Observations over one day

The image below shows the results of observations over one day. Coloured arrows indicate horizontal wind direction. They are plotted against altitude or height in kilometres (up the page) and time (across the page). An upward pointing arrow indicates wind blowing toward the north. An arrow pointing to the right indicates a wind blowing toward the east (as on a compass dial). The lack of an arrow indicates that no data was obtained for that height and time. The colour of the arrow indicates the wind speed as shown in the scale bar on the right hand side.

Wind speeds of 50 m/s (180 km/h) are not uncommon at these heights. The east-west winds (around the planet) are usually greater than the north-south winds (pole to pole). Although the radar measures returns from 40 km to 108 km, the analysis does not always yield an answer that passes various quality checks. The range over which we are currently able to measure winds is indicated by the presence of arrows. The base height of observations usually moves downwards in the southern winter months.

Horizontal velocity field, results of observations over a day.
Horizontal wind speed and direction recorded by the MFSA radar at Davis.

Observations over a few days

The image below shows a few days of Zonal (east-west, eastward positive) and Meridional (north-south, north positive) wind velocities at a height of 86 km. The horizontal axis shows time. The vertical axis shows velocity in m/s. The values have been averaged into hourly blocks.

Solar heating of the ozone in the stratosphere (around 25 to 55 km) and water vapour in the troposphere (up to approximately 12 km) causes variations in the wind in the mesosphere (86 km). The variations are of one-day period,or ‘harmonics’ such as 12 hour or 8 hour periods. These are known as atmospheric tides. Tidal variations of the wind can be seen in the velocity time series. There are also variations of shorter periods due to atmospheric gravity waves.

Davis MF radar winds, over 8 days

Observations over one year

Over the seasons, the circulation of this part of the atmosphere changes significantly. As shown in the graph below, the wind at 80 km blows toward the east during the southern winter (positive zonal velocity) and toward the west (negative zonal velocity) during the summer. The north-south winds are weaker but are more variable during the winter than through the summer. This is because wavelike motions that flow around the planet are only able to reach this height during the winter.

Davis MF radar winds for a height of 80 km

This graph shows data from 2019. The blue line is the zonal (east-west) wind speed. The green line is the meridional (north-south) wind speed. The units are metres per second (m/s).

MFSA radar signal-to-noise ratio

The signal-to-noise ratio is an important parameter in deciding if a wind determination is possible. During the summer, when the sun is illuminating the sampled height range throughout the day, there is little variation in the signal-to-noise ratio. This is less true in winter and during periods of high solar activity.

Signal-to-noise ratio as a function of height and time.
Signal-to-noise ratio as a function of height and time.