Energetic particle precipitation in ECHAM5/MESSy1 – Part 1: Downward transport of upper atmospheric NO_x produced by low energy electrons

Abstract. The atmospheric chemistry general circulation model ECHAM5/MESSy1 has been extended by processes that parameterise particle precipitation. Several types of particle precipitation that directly affect NO_y and HO_x concentrations in the middle atmosphere are accounted for and discussed in a series of papers. In the companion paper, the ECHAM5/MESSy1 solar proton event parametrisation is discussed, while in the current paper we focus on low energy electrons (LEE) that produce NO_x in the upper atmosphere. For the flux of LEE NO_x into the top of the model domain a novel technique which can be applied to most atmospheric chemistry general circulation models has been developed and is presented here. The technique is particularly useful for models with an upper boundary between the stratopause and mesopause and therefore cannot directly incorporate upper atmospheric NO_x production. The additional NO_x source parametrisation is based on a measure of geomagnetic activity, the A_p index, which has been shown to be a good proxy for LEE NO_x interannual variations. HALOE measurements of LEE NO_x that has been transported into the stratosphere are used to develop a scaling function which yields a flux of NO_x that is applied to the model top. We describe the implementation of the parametrisation as the submodel SPACENOX in ECHAM5/MESSy1 and discuss the results from test simulations. The NO_x enhancements are shown to be in good agreement with independent measurements. A_p index data is available for almost one century, thus the parametrisation is suitable for simulations of the recent climate.