Reduced Sea-Surface Roughness Length at a Coastal Site

Yuncheng He,Kang Zhou,Jiyang Fu,Qiusheng Li,Pak Wai Chan,Zhenru Shu

doi:10.3390/atmos12080991

Yuncheng He, Kang Zhou + Show 4 more

Open Access

https://doi.org/10.3390/atmos12080991

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Abstract

Sea-surface roughness length is a key parameter for characterizing marine atmospheric boundary layer. Although aerodynamic roughness lengths for homogeneous land and open water surfaces have been examined extensively, the extension of relevant knowledge to the highly inhomogeneous coastal area is problematic due to the complex mechanisms controlling coastal meteorology. This study presented a lidar-based observational analysis of sea-surface roughness length at a coastal site in Hong Kong, in which the wind data recorded from March 2012 to November 2015 were considered and analyzed. The results indicated the turning of wind near the land-sea boundary, leading to a dominative wind direction parallel to the coastline and an acceleration in wind. Moreover, the roughness lengths corresponding to two representative azimuthal sectors were compared, in which the roughness lengths for the onshore wind sector (i.e., 120°–240°) appear to be larger than the constant value (z0 = 0.2 mm) recommended in much existing literature, whereas the values for the alongshore wind sector (i.e., 60°–90°) are significantly smaller, i.e., about two orders of magnitude less than that of a typical sea surface. However, it is to be noted that the effect of atmospheric stability, which is of crucial importance in governing the marine atmospheric boundary layer, is not taken into account in this study.

Highlights

Proper understanding of marine atmospheric boundary layer (MABL) is meeting growing attention [1,2,3,4], taking into account the substantial increase in coastal and offshore activities, such as the construction of marine structures and implementation of ocean-based renewable energy technologies
A more detailed interpretation and discussion on the factors affecting coastal meteorology was presented by Barthelmie et al [9] and Barthelmie [10], which mainly consists of air-sea temperature differences, orientation of the coastline, prevailing wind speed and direction, water depth, latitude, distance from the coastal discontinuity, and fetch distance
Given the substantial importance of sea-surface roughness length for describing marine atmospheric boundary layer, this study examines and discusses the observations of coastal-site roughness length based on wind lidar measurement at an offshore platform in Hong Kong

Summary

Introduction

Proper understanding of marine atmospheric boundary layer (MABL) is meeting growing attention [1,2,3,4], taking into account the substantial increase in coastal and offshore activities, such as the construction of marine structures (e.g., oil platform) and implementation of ocean-based renewable energy technologies (e.g., offshore wind farm). When the mean wind speed lies in the range between 11 and14 m/s, the aerodynamic roughness length for the sector of 60◦–90◦ is estimated to be 0.002 mm, whereas the value for the sector of 120◦–240◦ is about 0.23 mm.

Results

Conclusion