The Impacts of Vegetation and Meteorological Factors on Aerodynamic Roughness Length at Different Time Scales

Mingzhao Yu,Hongwei Zeng,Bingfang Wu,Qiang Xing,Weiwei Zhu

doi:10.3390/atmos9040149

Abstract

The aerodynamic roughness length (z0m) is a crucial parameter for reliably simulating turbulent exchanges between the land surface and the atmosphere. Due to the large number of input variables related to vegetation growth and aerodynamic conditions near the surface, estimating z0m precisely is difficult and, to date, no universal model has been established. Understanding the z0m changes in time series data and the relative contributions of vegetation indices and meteorological factors is important to providing a basis for modelling z0m. In this paper, the main meteorological factors that influence z0m in different seasons are presented based on data from three automatic weather stations (AWSs) that represent various land surface patterns in the Heihe river basin. A correlation analysis identified the dominant factors that influence z0m changes at half-hour and daily scales; then, a factor analysis was performed to identify the different contributions of vegetation indices and meteorological factors to z0m at different time scales. The results show that meteorological factors (wind speed, wind direction and atmospheric stability) are the main driving factors for z0m at the Arou and Guantan sites, which are situated in grassland and forest mountain areas, respectively, and that the vegetation indices have no impact on the z0m variations in these areas. In contrast, for the Daman site, situated in flat farmland, the vegetation indices are the primary driving factors, while meteorological factors such as wind speed and atmospheric stability are secondary factors, and wind direction has no significant influence. Finally, a detailed analysis was conducted to detect the relationships between half-hourly z0m measurements and three dominant meteorological factors.

Highlights

The aerodynamic roughness length (z0m ) is the height above the ground where the wind speed is zero under neutral conditions [1]
The results of bivariate correlation analyses between half-hourly z0m, daily z0m and each possible driving factor are shown in Table 2, which indicates that the main driving factors for z0m differ among the three stations
For the Arou and Guantan sites, the half-hourly and daily z0m values are positively correlated with the wind deflection angle and negatively correlated with the wind speed, while no significant correlation exists with the normalized different vegetation index (NDVI) and the leaf area index (LAI), which represent the vegetative growth conditions

Summary

Introduction

The aerodynamic roughness length (z0m ) is the height above the ground where the wind speed is zero under neutral conditions [1]. Aerodynamic roughness length is a key variable in land surface flux simulations and atmospheric boundary studies [2,3]. The z0m metric is a virtual height under ideal atmospheric conditions that cannot be measured and quantified directly; it is important to develop a reliable parameterized method for estimating z0m at different spatial and temporal scales. Because modern studies involve land surface processes at a greater spatial scale, in recent years, an increasing number of models have been established to estimate regional z0m. Raupach developed an approach for estimating z0m in terms of height, width, spacing and wind-drag properties of surface roughness elements [6,7], Jasinsk simulated the parameters in the Raupach model by classifying four vegetation

Methods

Results

Discussion

Conclusion