Samir Khanna

Three-Dimensional Buoyancy- and Shear-Induced Local Structure of the Atmospheric Boundary Layer

Abstract Three-dimensional visualization together with statistical measures are used to describe the instantaneous local structure of the atmospheric boundary layer (ABL) under various stability states using large-eddy simulation (LES) data. To explore the relative roles of buoyancy and shear in ABL structure, a wide range of −zi/L ABL states, from 0.44 to 730, is analyzed. It is known that buoyancy-induced updrafts and downdrafts are primarily responsible for the upward flux of momentum, heat, and passive scalar, and strongly influence near-ground horizontal motions. These buoyancy-induced features of the convective boundary layer (CBL) are presented here in clearly observable 3D visual images of vertical velocity and temperature, showing large turbulent cell-like structure several zi in horizontal extent. The horizontal length scales of the temperature field near the ground are found to be of the order of the horizontal velocity length scales. It is noted by comparing visual structure with spectra that ...

Critical test of the validity of Monin-Obukhov similarity during convective conditions

A recent study of convective boundary layer characteristics performed with large eddy simulation technique (LES) has demonstrated unexpected influence of the depth of the boundary layer on surface layer characteristics. The present study tests some of the predictions from these simulations with field measurements from a summertime experiment in Sweden, which includes in addition to regular surface layer data also airborne measurements and numerous radio soundings, which enable accurate determination of boundary layer depth. It is found that the measurements strongly support most of the conclusions draws from the LES study and give additional information over a wider stability range. Thus, the normalized wind gradient fm is found to depend on both z/L, where z is height above the ground and L is the Monin‐Obukhov length, and zi/L, where zi is the height of the convective boundary layer. This additional dependence on zi/L explains much of the scatter between experiments encountered for this parameter. In the case of the normalized temperature gradient fh, the experimental data follow the generally accepted functional relation with z/L, but with an additional, slight ordering according to zi/L. Analyses of nondimensional variances show (i) the horizontal velocity variance scales on mixed layer variables and is a function only of zi/L, in agreement with the LES results and with previous measurements; (ii) the normalized vertical velocity variance depends on the large-scale pressure gradient length scale for slight instability and is primarily a function of z /L for moderate and strong instability; (iii) the normalized temperature variance is a function of z/L, with a possible slight dependence on zi/L; and (iv) whereas mean temperature gradient is characterized by local shear scales, temperature variances are normalized by local buoyancy-driven scales.

Spectra in the Unstable Surface Layer

Abstract A simple approach to modeling spectra in unstable atmospheric surface layers is presented. The authors use a single form for the two-dimensional spectrum of horizontal velocity, vertical velocity, and a scalar in the horizontal plane; it has two free constants, a length scale, and an intensity scale. Continuity is used to relate the vertical and horizontal velocity spectra. The two free constants are determined by matching the variance and the inertial-subrange spectral level with observations. The scales are chosen so that the spectra follow law of the wall and mixed-layer scaling in the neutral and free-convection limits, respectively. The authors model the stability dependence of the spectra by combining these two limiting forms. The one-dimensional spectra, obtained by integration over one wavenumber component, and their variances agree well with observations. Near the surface the vertical velocity variance follows Monin-Obukhov (M–O) similarity and shows a realistic local free-convection asy...

Resolvable- and Subgrid-Scale Measurement in the Atmospheric Surface Layer: Technique and Issues

A new technique for the measurement of two-dimensionally filtered resolvable- and subgrid-scale (SGS) turbulence in the atmospheric surface layer is studied. The technique uses an array of sensors to do spatial filtering in the direction transverse to the mean flow. Taylor’s hypothesis is used to approximate streamwise filtering with time filtering. The performance of this two-dimensional surrogate filter is evaluated with data from a high-resolution large-eddy simulation of the atmospheric boundary layer. In general, both resolvable- and subgrid-scale velocity and temperature fields obtained from a two-dimensional spectral filter and the surrogate filter exhibit high cross correlation (.0.85‐0.95). The correlation between the true and the surrogate SGS stress and temperature flux is somewhat lower than that for the velocities. A detailed analysis of the applicability of Taylor’s hypothesis to the energy-containing scales of vertical velocity shows that among the mechanisms that could limit its fidelity, only the effect of fluctuating convection velocity is nonnegligible, and its aliasing effects are more significant for stress and scalar-flux fluctuations than for velocity fluctuations. The authors suggest this is why the correlations were lower for stress and flux than velocities. The results suggest that the sensor array is a feasible technique for SGS measurement in the atmospheric surface layer.

