Diurnal variation of boundary layer extinction on the windward side of O´ahu, Hawai´i measured by a scanning lidar

In order to study the diurnal cycle of aerosol at a coastal marine site, the authors continuously operated a scanning lidar at Bellows Beach, on the northeast side of O´ahu Hawai´i, for four days in September, 1999 and three days in April, 2000. They obtained 2.5 km-radius vertical profiles of the atmospheric extinction at two to three minute intervals using a wavelength of 0.532 microns. The extinction is caused primarily by sea-spray generated aerosols. They simultaneously monitored wind speed and direction, relative humidity and temperature at the lidar site as well as wave height just outside a reef approximately 2 km offshore. The 1999 results show that under light-wind (<4 m/s) conditions, there appears to be a diurnal variation in the mean boundary layer extinction with maxima and minima occurring close to those in the air temperature at about 1400 hrs and 0600 hrs, local time. This diurnal variation is overlaid by laterally inhomogeneous scattering due to reef and shore based spray as well as to cloud formation. Under more typical tradewind speeds (>5 m/s) the diurnal variation is not as evident