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1.
The role of thermals in the convective boundary layer   总被引：1，自引：0，他引：1  
Detailed measurements of the structure of thermals throughout the convective boundary layer were obtained from the NCAR Electra aircraft over the ocean during the Air Mass Transformation Experiment (AMTEX). Humidity was used as an indicator of thermals. The variables were first high-pass filtered with a 5 km cutoff digital filter to eliminate mesoscale variations. Segments of the 5 min (30 km length) horizontal flight legs with humidity greater than half the standard deviation of humidity fluctuations for that leg were defined as thermals. This was found to be a better indicator of thermals than temperature in the upper part of the boundary layer since the temperature in a thermal is cooler than its environment in the upper part of the boundary layer. Using mixed-layer scaling, the normalized length and number of thermals were found to scale with the 1/3 and -1/3 powers, respectively, of normalized height, while vertical velocity and temperature scaled according to similarity predictions in the free convection region of the surface layer. The observational results presented here extend throughout the entire mixed layer. Using these results in the equation for mean updraft velocity of a field of thermals, the sum of the vertical pressure gradient and edge-effect terms can be estimated. This residual term is found to be important throughout most of the boundary layer. The magnitude of the divergence of vertical velocity variance within a thermal is found to be larger than the magnitude of the mean updraft velocity term throughout most of the mixed layer.Part of this work was completed while visiting Risø National Laboratory, Denmark.The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

2.
Spectral analysis was performed on aircraft observations of a convective boundary layer (CBL) that developed over a thermally inhomogeneous, well-marked mesoscale land surface. The observations, part of the GAME-Siberia experiment, were recorded between April and June 2000 over the Lena River near Yakutsk City. A special integral parameter termed the ‘reduced depth of the CBL’ was used to scale the height of the mixed layer with variable depth. Analysis of wavelet cospectra and spectra facilitated the separation of fluxes and other variables into small-scale turbulent fluctuations (with scales less than the reduced depth of the CBL, approximately 2 km) and mesoscale fluctuations (up to 20 km). This separation approach allows for independent exploration of the scales. Analyses showed that vertical distributions obeyed different laws for small-scale fluxes and mesoscale fluxes (of sensible heat, water vapour, momentum and carbon dioxide) and for other variables (wind speed and air temperature fluctuations, coherence and degree of anisotropy). Vertical profiles of small-scale turbulent fluxes showed a strong decay that differed from generally accepted similarity models for the CBL. Vertical profiles of mesoscale fluxes and other variables clearly showed sharp inflections at the same relative (with respect to the reduced depth of the CBL) height of approximately 0.55 in the CBL. Conventional similarity models for sensible heat fluxes describe both small-scale turbulent and mesoscale flows. The present results suggest that mesoscale motions that reach up to the relative level of 0.55 could be initiated by thermal surface heterogeneity. Entrainment between the upper part of the CBL and the free atmosphere may cause mesoscale motions in that region of the CBL.  相似文献   

3.
A theoretical approach suggests that the surface heterogeneity on a scale of tens of kilometres can generate mesoscale motions that are not in a quasi-stationary state. The starting point of the theoretical approach is the equations of horizontal velocity and potential temperature that are low-pass filtered with a mesoscale cut-off wavelength. The transition of the generated mesoscale motions from a quasi-stationary state to a non-stationary state occurs when horizontal advection is strong enough to level out the potential temperature gradient on the surface heterogeneity scale. Large-eddy simulations (LES) suggest that the convective boundary layer (CBL) changes to a non-stationary state when forced by a surface heat-flux variation of amplitude of 100W m−2 or higher and a wavelength of the order of 10 km. Spectral analysis of the LES reveals that when the mesoscale motions are in a quasi-stationary state, the energy provided by the surface heat-flux variation remains in organized mesoscale motions on the scale of the surface variation itself. However, in a non-stationary state, the energy cascades to smaller scales, with the cascade extending down into the turbulence scale when the wavelength of the surface heat-flux variation is on a scale smaller than 100 times the CBL height. The energy transfer from the generated mesoscale motions to the CBL turbulence results in the absence of a spectral gap between the two scales. The absence of an obvious spectral gap between the generated mesoscale motions and the turbulence raises questions about the applicability of mesoscale models for studies on the effect of high-amplitude surface heterogeneity on a scale of tens of kilometres. The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

