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1.
Sodar has been installed at Kharagpur (22.2 ° N, 87.3 ° E) as a part of the MONTBLEX-90 experiment and data were collected during the monsoon period. The variation of the nocturnal boundary layer (NBL) during the monsoon period is discussed. The height corresponding to the low-level wind maximum in the sodar wind profile during night time is identified as the NBL height. Mean monthly winds for July and August, plotted as time-height cross sections, reveal the height of the ground-based stable layer. The average NBL heights in the months of July and August are found to be 324 m and 296 m respectively. It is observed that the NBL height is relatively high in the month of July (active phase of monsoon) compared to that during August (weak phase). The months of July (total rainfall = 901 mm) and August (total rainfall = 134 mm) are associated with cloudy and relatively clear sky conditions. This indicates that clouds (through their effect on longwave cooling to space) play an important role in determining the NBL height during the monsoon.  相似文献   

2.
干旱地区大气与地表特征对辐射加热场的影响   总被引：1，自引：0，他引：1  
杨文  季国良 《高原气象》1994,13(3):266-273
本文利用美国犹他大学气象系的辐射和云参数化模式，对ＨＥＩＦＥ期间张掖地区１９９１年春、夏、秋、冬四季资料进行了计算，讨论了晴天条件下的大气状况态地表反射率与地表比辐射率等因子对地气系统的太阳辐射收支以及短波加热率与长波冷却率分布的影响；揭示了不同季节的整层大气反射、透过与吸收特征，分析了大气中各主要吸收成分对加热率与冷却率的贡献，同时就辐射模式的垂直分辨率对加热率与冷却率的影响亦作了讨论。  相似文献   

3.
The atmospheric katabatic flow in the foothills of the Front Range of the Rocky Mountains has been monitored by a network of towers and sodars for several years as part of the Atmospheric Studies in COmplex Terrain (ASCOT) program. We used three years of data from the network to explore the dependence on surface cooling and channeling by winds above the canyon of (1) profiles of the mean and variance of the vertical (perpendicular to the geopotential) component of motion and (2) the mean component of the wind perpendicular to the local terrain of Coal Creek Canyon. Previously we found that the magnitude of the near-surface temperature difference decreases with increasing surface cooling in light winds, apparently because of increasing turbulence caused when increasing drainage winds interact with surface topography. The variance of vertical velocity exhibits three types of vertical profiles, corresponding to different cooling rates and external wind speeds. The mean variance was found to depend strongly on a locally derived Richardson number.  相似文献   

4.
In this paper, a second-order model is proposed for the study of the evolution of the nocturnal boundary layer (NBL). The model is tested against the Wangara data on atmospheric boundary layer. The computer results show ihat the model can simulate some important characters observed in the NBL, and that a kind of sudden change may occur in the developing process of NBL.  相似文献   

5.
Observations obtained mainly from a research aircraft are presented of the mean and turbulent structure of the stably stratified internal boundary layer (IBL) over the sea formed by warm air advection from land to sea. The potential temperature and humidity fields reveal the vertical extent of the IBL, for fetches out to several hundred of kilometres, geostrophic winds of 20–25 m s–1, and potential temperature differences between undisturbed continental air and the sea surface of 7 to 17 K. The dependence of IBL depth on these external parameters is discussed in the context of the numerical results of Garratt (1987), and some discrepancies are noted.Wind observations show the development of a low-level wind maximum (wind component normal to the coast) and rotation of the wind to smaller cross-isobar flow angles. Potential temperature () profiles within the IBL reveal quite a different structure to that found in the nocturnal boundary layer (NBL) over land. Over the sea, profiles have large positive curvature with vertical gradients increasing monotonically with height; this reflects the dominance of turbulent cooling within the layer. The behaviour is consistent with known behaviour in the NBL over land where curvature becomes negative (vertical gradients of decreasing with height) as radiative cooling becomes dominant.Turbulent properties are discussed in terms of non-dimensional quantities, normalised by the surface friction velocity, as functions of normalised height using the IBL depth. Vertical profiles of these and the normalised wavelength of the spectral maximum agree well with known results for the stable boundary layer over land (Caughey et al., 1979).  相似文献   

6.
Planetary boundary-layer (PBL) structure was investigated using observations from a Doppler lidar and the 325-m Institute of Atmospheric Physics (IAP) meteorological tower in the centre of Beijing during the summer 2015 Study of Urban-impacts on Rainfall and Fog/haze (SURF-2015) field campaign. Using six fair-weather days of lidar and tower data under clear to cloudy skies, we evaluate the ability of the Doppler lidar to probe the urban boundary-layer structure, and then propose a composite method for estimating the diurnal cycle of the PBL depth using the Doppler lidar. For the convective boundary layer (CBL), a threshold method using vertical velocity variance \((\sigma _w^2 >0.1\,\hbox {m}^{2}\hbox {s}^{-2})\) is used, since it provides more reliable CBL depths than a conventional maximum wind-shear method. The nocturnal boundary-layer (NBL) depth is defined as the height at which \(\sigma _w^2\) decreases to 10 % of its near-surface maximum minus a background variance. The PBL depths determined by combining these methods have average values ranging from \(\approx \)270 to \(\approx \)1500 m for the six days, with the greatest maximum depths associated with clear skies. Release of stored and anthropogenic heat contributes to the maintenance of turbulence until late evening, keeping the NBL near-neutral and deeper at night than would be expected over a natural surface. The NBL typically becomes more shallow with time, but grows in the presence of low-level nocturnal jets. While current results are promising, data over a broader range of conditions are needed to fully develop our PBL-depth algorithms.  相似文献   

7.
不同污染条件下气溶胶对短波辐射通量影响的模拟研究   总被引：1，自引：0，他引：1  
将高光谱分辨率的气溶胶光学参数化方案应用于高精度的辐射传输模式BCC_RAD（974带）中,研究不同污染状况下气溶胶在地表与近地层大气中造成的直接辐射强迫与辐射强迫效率。发现气溶胶在地表产生的直接辐射强迫为负,在近地层大气中产生的直接辐射强迫为正,且随气溶胶浓度的升高变大,说明大气气溶胶的含量越高,单位气溶胶光学厚度产生的直接辐射强迫越大。将短波划分为3个波段:紫外、可见光和近红外,发现在紫外、可见光和近红外波段中,不同污染状况下气溶胶在地表造成的直接辐射强迫范围分别为:-1.36—-13.66、-3.03—-32.41和-2.74—-28.62 W/m2,在近地层大气中产生的直接辐射强迫范围分别为0.44—4.26、0.99—9.80和0.93—8.87 W/m2。通过进一步对比自然和人为气溶胶的影响,发现人为气溶胶在地表和大气层顶产生的负直接辐射强迫以及对整层和近地面大气造成的正直接辐射强迫均大于自然气溶胶的影响,且上述两种排放源的气溶胶对整层大气辐射收支的影响主要集中在800 hPa高度以下的大气中。按照地表直接辐射强迫大小来分析不同种类气溶胶的影响,结果为硫酸盐>有机碳>黑碳>海盐>沙尘;按照近地层大气直接辐射强迫大小排序则为黑碳>有机碳>沙尘>海盐>硫酸盐。最后,通过分析散射型气溶胶与吸收型气溶胶对辐射通量的影响,还探究了大气中散射与吸收过程的异同。   相似文献   

8.
In large-eddy simulations (LES) of the atmospheric boundary layer (ABL), near-surface models are often used to supplement subgrid-scale (SGS) turbulent stresses when a major fraction of the energetic scales within the surface layer cannot be resolved with the temporal and spatial resolution at hand. In this study, we investigate the performance of both dynamic and non-dynamic eddy viscosity models coupled with near-surface models in simulations of a neutrally stratified ABL. Two near-surface models that are commonly used in LES of the atmospheric boundary layer are considered. Additionally, a hybrid Reynolds- averaged/LES eddy viscosity model is presented, which uses Prandtl’s mixing length model in the vicinity of the surface, and blends in with the dynamic Smagorinsky model away from the surface. Present simulations show that significant portions of the modelled turbulent stresses are generated by the near-surface models, and they play a dominant role in capturing the expected logarithmic wind profile. Visualizations of the instantaneous vorticity field reveal that flow structures in the vicinity of the surface depend on the choice of the near-surface model. Among the three near-surface models studied, the hybrid eddy viscosity model gives the closest agreement with the logarithmic wind profile in the surface layer. It is also observed that high levels of resolved turbulence stresses can be maintained with the so-called canopy stress model while producing good agreement with the logarithmic wind profile.  相似文献   

