Satellites to study Antarctic atmosphere

Dr Adrian McDonald from the Physics and Astronomy Department, University of Canterbury, relies on satellites to study the atmosphere over Antarctica. His research focuses on factors that affect the Antarctic climate, the ozone hole and their interactions. Dr McDonald compares data from satellites with data from weather stations he sets up each year in Antarctica. His findings are helping to create more accurate models of how the Antarctic climate is changing and the factors contributing to this change.

Ozone depletion

Ozone is a molecule made up of three oxygen atoms. Most of the ozone found in our atmosphere is at an altitude of 10–15 km and is spread very thinly through this region. It plays a key role in absorbing harmful UV radiation. The amount of ozone over Antarctica becomes depleted over the spring and early summer period.

There are two main factors that contribute to this ozone depletion

• Chlorofluorocarbon pollution in the upper layers of the atmosphere. When this chemical breaks up in the upper atmosphere, an increase in the quantity of chemical compounds that destroy ozone occurs. Most of these polluting gases are produced in the northern hemisphere and can take 15–20 years to reach the upper layers of the atmosphere over Antarctica. Once damage occurs, it is estimated that it could take a hundred years or so for these pollutants to be removed from the atmosphere, so ozone depletion will continue for many years.
• Cold temperatures in the upper layers of the atmosphere. This creates wind patterns that keep the harmful manmade chemicals in contact with each other, which leads to rapid ozone destruction.

As more UV light passes through the atmosphere, the cooling effect causes stronger winds to flow. These stronger winds act to keep the cooler air over the Antarctic. Without ozone depletion, the effects of greenhouse gases would most likely have had more of an impact on the Antarctic surface climate. Satellites play a critical role in helping to analyse the gases and temperatures at different layers of the atmosphere over Antarctica.

How satellites measure atmospheric temperature

One system of satellites used to measure atmospheric temperatures is the COSMIC FORMOSAT-3 series of satellites (COSMIC stands for Constellation Observing System for Meteorology, Ionosphere and Climate). Taiwan and US scientists launched these six identical satellites in 2006. Each satellite has a mass of 92 kg. They were launched into a polar orbit at an initial altitude of 500 km. This was to increase to 800 km over the course of their 5-year expected mission duration.

COSMIC FORMOSAT satellites measure temperature at different layers of the atmosphere by using signals from GPS satellites. As the signals from the GPS satellites pass through a section of the atmosphere, they change direction slightly (refract). The amount that the signal changes direction (how much the path differs from a straight line) is used to calculate the temperature of the various layers of the atmosphere.

Other satellites have collected detailed information on the concentration (or quantity) of various atmospheric gases.

Validating satellite data

Measurements from satellites are useful, but ground validation efforts that compare measurements from weather stations set up in Antarctica with the satellite data is important in ensuring the accuracy of satellite observations. Each year, Dr McDonald travels to Antarctica to set up weather stations to monitor atmospheric phenomena. The data obtained shows that the satellites are very accurate, although there are slight differences in certain regions. Working out why these differences occur helps to make predictive models more accurate.

Dr McDonald’s work is contributing to a better understanding of the chemical processes that affect ozone and how temperatures at different levels of the atmosphere can influence wind patterns. If the temperature and the chemical composition are known, scientists can predict the flow of different sections of air. This information feeds in to climate models that help to build up a more complete picture of how climate is likely to change.