4.
In this study, the accuracy of a Pennsylvania State University-National Center for Atmospheric Research mesoscale model (PSU/NCAR MM5) for predicting heavy summer precipitation over the Korean Peninsula was investigated. A total of 1800 simulations were performed using this model for 30 heavy rainfall events employing four cumulus parameterization schemes (CPS), two grid-scale resolvable precipitation schemes (GRS), and two planetary boundary layer (PBL) schemes in three model resolutions (90 km, 30 km, and 10 km). The heavy rainfall events were mesoscale convective systems developed under the influence of mid-latitude baroclinic systems with low-level moisture transport from the ocean. The predictive accuracy for maximum rainfall was approximately 80% for 10-km resolution and was 60% for 30-km resolution. The predictive accuracy for rainfall position extended to ～150 km from the observed position for both resolutions. Simulated rainfall was most sensitive to CPS, then to PBL schemes, and then to GRS. In general, the Grell (GR) scheme and the Anthes and Kuo (AK) scheme showed a better prediction capability for heavy rainfall than did the Betts-Miller (BM) scheme and the Kain-Fritsch (KF) scheme. The GR scheme also performed well in the 24-h and 12-h precipitation predictions: the parameterized convective rainfall in GR is directly related to synoptic-scale forcing. The models without CPS performed better for rainfall amounts but worse for rainfall position than those with CPS. The MM5 model demonstrated substantial predictive capacity using synoptic-scale initial conditions and lateral boundary data because heavy summer rainfall in Korea occurs in a strong synoptic-scale environment.  相似文献   

5.
We investigate the cumulative added value of assimilating temperature, moisture, and wind observations in the three-dimensional non-hydrostatic Fifth-Generation Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model MM5 and use these forecasts to analyze the relationship between surface forcing and planetary boundary-layer (PBL) depth. A data assimilation methodology focused on the surface and the PBL, previously tested in a one-dimensional version of MM5, is applied to 29 May, 6 June, and 7 June 2002 during the International $\hbox {H}_{2}\hbox {O}$ Project over the Southern Great Plains. Model-predicted PBL depth is evaluated against PBL depth diagnosed from data across 4,800 km of airborne lidar data (flight tracks 100–300 km long). The forecast with data assimilation verifies better against observations and is thus used to investigate the environmental conditions that govern PBL depth. The spatial structure in PBL depth is found to be most affected by spatial variations in surface buoyancy flux and capping inversion strength. The spatial scales of surface flux forcing reflected in the PBL depth are found through Fourier analysis and multiresolution decomposition. Correlations are ${<}0.50$ at scales of 64 km or less and increase at larger scales for 29 May and 6 June, but on 7 June low correlations are found at all scales, possibly due to greater within-PBL wind speeds, a stronger capping inversion on this day, and clouds. The results suggest a minimum scale, a function of wind speed, below which heterogeneity in surface buoyancy fluxes is not reflected directly in PBL depth.  相似文献   

