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1.
The vertical velocity field and the convective plumes in the atmospheric boundary layer have been observed during morning hours with the acoustic Doppler sounder of the C.R.P.E. A method for plume determination using acoustic soundings in the well-mixed layer is presented. Using Telford's 1970 and Manton's 1975 models, a comparison is made between the predictions of the models and the plume properties as observed by the Doppler sodar. The mean plume velocity is found to be parabolic. It is shown, restricting Monin and Obukhov similarity to conditions inside plumes and using only vertical velocity within plumes, that the observed convective plumes carry nearly sixty percent of the sensible heat flux at the top of the surface layer. 相似文献
2.
《Dynamics of Atmospheres and Oceans》1997,26(2):95-130
A one-dimensional penetrative plume model has been constructed to parameterize the process of deep convection in ocean general circulation models (OGCMs). This research is motivated by the need for OGCMs to better model the production of deep and intermediate water masses. The parameterization scheme takes the temperature and salinity profiles of OGCM grid boxes and simulates the subgrid-scale effects of convection using a one-dimensional parcel model. The model moves water parcels from the surface layer down to their level of neutral buoyancy, simulating the effect of convective plumes. While in transit, the plumes exchange water with the surrounding environment; however, the bulk of the plume water mass is deposited at e level of neutral buoyancy. Weak upwelling around the plumes is included to maintain an overall mass balance. The process continues until the negative buoyant energy of the one-dimensional vertical column is minimized. The parameterized plume entrainment rate, which plays a central role in the parameterization, is calculated using modified equations based on the physics of entraining buoyant plumes. This scheme differs from the convective adjustment techniques currently used in OGCMs, because the parcels penetrate downward with the appropriate degree of mixing until they reach their level of neutral stability. 相似文献
3.
利用 1 998年第 2次青藏高原野外试验中的多普勒声雷达探测、低空探测观测以及卫星观测资料对高原大气边界层内的对流现象进行分析研究。声雷达探测到了高原边界层内有强烈的对流活动。这种对流泡中心的垂直速度可超过 1m/s,并存在尺度为 1个多小时的周期性 ,表现为中小尺度的有组织的湍流活动。高原边界层强对流得以发展和维持的物理机制是 :强辐射加热、复杂的地形地貌形成的下垫面不均一性造成边界层斜压性、边界层内的平流活动等 ,这些现象都有利于对流的发展。在这些条件的作用下 ,边界层内可以产生一系列有组织的强湍流大涡旋活动 ,这些大涡旋形成的热泡在向上发展的过程中有的能够发生合并 ,变得更大也更为猛烈 ,达到凝结高度以上可形成对流云 ,并发生充分的对流混合。成云过程凝结潜热释放更有利于对流运动进一步发展 ,使对流云逐步发展成更大的对流云团 ,从而产生卫星云图中显示的云团发展过程。 相似文献
4.
Bryan R. Kerman 《Boundary-Layer Meteorology》1979,16(3):395-408
A model of buoyancy- and momentum-driven industrial plumes in a freely convective boundary layer is proposed. The development combines the Lagrangian similarity models of Yaglom for non-buoyant releases in the convective surface layer with the Scorer similarity model for industrial plumes. Constraints on the validity of the extension of Yaglom’s model to the entire convective planetary boundary layer, arrived at by consideration of Batchelor’s formulation for diffusion in an inertial subrange, are often met in practice. The resulting formulation applies to an interval of time in which the entrainment of the atmosphere by the plume is balanced by the entrainment of the plume by the atmosphere. It is argued that during this interval, both maximum plume rise and ground contact are achieved. Further examination of the physical interrelationship with the Csanady-Briggs formulation serves to consolidate the model hypotheses, as well as to simplify the derivation of maximum ground-level concentrations. Experimental evidence is presented for the validity of the model, based on Moore’s published data. 相似文献
5.
