基于风廓线雷达的对流边界层高度的确定

大气边界层高度对于天气、气候和大气污染研究是一个至关重要的参量。对流边界层(Convective Boundary Layer,CBL)顶部的夹卷过程造成温度和湿度垂直梯度增强,导致这一层的折射率结构常数C■变高。C■的这种垂直分布特征经常被用来定位出CBL高度Z_i。本文利用2010年7—8月天津大港的风廓线雷达数据推断出CBL高度Z_i,对于多重C■峰值或不明确的C■峰值,本文改进了对Z_i的测定,分别讨论了C■最大后向散射法与C■和垂直速度方差(σ■)相结合的新方法的适用性。研究显示:(1)C■廓线具有单峰时,最大后向散射强度法能正确估计CBL高度,这种情况往往对应的是晴天。CBL上存在的残留层或云层引起的温湿起伏变化导致C■廓线具有双峰甚至多峰时,最大后向散射强度法可能会错误估计CBL高度;(2)C■和σ■结合的方法不仅与晴天时C■最大后向散射法有较好的一致性,而且可以将CBL造成的C■峰值从云层造成的C■峰值中区分出来,从而正确估计CBL高度;(3)一般而言,对流边界层中存在有明显的、破碎或者分散不明显的云时,C■和σ■结合的方法都能较好地识别出CBL对应的C■峰值。但由于边界层中的情况极为复杂,C■和σ■结合法也会因不同的原因而错误估计CBL高度。

Determination of connective boundary layer height based on wind profile radar

The atmospheric boundary layer height is a critical parameter for studies of weather, climate and air pollution. The entrainment process at the top of the Convective Boundary Layer (CBL) results in an increased vertical gradient of temperature and humidity, thus enhancing the refractive index structure constant C_n² for this layer. This vertical distribution feature of C_n² is often used to locate the CBL height Z_i. In this paper, the CBL height Z_i is deduced from the wind profile radar data of Grand Port District of Tianjin from July to August in 2010. For the multiple or unclear C_n² peaks, this paper improves the measurement of Z, and discusses the applicability of the maximum backscattered intensity method and the method combining C_n² and the vertical velocity variance σ_w². The results show that:(1) When the C_n² profile has a single peak, the maximum backscattered intensity method can correctly estimate the height of CBL. This situation often corresponds to a sunny day. The maximum backscattered intensity method may incorrectly estimate the height of CBL when the temperature and humidity fluctuation caused by the residual layer or the cloud layer in the CBL leads to a C_n² profile with double peaks or even multiple peaks; (2) The method combining C_n² and σ_w² is not only consistent with the maximum backscattered method at sunny days, but also distinguishes the C_n² peak caused by CBL from the C_n² peak caused by the cloud layer, thus correctly estimating the CBL height; (3) In general, the method combining C_n² and σ_w² can well identify the C_n² peak corresponding to CBL when there are obvious, broken or undisclosed clouds in the convective boundary layer. However, because the situation in the boundary layer is extremely complex, this method may also incorrectly estimate the CBL height for different reasons.