Analysis of the effect of the coastal discontinuity on near-surface flow

S C Pryor,R J Barthelmie

doi:10.1007/s005850050657

Abstract

Conditional sampling is used herein to examine the effect of fetch, stability, and surface roughness changes on wind speeds in the coastal zone. Using data from an offshore wind farm it is shown that at a distance of 1.2â1.7 km from the coast, up to a height of 20 m above the surface, differences in wind speed distributions from onshore and offshore masts are statistically significant for flow moving offshore under all stability conditions. In contrast, differences between the distribution of wind speeds at 38 and 48 m at masts located at the coast and in the coastal zone are not significant for flow moving offshore, indicating that flow at these heights is not fully adjusted to the change in surface roughness (land to sea). These findings are in accordance with calculations of the internal boundary layer (IBL) height which indicate that the IBL would frequently be below the two upper measurement heights at 1.2â1.6 km from the coast. The analyses presented here indicate that the wind speed distribution at a potential offshore wind farm site is not solely dependent on fetch (distance from the coast) but also depends on the stability climate.Key words. Meteorology and atmospheric dynamics Â· Mesoscale meteorology Â· Ocean-atmosphere interaction Â· Marine meteorology

Highlights

Introduction and objectivesModi®cation ofow in the coastal zone occurs as a result of stability and roughness changes, and has implications for applications such as oshore wind energy production and atmospheric dispersion and Correspondence to: S
Data used were collected at an oshore wind farm located at Vindeby, Denmark, on three masts shown in Fig. 1; one located at the coast (LM) and two located at minimum distances from the coast of 1270 and 1630 m (SMS and SMW, respectively)
Wind speed data at each measurement height (7, 20, 38, and 48 m) were selected by directional sector and stability class for all cases when stability conditions at the two masts (LM and SMS or land mast (LM) and SMW) were in the same class

Summary

Introduction and objectives

Modi®cation ofow in the coastal zone occurs as a result of stability and roughness changes, and has implications for applications such as oshore wind energy production and atmospheric dispersion and Correspondence to: S. To elucidate relationships between dierences in wind speeds measured on- and oshore andow direction (onshore or oshore), fetch (distance from the coastal discontinuity), stability (classes de®ned using the Monin-Obukhov length), and surface roughness changes (z0 over land and z0 over sea). Barthelmie: Analysis of the eect of the coastal discontinuity on near-surfaceow sectors are used: a north sector from 285° to 65° ( ̄ow directly over sea to all three masts) and a south sector from 135° to 195° [ ̄ow is over at least 10 km of land prior to observation at LM and has a sea fetch of between 1 and 2 km before reaching the sea masts (SMS and SMW)] This analysis has direct implications for siting of wind farms in the coastal zone to maximize power output

Conditional sampling of wind speed data

Quantitative analyses of the wind speed distributions

Findings

Discussion and conclusions