Stable extension of the unified model into the mesosphere and lower thermosphere

Abstract

A coupled Sun-to-Earth model is the goal for accurate forecasting of space weather. A key component of such a model is a whole atmosphere model – a general circulation model extending from the ground into the upper atmosphere – since it is now known that the lower atmosphere also drives variability and space weather in the upper atmosphere, in addition to solar variability. This objective motivates the stable extension of The Met Office’s Unified Model (UM) into the Mesosphere and Lower Thermosphere (MLT), acting as a first step towards a whole atmosphere model. At the time of performing this research, radiation and chemistry schemes that are appropriate for use in the MLT had not yet been implemented. Furthermore, attempts to run the model with existing parameterizations and a raised upper boundary led to an unstable model with inaccurate solutions. Here, this instability is examined and narrowed down to the model’s radiation scheme – its assumption of Local Thermodynamic Equilibrium (LTE) is broken in the MLT. We subsequently address this issue by relaxation to a climatological temperature profile in this region. This provides a stable extended UM which can be used as a developmental tool for further examination of the model performance. The standard vertical resolution used in the UM above 70 km is too coarse (approx. 5 km) to represent waves that are important for MLT circulation. We build on the success of the nudging implementation by testing the model at an improved vertical resolution. Initial attempts to address this problem with a 3 km vertical resolution and a 100 km lid were successful, but on increasing the resolution to 1.5 km the model becomes unstable due to large horizontal and vertical wind velocities. Increasing the vertical damping coefficient, which damps vertical velocities near the upper boundary, allows a successful year long climatology to be produced with these model settings. With the goal of a whole atmosphere model we also experiment with an increased upper boundary height. Increasing the upper model boundary to 120 and 135 km also leads to stable simulations. However, a 3 km resolution must be used and it is necessary to further increase the vertical damping coefficient. This is highly promising initial work to raise the UM into the MLT, and paves the way for the development of a whole atmosphere model.

The Space Weather Atmosphere Models and Indices (SWAMI) project: Overview and first results

“…The wind and temperature structures below 100 km are consistent for all runs. Griffith et al (2020) also show good agreement between the 85 km and 100 km runs below 85 km and also good agreement with the Microwave Limb Sounder temperature climatology and the Upper Atmosphere Research Satellite Reference Atmosphere Project (URAP) (Swinbank & Ortland, 2003) zonal wind climatology. However, easterly winds at equatorial uppermost levels in the 135 km run are too strong and the latitudinal and longitudinal structure of the models fields above around 100-110 km is increasingly poor compared to fields from other models (not shown).…”

“…Initial simulations did not employ any changes to the model to account for the MLT other than the use of a raised lid. It was quickly realised that use of a nudging scheme (Griffith et al, 2020) to approximate MLT radiative and chemical heating was required, and initial runs used this, with the UM SOCRATES radiative heating switched off above 70 km. The zonal mean wind and temperature fields from these runs compare quite well with climatology but the latitude/longitude structure above around 100 km is increasingly inaccurate, which reveals issues with nudging to a global mean temperature profile.…”

“…Two simulations were carried out, with September and March start dates, and both failed in the following solstice with excessive and unrealistic model winds seen near the summer pole. Further analysis indicated that these winds are caused by the inappropriate use of the SOCRATES radiation scheme at higher altitudes and the consequent production of excessive and erroneous heating rates at upper levels (see Griffith et al, 2020 for further details).…”

“…The nudging scheme developed here is a Newtonian relaxation to a globally uniform temperature profile over a given timescale (by default, 24 h is used). Further details appear in Griffith et al (2020).…”

“…One half of a scale height will not resolve all vertically propagating waves over this scale height, but will resolve larger scale waves in an acceptable manner. The analysis by Griffith et al (2020) indicated that the respective vertical resolutions used should be 1.5 km and 3 km. As well as testing different vertical resolutions, simulations with different UM lid heights were carried out.…”