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1.
Seasonal variations of the spectra of wind speed and air temperature in the mesoscale frequency range from 1.3 × 10-4 to 1.5 × 10-3 Hz (10 min to 2 h periods) have been studied through observations over land for one year. Spectrographs [time series of isopleths of spectral densities, f · S(f) vs f] of wind speed and air temperature contain occasional peaks that are attributed to short-lived mesoscale atmospheric activity with narrow frequency bands. Significant spectral peaks of wind speed were found in 19% of the total observations in winter, and in 15–16% in the other seasons; for air temperature, they occured in 12% of observations in autumn, and in 16–19% in the other seasons. The peaks most often occurred in the period range from 30 min to 1 h; most had durations less than 24 h. Mesoscale fluctuations of wind speed and air temperature were highly correlated, and in most cases, phase differences were 90–180 ° with air temperature leading wind speed. Significant spectral peaks of wind speed often occurred during northerly seasonal cold winds in winter, and accompanied tropical and/or mid-latitude cyclones in the other seasons. When the peaks occurred, wind speed was usually relatively high and the atmospheric surface layer was unstable.  相似文献   

2.
Observations of wind speed and direction, air and sea temperatures and solar radiation were obtained from an array of buoys in JASIN-1978 conducted in the area northwest of Scotland in the summer of 1978. The observations were analyzed to show spatial and temporal variability in the mesoscale fields. Spectra of wind speed and air and sea temperatures were computed to illustrate the distribution of variance over periods ranging from 3.5 min to 40 days. When plotted on log-log graphs, the spectral estimates generally decreased with slopes between –3/2 and –2 with increasing frequency. Spectra of air and sea temperatures had a peak at the diurnal period but not the wind speed spectrum. When plotted in variance-preserving form, the spectrum of wind speed was consistent with a spectral gap and was qualitatively similar to other observations of low-frequency spectra. On the basis of auto- and cross-correlation analyses, it appeared that mesoscale eddies propagated through the array of buoys with the mean wind speed except during times of frontal passages. The cross-correlation between wind speed and air temperature showed evidence of horizontal roll vortices or some other forms of organized convection.  相似文献   

3.
Currents and atmospheric parameters were measured in the East China Sea in February, 1975, as part of the AMTEX'75 program. These data were used to describe outbreaks of cold continental air over this warm and shallow sea. Particular emphasis was placed on describing the structure of mesoscale atmospheric cells embedded in the outbreaks and the effects of these cells on the water column.Two cold air outbreaks were recorded. Heat fluxes (latent plus sensible) as high as 1270 cal/cm2 day were calculated. Evidence of mesoscale atmospheric cells was found during outbreaks in satellite imagery and in solarimeter data. The development of mesoscale cells was described by correlating fluctuations in the air temperature and absolute humidity records. The cells were found to be best developed when satellite imagery showed that they were of the closed variety. The data suggest that cellular activity matures from open to closed cell types.During the period of greatest development, a representative closed cell was 24–30 km in diameter, moved at 8 m s-1 over the spar buoy, had a temperature fluctuation of 0.4 °C, an absolute humidity fluctuation of 0.4 g kg-1, and wind speed and heat flux fluctuations of - 12.5%.A non-dimensional index, formed from the fluctuations of the air temperature and absolute humidity records, was used to indicate the passage of mesoscale atmospheric cells over the measuring site. Using this index as input and the fluctuations in the oceanic parameters at a depth of 20 m as output, it was found that the passage of mature cells was significantly correlated with temperature fluctuations and current fluctuations aligned 25 ° to the right of the wind about 45 min later.  相似文献   

4.
Spectra of CO2 and water vapour fluctuations from measurements made in the marine atmospheric surface layer have been analyzed. A normalization of spectra based on Monin–Obukhov similarity theory, originally developed for wind speed and temperature, has been successfully extended also to CO2 and humidity spectra. The normalized CO2 spectra were observed to have somewhat larger contributions from low frequencies compared to humidity spectra during unstable stratification. However, overall, the CO2 and humidity spectra showed good agreement as did the cospectra of vertical velocity with water vapour and CO2 respectively. During stable stratification the spectra and cospectra displayed a well-defined spectral gap separating the mesoscale and small-scale turbulent fluctuations. Two-dimensional turbulence was suggested as a possible source for the mesoscale fluctuations, which in combination with wave activity in the vertical wind is likely to explain the increase in the cospectral energy for the corresponding frequency range. Prior to the analysis the turbulence time series of the density measurements were converted to time series of mixing ratios relative to dry air. Some differences were observed when the spectra based on the original density measurements were compared to the spectra based on the mixing ratio time series. It is thus recommended to always convert the density time series to mixing ratio before performing spectral analysis.  相似文献   