Bryan R. Kerman 《Boundary-Layer Meteorology》1979,16(4):395-408
A model of buoyancy- and momentum-driven industrial plumes in a freely convective boundary layer is proposed. The development combines the Lagrangian similarity models of Yaglom for non-buoyant releases in the convective surface layer with the Scorer similarity model for industrial plumes. Constraints on the validity of the extension of Yaglom’s model to the entire convective planetary boundary layer, arrived at by consideration of Batchelor’s formulation for diffusion in an inertial subrange, are often met in practice. The resulting formulation applies to an interval of time in which the entrainment of the atmosphere by the plume is balanced by the entrainment of the plume by the atmosphere. It is argued that during this interval, both maximum plume rise and ground contact are achieved. Further examination of the physical interrelationship with the Csanady-Briggs formulation serves to consolidate the model hypotheses, as well as to simplify the derivation of maximum ground-level concentrations. Experimental evidence is presented for the validity of the model, based on Moore’s published data. 相似文献
6.
Keith Julien Sonya Legg Jim McWilliams Joseph Werne 《Dynamics of Atmospheres and Oceans》1996,24(1-4)
Turbulent convection forced by a surface heat flux into a stably stratified region is a feature of both the atmospheric and oceanic planetary boundary layers. Of particular interest is the interface between the convective layer and the stable stratification, where the entrainment of fluid into the convective layer by penetrating plumes may lead to a reverse buoyancy flux, and an enhancement of the stable stratification. Whereas in the atmosphere the influence of rotation on this penetrative convection is negligible, oceanic convection may be subjected to lower Rossby numbers and hence greater rotational influence. To isolate the effects of rotation, we present three numerical solutions for turbulent penetrative convection, characterised by different rotation rates, with all other parameters being held constant. Our results indicate that at lower Rossby numbers the lateral scale of the plumes is reduced, whereas the vertical vorticity of the plumes is much enhanced. Vertical transports of buoyancy and kinetic energy across the convective layer are reduced, leading to less efficient penetration at the interface with the stratified layer, and hence less reverse buoyancy flux in this region. 相似文献
7.
An acoustic echo sounder situated in downtown Toronto has been used to detect convective plumes in the planetary boundary layer and to measure, by means of the Doppler effect, the vertical air motions associated with them. The plumes observed were the order of 390 m in horizontal extent, were detectable to a height of about 400 m, and were characterized by peak upward velocities in excess of 1 m s?1. The sounder measurements are shown to be consistent with surface meteorological parameters, and suggest that free convection over an urban area of considerable surface roughness and non‐uniformity is not greatly different from that over uniform land surfaces or water. 相似文献
8.
Experimental data for buoyant plumes released from high sources into layers having little ambient turbulence show that plume dispersion parameters vary in a manner similar to that during initial plume rise. This is consistent with general plume rise theory. Dispersion of plumes from tall stacks in a shoreline environment where a thermal internal boundary layer is formed often demonstrates this behaviour. 相似文献
9.
Nobuyuki Kinoshita 《Boundary-Layer Meteorology》2003,106(1):61-91
Turbulent flow data of wind velocity and temperature in the unstably stratifiedatmospheric boundary layer, derived from steel tower observations in the field and wind-tunnel experiments were used to study the relationship between the plumes and the small-scale eddies in the inertial subrange. Flow visualisation experiments in the wind tunnel were also conducted to observe the structure of the flow in the plumes, and time series data were analysed by using wavelet transforms. The results show that variances of velocity and temperature due to the small-scale eddies are large in the plumes and small outside of the plumes, and that the momentum and heat fluxes due to the small-scale eddies follow the same tendency as found in the variances. The ratios of the variances caused by the small-scale eddies in the plumes to the whole of the variances caused by the small-scale eddies in and out the plumes increase with non-dimensional height -z/L in which L is the local Obukhov length. Similar ratios of the fluxes caused by the small-scale eddies also show the same tendency. These ratios can be expressed as functions of -z/L for results based on field observationand the wind tunnel experiments. This relation hardly changes even if the wavelet function is changed. The flow visualisation experiments show that the plumes have a complicated structure in which mushroom type flows are stacked on top of each other. This characteristic structure seems to increase the energy of the small-scale eddies in the plumes. 相似文献
10.
Boundary-Layer Meteorology - The vertical eddy diffusion coefficient (K z) for five smoke plumes entrapped in a stable marine layer is estimated from the published LANDSAT imagery data sets of... 相似文献
11.
