Improved Estimates Of Turbulence From Mesosphere-Stratosphere-Troposphere (MST) Radars

MST（中间层-平流层-对流层）雷达测量的多普勒光谱由于波束中散射元素的湍流速度波动而展宽。因此，测量谱展是利用MST雷达估计湍流强度的方法之一。由于这类雷达的波束相当宽（通常是几度），因此由于波束横向风（所谓的波束增宽）和波束轴方向上径向风分量的切变，对多普勒传播有额外的贡献。最近的研究表明，目前用于纠正这些影响的方法是不准确的。这种方法通常基于这样的假设，即贡献体积均匀地充满湍流。事实上，湍流通常局限于一个或多个水平层，这些水平层比贡献体积的垂直范围更薄。基于假定填充光束的修正通常太大。目前的合同支持开发一种新的实验技术，该技术将改进雷达对湍流的估计。这种技术被称为双波束宽度技术，它的优点是可以直接获得湍流强度，而不需要对多普勒频谱宽度进行通常的修正。该方法的基本假设是波束展宽对多普勒扩频的贡献（通常未知）与波束宽度成正比。在同一距离上用两种波束宽度同时测量，这在日本和印度等先进的MST雷达上是可能的，这样就可以测量波束宽度对总多普勒扩频的贡献，并在估计湍流引起的剩余扩频时对其进行补偿。以这种方式确定的湍流强度将与采用“有效波束宽度”概念的单波束估计的湍流强度进行比较，有效波束宽度小于几何波束宽度，以允许湍流层的有限垂直范围。如果采用单一有效波束宽度的方法被证明是准确的，那么它将被应用于重新计算来自日本和新墨西哥州白沙的雷达数据的湍流强度，这些数据已经在文献中报道过。

Doppler spectra measured by MST (mesosphere-stratosphere-troposphere) radars are broadened by turbulent velocity fluctuations of the scattering elements in the beam. Measuring the spectral spread is therefore one of the ways of using MST radar to estimate the intensity of turbulence. Because the beams of such radars are fairly broad (typically several degrees) there are additional contributions to the Doppler spread caused by the cross-beam wind (so-called beam broadening) and by shear of the radial wind component in the direction of the beam axis. Recent studies indicate that the methods currently used to correct for these effects are inaccurate. Such methods are usually based on the assumption that the contributing volume is uniformly filled with turbulence. In fact, the turbulence is often confined to one or more horizontal layers that are thinner than the vertical extent of the contributing volume. Corrections based on the assumption of a filled beam are then generally too large.The present award supports development of a new experimental technique that will enable improved radar estimates of turbulence. Called the dual-beamwidth technique, it has the advantage of obtaining the turbulence intensity directly, without the need for the usual corrections to the Doppler spectral width. The assumption underlying the method is that the (generally unknown) contribution of beam broadening to the Doppler spread is proportional to the beamwidth. Simultaneous measurements at the same range with two beamwidths, which is possible with advanced MST radars such as those in Japan and India, then make it possible to measure the contribution of beam-broadening to the total Doppler spread and to compensate for it in estimating the remaining spread due to turbulence. The turbulence intensities determined this way will be compared with those estimated from a single beam employing the concept of an "effective beamwidth," which is less than the geometric beamwidth to allow for the limited vertical extent of the turbulent layers. If the method employing a single effective beamwidth is proven accurate, it will then be applied to recompute turbulence intensities for radar data from Japan and White Sands, New Mexico, that have already been reported in the literature.