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1.
Development and structure of a maritime continent thunderstorm   总被引:4,自引:0,他引:4  
Summary The evaluation of a maritime continent thunderstorm complex (Hector) occurring over Bathurst and Melville Islands north of Darwin, Australia (12° S, 131° E) is investigated primarily using Doppler radar data. Thunderstorm formation follows the development of sea breeze circulations and a period of shallow non-precipitating convection. Evidence exists for initiation of long-lived and organised convection on the sea breeze fronts, although short-lived, scattered convection is apparent earlier in the day. Merging of the convective systems is observed in regions of enhanced low-level convergence related to sea breeze circulations. The merged convective complex is initially aligned in an almost east-west direction consistent with the low-level forcing. The merged complex results in rapid vertical development with updraughts reaching 40 m s and echo tops reaching 20 km height. Maximum precipitation production occurs during this merger phase. On the perimeter of the merged convective complex, evidence exists for front-to-rear updraughts sloped over lower-level downdraughts with rear-to-front relative flow and forward propagating cold pools. The mature phase is dominated by this convection and the complex re-orientates in the prevailing easterly vertical shear to an approximate north-south direction, then moves westward off the islands with the classic multicellular squall-like structure.The one-dimensional cloud model of Ferrier and Houze (1989) used with a four class ice formulation reproduced the cloud top height, updraught structure and echo profile very well. To test the importance of ice physics upon thunderstorm development, several sensitivity tests were made removing the effects of the ice phase. All of these model clouds reached nearly 20 km, although simulations without the effects of ice had updraughts reduced from about 40 m s–1 to 30 m s–1. The simulated convection was more sensitive to changes in environmental conditions and parameterised cloud dynamics. The strong intensity of the convection was largely accounted for by increasing equivalent potential temperatures due to diurnal heating of the surface layer. The vertical velocity and radar structure of the island thunderstorm has more similarity with continental rather than oceanic convection. Maximum vertical velocities, in particular are almost an order of magnitude greater than typical of oceanic convection. With the intense updraughts, even in the low shear environment, there is evidence for mesoscale circulations within the convection.With 17 Figures  相似文献   

2.
Large-eddy simulation is used to study secondary circulations in the convective boundary layer modulated as a result of horizontally varying surface properties and surface heat fluxes over flat terrain. The presence of heat flux heterogeneity and its alignment with respect to geostrophic wind influences the formation, strength and orientation of organized thermals. Results show boundary-attached roll formation along heat flux maxima in the streamwise direction. The streamwise organization of the updrafts and downdrafts formed downwind of heterogeneities leads to counter-rotating secondary circulations in the crosswind plane. The distribution of resolved-scale pressure deviations shows large pressure gradients in the crosswind plane. Spanwise and vertical velocity variances and heat flux profiles depict considerable spatial variability compared to a homogeneous forest simulation. Secondary circulations are observed for various ambient wind scenarios parallel and perpendicular to heterogeneities. In the presence of increased wind speed, thermals emerging from the heat flux heterogeneity are elongated, and organize along and downwind of large-scale heterogeneity in the streamwise direction. Simulation with a reduced heat flux shows a shallower circulation with a lower aspect ratio. Point measurements of heat flux inside the roll circulation could be overestimated by up to 15–25% compared to a homogeneous case.  相似文献   

3.
Using data collected during 1975–1976 from a meteorological network operating in the vicinity of the Columbia Generating Site approximately 8 km south of Portage, Wis., the influence of the Baraboo Hills on the surface wind field is determined. Half-hour means of wind speed and direction measured at 9 m at three sites were used to compute divergence and vorticity using Bellamy's method. The data were grouped into 18 sectors each 20 deg wide and averages computed for each quantity. Results indicate that for wind directions perpendicular to the eastern edge of the Baraboo Hills, the surface (9m) wind field is significantly perturbed up to 4 km from the bluffs. The largest convergence of 2.1 × 10–4 s–1 occurs with 160 deg wind direction and the largest divergence of 1.2 × 10–4 s–1 with 290 deg wind direction. The maximum anticyclonic vorticity was 1.6 × 10–4 s–1 at 210 deg and the maximum cyclonic vorticity was 1.6 × 10–4 s–1 at 330 deg.  相似文献   

