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1.
High frequency measurements of near-surface meteorological data acquired in north Benin during the 2006 West African monsoon seasonal cycle, in the context of the African Monsoon Multidisciplinary Analysis (AMMA) experiment, offer insight into the characteristics of surface turbulence in relation to planetary boundary-layer (PBL) processes. A wide range of conditions is encountered at the lower and upper limits of the PBL: (i) from water-stressed to well-fed vegetation, and (ii) from small to large humidity and temperature jumps at the PBL top inversion, due to the Saharan air layer overlying the monsoonal flow. As a result, buoyant convection at the surface and entrainment at the PBL top play very different roles according to the considered scalar. We show that, when the boundary-layer height reaches the shear level between the monsoonal and Harmattan flows, the temperature source and humidity sink at the boundary-layer top are sufficient to allow the entrainment to affect the entire boundary layer down to the surface. This situation occurs mainly during the drying and moistening periods of the monsoon cycle and affects the humidity statistics in particular. In this case, the humidity turbulent characteristics at the surface are no longer driven solely by buoyant convection, but also by entrainment at the boundary-layer top. Consequently, the Monin–Obukhov similarity theory appears to fail for the parameterisation of humidity-related moments.  相似文献   

2.
Mean Profiles of Moisture Fluxes in Snow-Filled Boundary Layers   总被引：1，自引：0，他引：1  
Profiles of moisture fluxes have been examined for convective boundary layers containing clouds and snow, using data derived from aircraft measurements taken on four dates during the 1983/1984 University of Chicago lake-effect snow project. Flux profiles were derived from vertical stacks of aircraft cross-wind flight legs taken at various heights over Lake Michigan near the downwind shore. It was found that, if ice processes are taken into account, profiles of potential temperature and water content were very similar to those presented in past studies of convective boundary layers strongly heated from below. Profiles of total water content and equivalent potential temperature adjusted for ice were nearly invariant with height, except very near the top of the boundary layer, suggesting that internal boundary-layer mixing processes were rapid relative to the rates at which heat and vapour were transported into the boundary layer through entrainment and surface fluxes. Ice was found to play a significant, measurable role in boundary-layer moisture fluxes. It was estimated that 40 to 57% of the upward vapour flux was returned to the surface in the form of snow, converting about 45 to 64% of the surface latent heat flux into sensible heat in the snow-producing process. Assuming advective fluxes are relatively small (thought to be appropriate after the first few tens of km over the lake as suggested by past studies), the boundary layer was found to warm at a rate faster than could be explained by surface heat fluxes and latent heat releases alone, the remainder of the heating presumably coming from radiational processes and entrainment. Discussions of moisture phase change processes throughout the boundary layer and estimates of errors of these flux measurements are presented.  相似文献   

3.
We present a method to detect influences of boundary-layer processes on surface-layer measurements, using statistics and spectra of surface-layer variables only. We validated our detection method with boundary-layer measurements. Furthermore, we confirm that Monin–Obukhov similarity functions fit well to temperature-variance data obtained at two different homogeneous surfaces. However, we found that humidity variance measurements deviate from the universal functions above one of the two studied surfaces for days on which entrained air reached the surface layer. These results confirm that Monin–Obukhov similarity theory should be used with care in the analysis of surface-layer data. Finally, we propose the use of an extra term in flux-variance relations that depends on the entrainment ratio for humidity and on the boundary-layer height. If boundary-layer measurements are not available, we show how the entrainment ratio for humidity can be approximated from the skewness of the humidity distribution.  相似文献   

4.
We propose improvements in the “non-local” parameterization scheme of the convective boundary layer. The countergradient terms for components of the momentum fluxes are introduced in a form analogous to those for other scalars. The scheme also includes explicit expressions for entrainment fluxes of momentum, temperature, and humidity. A simplified procedure for calculating the boundary-layer height is proposed, consisting of two steps: the evaluation of the convection level, followed by the assessment of the depth of the interfacial layer.  相似文献   

