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.
Highlights
An important focus of many weather forecasting organisations is the development of a complete Sun-to-Earth model in order to enhance forecasting of space weather, and develop a fully coupled system describing the Earth’s atmosphere (e.g. Tóth et al, 2005)
This non-hydrostatic framework allows for vertical acceleration; an important consideration in the forcing of vertical winds, which are typically larger in the upper atmosphere than those seen in the lower atmosphere
We strongly suspect that it is the assumption of Local Thermodynamic Equilibrium (LTE) that is leading to the observed instability in the Unified Model (UM), and that non-LTE (NLTE) effects must be considered when extending the upper boundary to the Mesosphere and Lower Thermosphere (MLT)
Summary
An important focus of many weather forecasting organisations is the development of a complete Sun-to-Earth model in order to enhance forecasting of space weather, and develop a fully coupled system describing the Earth’s atmosphere (e.g. Tóth et al, 2005). Ineson et al (2011) indicates the impact of solar minimum and solar maximum on Northern Hemisphere winter surface temperatures via changes to the North Atlantic Oscillation These waves are influential in atmospheric dynamics at all heights, and so their inclusion is important as drivers of upper atmosphere circulation, and in terms of improving accuracy for current lower atmosphere models. We advance the development of a whole atmosphere model by investigating the instability when the model lid is raised into the MLT, raising the roof of the current 85 km lid This is achieved by first investigating the output produced using a raised upper boundary of 100 km and considering key prognostic variables such as winds and temperature, as well as diagnostic variables such as short wave radiative heating.
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