共查询到20条相似文献，搜索用时 31 毫秒
1.
D. G. Steyn 《Boundary-Layer Meteorology》1990,53(1-2):21-31
A slab mixed-layer model with zero-order entrainment for both temperature and humidity is developed in order to examine the relative magnitude of advective and turbulence flux convergence effects. The model formulation provides an analytic function for the ratio of surface-layer to entrainment-layer humidity flux. Model results are compared with measured mixed-layer properties over one day at a coastal location. It is concluded that the model is highly successful at simulating the mixed-layer depth, and mean mixed-layer humidity. It is suggested that a first-order model may be more appropriate for the latter half of the day when the mixed-layer depth is decreasing due to the dominance of advection over vertical turbulence flux convergence. 相似文献
2.
基于激光雷达遥感和参数化模式研究城市混合层的发展机制 总被引：3，自引：1，他引：3
利用北京大学的微脉冲激光雷达(MPL)观测的偏南气流条件下的混合层高度和夹卷层厚度探测资料,研究简单天气条件下城市混合层的发展机制并与GB94的参数化方案相互映证.通过激光雷达遥感的混合层高度和夹卷层厚度计算了混合层顶的夹卷率A,得到其平衡夹卷阶段的值为0.24.在不考虑机械混合前提下反演了地面感热通量,结果表明遥感的反演值与梯度法的计算值有系统性偏差,但总体上仍旧有较好的相关.偏差量的大小反映了影响混合层发展的机械湍流的参数B,进一步通过GB91模式的模拟确定该参数的最佳值约为3.5.在此基础上讨论了混 相似文献
3.
A model for the time and space variation of the internal boundary-layer height over a land area with an irregular coastline is presented. It is based on the analytical model of the boundary-layer height proposed by Gryning and Batchvarova (1990) and Batchvarova and Gryning (1991), The model accounts for the temperature jump and the mean vertical air motion at the top of the internal boundary-layer. Four cases from experiments in Nanticoke and Vancouver are used for model validation. The agreement between the calculated and measured internal boundary layer height at the observational sites is fairly good. The input information for the model consist of wind speed and direction, friction velocity and kinematic heat flux in time and space for the area, and the potential temperature gradient and the mean vertical air motion above the internal boundary layer. For the experiments used in the validation the effect of subsidence is relatively important in the afternoon under low wind speed high pressure conditions, lowering the height of the internal boundary layer by up to 10%, and it is negligible in the morning hours. The effect of the mixing height over the sea is found to be negligible. 相似文献
4.
Ekaterina Batchvarova Xiaoming Cai Sven-Erik Gryning Douw Steyn 《Boundary-Layer Meteorology》1999,90(1):1-20
The purpose of this paper is to test the ability of two quite different models to simulate the combined spatial and temporal variability of the internal boundary layer in an area of complex terrain and coastline during one day. The simple applied slab model of Gryning and Batchvarova, and the Colorado State University Regional Atmospheric Modelling System (CSU-RAMS) are tested by comparison with data gathered during a field study (called Pacific '93) of photochemical pollution in the Lower Fraser Valley of British Columbia, Canada. The data utilised here are drawn from tethered balloon flights, free flying balloon ascents, and downlooking lidar operated from an aircraft flown at roughly 3500 m above sea level. Both models are found to represent the temporal and spatial development of the internal boundary-layer depth over the Lower Fraser Valley very well, and reproduce many of the finer details revealed by the measurements. 相似文献
5.