4.
We investigate the impact of observed surface heterogeneities during the LITFASS-2003 experiment on the convective boundary layer (CBL). Large-eddy simulations (LES), driven by observed near-surface sensible and latent heat fluxes, were performed for the diurnal cycle and compare well with observations. As in former studies of idealized one- and two-dimensional heterogeneities, secondary circulations developed that are superimposed on the turbulent field and that partly take over the vertical transport of heat and moisture. The secondary circulation patterns vary between local and roll-like structures, depending on the background wind conditions. For higher background wind speeds, the flow feels an effective surface heat-flux pattern that derives from the original pattern by streamwise averaging. This effective pattern generates a roll-like secondary circulation with roll axes along the mean boundary-layer wind direction. Mainly the upstream surface conditions control the secondary circulation pattern, where the fetch increases with increasing background wind speed. Unlike the entrainment flux that appears to be slightly decreased compared to the homogeneously-heated CBL, the vertical flux of sensible heat appears not to be modified in the mixed layer, while the vertical flux of latent heat shows different responses to secondary circulations. The study illustrates that sufficient time averaging and ensemble averaging is required to separate the heterogeneity-induced signals from the raw LES turbulence data. This might be an important reason why experiments over heterogeneous terrain in the past did not give any clear evidence of heterogeneity-induced effects.  相似文献   

5.
Large-eddy simulations (LES) are performed to investigate the entrainment andthe structure of the inversion layer of the convective boundary layer (CBL) withvarying wind shears. Three CBLs are generated with the constant surface kinematicheat flux of 0.05 K m s-1 and varying geostrophic wind speeds from 5 to 15m s-1. Heat flux profiles show that the maximum entrainment heat flux as afraction of the surface heat flux increases from 0.13 to 0.30 in magnitude withincreasing wind shear. The thickness of the entrainment layer, relative to the depthof the well-mixed layer, increases substantially from 0.36 to 0.73 with increasingwind shear. The identification of vortices and extensive flow visualizations nearthe entrainment layer show that concentrated vortices perpendicular to the meanboundary-layer wind direction are identified in the capping inversion layer for thecase of strong wind shear. These vortices are found to develop along the mean winddirections over strong updrafts, which are generated by convective rolls and to appearas large-scale wavy motions similar to billows generated by the Kelvin–Helmholtzinstability. Quadrant analysis of the heat flux shows that in the case of strong windshear, large fluctuations of temperature and vertical velocity generated by largeamplitude wavy motions result in greater heat flux at each quadrant than that inthe weak wind shear case.  相似文献   

6.
A model is developed to simulate the potential temperature and the height of the mixed layer under advection conditions. It includes analytic expressions for the effects of mixed-layer conditions upwind of the interface between two different surfaces on the development of the mixed layer downwind from the interface. Model performance is evaluated against tethersonde data obtained on two summer days during sea breeze flow in Vancouver, Canada. It is found that the mixed-layer height and temperature over the ocean has a small but noticeable effect on the development of the mixed layer observed 10 km inland from the coast. For these two clear days, the subsidence velocity at the inversion base capping the mixed layer is estimated to be about 30 mm s–1 from late morning to late afternoon. When the effects of subsidence are included in the model, the mixed-layer height is considerably underpredicted, while the prediction for the mean potential temperature in the mixed layer is considerably improved. Good predictions for both height and temperature can be obtained when values for the heat entrainment ratio,c, 0.44 and 0.68 for these two days respectively for the period from 1000 to 1300 LAT, were used. These values are estimated using an equation including the additional effects on heat entrainment due to the mechanical mixing caused by wind shear at the top of the mixed layer and surface friction. The contribution of wind shear to entrainment was equal to, or greater than, that from buoyant convection resulting from the surface heat flux. Strong wind shear occurred near the top of the mixed layer between the lower level inland flow and the return flow aloft in the sea breeze circulation.Symbols c entrainment parameter for sensible heat - c p specific heat of air at constant pressure, 1010 J kg–1 K–1 - d 1 the thickness of velocity shear at the mixed-layer top, m - Q H surface sensible heat flux, W m–2 - u m mean mixed-layer wind speed, m s–1 - u * friction velocity at the surface, m s–1 - w subsidence velocity, m s–1 - W subsidence warming,oC s–1 - w e entrainment velocity, m s–1 - w * convection velocity in the mixed layer, m s–1 - x downwind horizontal distance from the water-land interface, m - y dummy variable forx, m - Z height above the surface, m - Z i height of capping inversion, m - Z m mixed-layer depth, i.e.,Z i–Zs, m - Z s height of the surface layer, m - lapse rate of potential temperature aboveZ i, K m–1 - potential temperature step atZ i, K - u h velocity step change at the mixed-layer top - m mean mixed-layer potential temperature, K  相似文献   

