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Parameters characterizing the atmospheric turbulence in a 16 km maritime optical link were measured for the months of January through June of 2007 on a continuous basis, as conditions allowed. Both the scintillation index [[sigma].sub.I.sup.2] and the atmospheric structure constant [C.sup.2.sub.n] are found to have a strong dependence on the air-minus-water temperature difference. There is no obvious diurnal variation of [C.sup.2.sub.n] or of [[sigma].sub.I.sup.2] akin to the reduction in turbulence level seen in terrestrial links in the hour before sunrise and the hour after sunset. Results from the analysis of these data highlight a need for new approaches to modeling beam propagation in a maritime environment. OCIS codes: 010.1300, 010.1330, 290.5930.

We develop a unified algorithm for performing blind deconvolution of a noisy degraded image. By incorporating a low-pass filter into the asymmetric multiplieative iterative algorithm and extending it to multiframe blind deconvolution, this algorithm accomplishes the blind deconvolution and noise removal concurrently. We report numerical experiments of applying the algorithm to the restoration of short-exposure atmosphere turbulence degraded images. These experiments evidently demonstrate that the unified algorithm has both good blind deconvolution performance and high-resolution image restoration. OCIS codes: 100.3020, 100.1455, 100.1830, 100.2000.

We analyze the effect of anisoplanatic atmospheric turbulence on the measurement accuracy of an extended-source Hartmann-Shack wavefront sensor (HSWFS). We have numerically simulated an extended-source HSWFS, using a scenery of the solar surface that is imaged through anisoplanatic atmospheric turbulence and imaging optics. Solar extended-source HSWFSs often use cross-correlation algorithms in combination with subpixel shift finding algorithms to estimate the wavefront gradient, two of which were tested for their effect on the measurement accuracy. We find that the measurement error of an extended-source HSWFS is governed mainly by the optical geometry of the HSWFS, employed subpixel finding algorithm, and phase anisoplanatism. Our results show that effects of scintillation anisoplanatism are negligible when cross-correlation algorithms are used. OCIS codes: 010.1080, 010.7350.

Lucky imaging, used with some success in astronomical and even horizontal-path imaging, relies on fleeting conditions of the atmosphere that allow momentary improvements in image quality, at least in portions of an image. Aperture synthesis allows a larger aperture and, thus, a higher-resolution imaging system to be synthesized through the superposition of image spatial-frequency components gathered by cooperative combinations of smaller subapertures. A combination of lucky imaging and aperture synthesis strengthens both methods for obtaining improved images through the turbulent atmosphere. We realize the lucky imaging condition appropriate for aperture synthesis imaging for a pair of rectangular subapertures and demonstrate that this condition occurs when the signal energy associated with bandpass spatial-frequency components achieves its maximum value. OCIS codes: 110.0115, 100.3175.

The main applications of adaptive optics are the correction of the effects of atmospheric turbulence on ground-based telescopes and the correction of ocular aberrations in retinal imaging and visual simulation. The requirements for the wavefront corrector, usually a deformable mirror, will depend on the statistics of the aberrations to be corrected; here we compare the spatial statistics of wavefront aberrations expected in these two applications. We also use measured influence functions and numerical simulations to compare the performance of eight commercially available deformable mirrors for these tasks. The performance is studied as a function of the size of the optical pupil relative to the actuated area of the mirrors and as a function of the number of modes corrected. In the ocular case it is found that, with the exception of segmented mirrors, the performance is greatly enhanced by having a ring of actuators outside the optical pupil, as this improves the correction of the pupil edge. The effect is much smaller in the case of Kolmogorov wavefronts. It is also found that a high Strehl ratio can be obtained in the ocular case with a relatively low number of actuators if the stroke is sufficient. Increasing the number of actuators has more importance in the Kolmogorov case, even for the relatively weak turbulence considered here. OCIS codes: 110.1080, 330.4460, 230.3990.