5.
Studies of entrainment across the top of the boundary layer rely to a great extent on identification of the boundary-layer top, inversion properties, entrainment-zone depth, and the temporal changes in all of these. A variety of definitions and techniques have been used to provide automated and objective estimates; however, direct comparisons between studies is made difficult by the lack of consistency in techniques. Here we compare boundary-layer depth, entrainment-zone thickness, and entrainment rate derived from several commonly used techniques applied to a common set of large-eddy simulations of the idealized, dry, convective boundary layer. We focus in particular on those techniques applicable to lidar backscatter measurements of boundary-layer structure. We find significant differences in all the quantities of interest, and further that the behaviour as functions of common scaling parameters, such as convective Richardson number, also differ, sometimes dramatically. The discretization of the possible values of some quantities imposed by the vertical grid is found to affect some of the results even when changes to model resolution does not affect the entrainment rate or scaling behaviour. This is a particular problem where entrainment parameters are derived from a single mean profile (e.g. the buoyancy-flux profile), but not where they are derived from the statistical properties of large numbers of individual profiles (e.g. the probability distribution of the local boundary-layer top at each model grid point).  相似文献   

6.
We examine daily (morning–afternoon) transitions in the atmospheric boundary layer based on large-eddy simulations. Under consideration are the effects of the stratification at the top of the mixed layer and of the wind shear. The results describe the transitory behaviour of temperature and wind velocity, their second moments, the boundary-layer height Z m (defined by the maximum of the potential temperature gradient) and its standard deviation σ m , the mixed-layer height z i (defined by the minimum of the potential temperature flux), entrainment velocity W e, and the entrainment flux H i . The entrainment flux and the entrainment velocity are found to lag slightly in time with respect to the surface temperature flux. The simulations imply that the atmospheric values of velocity variances, measured at various instants during the daytime, and normalized in terms of the actual convective scale w*, are not expected to collapse to a single curve, but to produce a significant scatter of observational points. The measured values of the temperature variance, normalized in terms of the actual convective scale Θ*, are expected to form a single curve in the mixed layer, and to exhibit a considerable scatter in the interfacial layer.  相似文献   

7.
The marine atmospheric boundary layer is characterized by cool temperatures and high humidity. Clouds are observed over most of the oceans. It is generally accepted that these overcast cloud decks break up into scattered fragments due to cloud-top entrainment instability. That is, if the air above the boundary layer is sufficiently cool and dry relative to cloud top, the buoyancy flux will be directed upwards and entrainment can occur freely.A boundary-layer model is used to test the sensitivity of the model atmosphere to the various processes which promote the onset of cloud-top entrainment instability. It is found that the transition from a solid cloud deck to scattered cumulus clouds depends on a rate process. The cloud cover is sensitive to mesoscale variations in sea surface temperature only if the cloud-top inversion is sufficiently weak.  相似文献   

8.
Atmospheric Boundary-Layer Dynamics with Constant Bowen Ratio   总被引：1，自引：1，他引：0  
Motivated by the observation that the diurnal evolution of sensible and latent heat fluxes tends to maintain a constant Bowen ratio, we derive approximate solutions of the ordinary differential equations of a simplified atmospheric boundary-layer (ABL) model. Neglecting the early morning transition, the potential temperature and specific humidity of the mixed layer are found to be linearly related to the ABL height. Similar behaviour is followed by the inversion strengths of temperature and humidity at the top of the ABL. The potential temperature of the mixed layer depends on the entrainment parameter and the free-atmosphere temperature lapse rate, while the specific humidity also depends on the free-atmosphere humidity lapse rate and the Bowen ratio. The temporal dynamics appear only implicitly in the evolution of the height of the boundary layer, which in turn depends on the time-integrated surface sensible heat flux. Studying the limiting behaviour of the Bowen ratio for very low and very large values of net available energy, we also show how the tendency to maintain constant Bowen ratio during midday hours stems from its relative insensitivity to the atmospheric conditions for large values of net available energy. The analytical expression for the diurnal evolution of the ABL obtained with constant Bowen ratio is simple and provides a benchmark for the results of more complex models.  相似文献   

