Three-Dimensional Calculation of Photolysis Frequencies in the Presence of Clouds

The photodissociation of tropospheric chemical species depends on the available solar flux reaching the troposphere. The propagation of sunlight deeper in the atmosphere is affected by various physical processes such as the multiple scattering. The actinic flux distribution is highly affected by the presence of clouds reflecting and diffusing the incoming solar flux inhomogeneously throughout the troposphere. In this paper, we explore the effects of a cumulonimbus cloud on the 3-D distribution of the spectral actinic flux and of various chemical species photolysis frequency. The simulation is based on a 3-D resolution of the UV-VIS radiative transfer equation solved by the Spherical Harmonic Discrete Ordinary Method (SHDOM). The solver (SHDOM) uses as input the 3-D cloud characteristics simulated by a dynamical cloud model (COMMAS). Results show that the distribution of the actinic flux overall the cloud domain is far from homogeneous and depends mainly on the cloud extinction correspondind to each type of hydrometeors. The actinic flux is enhanced by more than a factor 2 to maximum 5, above at the top edge and around the cloud when compared to a clear sky. In a second step, the 3-D actinic flux issued from this simulation is used to calculate the photolysis rates of some chemical speicies (e.g. N02, 03 and HCHO). Results show that the photolysis rates are as well distributed inhomogeneously throughout the cloud, maximums are calculated in regions where the actinic flux is largely enhanced. In a final step, the potential importance of this photolysis enhancement on the chemistry is studied using a box model simulation. Results show that a large OH concentration is calculated in the upper troposphere (120–200 %) over the cloud and a non negligible change in ozone production rate (+ 15%) is obtained on a “steady state condition”.