6.
The boundary layer in the warm sector of a moderately deepening winter cyclone during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) is studied near the cold front. Data from the National Center for Atmospheric Research Electra research aircraft are used to examine mean and turbulence quantities. The aircraft data and supplemental data from ships, drifting buoys and moored buoys reveal an equivalent-barotropic pressure field. The area is found to be dominated by gradients in temperature and in turbulent fluxes, with changes occurring over 100 km horizontally being comparable to changes over 350 m vertically. The horizontal components of the gradients are found to be a maximum in a direction perpendicular to the front. Cross-sections perpendicular to the front are used to illustrate boundary-layer structure. Profiles of wind speed, stress, wind direction and stress direction are estimated from an Ekman model that is modified to take into account the equivalent-barotropic pressure field. Comparison of profiles from the model to the aircraft-measured data show reasonable agreement far from the front (100 km) when the model uses a constant eddy viscosity of approximately 6 kg m–1 s–1. Near the front there is less agreement with the model. Profiles of turbulent fluxes of momentum, heat and latent heat are divergent, with along-wind momentum flux negative and decreasing upward, cross-wind momentum flux positive and increasing upward, and heat flux and latent heat flux small, positive and decreasing upward. Far from the front, the turbulent kinetic energy budget shows that dissipation balances shear production. However, near-front behavior has an imbalance at low altitude, with shear production appearing as a TKE sink.  相似文献   

7.
Questions related to the spatial integration of turbulent surface fluxes of heat and moisture up to horizontal scales of 100km are discussed. Results taken from the HAPEX-MOBILHY programme are presented, concerning either comparisons between surface-based and aircraft measurements, or estimates derived from numerical simulations of the large-scale hydrological balance, or finally area-averaged values obtained from atmospheric numerical mesoscale models. Conclusions are drawn from these results on how to estimate spatially-averaged surface fluxes efficiently.  相似文献   

8.
Air/sea flux variability on horizontal scales from 50 m to several km results, in part, from the presence of coherent convective structures within the atmospheric boundary layer. The horizontal distribution of fluxes within these convective updrafts and downdrafts is, therefore, central to studies of air/sea interaction and remote sensing of sea surface wind and wave fields. This study derives these flux patterns from observations of the Marine Atmospheric Surface Layer (MASL).Research aircraft flights through the MASL provide an optimal means for sampling large numbers of the above-mentioned coherent structures. The NCAR Electra flew numerous legs through the MASL at a height of 50 m during the 1987 stratocumulus phase of Project FIRE (First ISSCP (International Satellite Cloud Climatology Program) Regional Experiment).In situ measurements from these legs serve as the dataset for this paper. The data are processed in such a way as to retain only the turbulence fluctuations. Conditional sampling, based on the vertical velocity field, results in the isolation of convective updrafts and downdrafts. Compositing of the data for these two classes of convective drafts results in horizontal planviews of the vertical fluxes of buoyancy, absolute humidity, along-meanwind component of momentum, and vertical velocity. To ensure dynamical similarity, these horizontal planviews are oriented in a coordinate system aligned with the mean wind.  相似文献   

9.
I outline the general features of the vertical profile of the vertical flux of a conservative scalar in the planetary boundary layer, giving special emphasis to the convective case and emphasizing the importance of the Webb correction. After the influence of thermal stability on the structure of the turbulent eddies carrying this flux is reviewed, recent developments in parameterizing vertical transport in the convective boundary layer are discussed. I then survey three approaches to the numerical modeling of this transport — second-order closure, large-eddy simulation, and direct numerical simulation. Eddy-correlation, eddy-accumulation, and indirect techniques for measuring scalar fluxes are surveyed and contrasted. Finally, I discuss the physics of probe-induced flow distortion and its impact on scalar flux measurement, showing that it can be quite severe for trace species density fluxes measured from aircraft.Based on lectures given at the workshop Measurement and Parameterization of Land-Surface Evaporation Fluxes, Banyuls, France, October, 1988.The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

10.
Observations of low-level jets (LLJs) at the Howland AmeriFlux site in the USA and the jet’s impact on nocturnal turbulent exchange and scalar fluxes over a tall forest canopy are discussed. Low-frequency motions and turbulent bursts characterize moderately strong LLJs, whereas low-frequency motions are suppressed during periods with strong LLJs and enhanced shear. An analysis based on the shear-sheltering hypothesis seeks to elucidate the effect of LLJs on flux measurements. In the absence of shear sheltering, large eddies penetrate the roughness sublayer causing enhanced mixing while during periods with shear sheltering, mixing is reduced. In the absence of the latter, ‘upside-down’ eddies are primarily responsible for the enhanced velocity variances, scalar and momentum fluxes. The integral length scales over the canopy are greater than the canopy height. The variance spectra and cospectra from the wavelet analysis indicate that large eddies (spatial scale greater than the low-level jet height) interact with active canopy-scale turbulence, contributing to counter-gradient scalar fluxes.  相似文献   