Local Structure of the Convective Boundary Layer from a Volume-Imaging Radar

Abstract The local structure and evolution of the convective boundary layer (CBL) are studied through measurements obtained with a volume-imaging radar, the turbulent eddy profiler (TEP). TEP has the unique ability to image the temporal and spatial evolution of both the velocity field and the local refractive index structure-function parameter, C2n. Volumetric images consisting of several thousand pixels are typically formed in as little as 1 s. Spatial resolutions are approximately 30 m by 30 m by 30 m. CBL data obtained during an August 1996 deployment at Rocks Springs, Pennsylvania, are presented. Measurements of the vertical C2n profile are shown, exhibiting the well-known bright band near the capping inversion at zi, as well as intermittent plumes of high C2n. Horizontal profiles show coherent 100-m-scale C2n and vertical velocity (w) structures that correspond to converging horizontal velocity vectors. To quantify the scales of structures, the vertical and streamwise horizontal correlation dista...

LES in the Surface Layer: Surface Fluxes, Scaling, and SGS Modeling

Abstract The surface fluxes in the fine-mesh numerical codes used in small-scale meteorology are typically diagnosed from resolvable-scale variables through surface-exchange coefficients. This is appropriate if the aspect ratio (length/height) of the grid volume adjacent to the surface is very large, as in mesoscale models. The aspect ratio can approach unity in large-eddy simulation (LES) codes for the planetary boundary layer, however. In that limit the surface-exchange coefficients are random variables, and it is shown through analysis of surface-layer measurements and LES results that their fluctuation levels can be large. As an alternative to surface-exchange coefficients, the authors derive conservation equations for the surface scalar and momentum fluxes in LES. Scaling relations for resolvable-scale variables in the surface layer are developed and used to simplify these equations. It is shown that, as the grid aspect ratio decreases toward unity, local time change, horizontal advection, production...

Electromagnetic wave propagation through simulated atmospheric refractivity fields

Large-eddy simulation (LES) provides three-dimensional, time-dependent fields of turbulent refractivity in the atmospheric boundary layer on spatial scales down to a few tens of meters. These fields are directly applicable to the computation of electromagnetic (EM) wave propagation in the megahertz range but not in the gigahertz range. We present an approximate technique for extending LES refractivity fields to the smaller scales needed for calculating EM propagation at gigahertz frequencies. We demonstrate the technique by computing refractivity fields through 1283 LES, extending them to smaller scales in two dimensions, and using them in a parabolic equation EM propagation model. At 0.263 GHz the very small scale structure in the extended fields has a negligible effect on the predicted EM levels. At 2 GHz, however, it increases the predicted levels by 15–25 dB. We relate these results to the refractivity structure sampled by EM waves at 0.263 and 2 GHz. We also show that at long range an EM field calculated through an LES-based refractivity field is generally less coherent and significantly weaker than one computed from a “plywood” (i.e., stratified, range-independent) model of the small-scale refractivity field. We give a physical explanation for the differences in the EM fields computed in these two ways. Finally, although the plywood model gives results that fit the EM levels observed in the recent Variability of Coastal Atmospheric Refractivity (VOCAR) experiment, it is not physically realistic. The instantaneous top of the atmospheric boundary layer is known to be sharp and horizontally varying, and we show that using such a top also yields a fit to the VOCAR data.

Comparison of Kansas Data with High-resolution Large-eddy Simulation Fields

The surface layer of an atmospheric boundary layer (ABL) is most accessible to field measurements and hence its ensemble-mean structure has been well established. The Kansas field measurements were the first detailed study of this layer, providing numerous benchmark statistical profiles for a wide range of stability states. Large-eddy simulation (LES), in contrast, is most suitable for studying the mixed layer of the ABL where the energy-containing range of the vertical velocity field is well resolved. In the surface layer, typical large-eddy simulations barely resolve the energy-containing vertical-velocity fields and hence do not provide sufficient data for a detailed analysis.We carried out a nested-mesh simulation of a moderately convective ABL (-zi/L = 8) in which the lower 6% of the boundary layer had an effective grid resolution of 5123. We analyze the LES fields above the 6th vertical grid level (z = 23 m) where the vertical velocity field has a well formed inertial subrange, for a detailed comparison with the Kansas results. Various terms in the budgets of turbulent kinetic energy, temperature variance, Reynolds stress, temperature flux, and some higher order moments are compared. The agreement is generally quite good; however, we do observe certain discrepancies, particularly in the terms involving pressure fluctuations.