A warm rain parameterization has been developed by solving the stochastic collection equation with the use of turbulent collision kernels. The resulting parameterizations for the processes of autoconversion, accretion, and self-collection are functions of the turbulent intensity of the flow and are applicable to turbulent cloud conditions ranging in dissipation rates of turbulent kinetic energy from 100 to 1500 [cm.sup.2] [s.sup-3]. Turbulence has a significant effect on the acceleration of the drop size distribution and can reduce the time to the formation of raindrops. When the stochastic collection equation is solved with the gravitational collision kernel for an initial distribution with a liquid water content of 1 g [m.sup.-3] and 240 drops [cm.sup.-3] with a mean volume radius of 10 [micro]m, the amount of mass that is transferred to drop sizes greater than 40 [micro]m in radius after 20 min is 0.9% of the total mass. When the stochastic collection equation is solved with a turbulent collision kernel for collector drops in the range of 10-30 [micro]m with a dissipation rate of turbulent kinetic energy equal to 100 [cm.sup.2][s.sup.-3], this percentage increases to 21.4. Increasing the dissipation rate of turbulent kinetic energy to 500, 1000, and 1500 [cm.sup.2][s.sup.-3] further increases the percentage of mass transferred to radii greater than 40 [micro]m after 20 min to 41%, 52%, and 58%, respectively, showing a substantial acceleration of the drop size distribution when a turbulent collision kernel that includes both turbulent and gravitational forcing replaces the purely gravitational kernel. The warm rain microphysics parameterization has been developed from direct numerical simulation (DNS) results that are characterized by Reynolds numbers that are orders of magnitude smaller than those of atmospheric turbulence. The uncertainty involved with the extrapolation of the results to high Reynolds numbers, the use of gravitational collision efficiencies, and the range of the droplets for which the effect of turbulence has been included should all be considered when interpreting results based on these new microphysics parameterizations.

The effects of atmospheric turbulence on munition target scatter are determined from numerical simulations of ballistic trajectories through many realizations of realistic simulated turbulent wind fields. A technique is evaluated for correcting for the effects of turbulence on ballistic testing procedures by using a line of sonic anemometer measurements taken along the trajectory path. The metric used to evaluate the correction is the difference between the target impact scatter produced with and without the use of the anemometers in the trajectory calculations. The improvement in the testing procedure as measured by this metric is determined as a function of the number of sonic anemometers in the line and the sonic averaging time interval. The performance of the simulations is also compared with data from a field test for a standard small-caliber munition, and the predicted and observed target scatter are in good qualitative agreement, supporting the feasibility of the approach.

A multichannel free-space optical (FSO) communication system based on orbital angular momentum (OAM)-carrying beams is studied. We numerically analyze the effects of atmospheric turbulence on the system and find that turbulence induces attenuation and crosstalk among channels. Based on a model in which the constituent channels are binary symmetric and crosstalk is a Gaussian noise source, we find optimal sets of OAM states at each turbulence condition studied and determine the aggregate capacity of the multichannel system at those conditions. OAM-multiplexed FSO systems that operate in the weak turbulence regime are found to offer good performance. We verify that the aggregate capacity decreases as the turbulence increases. A per-channel bit-error rate evaluation is presented to show the uneven effects of crosstalk on the constituent channels. OCIS codes: 060.2605, 050.4865, 010.1300, 010.3310, 060.4230, 050.7330, 010.7060.

The present study is a continuation of the series of studies dedicated to the investigation of cloud droplet collisions in turbulent flow with characteristics that are typical of real clouds. Detailed tables of collision kernels and collision efficiencies calculated in the presence of hydrodynamic interaction of droplets are presented. These tables were calculated for a wide range of turbulent parameters. To illustrate the sensitivity of droplet size distribution (DSD) evolution to the turbulence-induced increase in the collision rate, simulations of DSD evolution are preformed by solving the stochastic kinetic equation for collisions. The results can be applied to cloud modeling. The tables of collision efficiencies and collision kernels are available upon request. Some unsolved problems related to collisions of droplets and ice hydrometeors in turbulent clouds are discussed in the conclusion.

An open question in cloud physics is how rain forms in warm cumulus as rapidly as it is sometimes observed. In particular, the growth of cloud droplets across the size gap from 10 to 50[micro]m in radius has not been fully explained. In this paper, the authors investigate the growth of cloud droplets by collision-coalescence, taking into account both the gravitational mechanism and several enhancements of the collision-coalescence rate due to air turbulence. The kinetic collection equation (KCE) is solved with an accurate bin integral method and a newly developed parameterization of turbulent collection kernel derived from direct numerical simulation of droplet-laden turbulent flows. Three other formulations of the turbulent collection kernel are also considered so as to assess the dependence of the rain initiation time on the nature of the collection kernel. The results are compared to the base case using the Hall hydrodynamical--gravitational collection kernel. Under liquid water content and eddy dissipation rate values typical of small cumulus clouds, it is found that air turbulence has a significant impact on the collection kernel and thus on the time required to form drizzle drops. With the most realistic turbulent kernel, the air turbulence can shorten the time for the formation of drizzle drops by about 40% relative to the base case, applying measures based on either the radar reflectivity or the mass-weighted drop size. A methodology is also developed to unambiguously identify the three phases of droplet growth, namely, the autoconversion phase, the accretion phase, and the larger hydrometeor self-collection phase. The important observation is that even a moderate enhancement of collection kernel by turbulence can have a significant impact on the autoconversion phase of the growth.