E. Barbaro J. Vilà-Guerau de Arellano M. C. Krol A. A. M. Holtslag 《Boundary-Layer Meteorology》2013,148(1):31-49
We investigated the impact of aerosol heat absorption on convective atmospheric boundary-layer (CBL) dynamics. Numerical experiments using a large-eddy simulation model enabled us to study the changes in the structure of a dry and shearless CBL in depth-equilibrium for different vertical profiles of aerosol heating rates. Our results indicated that aerosol heat absorption decreased the depth of the CBL due to a combination of factors: (i) surface shadowing, reducing the sensible heat flux at the surface and, (ii) the development of a deeper inversion layer, stabilizing the upper CBL depending on the vertical aerosol distribution. Steady-state analytical solutions for CBL depth and potential temperature jump, derived using zero-order mixed-layer theory, agreed well with the large-eddy simulations. An analysis of the entrainment zone heat budget showed that, although the entrainment flux was controlled by the reduction in surface flux, the entrainment zone became deeper and less stably stratified. Therefore, the vertical profile of the aerosol heating rate promoted changes in both the structure and evolution of the CBL. More specifically, when absorbing aerosols were present only at the top of the CBL, we found that stratification at lower levels was the mechanism responsible for a reduction in the vertical velocity and a steeper decay of the turbulent kinetic energy throughout the CBL. The increase in the depth of the inversion layer also modified the potential temperature variance. When aerosols were present we observed that the potential temperature variance became significant already around $0.7z_i$ (where $z_i$ is the CBL height) but less intense at the entrainment zone due to the smoother potential temperature vertical gradient. 相似文献
6.
A. G. M. Driedonks 《Boundary-Layer Meteorology》1982,22(4):475-480
Jump or slab models are frequently used to calculate the depth of the convectively mixed layer and its potential temperature during the course of a clear day. Much attention has been paid theoretically to the parameterization of the budget for turbulent kinetic energy that is required in these models. However, for practical applications the sensitivity of the solutions of the model equations to variations in the entrainment formulation and in the initial and boundary conditions is also very important. We analyzed this sensitivity on the basis of an analytical solution for the model which uses the well-known constant heat flux ratio. The initial conditions for the mixed-layer height (h) and potential temperature (
m
) quickly lose their influence. Only the initial temperature deficit is important. The mixed-layer temperature at noon on convective days is insensitive to the entrainment coefficient c. It is governed by the integral of the heat input and by the stable lapse rate. A change in c from 0.2 to 0.5 leads to a variation of 20% in h. This is not very much considering the accuracy in the determination of h from actual observations. 相似文献
7.
Observations and Large-Eddy Simulations of Entrainment in the Sheared Sahelian Boundary Layer 总被引：1，自引：1，他引：0
Guylaine Canut Fleur Couvreux Marie Lothon David Pino Frédérique Saïd 《Boundary-Layer Meteorology》2012,142(1):79-101
At the top of the planetary boundary layer, the entrainment of air, which incorporates dry and warm air from the free troposphere
into the boundary layer, is a key process for exchanges with the free troposphere since it controls the growth of the boundary
layer. Here, we focus on the semi-arid boundary layer where the entrainment process is analyzed using aircraft observations
collected during the African Monsoon Multidisciplinary Analysis experiment and large-eddy simulations. The role of the entrainment
is specifically enhanced in this region where very large gradients at the planetary boundary-layer top can be found due to
the presence of the moist, cold monsoon flow on which the dry, warm Harmattan flow is superimposed. A first large-eddy simulation
is designed based on aircraft observations of 5 June 2006 during the transition period between dry conditions and the active
monsoon phase. The simulation reproduces the boundary-layer development and dynamics observed on this day. From this specific
case, sensitivity tests are carried out to cover a range of conditions observed during seven other flights made in the same
transition period in order to describe the entrainment processes in detail. The combination of large-eddy simulations and
observations allows us to test the parametrization of entrainment in a mixed-layer model with zero-order and first-order approximations
for the entrainment zone. The latter representation of the entrainment zone gives a better fit with the conditions encountered
in the Sahelian boundary layer during the transition period because large entrainment thicknesses are observed. The sensitivity
study also provides an opportunity to highlight the contribution of shear stress and scalar jumps at the top of the boundary
layer in the entrainment process, and to test a relevant parametrization published in the recent literature for a mixed-layer
model. 相似文献
8.