9.
The morning transition between the stable nocturnal situation and the daytime convective boundary layer (CBL) is of interest both for basic understanding and for initializing prognostic models. While the morning growth phase of the CBL has been studied in detail, relatively little has been published on the transition itself. In this paper, conventional observations of surface temperature, humidity, and turbulent fluxes,and data from a meteorological tower, are combined with measurements of the onset of convection by boundary-layer wind profilers to explore the timing and behaviour of the transition period. The transition is defined here as the period between sunrise and the time at which the depth ofconvection reaches about 200 m AGL. Diagnostic relationships based on surface heat flux, the temperature difference between 2 m and 200 m, and bulk Richardson number are explored. The transition is foundto be enabled by surface heating relaxing the surface stability, while the warming of the layerbetween 2 m and 200 m is in large part due to shear-driven entrainment.  相似文献   

10.
The mean structure calculated by a three-dimensional numerical model of a heated planetary boundary layer, in simulation of DAY 33 of the Australian Wangara data, has been previously described. The present study supplements it by describing properties of the calculated turbulence.A major finding is the importance of entrainment upon turbulence statistics relating to specific humidity, relative to those for potential temperature. The variances, skewness and spectra of velocity, temperature and humidity are presented, as are budget equations for kinetic energy, temperature and humidity variances and heat/moisture fluxes. These are interpreted with regard to the relative importance of the surface flux vs the flux due to entrainment at the top of the mixed layer, and in regard to the structure which would occur if the entrainment were to vanish.The Rotte-type closure assumption is tested for the correlation between the pressure fluctuation and the vertical gradient of vertical velocity, potential temperature, or specific humidity, and found to be qualitatively correct except near the top of the mixed layer.NCAR is sponsored by the National Science Foundation (U.S.A.).  相似文献   

11.
Boundary-layer measurements made from the Swedish icebreaker Oden during the Arctic Ocean Experiment 2001 (AOE-2001) are analysed. They refer mainly to ice drift in the central Arctic during the period 2–21 August 2001. On board Oden a remote sensing array with a wind profiler, cloud radar and a scanning microwave radiometer, and a regular weather station operated continuously; soundings were also released during research stations. Turbulence and profile measurements on an 18-m mast were deployed on the ice, along with two sodar systems, a microbarograph array and a tethered sounding system. Surface flux and meteorological stations were also deployed on nearby ice floes. There is a clear diurnal cycle in radiation and also in wind speed, cloud base and visibility. It is absent in temperature and humidity, probably due to the very strong control by melting/ freezing ice and snow. In the advection of warm air, latent heat of melting maintains the surface temperature at 0 °C, while with a negative energy balance the latent heat of freezing of the salty ocean water acts to maintain the surface temperature > −2 °C. The constant presence of water at the surface maintains a relative humidity close to 100%, and this is also often facilitated by an increasing specific humidity through the capping inversion, making entrainment a moisture source. This ensures cloudy conditions, with low cloud and fog prevailing most of the time. Intrusions of warm and moist air from beyond the ice edge are frequent, but the local Arctic boundary layer remains at a relatively constant temperature, and is shallow and well mixed with strong capping inversions. Power spectra of surface-layer wind speed sometimes show large variance at low frequency. A scanning radiometer provides a monitoring of the vertical thermal structure with a spatial and temporal resolution not seen before in the Arctic. There are often two inversions, an elevated main inversion and a weak surface inversion, and occasionally additional inversions occur. Enhanced entrainment across the main inversion appears to occur during frontal passages. Variance of the scanning radiometer temperatures occurs in large pulses rather than varying smoothly, and the height to the maximum variance appears to be a reasonable proxy for the boundary-layer depth.  相似文献   