Numerical experiments are carried out for a plane wave propagating in the atmospheric turbulence for a weak to strong fluctuation condition, i.e., the Rytov index being in a large range of 2 x [10.sup.-3] to 20. Mainly two categories of propagation events are explored for the same range of Rytov index. In one category the propagation distance and also the Fresnel length are kept fixed with the turbulence strength changing. In the other the turbulence strength is kept fixed with the distance changing. The statistical characteristics of the scintillation index, the maximum and minimum of the intensity, the fractal dimension of the intensity image, and the number density of the phase singularity are analyzed. The behaviors of the fractal dimension and the density of the phase singularity present obvious differences for the two categories of propagation. The fractal dimension depends both on the Rytov index and the Fresnel length. In both weak and strong fluctuation conditions the dimension generally increases with the Rytov index, but is at minimum at the onset region. The phase singularity density is coincident with the theoretical results under a weak fluctuation condition, and has a slowly increasing manner with the Rytov index in the strong fluctuation condition. The dependence on the Fresnel size is confident and there is no saturation for the phase singularity. OCIS codes: 010.1300, 010.1330, 010.1290, 010.7060.

Parameterization of turbulent flux from bare-soil and undercanopy surfaces is imperative for modeling land-atmosphere interactions in arid and semiarid regions, where flux from the ground is dominant or comparable to canopy-sourced flux. This paper presents the major characteristics of turbulent flux transfers over seven bare-soil surfaces. These sites are located in arid. semiarid, and semihumid regions in Asia and represent a variety of conditions for aerodynamic roughness length ([z.sub.0m] from

Data obtained in downtown Oklahoma City, Oklahoma, during the Joint Urban 2003 atmospheric dispersion study have been analyzed to investigate the effects of upstream atmospheric stability on turbulence statistics in an urban core. The data presented include turbulent heat and momentum fluxes at various vertical and horizontal locations in the lower 30% of the street canyon. These data have been segregated into three broad stability classification regimes: stable (z/L > 0.2), neutral (-0.2 < z/L < 0.2), and unstable (z/L < -0.2) based on upstream measurements of the Monin-Obukhov length scale L. Most of the momentum-related turbulence statistics were insensitive to upstream atmospheric stability, while the energy-related statistics (potential temperatures and kinematic heat fluxes) were more sensitive. In particular, the local turbulence intensity inside the street canyon varied little with atmospheric stability but always had large magnitudes. Measurements of turbulent momentum fluxes indicate the existence of regions of upward transport of high horizontal momentum fluid near the ground that is associated with low-level jet structures for all stabilities. The turbulent kinetic energy normalized by a local shear stress velocity collapses the data well and shows a clear repeatable pattern that appears to be stability invariant. The magnitude of the normalized turbulent kinetic energy increases rapidly as the ground is approached. This behavior is a result of a much more rapid drop in the correlation between the horizontal and vertical velocities than in the velocity variances. This lack of correlation in the turbulent momentum fluxes is consistent with previous work in the literature. It was also observed that the mean potential temperatures almost always decrease with increasing height in the street canyon and that the vertical heat fluxes are always positive regardless of upstream atmospheric stability. In addition, mean potential temperature profiles are slightly more unstable during the unstable periods than during the neutral or stable periods. The magnitudes of all three components of the heat flux and the variability of the heat fluxes decrease with increasing atmospheric stability. In addition, the cross-canyon and along-canyon heat fluxes are as large as the vertical component of the heat fluxes in the lower portion of the canyon.

The effect of turbulence on the collision rate between droplets in clouds is investigated. Because of their inertia, water droplets can be shot out of curved streamlines of the turbulent airflow. The contribution of such a 'sling effect' in the collision rate of the same-size water droplets is described and evaluated. It is shown that already for turbulence with the dissipation rate [10.sup.3] [cm.sup.2] [s.sup.-3], the sling effect gives a contribution to the collision rate of 15-[micro]m droplets comparable to that due to the local velocity gradient. That may explain why the formulas based on the local velocity gradient consistently underestimate the turbulent collision rate, even with the account of preferential concentration.

