Temperature Structure Parameter CT2 Research Articles

Seasonal Changes in Atmospheric Optical Turbulence in a Near Maritime Environment Using Turbulence Flux Measurements

Light propagation through the atmosphere is affected by fluctuations in the refractive index along the path of propagation, called optical turbulence. In the atmospheric surface layer, these fluctuations are due mostly to turbulent mixing of variations in temperature. To this end, a characterization of the atmospheric surface layer above the water in Maryland, USA, is presented. Meteorological data were collected from a sensor array comprising two sonic anemometers and an infrared gas analyzer (IRGASON) over the 12-month period from January to December, 2021. The instruments were positioned vertically at heights of approximately 4 m, 6 m, and 8 m above the water. The temperature structure parameter CT2 was calculated and scaled using Monin–Obukhov assumptions. We find that the distribution of atmospheric stability presents as an approximately normal distribution during the winter and early spring, but is skewed negatively during the summer and late fall. We hypothesize that this could be due to a more stable stratification of the underlying water during summer months. Under unstable (−z/L) conditions, turbulent quantities including the temperature structure parameter scale in a manner consistent with the literature, but with generally better agreement during the summer months. Additionally, the CT2 estimations appear more consistent during the summer months across all values of (z/L). Under stable (+z/L) conditions, there are wide variations in CT2, which is also consistent with the literature.

Fine Structure, Instabilities, and Turbulence in the Lower Atmosphere: High-Resolution In Situ Slant-Path Measurements with the DataHawk UAV and Comparisons with Numerical Modeling

AbstractA new platform for high-resolution in situ measurements in the lower troposphere is described and its capabilities are demonstrated. The platform is the small GPS-controlled DataHawk unmanned aerial system (UAS), and measurements were performed under stratified atmospheric conditions at Dugway Proving Ground, Utah, on 11 October 2012. The measurements included spiraling vertical profiles of temperature and horizontal wind vectors, from which the potential temperature θ, mechanical energy dissipation rate ε, Brunt–Väsälä frequency N, temperature structure parameter CT2, Thorpe and Ozmidov scales LT and LO, and Richardson number Ri were inferred. Profiles of these quantities from ~50 to 400 m reveal apparent gravity wave modulation at larger scales, persistent sheet-and-layer structures at scales of ~30–100 m, and several layers exhibiting significant correlations of large ε, CT2, LT, and small Ri. Smaller-scale flow features suggest local gravity waves and Kelvin–Helmholtz instabilities exhibiting strong correlations, yielding significant vertical displacements and inducing turbulence and mixing at smaller scales. Comparisons of these results with a direct numerical simulation (DNS) of similar multiscale dynamics indicate close agreement between measured and modeled layer character and evolution, small-scale dynamics, and turbulence intensities. In particular, a detailed examination of the potential biases in inferred quantities and/or misinterpretation of the underlying dynamics as a result of the specific DataHawk sampling trajectory is carried out using virtual sampling paths through the DNS and comparing these with the DataHawk measurements.

Backscattering and reflection of acoustic waves in the stable atmospheric boundary layer

Among different applications of sodars in studies of the atmospheric boundary layer (ABL), the remote sensing of the temperature structure parameter CT2 and the closely associated structure parameter of refraction index Cn2 occupies an important place. These parameters are calculated from sodar return signal with the help of the theory of Bragg volumetric scattering of sound by small-scale turbulent irregularities of temperature. According to the Kolmogorov-Oboukhov conception, the irregularities are assumed to be locally homogeneous and isotropic. The theory was confirmed by experiments and universally recognized as a classical one. However, detailed comparisons are still being made between CT2 values derived from sodar data and from aircraft and meteorological-tower measurements. The results of such comparisons performed by different researchers under different conditions contradict to each other. In this paper, the results of comparisons between remote and in situ measurements of CT2 in the ABL are briefly reviewed. Some examples of a systematic overestimation of CT2 calculated from sodar data obtained under stable stratification of the ABL are given. Technical and physical causes of these discrepancies are discussed. The following factors are considered as possible physical factors: contribution of a partial (Fresnel) reflection of sound from layers with a sharp temperature jump, anisotropy of small-scale turbulence, intermittency, and contribution of quasi-wave air-density inhomogeneities. It is shown that these factors can explain some short-lived increase of the sodar return signal, but not a systematic overestimation of CT2. The role of sound-beam refraction (which regularly occurs in the stable ABL due to strong air-temperature and wind-speed gradients) is discussed. Due to a refraction turn of sodar beam, the angle of scattering changes, and scattering by turbulent wind-speed irregularities could permanently contribute to return signal of a monostatic sodar.

A Two-Regime Model For The Probability Density Function Of The Temperature Structure Parameter In The Convective Boundary Layer

The experimentally observed probability distribution of the acoustic echo-signal intensity associated with the temperature structure parameter CT2 was found to be markedly non-lognormal in the convective boundary layer. A two-regime model of thermal turbulence is suggested that combines a small-scale turbulence description with the concept of coherent structures. The probability density function of the logarithm of the temperature structure parameter CT2 is assumed to be approximated by a combination of two normal distributions taken with corresponding weights. It can be interpreted as the existence of two regimes of thermal turbulence, namely, light background turbulence and strong turbulence within convective plumes. This decomposition of the probability density function into ‘plume’ and ‘background’ parts provides an objective procedure for separating the statistics of these two regimes as well as to determine the probability of convective plume occurrence. It is shown that the conventionally measured overall mean CT2, averaged over a long enough time interval, is determined mainly by the probability of plume occurrence, while the mean CT2 value within plumes changes slightly. The decrease of the total mean CT2 with height is caused mainly by the decrease of the probability of the plume occurrence with height.

