The TROWARA Water Vapour Radiometer

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The Trowara (TRoposperic WAter vapour RAdiometer) began operating on the roof of the Institute of Applied Physics in Bern in late 1994. It senses microwave radiation at 21.3 and 31.5 GHz. The first channel is close to the water vapour absorption line and the second is in the water vapour continuum. Brightness temperature measurements in both channels can be used to calculate total column water and total liquid water. The photo on the right shows Trowara in its original position in the roof where it observed the sky at a constant elevation angle of 40 degrees. Every hour the instrument automatically viewed the sky at elevation angles of 20 to 70 degrees and used these measurements to correct the sky brightness temperatures. This is known as a tipping calibration.

Data were collected in this way until April 2002 when the instrument underwent a review. There were several reasons for this. Firstly the data acquisition software was outdated (a Pascal program was still running). Secondly the whole front part of the radiometer had to move during the hourly tipping calibrations and this caused a small but not insignificant disturbance to the measured brightness temperatures. Thirdly the measurements could be affected by rain collecting on the radiometer lens. Rain collecting on the instrument is a problem which affects most microwave radiometers.

Trowara in original position on roof
Trowara in new position indoors

The radiometer was reviewed during the spring and summer of 2002 and was eventually installed in a new position in November 2002. It is still on the roof of the IAP building but this time it is inside instead of outside and looking southeast at an elevation angle of 40 degrees instead of northeast. The photo on the left shows Trowara in its new position. It views the atmosphere through a microwave transparent window (the blue window in the photo). The view angle of the instrument can be changed between 20 and 45 degrees using the pulley on the left of the photo.

The instrument now has an up to date data acquisition program, less disturbance to the instrument. Because the instrument is inside, the temperature is much better controlled, which means that the instrument is a lot more stable. Tipping calibrations are carried out at intervals of one to several weeks (depending on the weather conditions) and so the disturbance to the instrument due to changing the view angle is minimal. Another advantage is that rain does not seem to collect on the microwave window, making Trowara one of the few microwave radiometers which can continue to collect observations during rain.

Here is a picture taken from the roof of the IAP building. It shows the outside of the Trowara microwave window (in blue). The large mirror in the right is used to make zenith sky observations for the tipping calibration. This calibration works best when the sky is cloud free, similar to the situation in the photo.

The Trowara microwave window from the outside
Water vapour and liquid water measured by Trowara on a clear day

The figure on the left shows total column water vapour and liquid water calculated from Trowara measurements on a clear day, 1st February 2003. At the start of the day, the water vapour value is very low, around 2 to 3 mm, but it increases during the day.

Here is an example of a cloudy day two weeks later. As often happens on the Swiss Central Plain, stratus builds up early in the morning and lingers all day..

Trowara water vapour and liquid water measurements on a cloudy day

Publications

Publications related to the TROWARA radiometer are available in the IAP publications database. These include: