Evaluation of doppler lidar measurements of momentum flux and wind variability along an upwelling coast

A research study, based on a comprehensive ocean-atmosphere dataset obtained along the northern Oregon coast, was conducted to understand the effects of wind stress variability on coastal upwelling.;The data for this study were collected during the Coastal Ocean Probing Experiment (COPE), which took place in September--October 1995. The objective of the research was two-fold, (1) to understand the complex interactions between the atmospheric boundary layer and the oceanic mixed-layer under a variety of wind conditions, and (2) to assess the applicability of using Doppler lidars to measure wind speed and wind stress in the coastal zone.;A number of tests were performed to assess the performance of the 10.59 mum Doppler lidar system in measuring wind stress in a coastal environment. Uncertainty in the lidar derived momentum fluxes were +/- 0.05 N m-2 in regions of good signal quality. Problems with water vapor attenuation and laser frequency stability necessitated the use of a velocity threshold technique. The standard deviations of the lidar derived wind stress were improved 25--50% after applying a signal threshold and a stability correction.;The COPE field measurements along with a ten-year wind climatology were analyzed to diagnose the long-term wind variability along the coast. The seasonal signal was synoptically dependent in both strength and direction in the near-shore zone. Wind rose frequency diagrams from the period 1987--96 showed that the coastal wind direction is oriented along-shore in the spring and summer, and more variable in direction and magnitude in fall and winter. Four primary wind patterns (wind regimes) were present in the buoy measurements along the coast: northerly alongshore flow, southerly along-shore flow, direct offshore flow and onshore sea breezes.;Case studies from COPE were used to assess the effects of the different wind regimes on Ekman transport and upwelling. Analysis of the wind and sea-surface temperature measurements showed that the relationship between wind stress variability and Ekman transport in the mixed-layer occurs on very short time-scales. A comparison between buoy, ship and Doppler lidar measurements of surface winds revealed that intense periods of upwelling are driven by high-frequency wind stress events that last for several minutes to an hour. The mixed layer response for the different wind events was often similar, with a decrease in sea-surface temperatures (SSTs) of 1--2°C during both southerly and northerly wind events. Strong upwelling was observed only during offshore flow, which was attributed to down-wind transport instead of Ekman transport in the mixed-layer.