Cloud Optical Depth Results from BAECC Finland

Description

The Three-Waveband Spectrally-agile Technique (TWST) sensor was deployed at Hyytiälä Finland from 16 July to 31 August 2014 as a guest instrument on the Biogenic Aerosols Effects on Climate and Clouds (BAECC) campaign. This data is currently available from the ARM Data Archive website and consists of the spectral radiance at 440 and 870 nm, the equivalent width of the oxygen A-band, and the Cloud Optical Depth (COD) measurements for the clouds directly overhead approximately every second during the daylight periods. A good range of cloud conditions were observed from clear sky to heavy rainfall. The TWST sensor directly collects calibrated spectral radiance at zenith for the spectral range of 350 – 1000 nm at ~2 nm spectral resolution. In stand-alone mode it retrieves Cloud Optical Depth (COD) at a sample rate of 1 Hz using the spectral radiance at 440 nm and the equivalent width of the oxygen A-band at 760 nm. By measuring the responsivity both before and after each test, we have determined that TWST has an excellent radiometric stability, on the order of a few percent. The absolute accuracy of the TWST COD values is better than 5% which is the estimated accuracy of our radiometric calibration procedure while the relative precision of our COD values is shown to be better than 0.001 with a theoretical noise limited sensitivity of OD=0.0001. Since typical Aerosol Optical Depths (AOD) fall in the range of 0 - 0.5, we are also able to present preliminary analyses of TWST data for aerosol dominated effects during periods of cloud-free conditions. The aerosol effects revealed strongly motivate future in-depth investigations by utilizing concurrent data from other instruments deployed for the BAECC campaign.