The processes of interaction between the atmospheric surface and mixed layers in daytime convective conditions over land are studied using a data set obtained during flights by an instrumented aircraft. Profiles of partitioned run-averaged statistics and examples of time series plots are discussed in the light of results from a recently published study by the authors, in which the average structure and flow within coherent eddies was reconstruced using a compositing technique. This evidence is used to support a conceptual model of the mechanisms of interaction between surface-layer plumes and mixed-layer thermal columns. The divergent flow created near the surface by the downdraft arms of the large-scale mixed-layer circulation patterns, forces the development of lines of convergence in the surface layer (the so-called thermal walls), which channel air into the bases of the mixed-layer thermals. Plumes progressively group and merge together with height in the surface and free convection layers, and move along these convergence lines toward large collector plumes at the intersection points, or hubs. Above the hubs are the thermals, and air parcels originating from plumes and their environment are strongly mixed as they rise, leading to an increased difficulty of the conditional sampling method to distinguish between them. The observed influence of mixed-layer convective processes far down into the surface layer, and the form of the averaged profiles, supports recent refinements of the theory of surface-layer structure suggested in Kader and Yaglom (1990).Notation CBL
convective boundary layer
- SL
surface layer
- FCL
free convection layer
- ML
mixed layer 相似文献
12.
Laboratory experiments were carried out to investigate the interaction between turbulent line buoyant plumes and sharp density interfaces, with the aim of using the results to interpret oceanic observations pertinent to crack openings in the polar ice-cap (leads). These openings take the form of long narrow channels, and are often modeled as line bouyant plumes. The plumes descend as in a homogenoous fluid, impinge on the density interface, and then spread horizontally as gravity currents. Depending on the Richardson number
, where Δb is the buoyancy jump across the interface, lD is the half-width of the plume before the impingement and q0is the buoyancy flux per unit length of the source, different flow patterns were identified. When Ri < 0.5, the plumes penetrate deep into the bottom layer, deflect horizontally and then spread while showing little vertical rise. When 0.6 < Ri < 5, the penetration is significant, but the fluid bounces back after entraining heavy fluid from the lower layer and then spreads horizontally above the interface as a gravity current. Appreciable mixing between this current and the lower layer was detected when Ri <1. When Ri > 10, the penetration was small and a sharp-nosed gravity current emerged some time after the impact. Measurements were made on the penetration depth, the velocities of the gravity current and the subsurface flow towards the plume, the entrainment rate and other wave parameters. Possible implications of the results for oceanic cases are also discussed. 相似文献
13.
Jason H. Middleton 《Boundary-Layer Meteorology》1986,36(1-2):187-199
Solutions are found describing the rise of turbulent buoyant plumes in a neutral or stably stratified crossflow from a source having finite fluxes of buoyancy, momentum and volume. Plumes from such sources are known as forced plumes. The solutions arise from a unified theory having one disposable parameter, the entrainment constant, and they describe a wide range of behaviour of plumes and jets in a crossflow. Solutions for buoyant plumes indicate that, for any given source, heights of rise are reduced with increased crossflow velocity and increased stratification while for constant environmental parameters, heights of rise are increased with increased fluxes of source buoyancy and momentum and are decreased with source radius. Maximum dilutions occur in plumes from small sources with relatively large buoyancy fluxes in light crossflows with small stratification. 相似文献
14.
Although it is well known that sea-ice regions are important components of the Earth's climate system, the exchanges of energy between ocean, ice and atmosphere are not well understood. The majority of past observational and modelling studies of atmosphere-surface interactions over sea-ice regions were primarily concerned with airflow over a single, isolated area of open water. The more realistic situations of multiple polynyas within a sea-ice field and different areal concentrations of sea ice were studied here. Spatial structure of the atmospheric boundary layer in response to this surface was simulated using a high-resolution numerical model. A sea-ice concentration of 80%, typical of the Southern Ocean sea-ice zone, was maintained within a 100-km wide domain. The effects of three polynya characteristics were assessed: their horizontal extent; local concentration of sea ice (LCI); and their arrangement with ice floes. Over polynyas of all sizes distinct plumes of upward heat flux, their width and height closely linked to polynya width, resulted in mixed layers 600 to 1000 m deep over and downwind of the polynyas, their depth increasing with polynya width. Mean surface heat flux (MSHF) increased with size in polynyas less than 30 km wide. The air-to-ice MSHF over the first 10 km of sea-ice downwind of each polynya and the domain-average surface heat flux increased linearly with polynya width. Turbulent kinetic energy plumes occurred over all polynyas, their heights and widths increasing with polynya widths. Downward flux of high momentum air in the plumes caused increased wind speeds over polynyas in the layer from about 300–1000 m above the surface, the depth varying directly with polynya width. MSHFs decreased as LCIs increased. The arrangement of polynyas had relatively little effect on the overall depth of the modified layer but did influence the magnitude and spatial structure of vertical heat transfer. In the two-polynya case the MSHF over the polynyas was larger when they were closer together. Although the MSHF over the sea ice between the polynyas decreased in magnitude as their separation increased, the percentage of the polynya-to-air heat recaptured by this ice floe increased fivefold. 相似文献
15.