11.
Buoyancy fluxes in the marine atmospheric boundary layer (MABL) for the cloud street regime, observed during the Genesis of Atlantic Lows Experiment (GALE), have been analyzed using the technique of joint frequency distribution. For the lower half of the MABL, the results suggest that the buoyancy flux is mainly generated by the rising thermals and the sinking compensating ambient air, and is mainly consumed by the entrainment and detrainment of thermals, penetrative convection, and the entrainment from the MABL top.The results are compared to those from previous studies of mesoscale cellular convection (Air-Mass Transformation Experiment, AMTEX), the dry convective boundary layer, and the trade-wind MABL. For the lower MABL, the quadrant buoyancy fluxes, fractional coverages, and flux intensities are in good agreement with those of mesoscale cellular convection (AMTEX) and the dry convective boundary layer. The results suggest that, if the buoyancy flux is primarily driven by the temperature flux, the physical processes for generating buoyancy flux mentioned above are about the same for the lower boundary layers over land and ocean, even with different convective regimes. For the trade-wind MABL, the buoyancy flux is mainly driven by the moisture flux; the quadrant flux intensities are stronger than those of the other three studies except for the buoyant updrafts (thermals). These results suggest that the entrainment and detrainment of thermals are more effective in the trade-wind MABL than in the boundary layers driven by the temperature flux.Scale analysis of the buoyancy flux is in good agreement with that of AMTEX. For the lower half of the MABL, the buoyancy flux is mainly generated by the intermediate scale (200 m to 2 km), which includes the dominant convective thermals in the surface layer and the mixed layer. The scale smaller than 200 m is important only in the surface layer. The scale larger than 2 km, which includes the roll vortices, increases its significance upward. While most of the positive and negative fluxes are associated with the updrafts for the intermediate scale, the downdrafts are as important as updrafts for the larger scale.ST Systems Corporation, Lanham, MD, 20706, U.S.A.  相似文献   

12.
Observations of fluxes over heterogeneous surfaces   总被引：1，自引：0，他引：1  
This study analyzes data collected from repeated aircraft runs 30 m over alternating regions of irrigated and dry nonirrigated surfaces, each region on the order of 10 km across, during the California Ozone Deposition Experiment (CODE). After studying the scale dependence of the flow, the variables and their fluxes are decomposed into means for sublegs defined in terms of irrigated and nonirrigated regions and deviations from such subleg means. Since the repeated runs were flown over the same track, compositing the eight flight legs for each of the two days allows partial isolation of the influences of surface heterogeneity and transient mesoscale motions.A variance analysis is carried out to quantify the relative importance of surface heterogeneity and transient mesoscale motions on the variability of the turbulence fluxes. The momentum and ozone fluxes are more influenced by transient mesoscale motions while fluxes of heat, moisture and carbon dioxide are more influenced by surface heterogeneity. The momentum field is also influenced by a quasi-stationary mesoscale front and larger scale velocity gradients.For the present case, the mesoscale modulation of the turbulent flux is numerically more important than the direct mesoscale flux. This spatial modulation of the turbulent fluxes leads to extra Reynolds terms which act to reduce the area-averaged turbulent momentum flux and enhance the area-averaged turbulent heat flux.  相似文献   

13.
Scalar fluxes measured through the eddy-correlation technique are prone to two types of errors caused by the sensor-induced flow distortion: those due to crosstalk from the horizontal flux, and those due to amplification or attenuation due to flow blocking. We show that the crosstalk error can be eliminated by designing the sensor array to be vertically symmetric about its horizontal midplane. In such an array, the flow-blocking effect causes the scalar flux to be overestimated, but this error can be made negligible by designing an array with minimal stagnation loss in streamwise speed at the flux-measurement point.The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

