AMSOS - Airborne Microwave Stratospheric Observing System

Airborne Microwave Stratospheric Observing System (AMSOS)

Measurement principle

The instrument does not measure the water vapor or ozone content directly at the flight altitude but rather from about 1-2 km above the aircraft (upper troposphere) up to the mesosphere. The water vapor and ozone molecules in the atmosphere produce characteristic spectral lines at frequencies near 183 GHz (water) and 176 GHz (ozone). The shape of the spectral line that is emitted by molecules at a certain altitude depends on the temperature and pressure at that altitude. The instrument below sees the sum of all spectral lines emitted by all molecules at all altitudes above the instrument. From this total spectrum, we can derive the altitude distribution of the molecules through a complicated mathematical process called inversion. One has to know the vertical distribution of temperature and pressure to do this.

There are two main reasons to operate this instrument from an aircraft. One reason is that the water vapor signal at lower altitudes becomes too strong because of the large amount of water vapor in the troposphere. This blocks the much weaker stratospheric signal at low altitudes. The second reason is that by flying the instrument around, we get a good look at the distribution of these important trace gases over a large geographical range.

H2O spectra measured at different altitudes
Fig. Set of H2O spectra measured at different altitudes during an ascent in the tropics