7.
Saline circulation forced by fresh water alone is studied for a broad region of parameter space by varying the amplitude and profile of evaporation minus precipitation, the vertical and horizontal mixing of salt, vertical and horizontal dissipation of momentum, and the horizontal resolution. The model is a modified Bryan-Cox model with a freshwater flux as the natural boundary condition for the salinity balance. For a model forced by a linear freshwater flux profile, as the amplitude of freshwater flux is increased from 0.01 m year –1 to 1 m year–1 with other parameters fixed, the system evolves from a steady state of no oscillation to a state of periodic oscillation whose frequency increases almost linearly with the amplitude of freshwater flux. When the freshwater flux is fixed and the vertical mixing coefficient is increased from 0.5 to 2.5 cm2s–1, the system evolves from a steady state to a state of single-period oscillation, chaotic, a single period, and finally to a chaotic state when the vertical mixing coefficient is larger than 2 cm2 s1. One set of numerical experiments forced by a cosine shape of freshwater flux clearly reveals the transition from a state of single period oscillation to period doubling, period quadrupling, and a state of chaotic oscillation. Simple scaling analysis and numerical experiments indicate that the strength of the meridional overturning increases with the square-root of the vertical mixing and the 1/4 power of the freshwater flux. The mean sea surface salinity (deviation from 35 psu) increases with the 3/4 power of the freshwater flux and decreases with the 1/2 power of the vertical salt mixing.Contribution No. 8191 from the Woods Hole Oceanographic Institution  相似文献   

8.
Abstract

The effects of small‐scale surface inhomogeneities on the turbulence structure in the convective boundary layer are investigated using a high‐resolution large‐eddy simulation model. Surface heat flux variations are sinusoidal and two‐dimensional, dividing the total domain into a checkerboard‐like pattern of surface hot spots with a 500‐m wavelength in the x and y directions, or 1/4 of the domain size. The selected wind speeds were 1 and 4 m s‐l, respectively. As a comparison, a simulation of the turbulence structure was performed over a homogeneous surface.

When the wind speed is light, surface heat flux variations influence the horizontally averaged turbulence statistics, including the higher moments despite the small characteristic length of the surface perturbation. Stronger mean wind speeds weaken the effects of inhomogeneous surface conditions on the turbulence structure in the convective boundary layer.

Results from conditional sampling show that when the mean wind speed is small, weak mean circulations occur, with updraft branches above the high heat flux regions and down‐draft branches above the low heat flux regions. The inhomogeneous surface induces significant differences in the turbulence statistics between the high and low heat flux regions. However, the effect of the surface perturbations weaken rapidly when the mean wind speed increases. This research has implications in the explanation of the large‐scale variability commonly encountered in aircraft observations of atmospheric turbulence.  相似文献   

9.
An observational study of wind-induced waving of plants   总被引:1,自引:0,他引:1  
The motions of individual plants and the turbulence statistics of surface winds measured near the top of a canopy are obtained over a wheat field and a rush field. Two typical cases of motions of individual plants are presented. The displacements of the ear of wheat (the plant height is 1.0 m) showed a natural oscillation in wind speeds of 1.6 m s–1 measured at a height of 30 cm over a wheat canopy, while displacements of the stem of a rush plant were closely related to the fluctuations of surface winds in wind speeds of 1.7 m s–1 measured at the top of the rush plant. The power spectra of displacements of a rush plant seem to support the negative seven-third power hypothesis proposed by Inoue. The frequency responses of displacements of plants to fluctuations of the instantaneous momentum flux are also presented.  相似文献   

