Atmospheric Modelling

The RSPP group has developed a series of atmospheric models, designed to study a wide range of dynamical processes in the middle and upper atmosphere.

The STEVE Model

The Stratosphere to Thermosphere Energy Variability Experiment has been developed to study the impact of the solar cycle on the middle atmosphere, specifically its effect on atmospheric gravity waves in this region.  This relatively simple 2D model is ideal for running simulations of the atmosphere over several decades and reproduces many of the observed features of the middle atmosphere.  The inclusion of the Medvedev and Klaassen (2000) gravity wave parameterisation has allowed realistic gravity wave processes to be studied in detail.

Figure 1, Zonal mean zonal winds in the STEVE model during December.  Positive values indicate eastward winds.

Figure 2, Zonal mean zonal winds in the MSIS-E90 Empirical model during December (Hedin 1991).  Positive values indicate eastward winds.

Animation 1, Annual cycle of the temperature in the STEVE model

The STEVE 3D Model

An extension on the STEVE model, the STEVE 3D model has been developed to study the interactions of planetary waves (Rossby waves) in the stratosphere with atmospheric gravity waves.  Studying this interaction is believed to be an important part of understanding how solar variability can affect our climate system.

Figure 3, Zonal winds in the STEVE 3D model at 50km altitude during January.  Planetary wave structures can be seen in the winter polar region.

The UCL CMAT model

In conjunction with the Atmospheric Physics Laboratory at University College London, we have been working on further developing the Coupled Middle Atmosphere Thermosphere General Circulation Model by improving the representation of atmospheric gravity wave processes within the model.  We have used the Medvedev and Klaassen 2000 scheme, developed for the STEVE model, to investigate interactions between tides and gravity waves in the middle atmosphere.

Figure 4, Zonal mean zonal winds in the CMAT model during March.

Figure 5, Zonal mean zonal winds in the HWM empirical model (Hedin et al. 1993)

Animation 2, Diurnal tide in the northward winds in the CMAT model during March

Chemical transport processes and impact on climate

The middle atmosphere contains 90% of the ozone layer that acts as a shield from the solar U.V. radiation. Solar forcing causes a variation in the concentration of ozone and a consequence variation in the heating that reaches the Earth’s surface.  

We use the STEVE model and a more basic DYNAMIC model to investigate how chemical transport processes can affect ozone and modify the global circulation in the atmosphere. Positive feedback processes are expected to arise and have an impact on the troposphere and thus on climate. 

Preliminary studies have concentrated on the use of passive tracers (i.e. with no chemical reaction involved) to investigate transport processes in the middle atmosphere. The two following examples show the dispersion of the original distribution of a passive tracer in the longitudinal direction.

Figure 6, a column distribution of a passive tracer modified by the global circulation after one day and after 10 days.  The vertical scale is in km.

Animation 3, evolution in time of the column of passive tracer

Figure 7, a simulation of an exceptional volcano explosion. Volcano dust is used as passive tracer and modified by the global circulation. The dust concentration is shown after one and10 days from the explosion. The vertical scale is in km.

Animation 4, evolution in time of the volcano dust

Coupling of the middle atmosphere to the troposphere is fundamental in order to quantify the impact of middle-atmosphere processes on the troposphere and on climate. A collaboration with the Danish Meteorological Institute is seeking to asses the response of the global circulation model to changes in the heating and chemical constituents (such as NO of the middle atmosphere. This collaboration is part of the CAL–Coupling of Atmospheric Layer consortium, which aims to investigate the contribution of sprites as means of generating NO, and assessing their importance on local and global scale.