Positive and negative turbulent heat diffusivity observed on a 325-m meteorological tower in Beijing 基于北京325米气象塔观测的正, 负湍流热扩散系数

Abstract

Turbulent diffusion efficiently transports momentum, heat, and matter and affects their transfers between the atmosphere and the surface. As a key parameter in describing turbulent diffusion, the turbulent heat diffusivity KH has rarely been studied in the context of frequent urban pollution in recent years. In this study, KH under urban pollution conditions was directly calculated based on K-theory. The authors found an obvious diurnal variation in KH, with variations also in the vertical distributions between each case and over time. Interestingly, the height corresponding to the high occurrence frequency of negative KH rises gradually after sunrise, peaks at noon, falls near sunset, and concentrates around 140 m during most of the night. The KH magnitude and fluctuation are smaller in the pollutant accumulation stage (CS) at all levels than in the pollutant transport stage and pollutant removal stage. Turbulent diffusion may greatly affect PM2.5 concentrations at the CS because of the negative correlation between PM2.5 concentrations and the absolute value of KH at the CS accompanied by weak wind speeds. The applicability of K-theory is not very good during either day or at night. These problems are inherent in K-theory when characterizing complex systems, such as turbulent diffusion, and require new frameworks or parameterization schemes. These findings may provide valuable insight for establishing a new turbulence diffusion parameterization scheme for KH and promote the study of turbulent diffusion, air quality forecasting, and weather and climate modeling.摘要本文基于K理论和北京325m铁塔湍流观测资料直接计算了城市污染条件下的湍流热扩散系数KH.KH具有明显的日变化特征, 其垂直分布随个例和时间而变化.负KH较高发生频率对应高度在日出后逐渐升高, 在中午达到峰值, 在日落附近下降, 而在夜间大部分时间集中在140 m左右.和传输和清除阶段相比, 污染物累积阶段的KH大小和波动程度均较小.此阶段PM2.5浓度与KH绝对值之间的负相关关系表明湍流扩散可能会极大地影响PM2.5浓度.这些发现可能为建立新湍流扩散参数化方案提供有价值的启示.