Ting-i Wang

Wind measurements by the temporal cross-correlation of the optical scintillations

Various methods of correlation analysis that have been used to deduce crosswind from a drifting scintillation pattern are briefly described and then compared with regard to their immunity to noise and their accuracy when faced with nonuniformities along the propagation path or changes in the characteristics of the turbulence. Of the techniques considered, none is ideal; but a new technique, using complete knowledge of the cross-covariance function, proves to be advantageous in a wide variety of situations.

Refractive-turbulence profiles measured by one-dimensional spatial filtering of scintillations

Stellar scintillations, when appropriately analyzed, yield information about the turbulence throughout the atmosphere. We describe an instrument involving a 36-cm telescope and an on-line minicomputer that provides, after 20 min of observation, the refractive-turbulence profile of the atmosphere. The height resolution is sufficient to divide the atmosphere into about four independent regions. The principal limitation to greater accuracy and resolution is the nonstationary behavior of the atmosphere during the 20-min observing period.

Optical wind sensing by observing the scintillations of a random scene

We demonstrate the feasibility of using a naturally illuminated scene, such as a hillside or forest, as a passive optical source to measure the path-averaged crosswind between the scene and the observer. The resultant path weighting function for the crosswind cannot be varied arbitrarily, but we can obtain a useful range of weighting functions by adjusting the geometry of the receiver.

Finite aperture optical scintillometer for profiling wind and Cn2

A new optical technique is described for measuring the path profiles of crosswind and of a refractive-index structure parameter C(2)(n) along a line-of-sight path. Different sizes of transmitters and receivers are used to control the path-weighting function so that it will peak at different path locations. Various linear combinations of these measurements yield the path profile of crosswind and C(2)(n). A prototype instrument has been built and tested. Experimental results show good agreement with the theoretical predictions.

Laser wind sensing: the effects of saturation of scintillation

We have developed a physically based extension of the first-order perturbation theory of optical scintillation that accounts for the observed variance and covariance of the amplitude fluctuations in strong integrated turbulence. We use this model to analyze the experimentally observed changes in the operation of our laser wind sensor. The theory suggests a transmitter-receiver configuration that can nearly eliminate the performance-degrading effects of strong turbulence. Based on this analysis, we have developed a saturation-resistant optical wind sensor that maintains its calibration and wind-weighting function throughout the observed range of integrated-turbulence values.

Measurement of rain parameters by optical scintillation

We describe a technique for measuring path-averaged rain parameters by analyzing the rainfall-induced scintillations of a laser beam. From the time-lagged covariance function of two vertically spaced line detectors, we determine the average rainfall rate and drop-size distribution along the optical path. This technique requires no prior assumption of the form of the drop-size distribution. Sample measurements on a 140-m path confirm that the path-averaged drop-size distribution of a steady rain follows a Marshall-Palmer distribution. The optically measured path-averaged rain rate also shows good agreement with conventional tipping-bucket rain-gauge data.

Simplified optical path-averaged rain gauge

It has previously been shown that the scintillations produced by raindrops falling through a collimated laser beam can be used to measure the drop-size distribution and the rainfall rate, both averaged over the path. We now present a theoretical analysis, verified by observation, showing that the variance of the scintillations detected by a line-detector measured at frequencies near 1 kHz is closely related to rain rate and is nearly independent of drop-size distribution. If only rain rate is desired, the variance type of optical rain gauge has several advantages over the earlier model. It could use a diverging beam, thus eliminating the practical difficulties of maintaining adjustment and pointing of a collimated beam. Furthermore, it is less sensitive to the presence of updrafts and downdrafts along the beam and can thus be used over rough terrain.

Optical rain gauge using a divergent beam

We have shown that path-averaged rain rates can be obtained from the raindrop-induced amplitude scintillations of a divergent laser beam (spherical wave case). We found that the rain rate obtained from a divergent beam is less sensitive to drop-size distribution than that from a collimated beam. However, the path-weighting function is heavily weighted toward the receiving end in the spherical wave case, whereas in the plane wave case, it is almost uniformly weighted along the optical path. The theory was confirmed by observations on two optical paths, one using a collimated beam on a 200-m path, the other using a divergent beam on a 1000-m path. The results for the longer path show a saturation effect for rain rates higher than 12 mm/h.

Measurement of rain parameters by optical scintillation: computer simulation of the correlation method

Earlier analysis of the use of laser-beam scintillations to measure path-averaged rainfall rate and drop-size distribution has been well verified for pathlengths up to 140 m even though, for such a path, overlapping of the scintillation patterns violates a simplifying assumption of the analyses. Analytic extension of the theory to the case where the scintillation patterns overlap appears intractable, so a computer simulation has been used to investigate that limitation of the theory. That simulation, presented here, verifies that the original scintillation-covariance technique for measuring rainfall parameters is, with only a slight modification, still applicable in the presence of overlapping scintillation patterns from many raindrops.

Simple inversion technique to obtain cloud droplet size parameters using solar aureole data.

A simple inversion technique in the single scattering regime has been developed to deduce cloud droplet size parameters by using the measurement of the radiance of near-forward scattered solar radiation as a function of angle. Compared with the numerical inversion technique that uses exact Mie scattering calculations, the new technique is much less time-consuming and hence should be usable in an on-line real time analysis. To test the effectiveness of the new technique, we use the results of polydispersed cloud size distribution calculated by Deirmendjian to retrieve the model size parameters. The agreement is excellent. We also generalize the theory to include the broadband source. A typical experimental example is given. Its comparison with time-consuming Mie scattering inversion technique again shows excellent agreement.