Reduced Sea-Surface Roughness Length at a Coastal Site

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.

Enhanced Dual Filter for Floating Wind Lidar Motion Correction: The Impact of Wind and Initial Scan Phase Models

“…The higher roughness of the urban terrain was also responsible for larger spatial and temporal variations in the wind field, which resulted in higher turbulence. The opposite was true for winds coming from the sea (WDs between 0 and 240 deg) because of the lower roughness of the sea surface [44]. It is known that lower roughness is directly related to higher wind speeds [45].…”

Considering the effects of sea waves on offshore wind simulations in the Weather Research and Forecasting model

“…In NWP models, interactions between wind and waves are simulated through drag coefficients or roughness length parameterizations. Numerous studies have shown key role of precise roughness length parameterization in accurately computing surface fluxes over the sea [e.g., 8,9]. In most NWP models, the sea surface is considered a smooth surface, but the sea surface constantly changes due to the propagation of waves with different amplitudes.…”