b 2 empirical constant (J/m2/sec) - c 2 empirical constant (J/m2/sec) - e 2 nondimensional empirical constant - f 2 empirical constant (J/m2/sec) - factor proportional to the conductive capacity of the surface medium - a s constant used in Sellers model - b s positive constant of proportionality used in the Sellers model (kg m2/J/sec2) - K HT coefficient for eddy diffusivity of heat (m2/sec) - K HE exchange coefficient for water vapor (m2/sec) - h depth of the water column (m) - z height (m) - V 0ws meridional component of surface current (m/sec) - n cloud amount - G 0,n long wave radiation form the atmosphere for cloud amount n (W/m2) - B 0 long wave radiation from the surface (W/m2) - S 0,n short wave radiation from the atmosphere for cloud amount n (W/m2) - A n albedo factor for a cloud amount n - R f1 large scale rainfall (mm/day) - R f2 small scale rainfall (mm/day) With 22 Figures  相似文献   

19.
Summary The effect of the Alpine orography on prototype cold fronts approaching from the west is investigated by three-dimensional numerical model simulations. The numerical experiments cover a range of parameter constellations which govern the prefrontal environment of the front. Especially, the appearance and intensity of prefrontal northern Alpine foehn varies from case to case.The behaviour of a cold front north of the Alps depends much on the prefrontal condition it encounters. It is found that prefrontal foehn can either accelerate or retard the approaching front.An important feature is the pressure depression along the northern Alpine rim that results from the southerly foehn flow. In cases where this depression compensates the eastward directed pressure gradient associated with the largescale flow, the front tends to accelerate and the foehn breaks down as soon as the front passes. In contrast, the foehn prevents the front from a rapid eastward propagation if it is connected with a strong southerly wind component.No-foehn experiments are performed for comparison, where either the mountains are removed, or the static stability is set to neutral. Also shown are effects of different crossfrontal temperature contrasts.List of Symbols c F propagation speed of a front - x, y horizontal grid spacing (cartesian system) - , horizontal grid spacing (geographic system) - t time step - z vertical grid spacing (cartesian system) - cross-frontal potential temperature difference - i potential temperature step at an inversion - E turbulent kinetic energy - f Coriolis parameter - FGP frontogenesis parameter (see section 2.2) - g gravity acceleration (g=9.81 m s–2) - vertical gradient of potential temperature - h terrain elevation (above MSL) - h i height of an inversion (h i =1000 m MSL) - H height of model lid (H=9000 m MSL) - K M exchange coefficient of momentum - K H exchange coefficient of heat and moisture - longitude - N Brunt-Väisäla-frequency - p pressure - Exner function (=T/) - latitude - q v specific humidity - R d gas constant of dry air (R d =287.06 J kg–1 K–1) - density of dry air - t time - T temperature - potential temperature - TFP thermal front parameter (see section 2.2) - u, v, w cartesian wind components - u g ,v g geostrophic wind components - horizontal wind vector - x, y, z cartesian coordinates Abbreviations GND (above) ground level - MSL (above) mean sea level - UTC universal time coordinated With 20 Figures  相似文献   

20.
In this paper we analyse diabatic wind profiles observed at the 213 m meteorological tower at Cabauw, the Netherlands. It is shown that the wind speed profiles agree with the well-known similarity functions of the atmospheric surface layer, when we substitute an effective roughness length. For very unstable conditions, the agreement is good up to at least 200 m or z/L–7(z is height, L is Obukhov length scale). For stable conditions, the agreement is good up to z/L1. For stronger stability, a semi-empirical extension is given of the log-linear profile, which gives acceptable estimates up to ~ 100 m. A scheme is used for the derivation of the Obukhov length scale from single wind speed, total cloud cover and air temperature. With the latter scheme and the similarity functions, wind speed profiles can be estimated from near-surface weather data only. The results for wind speed depend on height and stability. Up to 80 m, the rms difference with observations is on average 1.1 m s–1. At 200 m, 0.8 m s–1 for very unstable conditions increasing to 2.1 m s–1 for very stable conditions. The proposed methods simulate the diurnal variation of the 80 m wind speed very well. Also the simulated frequency distribution of the 80 m wind speed agrees well with the observed one. It is concluded that the proposed methods are applicable up to at least 100 m in generally level terrain.  相似文献   

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