Raising the Roof: Extending the Met Office's Unified Model into the Mesosphere and Lower Thermosphere

Abstract

In this thesis, we extend the UK Met Office's Unified Model into the Mesosphere and Lower Thermosphere (MLT), and validate the realism of the model by comparison to meteor radar observations and other high-top atmospheric models. This thesis therefore "raises the roof" of current weather forecasting at the Met Office.

The first goal of this thesis is to stabilise the previously unstable Extended Unified Model (ExUM). We perform a thorough and systematic analysis of the UM with an extended upper boundary, and show that the incorrect assumption of Local Thermodynamic Equilibrium (LTE) in the MLT is the primary driver of the model instability. The implementation of a temperature relaxation, or "nudging", scheme to replace the radiation scheme above 70 km stabilises the model with upper boundaries of 100, 120 and 135 km.

The second goal of this thesis is to validate the realism of the modelled fields produced by the ExUM in the MLT. Comparisons are made with meteor-radar observations as well as with other atmospheric models and observational datasets of this region. These show that the ExUM has a good natural capability to model the MLT, but that developments to the model's physical schemes - such as the non-orographic gravity wave and chemistry schemes - are required in this region to obtain more realistic fields.

The work performed in this thesis gives insight into the work required to successfully extend an atmospheric model into the MLT and lays the foundation for the future development of a Whole Atmosphere UM.

Date of Award	8 Sept 2021
Original language	English
Awarding Institution	University of Bath
Supervisor	Chris Budd (Supervisor), Nicholas Mitchell (Supervisor) & David Jackson (Supervisor)