5.
Buoy and satellite observations of mesoscale cellular convection (MCC) over the East China Sea in the vicinity of the Kuroshio current have been made during 14–18 February 1975, as a part of the Air Mass Transformation Experiment (AMTEX). Surface observations of solar radiation from spar buoys indicate the distinct passage of open and closed MCC that formed and continued for three consecutive days during an outbreak of cold polar air over the much warmer Kuroshio. A critical air-sea temperature of –5 °C for the occurrence of MCC has been substantiated. The time required for the passage of solar radiation peaks coupled with the buoy wind speed gave a computed closed cell diameter of 28 km, comparable to estimates from satellite photographs.The horizontal component of wind beneath the cloudy portion of a closed cell, due to convection, has been estimated as 0.6 m s–1. This represents the speed at which air near the sea surface moves from the edge toward the center of a closed cell. Also, the temperature difference obtained near the sea surface between the relatively cold descending branch and the warm ascending branch is 0.2°C. Similarly, the specific humidity difference of the less moist descending air near the edge and the moist ascending air near cell center is 9% (0.4 g/kg). Some indications were also found in the variation of horizontal wind direction with the passage of closed cells, since wind variations at the edge of passing cells exceeded the mean sequential variability (10.6 ° compared to 9.4 °).Sensible heat flux calculations associated with closed MCC suggest that strong surface heating can be associated with closed cells, previously reported by Hubert (1966) to be a characteristic of only open cells.Finally, the results of this study should remove any disclaimers that MCC appear in satellite photography simply because of a resolution bias and that the consideration of all visible clouds actually present would remove any periodicity one might expect to see in surface observations.  相似文献   

6.
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   

7.
Experimental observations on the temperature and wind fields above flat grassy terrain have been obtained with an instrumented 92-m tower during intervals of strong insolation about midday. The turbulence characteristics of the air confirm that free convection prevailed at heights between 16 and 48 m, with some tendency for departure at higher levels. The spectra of temperature and vertical velocity contain gaps at wave numbers in the range 0.01–0.025 m–1. These are attributed to natural thermal plumes that act as sources of extra energy input to the Kolmogorov-Obukhov-Corrsin scheme of turbulence in or at the low-wave number limit of the inertial subrange. Modified forms of the K-O-C spectral laws for thermally unstable air are derived which agree with the observed spectra over the whole range of wave numbers examined, and which contain the spectral gap at wave numbers corresponding to the thermal plume diameters.  相似文献   

8.
The multiscaling statistics of atmospheric surface-layer winds at low wavenumbers above farmland and in the lee of a mountain range were examined using a hot-wire and lightweight cup anemometer. It was found that the horizontal velocity spectra could be broken into high and low-wavenumber regimes according to the parameters given by this analysis. The low-wavenumber end of the spectrum possessed a spectral slope parameter that varied between values of 0.8 and 1.35 at the farmland site during the period of the experiment, and the high-wavenumber end – corresponding to the inertial range – possessed a spectral slope slightly greater than -5/3. The larger values for this parameter for the low-wavenumber end appeared to coincide with unstable conditions. In the lee of the mountain range, the low-wavenumber spectral slope parameter was larger still, at 1.45. The low-wavenumber signals over farmland were much less intermittent than inertial-range signals, but in the lee of the mountain range the intermittency increased. From this analysis, it was shown that the statistical properties of the recorded wind signal could be reproduced using a bounded random multiplicative cascade. The model was successfully used to simulate the wind velocity field directly, rather than simulating the energy dissipation field. Since the spectral slope parameter for low wavenumbers appeared to be a function of atmospheric stability, the method presented is a simple way of generating wind signals characteristic of a variety of atmospheric conditions.  相似文献   