12.
Summary Water vapour flux profiles in the atmospheric boundary layer have been derived from measurements of water vapour density fluctuations by a ground-based Differential Absorption Lidar (DIAL) and of vertical wind fluctuations by a ground-based Doppler lidar. The data were collected during the field experiment LITFASS-2003 in May/June 2003 in the area of Lindenberg, Germany. The eddy-correlation method was applied, and error estimates of ±50 W/m2 for latent heat flux were found. Since the sampling error dominates the overall measurement accuracy, time intervals between 60 and 120 min were required for a reliable flux calculation, depending on wind speed. Rather large errors may occur with low wind speed because the diurnal cycle restricts the useful interval length. In the lower height range, these measurements are compared with DIAL/radar-RASS fluxes. The agreement is good when comparing covariance and error values. The lidar flux profiles are well complemented by tower measurements at 50 and 90 m above ground and by area-averaged near surface fluxes from a network of micrometeorological stations. Water vapour flux profiles in the convective boundary layer exhibit different structures mainly depending on the magnitude of the entrainment flux. In situations with dry air above the boundary layer a positive entrainment flux is observed which can even exceed the surface flux. Flux profiles which linearly increase from the surface to the top of the boundary layer are observed as well as profiles which decrease in the lower part and increase in the upper part of the boundary layer. In situations with humid air above the boundary layer the entrainment flux is about zero in the upper part of the boundary layer and the profiles in most cases show a linear decrease.  相似文献   

13.
The dissimilarity of temperature and humidity transfer in the marine surface layer (MSL) is investigated through the relative transport efficiency and correlation coefficient of these two scalars. We examine their variability and relationship with mean values, as well as spectral characteristics. It is shown that the dissimilarity between these two scalars in the MSL is a function of stability, the boundary-layer depth, and flow steadiness. In general the temperature and humidity are less correlated in shallow marine boundary layers compared to deep marine boundary layers, due to the stronger impact of the boundary-layer scale in breaking the “same source, same sink” assumption for scalar similarity. This is supported by the combination of our spectral analysis of scalar fluxes and corresponding measured and modelled boundary-layer depth. This assumption is also broken in near-neutral conditions, when there is an efficient latent heat transfer but negligible sensible heat transfer. Our data suggest that parametrization of humidity fluxes via similarity theory could still be reliable when the correlation coefficient $>$ 0.5, and in near-neutral conditions the humidity flux can be estimated without use of the sensible heat flux.  相似文献   

14.
For the presentation and analysis of atmospheric boundary-layer (ABL) data, scales are used to non-dimensionalise the observed quantities and independent variables. Usually, the ABL height, surface sensible heat flux and surface scalar flux are used. This works well, so long as the absolute values of the entrainment ratio for both the scalar and temperature are similar. The entrainment ratio for temperature naturally ranges from −0.4 to −0.1. However, the entrainment ratio for passive scalars can vary widely in magnitude and sign. Then the entrainment flux becomes relevant as well. The only customary scalar scale that takes into account both the surface flux and the entrainment flux is the bulk scalar scale, but this scale is not well-behaved for large negative entrainment ratios and for an entrainment ratio equal to −1. We derive a new scalar scale, using previously published large-eddy simulation results for the convective ABL. The scale is derived under the constraint that scaled scalar variance profiles are similar at those heights where the variance producing mechanisms are identical (i.e., either near the entrainment layer or near the surface). The new scale takes into account that scalar variance in the ABL is not only related to the surface flux of that scalar, but to the scalar entrainment flux as well. Furthermore, it takes into account that the production of variance by the entrainment flux is an order of magnitude larger than the production of variance by the surface flux (per unit flux). Other desirable features of the new scale are that it is always positive (which is relevant when scaling standard deviations) and that the scaled variances are always of order 1–10.  相似文献   

15.
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16.
Different methods to determine the height of the convective boundary layer from lidar measurements are described and compared. The differences in either aerosol backscatter or in humidity between the boundary layer and the free troposphere are used, and either the variance or the gradient profile of the parameter under study is evaluated. On average the different methods are in very good agreement. Temporal resolution of the gradient methods is very high, on the order of seconds, but often there is an ambiguity in the choice of the “relevant” minimum in the gradient that corresponds to the boundary-layer height. This is avoided by combining the variance and the gradient methods, using the result of the variance analysis as an indicator for the region where the minimum of the gradient is sought. The combined method is useful for automated determination of the boundary-layer height at least under convective conditions. Aerosol backscatter is found to be as good an indicator for boundary-layer air as humidity, so a relatively simple backscatter lidar is sufficient for determination of the boundary-layer height.  相似文献   

