Thursday, 6 June 2002: 3:45 PM
Scales of cloud's organisation: implications for radiative transfer calculation
It is not currently understood what magnitude of error is involved when a two stream approximation is used to describe the radiative transfer through optically and/or geometrically complex cloud fields.The close connection between the spatial distribution of the radiation field and the photon path length would suggest that the importance of 3D processes, such as horizontal transport of radiation and side illumination, could be conditioned by the different horizontal and vertical scales of organisation of the clouds within the system in question. The aim here is to relate the changes in the 3D/1D radiative calculations with the spatial variability of the liquid/ice water. A new system, based on a statistical technique has been developed, such that realistic cloud fields with variability on many spatial scales can be obtained and controlled by a small number of parameters, which relate directly to the dynamical and thermodynamical meteorology of the situation to be examined. This initial study deals with a single layer stratocumulus case since there exists a wide bibliography for these kind of clouds for verification of the results. Nevertheless the extension of the model to more complex, multi layer cloud situations could be performed and will be the subject of a further investigation. The results show that when conditions are not overcast, 3D effects can be relevant, affecting both the atmospheric absorption of the clear and cloudy columns and the mean albedo. The understanding of the mechanisms involved in the main 3D/1D differences is the starting point for the future attempt to develop a parameterisation procedure.