A series of numerical experiments on two-dimensional decaying turbulence is performed for a barotropic fluid on a rotating sphere. Numerical calculations have confirmed two important asymptotic features: emergence of the banded structure of zonal flows and their extreme latitudinal inhomogeneities in which kinetic energy is accumulated into the easterly circumpolar jets. The banded structure of zonal flows is established relatively early on in the initial stage. Later, afler extended periods of time integration, only the circumpolar jets are intensified gradually, while there is no further evolution in the banded structure in the low and midlatitudes. Wave activity flux analysis illustrates that the initial vortices in the low and midlatitudes propagate poleward as Rossby waves and converge to produce easterly circumpolar flows. In association with this convergence, accumulation of the mean zonal component of kinetic energy proceeds. The tendency for the accumulation becomes strong as the rotation rate is increased, and nearly all of the kinetic energy is concentrated to the circumpolar flows in cases of rapid rotation. A theoretieal model is constructed under the assumption that a circumpolar jet emerges around the latitude where the local Rhines scale is equal to the distance from the Pole, and that initial vortices at the lower latitudes contribute to the generation of the jets. The model describes the mean zonal component of kinetic energy and the averaged speed and width of the circumpolar jets as functions of the rotation rate, which agree satisfactorily with the numerical results.

To assess the coherent detection of an optical signal perturbed by atmospheric turbulence, the loss in the mean signal-to-noise ratio (SNR) is usually invoked although it constitutes a limited description of the signal fluctuations. To produce statistical distributions of the SNR, we generate random optical fields. A 5/3-power law for the phase structure function is considered. The benefit of a wavefront tilt correction is assessed. Based on the 1%-probability fade, an optimum receiver size is found. For phase fluctuations only, a similarity between the signal distribution and the beta distribution is observed. Phase and amplitude are assumed independent, and the influence of amplitude perturbations is assessed with a scintillation index of 2. Turbulence impairments are compared for a coherent receiver and a direct-detection receiver. OCIS codes: 010.1300, 010.1330, 060.1660, 060.4510.

The problem of estimating mechanical boresight and jitter performance of a laser pointing system in the presence of atmospheric turbulence is considered. A novel estimator based on maximizing an average probability density function (pdf) of the received signal is presented. The proposed estimator uses a Gaussian far-field mean irradiance profile, and the irradiance pdf is assumed to be lognormal. The estimates are obtained using a sequence of return signal values from the intended target. Alternatively, one can think of the estimates being made by a cooperative target using the received signal samples directly. The estimator does not require sample-to-sample atmospheric turbulence parameter information. The approach is evaluated using wave optics simulation for both weak and strong turbulence conditions. Our results show that very good boresight and jitter estimation performance can be obtained under the weak turbulence regime. We also propose a novel technique to include the effect of very low received intensity values that cannot be measured well by the receiving device. The proposed technique provides significant improvement over a conventional approach where such samples are simply ignored. Since our method is derived from the lognormal irradiance pdf, the performance under strong turbulence is degraded. However, the ideas can be extended with appropriate pdf models to obtain more accurate results under strong turbulence conditions. OCIS codes: 010.1330, 120.0120, 140.0140, 030.6600.

The intensity fluctuations of incoherent flat-topped Gaussian beams are evaluated when such sources are used in weakly turbulent horizontal atmospheric links. The formulation is developed for a detector having a response time much longer than the source coherence time. The flat-topped Gaussian profile is obtained by superposing many Gaussian beams, then the incoherence is introduced through delta correlation in space. The scintillation index of the incoherent flat-topped Gaussian beams is found to be smaller than the scintillation index of the corresponding incoherent Gaussian beams at the same link length, source size, and wavelength. When compared with the coherent counterparts, the intensity fluctuations of the incoherent flat-topped Gaussian beams are much smaller, yielding the same value only at the spherical wave limit, as expected. Transmitter aperture averaging is a special case of our solution. OCIS codes: 010.1330, 010.1300, 290.5930, 030.7060.

The performance of lasercom systems operating in the atmosphere is reduced by optical turbulence, which causes irradiance fluctuations in the received signal. The result is a randomly fading signal. Fade statistics obtained from experimental data were compared to theoretical predictions based on the lognormal and gamma-gamma distributions. The probability of fade, the expected number of fades per second, and the mean fade time were calculated from the irradiance fluctuations of a Gaussian beam wave propagating through the atmosphere along a horizontal path, near ground, in the moderate-to-strong optical turbulence regime. Irradiance data were collected simultaneously at three receiving apertures, each with a different size. Atmospheric propagation parameters were inferred from the measurements and were used in calculations for the theoretical distributions. It was found that fade predictions made by the gamma-gamma and lognormal distributions provide an upper and lower bound, respectively, for the probability of fade and the number of fades per second for the irradiance data collected in the moderate-to-strong fluctuation regime. What is believed to be a new integral expression for the expected number of fades based on the gamma-gamma distribution was developed. This new expression tracked the gamma-gamma distributed data more closely than the existing approximation and resulted in a higher number of fades. OCIS codes: 010.1300, 010.1330, 290.5930.