14.
Most atmospheric boundary-layer theories are developed over vegetative surfaces and their applicability at urban sites is questionable. Here, we study the intra-city variation of turbulence characteristics and the applicability of boundary-layer theory using building-morphology data across Helsinki, and eddy-covariance data from three sites: two in central Helsinki (400 m apart) and one 4 km away from the city centre. The multi-site measurements enable the analysis of the horizontal scales at which quantities that characterize turbulent transport vary: (i) Roughness characteristics vary at a 10-m scale, and morphometric estimation of surface-roughness characteristics is shown to perform better than the often used rule-of-thumb estimates (average departures from the logarithmic wind profile are 14 and 44 %, respectively). (ii) The drag coefficient varies at a 100-m scale, and we provide an updated parametrization of the drag coefficient as a function of z/z H (the ratio of the measurement height to the mean building height). (iii) The transport efficiency of heat, water vapour and CO2 is shown to be weaker the more heterogeneous the site is, in terms of sources and sinks, and strong scalar dissimilarity is observed at all sites. (iv) Atmospheric stability varies markedly even within 4 km across the city: the median difference in nocturnal sensible heat fluxes between the three sites was over 50W m?2. Furthermore, (v) normalized power spectra and cospectra do not vary between sites, and they follow roughly the canonical theory as developed over vegetated terrain.  相似文献   

15.
Two surface layer parameterization schemes along with five planetary boundary layer (PBL) schemes in the Weather Research and Forecasting model (WRF) are analyzed in order to evaluate the performance of the WRF model in simulating the surface variables and turbulent fluxes over an Indian sub-continent region. These surface layer schemes are based on the fifth-generation Pennsylvania State University—National Center for Atmospheric Research Mesoscale Model (MM5) parameterization; (a) Old MM5 scheme having Businger-Dyer similarity functions and (b) revised MM5 scheme utilizing the functions that are valid for full ranges of atmospheric stabilities. The study suggests that each PBL scheme can reproduce the diurnal variation of 2 m temperature, momentum flux and sensible heat flux irrespective of the surface layer scheme used for the simulations. However, a comparison of model-simulated values of surface variables and turbulent fluxes with observed values suggests that each PBL scheme is found to systematically over-estimate the nocturnal 2 m temperature and 10 m wind speed with both the revised and old schemes during stable conditions.  相似文献   

16.
Summary  High resolution aircraft observations made along flight tracks over inhomogeneous surface in the late wintertime boreal zone are described and compared to 2D mesoscale model simulations with surface properties defined at 2 km resolution from maps. All observations displayed the expected small-scale turbulence. On top of that, the near-surface wind speeds (but not directions) showed mesoscale variations related to local topography and roughness. Upward (but not downward) SW and LW radiative fluxes and ground temperature also displayed mesoscale variability; in SW radiation this was clearly due to local albedo changes. In the sensible heat flux there was strong horizontal variation near the surface in correlation with surface types. The above observed mesoscale along-track variations were reasonably well represented by the mesoscale model simulation. The track-averaged observed sensible and latent heat flux profiles were in rough agreement with a mixing length approach, which used the track-averaged wind, temperature and moisture profiles as input (mimicking a first-order turbulence closure scheme of a GCM). Received September 20, 1999 Revised January 21, 2000  相似文献   

