Current problems in scattering radiative transfer modelling for data assimilation

This article assesses current issues related to scattering radiative transfer (RT) in data assimilation (DA) and proposes possible ways to solve or mitigate these issues. Emphasis is put not so much on fundamental issues related to RT but on the practical application within a framework of operational numerical weather prediction and DA with their tight constraints on computational efficiency. In particular, three potentially critical open issues are studied: firstly, the trade-off between speed and accuracy in RT schemes for DA. A numerically efficient method is proposed to determine beforehand whether scattering needs to be accounted for. Secondly, the impact of spectrally highly variable gaseous absorption coefficients within a given instrument's bandpass and its implications on scattering RT is studied. Results of this second part are also put in context with uncertainties caused by the lack of knowledge of scattering optical properties. Finally, model errors due to, for example, the assumption of plane-parallel RT are studied. It is argued that errors caused by plane-parallel RT will likely continue to dominate the error budget both in terms of biases and random errors. Copyright (C) 2011 Royal Meteorological Society