William H. Snyder Robert E. Lawson Jr. Michael S. Shipman Jie Lu 《Boundary-Layer Meteorology》2002,102(3):335-366
A laboratory convection tank has been established following thepioneering work of Willis and Deardorff, but with many improvements and enhancements thattake advantage of modern technology. The main emphasis in the current design was toprovide the ability to conduct a virtually unlimited number of realizations under essentiallyidentical conditions in order to obtain reliable statistics on the dispersion of plumes and puffsreleased within the simulated atmospheric convective boundary layer. Described herein is the tankitself and its auxiliary systems, including a laser-induced-fluorescence and video-imaging system for makingnon-intrusive, full-field measurements of concentrations, and the interfacing of varioussubsystems with a master controller that automates essentially all operation and measurement functions.The current system provides unprecedented resolution, control, and data volumes. Exampleresults are presented from two types of releases: continuous plumes and instantaneous puffs.These data sets clearly show penetration of the highly buoyant plumes and puffs into theinversion above the convective boundary layer, gravity spreading within the inversion, andrapid diffusion within the mixed layer. They also show extreme `spottiness' in the instantaneousconcentration cross-sections. 相似文献
16.
Interaction between wind and temperature fields in the planetary boundary layerfor a spatially heterogeneous surface heat flux has been investigated using large-eddysimulation. It is shown that a substantial difference exists in the wind and temperaturefields, depending on whether the directions of the background wind and the surfaceheat flux variation are parallel or perpendicular.When they are parallel to each other, two-dimensional plumes induced by theheterogeneous surface heat flux are easily destroyed by the background wind,and the velocity field is strongly modified by convective eddies compared tothe case when they are perpendicular to each other. This leads to a substantialdifference in the profiles of turbulent kinetic energy and its flux.It also results in a difference between the two cases in the bulk properties of theplanetary boundary layer, such as the entrainment at the top of the planetary boundarylayer and the drag at the bottom, which have important implications for boundary-layermodelling. The difference between the two cases exists even when the background windspeed is as large as 15.0 m s-1. Meanwhile, the contrast between two cases is weakened by the Coriolis force. 相似文献
17.
Abstract A simplified cumulus parameterization scheme, suitable for use in GCMs, is presented. This parameterization is based on a plume ensemble concept similar to that originally proposed by Arakawa and Schubert (1974). However, it employs three assumptions which significantly simplify the formulation and implementation of the scheme. It is assumed that an ensemble of convective‐scale updrafts with associated saturated downdrafts may exist when the atmosphere is locally conditionally unstable in the lower troposphere. However, the updraft ensemble is comprised only of those plumes which are sufficiently buoyant to penetrate through this unstable layer. It is assumed that all such plumes have the same upward mass flux at the base of the convective layer. The third assumption is that moist convection, which occurs only when there is convective available potential energy (CAPE) for reversible ascent of an undiluted parcel from the sub‐cloud layer, acts to remove CAPE at an exponential rate with a specified adjustment time scale. The performance of the scheme and its sensitivity to choices of disposable parameters is illustrated by presenting results from a series of idealized single‐column model tests. These tests demonstrate that the scheme permits establishment of a quasi‐equilibrium between large‐scale forcing and convective response. However, it is also shown that the strength of convective downdrafts is an important factor in determining the nature of the equilibrium state. Relatively strong down‐drafts give rise to an unsteady irregularly fluctuating state characterized by alternate periods of deep and shallow convection. The effect of using the scheme for GCM climate simulations is illustrated by presenting selected results of a multi‐year simulation carried out with the Canadian Climate Centre GCM using the new parameterization (the CONV simulation). Comparison of these results with those for a climate simulation made with the standard model (the CONTROL simulation, as documented by McFarlane et al., 1992) reveals the importance of other parameterized processes in determining the ultimate effect of introducing the new convective scheme. The radiative response to changes in the cloudiness regime is particularly important in this regard. 相似文献
18.
