Programme by Session

A goal of Met Office space weather research is to develop an operational coupled Sun to Earth model forecast system, including a model spanning the Earth’s surface to the thermosphere. The first step is to extend the Met Office Unified Model (UM) from its current upper boundary near 85 km to an upper boundary near 150 km. This work is also an important part of the EU project SWAMI (Space Weather Atmosphere Model and Indices).

We focus on the changes to the representation of radiative transfer in the UM to make the model fit for purpose in the mesosphere and lower thermosphere (MLT). The existing UM radiation scheme does not represent non-local thermodynamic equilibrium (NLTE), which is needed for accurate heating rates at MLT altitudes. Here, we describe work to replace pre-existing heating rates with NLTE heating rates from the Fomichev scheme at such altitudes.

The UM scheme does not address radiative transfer from wavelengths less than 200 nm. This approach is acceptable in the lower atmosphere where these wavelengths do not penetrate, but in the MLT shorter wavelengths in the Extreme Ultraviolet (EUV) and Far Ultraviolet (FUV) also need to be included. We describe work to extend absorption calculations to these wavelengths, using the correlated-k approach to ensure accuracy while maintaining computational speed. The FUV / EUV actinic fluxes derived from two-stream calculations are used to calculate photolysis rates in order to drive the exothermic chemical reactions responsible for the large increases of temperature with altitude in the MLT.