Lidar measurements of the atmospheric entrainment zone and the potential temperature jump across the top of the mixed layer 总被引：10，自引：0，他引：10
Lidar measurements of the thickness of the atmospheric entrainment zone are presented. The measurements were obtained in central Illinois during 6 days of clear-air convection.A new method was developed to monitor the potential temperature jump across the entrainment zone. A single early morning temperature sounding and continuous lidar measurements of the mixed-layer height provide potential temperature jump values which agree well with in situ observations.Lidar measurements of the thickness of the entrainment zone normalized by mixed-layer depth are presented as a function of a convective Richardson number; these values show reasonable agreement with published laboratory results. The lidar observations span a wider range of mixed-layer depths and contain higher values of the normalized entrainment rate (dh/dt)/w
* than those observed in tank studies. Both lidar and tank results show that simple parcel theory does not properly predict entrainment-zone thickness. During this experiment which examined mostly high entrainment conditions, the normalized entrainment-layer thickness was linearly dependent on entrainment rate. 相似文献
9.
10.
Recently a range of sophisticated large-eddy simulations of thecloud-topped boundary layer have been intercompared and furthercompared with observations and single column models. Here we comparethese results with perhaps the simplest model of the cloud-toppedboundary layer, namely a mixed-layer model. Results from the model aredescribed with two aims in mind. Firstly, the good results act as areminder of the success of simple models, and, secondly, we suggestthat a simple mixed-layer model could be used as a baseline for futuremodel intercomparisons.The mixed-layer model is based on two assumptions that follow previousstudies. Firstly, the liquid-water potential temperature and the total waterspecific humidity are assumed to be constant with height in the boundarylayer. Secondly, turbulence entrains air across the inversion into the boundarylayer at a rate that is assumed to be proportional to the jump in radiative flux at the cloud top and inversely proportional to the jump in buoyancy at the inversion. The constant of proportionality is called the entrainment efficiency.Results from the model for the entrainment rate and height evolutionof the boundary layer are compared with the observations and modelsconsidered in a EUCREM intercomparison study. Thepresent mixed-layer model accurately predicts the observed heightevolution of the boundary layer, but over-estimates the entrainmentrate to a similar degree as the large-eddy simulations. We show that,if the subsidence rate is reduced to the value given by observationsrather than the value used in the EUCREM intercomparison study,then the model agrees well with observed value of the entrainment rateif the entrainment efficiency is taken to be 0.6. With this value, themodel also agrees well with a further case study byBechtold et al. An entrainment efficiency of 0.6 is a little higherthan suggested by large eddy simulations, but such simulations do notcurrently resolve the entrainment events explicitly. Hence this pointdeserves further study. 相似文献
11.
Twenty-six months of continuous ceilometer data are used to estimate the convective mixed-layer height for 710 days by identifying
backscatter gradients associated with the entrainment zone. To accomplish this, a semi-automatic procedure is developed that
removes all non-applicable data before applying a mixed-layer height algorithm to the backscatter profiles. Two different
algorithms for estimating the mixed-layer height are assessed: the minimum-gradient method and the ideal-profile method. The
latter of these two algorithms is found to be more robust. Comparisons of mixed-layer height values estimated from the ceilometer
agree with previous observations with slightly higher estimates in the mornings and evenings. For clear days with no cumulus
cloud formation, the seasonal cycle in mixed-layer heights peaks in late June to early July. Daily maximum values are suppressed
by the site’s coastal location, remaining below 800 m for all but a few days. The mean daily maximum mixed-layer height increases
by 384 m for days with boundary-layer clouds. The mean summer diurnal trend is found not to differ greatly from that in spring
on clear days, while days with boundary-layer clouds have higher spring values than in summer. Net surface heat flux and synoptic
stability likely have the largest influence on the mixed-layer heights. Additionally, large intra-monthly variability suggests
a strong influence from regional dynamics. 相似文献
12.
Some of the most widely used slab model formulations for applications in the convective boundary layer are analysed and discussed. Three main classes are identified based on different approximations of the turbulent kinetic energy equation. The models appear to be quite insensitive to the initial values for boundary-layer height, and temperature discontinuity at the boundary-layer top. The slab models are applied to a case of sea-land transition from the literature, and a case of convective boundary layer time evolution over a homogeneous terrain at San Pietro Capofiume (Bologna, Italy). The different parameterisations turn out to be almost equivalent for the cases studied. The models generally underpredict the value for the height, while all give very good estimates for the mean mixed-layer temperature. 相似文献
13.