9.
Summary An attempt has been made in this paper to examine different modes of oscillation in the wind field during different seasons over Thiruvananthapuram (lat. 8.29° N, long. 76.59° E, located at the extreme southwest coast of India) based on daily upper air observations for the period from January 1997 to December 1999. A power spectral analysis is carried out with the upper air data of the station. The study shows that one and half cycle of Quasi–Biennial Oscillation (QBO) and the power spectra of the meridional wind component exhibit peaks between the period of four days and seven days (corresponding frequency range between 0.25 day−1 and 0.15 day−1) during all seasons. The seasonal variation of these large-scale oscillations over the station depends upon the background mean zonal flow, which in turn closely related to the QBO structure. The time sequence of power spectra shows that the disturbances with periods between four days and seven days dominantly prevail in the upper troposphere and lower stratosphere throughout the year. The regimes of high power spectral intensity in this period range are maintained in the levels where the mean zonal flow (westerly or easterly) weakens and changes with height. The study establishes the fact that disturbances (mixed Rossby-gravity waves) acquire maximum power in the winter season whereas the south-west monsoon exhibits minimum spectral intensity when spreading of energy over a frequency range takes place.  相似文献   

10.
On the contribution of atmospheric moisture to dew formation   总被引：4，自引：0，他引：4  
The relative contributions of dewfall (a flux of water vapour from air to surface) and distillation (a flux of water vapour from soil to canopy) to dew formation on closed canopy and bare soil surfaces are assessed, and the dependence of dew amount upon wind speed, absolute temperature, atmospheric stability, relative humidity, soil characteristics and cloudiness, all of which are significant factors, is evaluated. Some of these evaluations provide refinements to similar ones given in Monteith (1961). High dewfall rates are usually 0.06 mm hr–1 over canopy or bare soil, though upon a canopy under soil-saturated and air-saturated conditions, rates of dew formation may reach 0.07–0.09 mm hr–1 with contributions from distillation. Various sets of observations are reanalyzed to illustrate the importance of the horizontal advection of moisture in the nocturnal boundary layer (NBL) to observed high rates of dew formation arising from the atmospheric contribution of water vapour (dewfall). These locally observed high dewfall rates must be the result of small-scale or mesoscale horizontal advection of moisture in the NBL, since the humidity changes within the typically shallow NBL required to balance the loss of water at the surface are not observed. Over extensive areas of uniform surface (horizontal scales 10 km), such continuously high dewfall rates could only be balanced by a local supply of atmospheric moisture since advection of moisture would necessarily be small.  相似文献   

11.
Surface measurements of cloud condensation nuclei (CCN) number concentration (cm−3) are presented for unmodified marine air and for polluted air at Mace Head, for the years 1994 and 1995. The CCN number concentration active at 0.5% supersaturation is found to be approximately log-normal for marine and polluted air at the site. Values of geometric mean, median and arithmetic mean of CCN number concentration (cm−3) for marine air are in the range 124–135, 140–150 and 130–157 for the two years of data. Analysis of CCN number concentration for high wind speed, U, up to 20 m s−1 show enhanced CCN production for U in excess of about 10–12 m s−1. Approximately 7% increase in CCN per 1 m s−1 increase in wind speed is found, up to 17 m s−1. A relationship of the form log10CCN=a+bU is obtained for the periods March 1994 and January, February 1995 for marine air yielding values a of 1.70; 1.90 and b of 0.035 for both periods.  相似文献   

12.
利用2013年6月巴丹吉林沙漠拐子湖地区流沙下垫面的陆气通量观测资料,计算并分析了该研究区不同大气稳定层结条件下的湍流速度各分量谱和温度谱及湍流的局地各向同性特征。结果表明:该研究区风速各分量的湍流强度随平均风速的增加而逐渐减小,当平均风速2 m/s时,风速各分量的湍流强度逐渐稳定且基本限定在0.5以内。在不同大气稳定度下,湍流速度和温度能谱曲线在惯性副区内逐渐有合并趋势且遵循Kolmogorov提出的-2/3定律,速度各分量谱在高频段均满足各向同性且符合低频限制理论。随着稳定度的增加,风速分量的能谱曲线逐渐降低且向高频端移动,风速分量和温度能谱对应的谱峰长度则逐渐减小。该研究区水平湍流尺度范围为9.0~600 m,垂直湍流谱峰波长为10.79~75 m。该结果介于草地和森林下垫面之间,与塔克拉玛干沙漠腹地的试验结果较为接近。  相似文献   

