Tethered‐balloon measurements of actinic flux in a cloud‐capped marine boundary layer

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Abstract
As part of the Atlantic Stratocumulus Experiment (ASTEX), more than 34 tethered‐balloon soundings were carried out on Santa Maria Island, Azores. The main purpose of the soundings was to measure vertical profiles of actinic flux under different meteorological conditions. In addition, vertical profiles of temperature, relative humidity, and wind speed and direction were measured. The actinic flux was measured with a photoelectrical detector with a spectral response that ranges from 330 nm to 390 nm. It is in this region where the photodissociation of nitrogen dioxide, the main precursor of ozone in the troposphere, takes place. Several vertical profiles of actinic flux under clear and cloudy conditions are presented. These profiles show the different behavior of the actinic flux below, in, and above clouds. These observations are complemented with detailed measurements of cloud characteristics; relevant cloud properties for this study are the cloud‐base and cloud‐top heights and the cloud optical depth. A slightly increasing tendency for the actinic flux in the whole atmospheric boundary layer was observed under clear sky conditions. Under the presence of clouds and compared with clear sky values measured at the same solar zenith angle and at the surface, lower values were observed below cloud (ratio ranging from 0.39 to 0.92) and higher at the top of the cloud (ratio ranging from 1.68 to 2.32). In the cloud an increase in the values of actinic flux with height was measured. The actinic flux measurements are evaluated against values obtained from a multilayer delta‐Eddington model. An excellent agreement is found for the soundings made under total overcast conditions.