9.
A one-dimensional model of the nocturnal boundary layer (NBL) has been used to investigate the time variation of the NBL height for stationary and horizontally homogeneous synoptic conditions. The time variation of the well known quantity = hflu * has been shown to be related to the wind variation at the top of the NBL. For the simple simulated conditions, this variation depends only on the roughness length and the Coriolis parameter. The value of averaged over the whole night is a function of the friction velocity. An expression is proposed for which is compared with observations. Under stationary external conditions, the new relation improves the determination of the NBL height if compared with the classical relation using a constant value of .  相似文献   

10.
The ongoing loss of Arctic sea-ice cover has implications for the wider climate system. The detection and importance of the atmospheric impacts of sea-ice loss depends, in part, on the relative magnitudes of the sea-ice forced change compared to natural atmospheric internal variability (AIV). This study analyses large ensembles of two independent atmospheric general circulation models in order to separate the forced response to historical Arctic sea-ice loss (1979–2009) from AIV, and to quantify signal-to-noise ratios. We also present results from a simulation with the sea-ice forcing roughly doubled in magnitude. In proximity to regions of sea-ice loss, we identify statistically significant near-surface atmospheric warming and precipitation increases, in autumn and winter in both models. In winter, both models exhibit a significant lowering of sea level pressure and geopotential height over the Arctic. All of these responses are broadly similar, but strengthened and/or more geographically extensive, when the sea-ice forcing is doubled in magnitude. Signal-to-noise ratios differ considerably between variables and locations. The temperature and precipitation responses are significantly easier to detect (higher signal-to-noise ratio) than the sea level pressure or geopotential height responses. Equally, the local response (i.e., in the vicinity of sea-ice loss) is easier to detect than the mid-latitude or upper-level responses. Based on our estimates of signal-to-noise, we conjecture that the local near-surface temperature and precipitation responses to past Arctic sea-ice loss exceed AIV and are detectable in observed records, but that the potential atmospheric circulation, upper-level and remote responses may be partially or wholly masked by AIV.  相似文献   

11.
A scheme for computing surface fluxes from mean flow observations   总被引：3，自引：0，他引：3  
A computational scheme is developed for estimating turbulent surface stress, sensible heat flux and humidity flux from mean velocity, temperature and humidity at a single height in the atmospheric surface layer; conditions at this reference level are presumed known from observations or from a numerical atmospheric circulation model. The method is based on coupling a Monin-Obukhov similarity profile to a force-restore formulation for the evolution of surface soil temperature to yield the local values of shear stress, heat flux and surface temperature. A self-contained formulation is presented including parameterizations for solar and infrared radiant flux at the surface.In addition to reference-level mean flow properties, the parameters needed to implement the scheme are thermal heat capacity of the soil, surface aerodynamic roughness, latitude, solar declination, surface albedo, surface emissivity and atmospheric transmissivity.Sample calculations are presented for (a), constant atmospheric forcing at the reference level, and (b) variable atmospheric forcing corresponding to Kahle's (1977) measurements of windspeed, air temperature and radiometer soil surface temperature under dry vegetatively sparse conditions in the Mohave Desert in California. The latter case simulated the observed diurnal variations resonably well for the parameters used.Consultant, Atmospheric Sciences Division, Department of Energy and Environment, Brookhaven National Laboratory, Upton, N.Y., pc11973, U.S.A.  相似文献   

