Stratospheric and mesospheric water vapor distribution over Northern Europe measured with an airborne mm-wave sensor

From April 13th to 15th 1993, a modified Learjet (operated by the Swiss Air Force) measured continuously the 183.31 GHz rotational transition of H₂O along a meridian of approximately 10° East and at a flight altitude of 12.5 km. From the spectral measurements of the Airborne Mm-wave Sensor (AMS), the stratospheric and mesospheric water vapor distribution has been determined in a latitude range from 46°N to 78°N. Since the observed line intensity of water vapor above the tropopause with an elevation angle of 15° is very high (≈140 K brightness temperature), a sufficient signal to noise ratio is obtained after 20 minutes of data integration allowing a latitude resolution of approximately 2°. The determination of H₂O profiles along the flight path revealed a low latitudinal variation and an unusual local maximum of 5 to 6 ppm VMR at 30 km for this specific area and period. Although a similar structure has also been observed earlier by other instruments, the reason for this springtime humid layer in the stratosphere is not yet clear and needs more precise investigations