In the near-infrared and visible bandpasses optical propagation theory conventionally assumes that humidity does not contribute to the effects of atmospheric turbulence on optical beams. While this assumption may be reasonable for dry locations, we demonstrate that there is an unequivocal effect owing to the presence of humidity upon the strength of turbulence parameter, [C.sub.n.sup.2], from data collected in the Chesapeake Bay area over 100 m length horizontal propagation paths. We describe and apply a novel technique, Hilbert phase analysis, to the relative humidity, temperature, and [C.sub.n.sup.2] data to show the contribution of the relevant climate variable to [C.sub.n.sup.2] as a function of time. OCIS codes: 010.1300, 010.7060, 030.7060.

The purpose of the study is to investigate the probability density function of turbulent boundary layer fluctuating pressures measured on the outer sidewall of a supersonic transport aircraft and to approximate these probability density functions by analytical models. Experimental flight results show that the fluctuating pressure probability density functions differ from the Gaussian distribution even for standard smooth surface conditions. The probability density function tails are wider and longer than those of the Gaussian model. For pressure fluctuations in front of forward-facing step discontinuities, deviations from the Gaussian model are more significant and the probability density functions become asymmetrical: There is a certain spatial pattern of the skewness and kurtosis behavior depending on the distance upstream from the step. All characteristics related to non-Gaussian behavior are highly dependent upon the distance from the step and the step height, less dependent on aircraft speed, and not dependent on the fuselage location. A Hermite polynomial transform model and a piecewise-Gaussian model fit the flight data well both for the smooth and stepped conditions. The piecewise-Gaussian approximation can be additionally regarded for convenience in usage after the model is constructed. DOI: 10.2514/1.18294

A methodology to predict aircraft transient motion resulting from flight within a turbulent atmospheric environment, coupled with validation using flight-test data, is explored. A family of four linear dynamic models is developed for describing the normal acceleration throughout an aircraft cabin due to vertical gust excitation. The four models successively build upon each other by incorporating higher fidelity gust penetration effects while simultaneously maintaining a unified modeling framework. Six wind fields reconstructed from flight-test data are used to excite the vehicle models. Simulation responses are compared with forward, center, and aft accelerometer data streams recorded during the test. Although significant differences are known to exist between the simulated dynamics and flight vehicle, all models tend to approximate adequately primary features in the nonlinear flight data. However, higher-order features in the flight data can not be reliably replicated in the simulations due to these differences. One model consistently tends to out perform the other models, both qualitatively and quantitatively, and corresponds to the highest fidelity gust penetration model. This particular model is recommended as an acceptable alternative to a higher fidelity, nonlinear full simulation when approximating the acceleration response for the intended application.

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The dependence of dynamic responses of airplanes on atmospheric turbulence has stimulated the development of gust models to study the dependence. The models provide information on the structural load determination of the airplanes and enable the development of better aeronautic designs. The research in the model was initiated by Wagner and Jones during the 1920s.

Aeroservoelastic state-space modeling of the continuous gust responses and the sensitivity derivatives in flight vehicles shows that air turbulence evokes gust responses, while root-mean-square values and power spectral density functions help define the responses as statistical expressions. The minimum-state approximation method and the physical aerodynamic data weighting method enable minimization of the number of aerodynamic states for higher accuracy. Augmentation of the aeroelastic states by gust filters results in a white noise-stimulated linear system, with a matrix Lyapunov equation providing solutions for the rms values of the responses.

Byline: Stefano Alessandrini, Enrico Ferrero, Silvia Trini Castelli, Domenico Anfossi A dispersion simulation test for the RAMS-MIRS-SPRAY (RMS) modelling system in real complex terrain is presented. As a test case, the TRACT campaign was chosen because this data set allows the comparison of both the meteorological variables and the tracer concentrations. Particular attention was focused on testing alternative turbulence closures and assessing their influence on modelling both the meteorology and the dispersion. Firstly, the different turbulence closures introduced in the meteorological model are compared. Secondly, the dispersion simulations are performed using the turbulence provided by the different models. It is demonstrated that using different closure models for turbulence modifies also the mean flow. Concerning the tracer dispersion, the new closures utilising a fully 3D equation for the turbulent kinetic energy seem to give promising results.