Acoustic calibration of sodars

Measurements of calibration parameters of sodars normally require either a high mast, a tethered balloon and calm weather, or a very large anechoic chamber for precise results. A simple method for the measurement of calibration parameters using a normal size anechoic chamber and a theoretical antenna directivity pattern is suggested. The method has been used for the two sodars belonging to the Institute of Atmospheric Physics, Moscow (Sodar LATAN-1) and the National Physical Laboratory, New Delhi. The calibrated sodar parameters determined for the Russian Sodar LATAN-1 have also been used to obtain values of the temperature structure parameter CT2 which have been found to be comparable with the simultaneously measured in situ microthermal values of CT2.

Surface-Layer Fluxes Measured Using theCT2-Profile Method

Abstract The first experimental test of obtaining heat and momentum fluxes from measurements of the profile of the temperature structure parameter CT2 is performed. The parameter CT2 is obtained from resistance-wire thermometers as well as from optical-scintillation measurements; the latter produces averaged values over a 606-m propagation path. Use of optical scintillation can produce spatial averaging over propagation paths as long as several kilometers. Deducing the fluxes requires the Monin-Obukhov similarity theory of the horizontally homogeneous atmospheric surface layer. The results show agreement with eddy-correlation measurements of the fluxes for homogeneous and stationary micrometeorological conditions. The accuracy of optical scintillometers must, however, be improved to obtain reliable fluxes by this method.

Characteristics of atmospheric turbulence in the surface layer over Antarctica

This paper presents meteorological measurements made during the antarctic summer period, on two 9 m and 3 m towers, on the rocky and ice shelf terrains of the Indian antarctic stations Maitri and Dakshin Gangotri, respectively. The measurements of fluctuations in temperature and wind speed made with relatively lesser precision instrumentation pertain to smaller wave numbers ~10-2 m-1 appropriate to outer scale L 0 of the atmospheric turbulence spectrum. Autocorrelation analysis of the fluctuations in temperature and wind speed has been performed. A new autoregressive scheme has been developed to represent the computed autocorrelation functions by a Yule statistical model, and to estimate the correlation period T 0 of the turbulent medium. Height profiles of outer scale L 0 of turbulence may be given in terms of T 0 and mean wind speed u. Further, the similarity theory of Monin-Obukhov has been used to compute height profiles of temperature structure parameter C T 2. At Maitri, values of L 0 and C T 2 are higher between 03–22 h local time than between 22–03 h. Values of L 0 and C T 2 are smaller over the ice shelf terrain of the Dakshin Gangotri station, compared to those over the rocky terrain of the Maitri station.

A sodar study of the temperature structure parameter in the convective boundary layer

From sodar measurements gathered during the Voves experiment (France, summer 1977), the variations of the temperature structure parameter CT2 were studied in the morning planetary boundary layer. Dimensionless profiles of CT2 are consistent with the mixed-layer scaling of Kaimal et al. (1976); however, for z < 0,5 zi, the decrease of CT2 as z4/3 should be weighted according to Frisch and Ochs (1975).

Comparison of Structure Parameter Scaling Expressions with Turbulence Closure Model Predictions

Abstract The convective boundary-layer scaling expressions presented by Wyngaard and LeMone (1980) are compared with predictions from a turbulence closure model. We first examine a model experiment involving a clear-air, convectively driven boundary layer overland. The model results agree well with scaling expressions and observations in the lower boundary layer and near the inversion. In the mid-boundary layer region, however, the closure model underestimates the temperature structure parameter CT2 and overestimates the humidity structure parameter Cq2. A cloud-topped marine boundary layer is examined in a second experiment which uses AMTEX data. Order-of-magnitude differences are found here between interfacial-layer scaling expressions and closure model predictions. Potential sources of this disagreement are discussed.

Vertical refraction angle derived from the variance of the angle-of-arrival fluctuations

A theory is developed for evaluating the vertical refraction angle from the variance of the angle-of-arrival fluctuations, assuming a horizontally homogeneous turbulent atmospheric surface layer. The vertical refraction angle is mainly a function of the vertical temperature gradient, and the variance of the angle-of-arrival is related to the temperature structure parameter CT2. However, surface layer similarity theory states that both the mean vertical temperature gradient and CT2are functions of the same scaling temperature T*and a thermal stability parameter. This therefore provides an indirect method of determining the vertical refraction angle from a measurement of the variance of the angle-of-arrival and an estimate of the thermal stability parameter. Advantages of this method over other techniques of evaluating vertical refraction are discussed.

Estimating Momentum, Heat and Moisture Fluxes from Structure Parameters

Abstract A theory is developed for obtaining the vertical fluxes of momentum, heat and moisture in a quasi-steady, locally-homogeneous surface layer from measurements of the structure parameters of velocity, temperature and humidity. The momentum flux is shown to be generally more sensitive to measurement errors in the structure parameters and to uncertainties in certain turbulence parameters than are the heat and moisture fluxes. The structure parameters can, in principle, be obtained from path-averaged wave propagation measurements; for example, the temperature structure parameter CT2, is readily obtained from optical scintillations. It is estimated that in the case of CT2 a path-averaged optical measurement requires about 1% of the averaging time of a conventional measurement of CT2 under typical conditions.