12.
The interaction between radiation and turbulence in the stable boundary layer over land is explored using an idealized model, with a focus on the surface layer after the evening transition. It is shown that finer vertical resolution is required in transitional boundary layers than in developed ones. In very light winds radiative cooling determines the temperature profile, even if similarity functions without a critical Richardson number are used; standard surface similarity theory applied over thick layers then yields poor forecasts of near-surface air temperatures. These points are illustrated with field data. Simulations of the developing nocturnal boundary layer are used to explore the wider role of radiation. Comparatively, radiation is less significant within the developed stable boundary layer than during the transition; although, as previous studies have found, it remains important towards the top of the stable layer and in the residual layer. Near the ground, reducing the surface emissivity below one is found to yield modest relative radiative warming rather than intense cooling, which reduces the potential importance of radiation in the developed surface layer. The profile of the radiative heating rate may be strongly dependent on other processes, leading to quite varied behaviour.  相似文献   

13.
The impacts of background (or ambient) and local atmospheric thermal stabilities, and slope steepness, on nighttime thermally induced downslope flow in meso- domains (i.e., 20–200 km horizontal extent) have been investigated using analytical and numerical model approaches. Good agreement between the analytical and numerical evaluations was found. It was concluded that: (i) as anticipated, the intensity of the downslope flow increases with increased slope steepness, although the depth of the downslope flow was found to be insensitive to slope steepness in the studied situations; (ii) the intensity of the downslope flow is generally independent of background atmospheric thermal stability; (iii) for given integrated nighttime cooling across the nocturnal boundary layer (NBL), Q s the local atmospheric thermal stability exerts a strong influence on downslope flow behavior: the downslope flow intensity increases when local atmospheric thermal stability increases; and (iv) the downslope flow intensity is proportional to Q s 1/2.  相似文献   

14.
河南省非降水云中液态水的卫星微波反演试验研究   总被引：1，自引：1，他引：0       下载免费PDF全文
云中液态水分布对全球气候和局地天气变化有重要影响, 是判别人工影响天气作业潜力区的重要依据。利用TRMM卫星微波成像仪 (TMI) 85.5 GHz通道垂直极化亮温资料与NCEP再分析资料, 结合VDISORT模式采用逐步逼近方法反演了河南地区地表比辐射率; 再利用TRMM/TMI 85.5 GHz通道垂直极化亮温资料、TRMM/VIRS红外辐射资料及NCEP再分析资料, 结合VDISORT模式采用迭代的方法反演了河南地区云中液态水的垂直积分总含量。与红外卫星云图、TRMM卫星2A12产品及NCEP资料对比分析表明:该研究提出的反演陆地上空非降水云中液态水方法是可行的, 且对云中液态水垂直积分总含量水平分布的反演结果较对比产品结果更好。  相似文献   

15.
During the international La Crau-experiment in 1987 the surface temperature was measured using an infrared radiometer. Sensitivity of the derived temperature to variations of surface emissivity and atmospheric radiation are investigated in this paper. A 1% error in emissivity would bias the derived surface temperature about 0.5 K. The atmospheric radiation was calculated using Lowtran-7 and radiosonde profiles. A 10% error in atmospheric radiation changes the derived surface temperature by about 0.1 K if the emissivity is 0.96. Comparison with atmospheric radiation values derived from equations by Idso (1981) showed general agreement, but also a few very exceptional cases. Equations by Svendsenet al. (1990) yield values for surface temperature corrections that are about 5 times smaller than our values.  相似文献   

16.
On calm clear nights, air at a height of a few decimetres above bare soil can be cooler than the surface by several degrees in what we shall call the Ramdas layer (Ramdas and Atmanathan, 1932). The authors have recently offered a logical explanation for such a lifted temperature minimum, together with a detailed numerical model. In this paper, we provide physical insight into the phenomenon by a detailed discussion of the energy budget in four typical cases, including one with a lifted minimum. It is shown that the net cooling rate near ground is the small difference between two dominant terms, representing respectively radiative upflux from the ground and from the air layers just above ground. The delicate energy balance that leads to the lifted minimum is upset by turbulent transport, by surface emissivity approaching unity, or by high ground cooling rates. The rapid variation of the flux emissivity of humid air is shown to dominate radiative transport near the ground.  相似文献   

