SENSITIVITY ANALYSIS OF A RADIATION PARAMETERIZATION

An assessment of the forecast sensitivity to model and radiation param eters is critical to validating climate change studies. This paper qua ntifies forecast first-order sensitivity to the input to the radiation calculations in a numerical weather prediction model. The forecast as pects of interest are the temperature at the surface and near the trop opause. The model used here is a single column model, the AER Local Fo recast and Assimilation (ALFA) model, which emulates a grid box of a g eneral circulation model. It includes the NASA Ames rapid model as a b roadband radiation parameterization. The sensitivities are calculated for a single time step (tendency) by the adjoint method. They reflect interactions of parameterized physical processes, since they are deriv ed from a single iteration of the forward model and its adjoint. The s ensitivities are distinguished by their sign as positive and negative responses to initial perturbation. When the atmosphere is cloudy, the surface temperature tendency shows the highest sensitivity to the sing le-scattering albedo in the band centered at 0.69 mu m (14,492 cm(-1)) in the visible window region. When the atmosphere is clear, the highe st sensitivity is to the water vapor path in the continuum band, but 1 order of magnitude smaller. Also, in the clear case, the temperature tendency just below the tropopause is most sensitive to the water vapo r path in the rotational absorption band. Again, this sensitivity is s maller than the sensitivity to a single-scattering albedo in a cloud a t the same altitude.