An equivalent neutral wind observation operator for variational assimilation of scatterometer ocean surface wind data

摘要

This article describes a new observation operator developed for improved variational data assimilation of scatterometer ocean wind vectors. The forward operator is designed explicitly to take into account the equivalent neutral calibration of scatterometer wind retrievals. The corresponding tangent linear and adjoint (TL/AD) operators are developed for the calculation of near-surface increments based explicitly on parametrizations ofsurface-layer turbulent momentum transfer. Tangent linear and adjoint operators are formulated using the Jacobian of the forward operator estimated using the perturbation method. Various TL/AD configurations are investigated in more detail to assess theimpact of sensitivities affecting the vertical gradient of surface-layer temperature during assimilation of ocean surface wind data. It is argued that these sensitivities should be neglected in the context of a stand-alone atmospheric prediction system.The complete form of the operator has, however, the potential to be a key component of a coupledatmosphere-ocean data assimilation system. Two data assimilation experiments were performed using distinct scatterometer observation operators, i.e. new equivalent neutral versus the simpler operational configuration using stability-dependent (real) background winds. Associated medium-range forecasts were run for a complete impact assessment. The impact of the new operator is tracked through the entire data assimilation and forecast process by comparing analyses and forecasts from both experiments. A significant sensitivity of the data assimilation system to the configuration of the observation operator has been found. Modest but beneficial impacts are obtained with the equivalent neutral wind observation operator with respect to short-range forecastsof ocean surface winds and medium-range forecasts of extratropical geopotential heights.