10.
A numerical investigation of wind speed effects on lake-effect storms   总被引:2,自引:0,他引:2  
Observations of lake-effect storms that occur over the Great Lakes region during late autumn and winter indicate a high sensitivity to ambient wind speed and direction. In this paper, a two-dimensional version of the Penn State University/National Center for Atmospheric Research (PSU/NCAR) model is used to investigate the wind speed effects on lake-effect snowstorms that occur over the Great Lakes region.Theoretical initial conditions for stability, relative humidity, wind velocity, and lake/land temperature distribution are specified. Nine different experiments are performed using wind speeds ofU=0, 2, 4,..., 16 m s–1. The perturbation wind, temperature, and moisture fields for each experiment after 36 h of simulation are compared.It is determined that moderate (4–6 m s–1) wind speeds result in maximum precipitation (snowfall) on the lee shore of the model lake. Weak wind speeds (0U<4 m s–1) yield significantly higher snowfall amounts over the lake along with a spatially concentrated and intense response. Strong wind speeds (6<U16 m s–1), yield very little, if any, significant snowfall, although significant increases in cloudiness, temperature, and perturbation wind speed occur hundreds of kilometers downwind from the lake.  相似文献   

11.
The hydrodynamic equations governing the water-level response of a lake to wind stress are inverted to determine wind stress from water-level fluctuations. In order to obtain a unique solution, the wind-stress field is represented in terms of a finite number of spatially dependent basis functions with time-dependent coefficients. The discretized version of the inverse equation is solved by a least-squares procedure to obtain the coefficients, and thereby the stress. The method is tested for several ideal cases with Lake Erie topography. Real water-level data is then used to determine hourly values of vector wind stress over Lake Erie for the period 5 May–31 October, 1979. Results are compared with measurements of wind speed and direction from buoys deployed in the lake. Calculated stress direction agrees with observed wind direction for wind speeds > 7.5 m s−1. Under neutral conditions, calculated drag coefficients increase with the wind speed from 1.53 × 10−3 for 7.5−10 m s−1 winds to 2.04 × 10−3 for 15−17.5 m s−1 winds. Drag coefficients are lower for stable conditions and higher for unstable conditions.  相似文献   

12.
Observations from research ships which took part in the Indo-Soviet Monsoon Experiment of 1977 (MONSOON 77) and the International Monsoon Experiments (MONEX 79) over the central Arabian Sea and the north central Bay of Bengal were analyzed to study the mean wind and temperature structure of the monsoon boundary layer during active and break conditions. Mean profiles of wind speed and direction along with virtual potential temperature obtained by averaging data from several research ships during 1977 and 1979 indicate that onset conditions were associated with substantial increases in wind speed over the Arabian Sea and a shift to strong southwest flow. Monsoon onset was also characterized by near-neutral to slightly unstable temperature profiles in the lowest kilometer. Break conditions in 1977 in which the monsoon trough moved northward and substantial (5 mb) pressure rises were noted over the Arabian Sea show wind speeds typically decreasing from approximately 18 m s–1 during active conditions to roughly 8 m s –1. Temperature profiles during break conditions are similar to those observed in pre-monsoon conditions in that the boundary layer is observed to be generally much more stable up to 900 mb. Above 900 mb, profiles of virtual potential temperature show little variation.Analysis of latent and sensible heat fluxes during June 1977 calculated by the bulk aerodynamic method indicates values of latent heat flux during active conditions to be roughly two to three times larger than those during break conditions. Sensible heat flux shows an increase from approximately 20 to 80 W m –1 during the onset of the monsoon. Surface fluxes of water vapor indicate the importance of water vapor transport over the ship observation region in the central Arabian Sea during active conditions. Onset of the monsoon over the Arabian Sea is accompanied by an increase in the surface moisture flux by a factor of about two. Time histories of precipitable water show decreases of approximately 15% from active to break periods.  相似文献   

13.
Summer weather conditions along the west coast of Africa near 34 ° S, 18 ° E are investigated using doppler acoustic sounder profiles. Case studies were selected from a two-year record to form composite analyses over the diurnal cycle. The SE trade wind exhibited a low level jet at the level of the temperature inversion due to a sharp reversal in the thermal wind vector aloft. Mean wind speeds reached 14 m s–1 just before midnight as the surface and upper inversions strengthened. Seabreezes were categorised by the supporting gradient wind and found to have mean depths of 400 m, speeds of over 6 m s–1 at the 200 m level, and advance/retreat times of 09 hr and 16–20 hr. During seabreezes and weak on-shore gradient flow conditions, the thermal internal boundary layer (TIBL) was monitored with sounder transects in the first 12 km of the coastal zone. The growth height was observed to be 1:20 in the first 5 km and 1:50 farther inland. The sounder climatology, together with surface network and aerial survey results, illustrate the four-dimensional characteristics of trade winds and seabreezes near Cape Town.  相似文献   