13.
Measurements of the small-, intermediate-, and large-ion concentrations and the air–earth current density along with simultaneous measurements of the concentration and size distribution of aerosol particles in the size ranges 4.4–163 nm and 0.5–20 μm diameter are reported for a drifting snow period after the occurrence of a blizzard at a coastal station, Maitri, Antarctica. Ion concentrations of all categories and the air–earth current simultaneously decrease by approximately an order of magnitude as the wind speed increases from 5 to 10 ms− 1. The rate of decrease is the highest for large ions, lowest for small ions and in-between the two for intermediate ions. Total aerosol number concentration decreases in the 4.4–163 nm size range but increases in the 0.5–20 μm size range with wind speed. The size distribution of the nanometer particles shows a dominant maximum at ~ 30 nm diameter throughout the period of observations and the height of the maximum decreases with wind speed. However, larger particles show a maximum at ~ 0.7 μm diameter but the height of the maximum increases with increasing wind speed. The results are explained in terms of scavenging of atmospheric ions and aerosols by the drifting snow particles.  相似文献   

14.
The three-dimensional mesoscale model FITNAH has been modified to simulate effects of a tall tree canopy on airflow in complex terrain. Numerical experiments show the general features of meteorological variables inside a plant stand with low wind speeds and a nearly neutral thermal stratification during night. Available observations from the Finkenbach valley and the simulated temperatures near the ground show good agreeement.Comparison of model results for a nighttime situation for cases with and without a canopy (after complete deforestation) leads to the main results, viz., that surface wind speed will increase and the atmosphere near the ground will become colder after deforestation.However, the production rate of cooled air (expressed in m3m–2h–1) decreases; that means that forested slopes are more effective in ventilating a city than slopes covered with short vegetation.  相似文献   

15.
台风“珍珠”登陆期间动量通量的多尺度分析   总被引：2，自引：1，他引：1  
利用台风"珍珠"登陆前后的近地层湍流观测资料,分析了该台风经过观测场地前后地面气象要素的变化及其动量通量特征。结果表明,台风"珍珠"经过观测场地前后的近地层气象要素发生了急剧的变化,并且在台风前部存在强的中尺度对流系统,反映在风速能谱密度结构上,频率f在3×10-4~2×10-3Hz之间的中尺度信号对能谱的贡献比平稳天气形势下的能谱贡献大很多,尤其是顺风方向风速的能谱密度的峰值与湍流信号的峰值相当;动量通量分析结果表明,台风中心经过观测场地的前后三小时,近地层通量以向下输送的中尺度通量为主,湍流通量的贡献相对于中尺度通量较小,也是向下输送的;而在其他时段,近地层通量主要以向上输送的湍流通量为主,中尺度通量量值很小,可以忽略。  相似文献   

16.
Vertical wind shear at a temperature inversion can be caused by baroclinicity associated with a tilt of the inversion. Four observational cases of tilted inversions are presented. The tilts on horizontal scales of 20–100 km range from 2–10 × 10-3 and the vertical wind shear is between 1 and 25 m/s per 100 m. In general, there is remarkable agreement between observed and geostrophic wind shear.The observations show that the inversion tilt is particularly strong at the edges of mesoscale cloud fields. The Richardson number can reach subcritical values. Cloud fields may be surrounded by a cyclonically rotating wind field and cloud gaps by an anticyclonically rotating wind field.  相似文献   

17.
Summary The seasonal and diurnal variations in the vertical component of the atmospheric electric field, air temperature, relative humidity and horizontal wind speed were studied using the surface data for the two periods (1936–40) and (1962–66) recorded at the tropical urban station, Colaba, Bombay (18°51N, 72°49E, 11 m ASL), located on the west coast of India.The atmospheric electric field during the latter period (1962–66) is significantly higher (up to 42.3%) than the earlier period (1936–40). This has been attributed to the enhanced particulate concentrations in the atmosphere. The increase noticed in the atmospheric electric field is a maximum during winter and minimum during the monsoon. The atmospheric electric field exhibited a marked semi-diurnal oscillation with peaks at 0900 LST and 2200 LST during winter, premonsoon and post-monsoon seasons of both the periods. During the monsoon season the double oscillation is not marked.The variations noticed in the surface air temperature and the relative humidity are in agreement with those observed in the atmospheric electric field. The horizontal wind speed showed a decrease which has been attributed to the surface roughness resulting from urbanization.With 7 Figures  相似文献   