By ALBERT ANSMANN MATTHIAS TESCHE PETER KNIPPERTZ EIKE BIERWIRTH DIETRICH ALTHAUSEN DETLEF MÜLLER OLIVER SCHULZ 《Tellus. Series B, Chemical and physical meteorology》2009,61(1):340-353
Lifting of dust particles by dust devils and convective plumes may significantly contribute to the global mineral dust budget. During the Saharan Mineral Dust Experiment (SAMUM) in May–June 2006 vertical profiling of dusty plumes was performed for the first time. Polarization lidar observations taken at Ouarzazate (30.9°N, 6.9°W, 1133 m height above sea level) are analyzed. Two cases with typical and vigorous formation of convective plumes and statistical results of 5 d are discussed. The majority of observed convective plumes have diameters on order of 100–400 m. Most of the plumes (typically 50–95%) show top heights <1 km or 0.3DLH with the Saharan dust layer height DLH of typically 3–4 km. Height-to-diameter ratio is mostly 2–10. Maximum plume top height ranges from 1.1 to 2.9 km on the 5 d. 5–26 isolated plumes and clusters of plumes per hour were detected. A low dust optical depth (<0.3) favours plume evolution. Observed surface, 1 and 2–m air temperatures indicate that a difference of 17–20 K between surface and 2-m air temperature and of 0.9–1 K between the 1 and 2-m temperatures are required before convective plumes develop. Favourable horizontal wind speeds are 2–7 m s−1 . 相似文献
19.
Comparison of meteorological tower measurements with a high resolution CW-FM radar indicates that the device is applicable to studies of structure of temperature inversions, stable and unstable waves upon such inversions, thermal plumes and convective processes within the boundary layer. The radar also appears to follow the transport of insects by such processes. 相似文献
20.
Frédéric Hourdin Jean-Yves Grandpeix Catherine Rio Sandrine Bony Arnaud Jam Frédérique Cheruy Nicolas Rochetin Laurent Fairhead Abderrahmane Idelkadi Ionela Musat Jean-Louis Dufresne Alain Lahellec Marie-Pierre Lefebvre Romain Roehrig 《Climate Dynamics》2013,40(9-10):2193-2222
Based on a decade of research on cloud processes, a new version of the LMDZ atmospheric general circulation model has been developed that corresponds to a complete recasting of the parameterization of turbulence, convection and clouds. This LMDZ5B version includes a mass-flux representation of the thermal plumes or rolls of the convective boundary layer, coupled to a bi-Gaussian statistical cloud scheme, as well as a parameterization of the cold pools generated below cumulonimbus by re-evaporation of convective precipitation. The triggering and closure of deep convection are now controlled by lifting processes in the sub-cloud layer. An available lifting energy and lifting power are provided both by the thermal plumes and by the spread of cold pools. The individual parameterizations were carefully validated against the results of explicit high resolution simulations. Here we present the work done to go from those new concepts and developments to a full 3D atmospheric model, used in particular for climate change projections with the IPSL-CM5B coupled model. Based on a series of sensitivity experiments, we document the differences with the previous LMDZ5A version distinguishing the role of parameterization changes from that of model tuning. Improvements found previously in single-column simulations of case studies are confirmed in the 3D model: (1) the convective boundary layer and cumulus clouds are better represented and (2) the diurnal cycle of convective rainfall over continents is delayed by several hours, solving a longstanding problem in climate modeling. The variability of tropical rainfall is also larger in LMDZ5B at intraseasonal time-scales. Significant biases of the LMDZ5A model however remain, or are even sometimes amplified. The paper emphasizes the importance of parameterization improvements and model tuning in the frame of climate change studies as well as the new paradigm that represents the improvement of 3D climate models under the control of single-column case studies simulations. 相似文献