14.
The local sea-breeze circulation in Venice is found to have typical characteristics but the nocturnal flow is found to be quite complex. This latter result is due to unequal cooling of the Alps and adjacent low-lying regions. The horizontal and vertical structure of the flow has been investigated using data obtained from surface anemometers and pilot balloon observations. Results show that the circulation is about 500 m in depth and achieves a maximum speed of 6 m s–1  相似文献   

15.
A two-dimensional mesoscale model has been developed to simulate the air flow over the Gulf Stream area where typically large gradients in surface temperature exist in the winter. Numerical simulations show that the magnitude and the maximum height of the mesoscale circulation that develops downwind of the Gulf Stream depends on both the initial geostrophic wind and the large-scale moisture. As expected, a highly convective Planetary Boundary Layer (PBL) develops over this area and it was found that the Gulf Stream plays an important role in generating the strong upward heat fluxes causing a farther seaward penetration as cold air advection takes place. Numerical results agree well with the observed surface fluxes of momentum and heat and the mesoscale variation of vertical velocities obtained using Doppler Radars for a typical cold air outbreak. Precipitation pattern predicted by the numerical model is also in agreement with the observations during the Genesis of Atlantic Lows Experiment (GALE).List of Symbols u east-west velocity [m s–1] - v north-south velocity [m s–1] - vertical velocity in coordinate [m s–1] - w vertical velocity inz coordinate [m s–1] - gq potential temperature [K] - q moisture [kg kg–1] - scaled pressure [J kg–1 K–1] - U g the east-south component of geostrophic wind [m s–1] - V g the north-south component of geostrophic wind [m s–1] - vertical coordinate following terrain - x east-west spatial coordinate [m] - y north-south spatial coordinate [m] - z vertical spatial coordinate [m] - t time coordinate [s] - g gravity [m2 s–1] - E terrain height [m] - H total height considered in the model [m] - q s saturated moisture [kg kg–1] - p pressure [mb] - p 00 reference pressure [mb] - P precipitation [kg m–2] - vertical lapse rate for potential temperature [K km–1] - L latent heat of condensation [J kg–1] - C p specific heat at constant pressure [J kg–1 K–1] - R gas constant for dry air [J kg–1 K–1] - R v gas constant for water vapor [J kg–1 K–1] - f Coriolis parameter (2 sin ) [s–1] - angular velocity of the earth [s–1] - latitude [o] - K H horizontal eddy exchange coefficient [m2 s–1] - t integration time interval [s] - x grid interval distance inx coordinate [m] - y grid interval distance iny coordinate [m] - adjustable coefficient inK H - subgrid momentum flux [m2 s–2] - subgrid potential temperature flux [m K s–1] - subgrid moisture flux [m kg kg–1 s–1] - u * friction velocity [m s–1] - * subgrid flux temperature [K] - q * subgrid flux moisture [kg kg–1] - w * subgrid convective velocity [m s–1] - z 0 surface roughness [m] - L Monin stability length [m] - s surface potential temperature [K] - k von Karman's constant (0.4) - v air kinematic viscosity coefficient [m2 s–1] - K M subgrid vertical eddy exchange coefficient for momentum [m2 s–1] - K subgrid vertical eddy exchange coefficient for heat [m2 s–1] - K q subgrid vertical eddy exchange coefficient for moisture [m2 s–1] - z i the height of PBL [m] - h s the height of surface layer [m]  相似文献   

16.
Moderating effects of Lake Apopka, Florida on downwind surface temperatures were evaluated under cold-air advective conditions. Point temperature measurements north and south of the lake and data obtained from a thermal scanner flown at 1.6 km indicate that surface temperatures directly downwind may be higher than surrounding surface temperatures by as much as 5 °C under conditions of moderate winds (~4 m s–1). No substantial temperature effects were observed with surface wind speed less than 1 m s–1. Fluxes of sensible and latent heat from Lake Apopka were calculated from measurements of lake temperature, net radiation, relative humidity and air temperature above the lake. Bulk transfer coefficients and the Bowen ratio were calculated and found to be in agreement with reported data for non-advective conditions.IFAS Journal Series No. 1006.  相似文献   

