A Spectral Simple Adjoint Method for Retrieving Low-Altitude Winds from Single-Doppler Data

Abstract

A spectral simple adjoint (SSA) method for retrieving, low-attitude time-mean horizontal winds from single-Doppler wind measurements is developed and tested with the Phoenix II held experimental data. The SSA method is similar to the previous grid simple adjoint (GSA) methods of Qiu and Xu and Xu et al., except that a truncated spectral expression is used to replace the grid representation of the wind field. In the previous GSA methods the retrieved lime-mean winds were resolved on the same full spatial grid as the data. This full spatial resolution was often excessive and unnecessary as the time-mean wind field was quite smooth. When a truncated spectral expression is used to reduce the excessive and unnecessary spatial resolution, the SSA method not only gains temporal resolution (e.g., the retrieving period can be reduced to 130 s?a single time interval of radar scan) but also improves the accuracy of the retrieved winds (because short-wave noise is filtered). The merits and limitations of the SSA method are also examined with idealized examples.