The parameterization of the dimensionless entrainment rate (w
e
/w
*) versus the convective Richardson number (Ri
δθ
) is discussed in the framework of a first-order jump model (FOM). A theoretical estimation for the proportionality coefficient
in this parameterization, namely, the total entrainment flux ratio, is derived. This states that the total entrainment flux
ratio in FOM can be estimated as the ratio of the entrainment zone thickness to the mixed-layer depth, a relationship that
is supported by earlier tank experiments, and suggesting that the total entrainment flux ratio should be treated as a variable.
Analyses show that the variability of the total entrainment flux ratio is actually the effect of stratification in the free
atmosphere on the entrainment process, which should be taken into account in the parameterization. Further examination of
data from tank experiments and large-eddy simulations demonstrate that the different power laws for w
e
/w
* versus Ri
δθ
can be interpreted as the variability of the total entrainment flux ratio. These results indicate that the dimensionless
entrainment rate depends not only on the convective Richardson number but also upon the total entrainment flux ratio. 相似文献
14.
A simple mixed-layer model is developed to describe evaporation into a convective planetary boundary layer (PBL). The model comprises volume budget equations for temperature and humidity, equations to describe transport through the surface layer which is treated as part of the lower boundary, and equations to describe entrainment at the top of the PBL. The ground surface is modelled as a canopy resistance. The model was integrated with canopy resistance, surface-layer resistance and available energy, (R
n – G), input as given functions of time, and the simulated PBL was allowed to grow into an atmosphere with known temperature and humidity profiles.Two variants of the mixed-layer model were tested using data from the KNMI tower site at Cabauw in the Netherlands. These variants differed only in the formulation of entrainment: one used a formulation developed by Driedonks (1982) while the other was a simpler formulation. Simulated evaporation agreed very well with observations irrespective of which entrainment formulation was used, despite discrepancies between simulated and observed PBL height growth which were sometimes quite large for the simpler formulation. Sensitivity analysis of the model confirms that good PBL height-growth predictions are not always a prerequisite for good evaporation predictions. 相似文献
15.
Based on the prognostic equations of mixed-layer theory assuming a zeroth order jump at the entrainment zone, analytical solutions for the boundary-layer height evolution are derived with different degrees of accuracy. First, an exact implicit expression for the boundary-layer height for a situation without moisture is analytically derived without assuming any additional relationships or specific initial conditions. It is shown that to expand the solution to include moisture, only minor approximations have to be made. Second, for relatively large boundary-layer heights, the implicit representation is simplified to an explicit function. Third, a hybrid expression is proposed as a reasonable representation for the boundary-layer height evolution during the entire day. Subsequently, the analysis is extended to present the evolution of any boundary-layer averaged scalar, either inert or under idealized chemistry, as an analytical function of time and boundary-layer height. Finally, the analytical solutions are evaluated. This evaluation includes a sensitivity analysis of the boundary-layer height for the entrainment ratio, the free tropospheric lapse rate of the potential temperature, the time-integrated surface flux and the initial boundary-layer height and potential temperature jump. 相似文献
16.