17.
In weak wind stable conditions, eddy-correlation fluxes calculated using conventional averaging times of 5 min or longer to define the perturbations are severely contaminated by poorly sampled mesoscale motions. A method is developed to identify the averaging time for each individual data record that captures the turbulence while excluding most of the mesoscale motions. The method is based on multiresolution decomposition of the heat flux, and provides an objective procedure for selecting the averaging time for calculating eddy-correlation fluxes. Eddy-correlation data collected in weak turbulence conditions over grass, snow, a pine forest and the ocean are used to demonstrate the approach.When the small-scale turbulence and mesoscale motions are clearly separated by a gap region in the heat flux cospectra, the variable window width reduces the influence of nonstationarity by more effectively filtering out mesoscale motions compared to traditional methods using constant averaging time. For records where turbulence and mesoscale motions overlap in scale, the method is not well posed, although such records occur infrequently for our datasets. These ambiguous cases correspond to significant nonstationarity at scales that overlap with turbulence scales. The improved turbulence fluxes calculated with the proposed method are the appropriate fluxes for evaluating flux-gradient relationships and Monin–Obukov similarity theory for developing improved model parameterizations of turbulence for weakly turbulent flows  相似文献   

18.
A case study of a particularly intense cold air outbreak over the northAtlantic Ocean extending from the northeast coast of the UnitedStates to the Gulf Stream is described. A RADARSAT satellite synthetic apertureradar (SAR) image of this outbreak dramatically illustrates the spatialevolution of convection. Nearly coincident images from the National Oceanic and Atmospheric Administration's Advanced Very HighResolution Radiometer are used to compare many interesting features.In addition, National Weather Service rawinsonde data, National Data Buoy Center buoy data, and Woods Hole Oceanographic Institute Coastal Mixing and Optics mooring data arepresented. We use these data to help describe the spatial evolution of the atmospheric boundary-layer processes involved in this outbreak.Rows of cellular convective clouds begin to appear some distance offshore and then slowly increase in horizontal diameter and wavelength in the downwind direction, with a subsequent jump in cloud diameter downwind of the Gulf Stream North Wall (GSNW). The SAR image shows a similar evolution of sea-surface footprints of these boundary-layer features. This change in boundary-layer structure is attributed to corresponding changes in static stability. About 300 km south of the GSNW in the SAR image, an even larger jump in cell diameter appears and the cells becomenon-uniform with bright crescents and filled semi-circles on thedownwind sides of the cells. These are believed to be surface effectsof gust fronts induced by the mesoscale cellular convection and enhanced by the overall northwesterly flow.  相似文献   

19.
A large set of tower data was used to identify the gap that separates small-scale turbulence and mesoscale structures in the cospectra of surface fluxes. The cospectra were obtained using a multi-resolution decomposition algorithm. The gap time scale τ g was found by fitting a fifth-order polynomial to the cospectra and identifying special points occurring after the peak at small scales. In unstable conditions (day) τ g was found to fall as the mean wind speed increased, while no such dependence was observed in stable conditions (night). The gap scale was found to change very weakly with stability both in moderately stable and moderately unstable conditions, with a sharp drop from about 1100 to 250 s occurring in near-neutral conditions. The vertical fluxes computed at different averaging intervals were found to correlate exceptionally well with each other, the scatter being somewhat larger during the night. Although considerable discrepancy may occur for individual records, when averaged over 10 months, the difference in the flux estimated at 7 to 109 min intervals never exceeded 4%, which is comparable or less than the instrumental error.  相似文献   

20.
The stably stratified boundary layer over the great plains   总被引：2，自引：2，他引：0  
Airplane measurements of the stably stratified boundary layer obtained during the Severe Environmental Storms and Mesoscale Experiment (SESAME) over rolling terrain in south-central Oklahoma indicate that considerable horizontal variability exists in the flow on scales of several kilometers. Much of this wave-like structure appears to be tied to the terrain. The criteria for existence of stationary gravity waves indicate that these waves can exist under the observed conditions. The spectrum of terrain variations also supports the existence of these waves. Observed spectra of the vertical velocity have two peaks: one at wavelengths of several kilometers, which is due to waves and the other at wavelengths of about 100 m, which is due to turbulence. The variance at several kilometers wavelength increases somewhat with height at least up to about 800 m, but the variance contributed by turbulence decreases rapidly with height.The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

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