17.
We used a set of large-eddy simulations to investigate the effect of one-dimensional stripe-like surface heat-flux heterogeneities on mixed-layer top entrainment. The profiles of sensible heat flux and the temporal evolution of the boundary-layer depth revealed decreased entrainment for small heat-flux amplitudes and increased entrainment for large heat-flux amplitudes, compared to the homogeneously-heated mixed layer. For large heat-flux amplitudes the largest entrainment was observed for patch sizes in the order of the boundary-layer depth, while for significantly smaller or larger patch sizes entrainment was similar as in the homogeneous case. In order to understand the underlying physics of this impact, a new approach was developed to infer local information on entrainment by means of the local flux divergence. We found an entrainment maximum over the centre of the stronger heated surface patch, where thermal energy is accumulated by the secondary circulation (SC) that was induced by the surface heterogeneity. Furthermore, we observed an entrainment maximum over the less heated patch as well, which we suppose is to be linked to the SC-induced horizontal flow convergence at the top of the convective boundary layer (CBL). For small heat-flux amplitudes a counteracting effect dominates that decreases entrainment, which we suppose is the horizontal advection of cold air in the lower, and warm air in the upper, CBL by the SC, stabilizing the CBL and thus weakening thermal convection. Moreover, we found that a mean wind can reduce the heterogeneity-induced impact on entrainment. If the flow is aligned perpendicular to the border between the differentially-heated patches, the SC and thus its impact on entrainment vanishes due to increased horizontal mixing, even for moderate wind speeds. However, if the flow is directed parallel to the border between the differentially-heated patches, the SC and thus its impact on entrainment persists.  相似文献   

18.
The heat and mass transfer coefficients for exchange across the fluid dynamic boundary layer over tree leaves were simultaneously determined in a controlled environment chamber. The mass transfer coefficients were calculated from measured values of evaporation, air specific humidity and a value of leaf specific humidity at leaf temperature. The heat transfer coefficients were calculated from measured values of air temperature, leaf temperature and an estimate of the sensible heat flux density calculated as the measured net radiation at the leaf surfaces minus the latent heat flux density. The experiments described in this paper indicate that the equations based on laminar boundary-layer theory can give reasonable estimates of the transfer coefficients of real tree leaves for the velocities most commonly experienced in plant canopies, if they are adjusted by a constant multiplier greater than one. Calculations of local mass transfer coefficients based on temperature measurements at three locations at different distances from the leading edge of the leaves, indicate that the deviation from theory is probably the result of transition to turbulent boundary-layer flow at some distance from the leading edge.  相似文献   

19.
The impact of mesoscale moisture variability on the vertical energy transfer through a pre-frontal boundary layer is studied with NOPEX aircraft data. The moisture variability relates to a cold front that passed the area 2 1/2 hours after the observations. We find a density front ahead of the cold front. The large vertical divergence of the turbulent moisture flux in the surface layer is partly related to this moisture variability. Large scale horizontal advection contributes to the observed vertical turbulent flux divergence. The estimated horizontal mesoscale advection term in the budget of sensible heat and moisture is on average small but locally it can be large. This term acts to re-distribute moisture in the boundary layer and leads to sub-grid variations of relative humidity, which is an important quantity for boundary-layer cloud models. The distinct spatial variations of specific humidity are mainly related to synoptic forcing and not to heterogeneity in the surface energy balance.  相似文献   

20.
Summary The development of a convective boundary layer over the Antarctic Plateau is documented by a Doppler minisodar data-set recorded during a 10 day campaign in January 1997. The vertical velocities associated with thermals do not exceed 1 m/s, while the depth of the convective layer, usually less than 200 m, never surpasses 300 m. Measurements of momentum flux, sensible heat flux, wind speed and radiation budget show characteristics that are typical of a convective boundary layer evolution. The diurnal behaviour of absolute humidity, however, exhibits features that are not expected, e.g. anticorrelation with incoming net radiation and air temperature. Received October 30, 1998 Revised May 26, 1999  相似文献   

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