A. G. M. Driedonks 《Boundary-Layer Meteorology》1982,23(3):283-306
The evolution of the mixed layer during a clear day can be described with a slab model. The model equations have to be closed by a parameterization of the turbulent kinetic energy budget. Several possibilities for this parameterization have been proposed. In order to assess the practical applicability of these models for the atmosphere, field experiments were carried out on ten clear days in 1977 and 1978. Within the accuracy of the measurements the mixed-layer height in fully convective conditions (at noon on clear days) is well predicted taking a constant heat flux ratio % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyOeI0Yaa0% aaaeaacqaH4oqCcaWG3bWaaSbaaSqaaiaadIgaaeqaaaaakiabg2da% 9iaaicdacaGGUaGaaGOmamaanaaabaGaeqiUdeNaam4DamaaBaaale% aacaWGZbaabeaaaaaaaa!41D4!\[ - \overline {\theta w_h } = 0.2\overline {\theta w_s } \]. In the early morning hours mechanical entrainment is also important. Good overall results are obtained with the entrainment formulation % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyOeI0Yaa0% aaaeaacqaH4oqCcaWG3bWaaSbaaSqaaiaadIgaaeqaaaaakiabg2da% 9iaaicdacaGGUaGaaGOmamaanaaabaGaeqiUdeNaam4DamaaBaaale% aacaWGZbaabeaaaaGccqGHRaWkcaaI1aGaamyDamaaDaaaleaacqGH% xiIkaeaacaaIZaaaaOGaamivaiaac+cacaWGNbGaamiAaaaa!49C1!\[ - \overline {\theta w_h } = 0.2\overline {\theta w_s } + 5u_ * ^3 T/gh\].Only large differences in the entrainment coefficients lead to significantly different results. Making the entrainment model more complex does not lead to substantial improvement.The mean potential temperature in the mixed layer is reproduced within 0.5 °C. This temperature is insensitive to the choice of a particular entrainment formulation and depends more on the surface heat input and the temperature gradient in the stable air aloft. 相似文献
17.
18.
On the Effect of Surface Heat-Flux Heterogeneities on the Mixed-Layer-Top Entrainment 总被引：1，自引：1，他引：0
Matthias Sühring Björn Maronga Florian Herbort Siegfried Raasch 《Boundary-Layer Meteorology》2014,151(3):531-556
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
19.
Cyrille Flamant Jacques Pelon Bart Brashers Robert A. Brown 《Boundary-Layer Meteorology》1999,93(1):47-73
The internal thermal boundary layer developing over the Mediterranean during a cold-air outbreak associated with a Tramontane event has been studied by means of airborne lidar, in situ sensors, and a modelling approach that consisted of nesting the University of Washington (UW) planetary boundary-layer (PBL) model in an advective zero-order jump model. This approach bypasses some of the deficiencies associated with each model: the absence of the dynamics in the mixed layer for the zero-order jump model and the lack of an inversion at the PBL top for the UW PBL model. Particular attention is given to the parameterization of the entrainment flux at the PBL top. Values of the entrainment closure parameter derived with the model when matching PBL structure observations are much lower than those derived with standard zero-order jump models. They also are in good agreement with values measured in different meteorological situations by other studies. This improvement is a result of the introduction of turbulent kinetic energy production in the mixed layer. 相似文献
20.
Understanding Madden-Julian-Induced sea surface temperature variations in the North Western Australian Basin 总被引：1，自引：1，他引：0
J. Vialard K. Drushka H. Bellenger M. Lengaigne S. Pous J. P. Duvel 《Climate Dynamics》2013,41(11-12):3203-3218
The strongest large-scale intraseasonal (30–110 day) sea surface temperature (SST) variations in austral summer in the tropics are found in the eastern Indian Ocean between Australia and Indonesia (North-Western Australian Basin, or NWAB). TMI and Argo observations indicate that the temperature signal (std. ~0.4 °C) is most prominent within the top 20 m. This temperature signal appears as a standing oscillation with a 40–50 day timescale within the NWAB, associated with ~40 Wm?2 net heat fluxes (primarily shortwave and latent) and ~0.02 Nm?2 wind stress perturbations. This signal is largely related to the Madden-Julian Oscillation. A slab ocean model with climatological observed mixed-layer depth and an ocean general circulation model both accurately reproduce the observed intraseasonal SST oscillations in the NWAB. Both indicate that most of the intraseasonal SST variations in the NWAB in austral winter are related to surface heat flux forcing, and that intraseasonal SST variations are largest in austral summer because the mixed-layer is shallow (~20 m) and thus more responsive during that season. The general circulation model indicates that entrainment cooling plays little role in intraseasonal SST variations. The larger intraseasonal SST variations in the NWAB as compared to the widely-studied thermocline-ridge of the Indian Ocean region is explained by the larger convective and air-sea heat flux perturbations in the NWAB. 相似文献