17.
The Signature of Sea Spray in the Hexos Turbulent Heat Flux Data   总被引:7,自引:0,他引:7  
The role of sea spray intransferring heat and moisture across the air-sea interface has remained elusive. Some studies have reported that sea spray does not affect the turbulent air-sea heat fluxes for 10-m wind speeds up to at least 25 m s-1, while others have reported important spray contributions for wind speeds as low as 12 m s-1. One goal of the HEXOS (Humidity Exchange over the Sea) program was to quantify spray's contribution to the turbulent air-sea heat fluxes, but original analyses of the HEXOS flux data found the spray signal to be too small to be reliably identified amid the scatter in the data. We look at the HEXOS data again in the context of the TOGA-COARE bulk flux algorithm and a sophisticated microphysical spray model. This combination of quality data andstate-of-the-art modelling reveals a distinct spray signature in virtually all HEXOS turbulent heat flux data collected in winds of 15 m s-1 and higher. Spray effects are most evident in the latent heat flux data, where spray contributes roughly 10% of the total turbulent flux in winds of 10 m s-1 and between 10 and 40% in winds of 15–18 m s-1. The spray contribution to the total sensible heat flux is also at least 10% in winds above 15 m s-1. These results lead to a new, unified parameterization for the turbulent air-sea heat fluxes that should be especially useful in high winds because it acknowledges both the interfacial and spray routes by which the sea exchanges heat and moisture with the atmosphere.  相似文献   

18.
On the cool skin of the ocean   总被引:1,自引:0,他引:1  
Previous data relating sea-surface temperature to heat flux across the air-sea interface were reanalyzed with a common formula for the wind-stress coefficient. An expression is proposed for the nondimensional thickness of the thermal sublayer: the expression increases with wind velocity at light winds and has a value of 7 when the wind velocity reaches 7 m s–1.  相似文献   

19.
Shallow Drainage Flows   总被引:1,自引:1,他引:0  
Two-dimensional sonic anemometers and slowresponse thermistors were deployedacross a shallow gully during CASES99. Weak gully flow of a few tenths of m s-1 anda depth of a few metres develops in the earlyevening on most nights with clear skies.Flow down the gully developed sometimes evenwhen the opposing ambient wind exceeded10 m s-1 at the top of the60–m tower. Cold air drainage fromlarger-scale slopes flows over the top ofthe colder gully flow. The gully flowand other drainage flows are generally eliminated in the middle of the night in conjunctionwith flow acceleration abovethe surface inversion layer and downwardmixing of warmer air and highermomentum. As the flow decelerates later inthe night, the gully flow may re-form.The thin drainage flows decouple standard observational levels of3–10 m from the surface.Under such common conditions, eddy correlationflux measurements cannot be used toestimate surface fluxes nor even detect thethin gully and drainage flows. The gentlegully system in this field program is typical ofmuch of the Earths land surface.  相似文献   

20.
Observations of a single boundary-layer event — the generation of an atmospheric gravity wave by an unstable shear flow at Haswell, Colorado on November 12, 1971 — are briefly described and discussed. The observations were made using: (a) an acoustic echo sounder, (b) anemometers mounted at two fixed levels on a 150-m tower, (c) an anemometer and a thermometer mounted on a movable carriage on the tower, and (d) a microbarograph array, including one microbarograph mounted atop the tower. The wave phase velocity (–3.5–4.0 m s–1) was found to equal the wind velocity in the middle of the shear flow, as assumed by other authors. The wave-associated vertical fluxes of momentum and energy measured just above the wave critical layer were estimated to be –5 dyn cm–2 and –800 erg cm–2 s–1, respectively. These are large values. The annual average vertical flux of momentum at temperate and high latitudes is –0.25 dyn cm–2, while the average kinetic energy dissipation rate in a unit column of atmosphere is –5 × 103 erg cm–2 s–1. If the region of wave generation was itself propagating horizontally, its propagation velocity was large compared with the horizontal phase speed of the small-scale waves generated. Wave generation appeared to occur over an area large compared with the size of the microbarograph array (i.e., 2 km).  相似文献   

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