18.
Profile and eddy-correlation (heights of 4 and 10 m) measurements performed on the Pasterze glacier (Austria) are used to study the characteristics of the stable boundary layer under conditions of katabatic and large-scale forcing. We consider cases where large-scale forcing results in a downslope (or following) ambient wind. The analysis of averaged spectra and cospectra reveals low frequency perturbations that have a large influence on the variances of temperature and horizontal wind components and also alter the cospectra of momentum and sensible heat flux. Only the spectrum of the vertical wind speed is comparable to universal spectra. The low frequency perturbations occur as brief intermittent events and result in downward entrainment of ambient air thereby producing enhanced downward sensible heat fluxes and downward as well as upward momentum fluxes with various magnitudes and timescales. After the variances were high pass filtered, the normalised standard deviations of wind speed and temperature compare favourably to findings in the literature within the range 0>z/L>0.5. For larger z/L they deviate as a result of an increased influence from low frequency perturbations and thus non-stationarity. In line with this, the turbulent kinetic energy budget (at 4 m height) indicates that production (shear) is in balance with destruction (buoyancy and dissipation) within the range 0>z/L>0.3. Non-dimensional gradients of wind speed within the range 0>z/L>0.3 have a slope of about 3.5. The scatter for the dimensionless temperature gradient is quite large, and the slope is comparable to that for wind speed gradients. For z/L>0.3 the imbalance in the turbulent kinetic energy budget grows and non-dimensional gradients for wind speed and temperature deviate considerably from accepted values as a result of increased non-stationarity. Average roughness lengths for momentum and sensible heat flux derived from wind speed and temperature profiles are respectively 1 × 10-3 m and 6 × 10-5 m, consistent with the literature. The ratio (z0h/z0m) compares to those predicted by surface renewal models. A variation of this ratio with the roughness Reynolds number is not indicated by our data.  相似文献   

19.
We analyzed the frequency distribution characteristics of wind speeds occurring at different offshore sites within a range of 0–200 km based on the sea surface wind data captured via buoys and oil platforms located along the east coast of Guangdong Province. The results of the analysis showed that average wind speed measured for each station reached a maximum in winter while minima occurred in summer, corresponding to obvious seasonal variation, and average wind speed increased with offshore distance. The prevailing wind direction at the nearshore site is the easterly wind, and the frequency of winds within 6–10 m s–1 is considerable with that of winds at > 10 m s–1. With the increase of the offshore distance, the winds were less affected by the land, and the prevailing wind direction gradually became northerly winds, predominately those at > 10 m s–1. For areas of shorter offshore distance (< 100 km), surface wind speeds fundamentally conformed to a two-parameter Weibull distribution, but there was a significant difference between wind speed probability distributions and the Weibull distribution in areas more than 100 km offshore. The mean wind speeds and wind speed standard deviations increased with the offshore distance, indicating that with the increase of the wind speed, the pulsation of the winds increased obviously, resulting in an increase in the ratio of the mean wind speed to the standard deviation of wind speed. When the ratio was large, the skewness became negative. When a relatively great degree of dispersion was noted between the observed skewness and the skewness corresponding to the theoretical Weibull curve, the wind speed probability distribution could not be adequately described by a Weibull distribution. This study provides a basis for the verification of the adaptability of Weibull distribution in different sea areas.  相似文献   

20.
Near-surface wind velocities were measured on an array of anchored spar buoys in the East China Sea in February 1975 as a part of the Japanese AMTEX '75 (Air Mass Transformation Experiment), a subprogram of GARP. These data were used to determine the effects of atmospheric convection associated with mesoscale convective cells (MCC) on near-surface winds over the ocean. When MCC were present, a peak occurred in the near-surface wind spectrum in the so-called spectral gap.  相似文献   

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