The Pico and Graciosa cloud optical thickness experiment

Description

Deployment of the first ARM Mobile Facility 1 (AMF1) at Graciosa, Azores, Portugal and a portable radiation package near the summit of the nearby Pico Island volcano afforded a rare opportunity to directly measure radiation transmission in a variety of boundary-layer cloud configurations. Pico's summit, which extends above the marine boundary layer, was instrumented with a multifilter shadowband radiometer (MFRSR), pyrgeometer, and pyranometer. Beneath the clouds near the ocean surface at nearby Graciosa Island was an identical set of radiometers deployed as part of AMF1 operating alongside the full complement of AMF1 active remote sensors. This geographic region often lacks the aerosol loads associated with continental locations whereupon aerosol radiative influences are minimized, enabling the radiative impacts of the clouds to be isolated. Relatively unambiguous direct and continuous measurements of radiation transmission were possible in broken clouds and in overcast conditions. These transmission measurements are classified according to cloud and boundary-layer structure using the AMF1 active remote sensors, and important benchmarks such as the obscuration of the sun's disk at optical thickness eight are directly observed, lending credibility to the accuracy of this measurement approach. Basic cloud and boundary-layer morphology are related to transmission, revealing interesting aspects of the radiation field.