17.
Measurements of atmospheric composition have been made over a remote rainforest landscape. A box model has previously been demonstrated to model the observed daytime chemistry well. However the box model is unable to explain the nocturnal measurements of relatively high [NO] and [O3], but relatively low observed [NO2]. It is shown that a one-dimensional (1-D) column model with simple O3-NOx chemistry and a simple representation of vertical transport is able to explain the observed nocturnal concentrations and predict the likely vertical profiles of these species in the nocturnal boundary layer (NBL). Concentrations of tracers carried over from the end of the night can affect the atmospheric chemistry of the following day. To ascertain the anomaly introduced by using the box model to represent the NBL, vertically-averaged NBL concentrations at the end of the night are compared between the 1-D model and the box model. It is found that, under low to medium [NOx] conditions (NOx?<?1 ppbv), a simple parametrisation can be used to modify the box model deposition velocity of ozone, in order to achieve good agreement between the box and 1-D models for these end-of-night concentrations of NOx and O3. This parametrisation would could also be used in global climate-chemistry models with limited vertical resolution near the surface. Box-model results for the following day differ substantially if this effective nocturnal deposition velocity for ozone is implemented; for instance, there is a 9% increase in the following days peak ozone concentration. However under medium to high [NOx] conditions (NOx > 1 ppbv), the effect on the chemistry due to the vertical distribution of the species means no box model can adequately represent chemistry in the NBL without modifying reaction rate coefficients.  相似文献   

18.
江灏  王可丽 《高原气象》1997,16(3):250-257
针对辐射传输模式在青藏高原地区的应用问题，使用Ｌｉｏｕ－Ｏｕ一维辐射传输模式及１９８２年８月￣１９８３年７月青藏高原热源观测实验期间青藏高原地面、高空与卫星观测资料，在高原辐射传输模式中区分了下垫面温度与地表空气温度的作用，并利用卫星观测资料对模式改进后的实际效果进行了验证；分析了地表温度的日变化和季节变化硬度，得到了下垫面温度的简单参数化方法。  相似文献   

19.
SENSITIVITY OF SEA SURFACE TEMPERATURE RETRIEVAL TO SEA SURFACE EMISSIVITY   总被引：1，自引：0，他引：1  
To estimate sea surface temperature(SST)with high accuracy from radiometrie measure-ments,it is no longer acceptable to assume that sea surface emissivity is unity or any other con-stant.This note presents an investigation of the desirable emissivity accuracy in relation to re-trieval.It was found that 1% error in surface emissivity can cause up to 0.7 K error in the re-trieved SST,although this sensitivity is often reduced to about 0.5 K on average because of thedownward atmospheric radiation at surface partially compensates for the emissivity error.Since thedownward atmospheric radiation ratio is controlled to a large extent by the integrated water vaporin the atmosphere and,secondarily,by view angle,the sensitivity of SST retrieval to surface emis-sivity has been computed as a function of these two parameters.  相似文献   

20.
Because the atmosphere and ocean are interacting systems, it is inappropriate to specify sea surface temperature when dealing with the atmosphere, or atmospheric anemometer level temperature and moisture when dealing with the ocean. All of these quantities should be determined interactively in terms of the external forcing: the solar constant.In the tropics, it is shown that the (cumulus) convective processes may be described by a one-dimensional cloud model. The near-surface ocean may similarly be described by a one-dimensional mixed-layer model. The coupling is achieved through a sea surface flux budget combined with the flux parameterizations implied by Monin-Obukhov similarity theory.The coupled one-dimensional atmosphere-ocean model is applied to the equilibrium situation in which all temperatures reach a steady state. Since the ocean, lacking an internal heating or cooling mechanism, can only be heated or cooled through sensibleheat fluxes through the sea surface, in equilibrium these fluxes must vanish. The atmosphere, however, maintains a stable lapse rate by balancing cumulonimbus heating against net radiative cooling. All water precipitated from cumulonimbus clouds must have evaporated from sea surface. It is shown that this equilibrium system is closed and determinable solely in terms of the solar constant.For various values of the solar constant, the sea surface temperature, the flux of latent and sensible heat from the surface, the height of the tropopause, mixed layer, and trade inversion layer, and generally, the entire vertical structure of the tropical atmosphere and near-surface ocean can be determined. The equilibrium sea surface temperature is shown to be relatively insensitive to changes in the solar constant, additional solar flux being compensated mainly by additional evaporation. Finally, the usefulness and limitations of the model are pointed out.  相似文献   

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