Measurements of Water (Vapor and Cloud)

Measurements of water vapor and cloud water/ice

Since May 2005 we deploy an instrument that simultaneously measures water vapor H2Ogas and cloud water/ice (H2Ocloud) onboard the CARIBIC passenger aircraft.

It is based on two different measurement techniques:

  • a two-channel photo-acoustic laser-spectrometer (PAS) developed at the Department of Optics and Quantum Electronics at the University Szeged (Hungary). It simultaneously measures H2Ogas and total water (H2Otot = H2Ogas + H2Ocloud). In two 4 cm long cells modulated laser light excites water molecules in the inflowing sample air and their thermal relaxation is measured by a microphone. This technique is (with ~2 s measurement time) fast, but is susceptible to (usually temperature driven) drifts in sensitivity.
  • a commercial chilled-mirror frost point hygrometer FPH (CR2, Buck Research, Boulder, USA) that controls the temperature of a tiny mirror (diameter: ~7 mm) to the dew/frost point temperature above/below 0°C. It is a well established and absolute method and serves as calibration unit for the laser-spectrometer.

The FPH and one PAS channel are supplied with sample air from a forward facing (H2Otot) inlet line; the second PAS channel receives air via a sideward facing inlet line for H2Ogas. The sampling conditions of the non-isokinetic sampling line for H2Otot lead to a (simultaneously measured) enhancement of cloud particles by a factor of ~4.

A cross-section of water vapor and cloud ice during a flight from Frankfurt (Germany) to Vancouver (Canada) in October 2012 is shown in Fig. 1. The flight took ~10 hours spanning almost 9000 km. H2Ogas from the PAS in light green, H2Otot from the PAS in blue and from the FPH in red, and the retrieved cloud ice H2Ocloud in dark green (right y-axis). H2O fastly dropped from >5000 ppmv at the ground to below 100 ppmv in the upper troposphere. Below ~20 ppmv the aircraft flew in the stratosphere (to ~30% of the flight time). Here the relative humidity is very low and dropped to values of as low as ~2 %.

The intersection of clouds is identified by total water measurements in excess of the water vapour measurements (H2Otot > H2Ogas), e.g. between 12:05 and at 13:00 o'clock. Here the water vapour measurements (light green) just reach 100% saturation versus ice (lower boundary of grey area) and thus indicate the presence of ice clouds. The two clouds intersected around 16:00 and 19:30 are supersaturated vs. ice, i.e.


Fig. 1: Cross-section of water vapor and cloud ice during a flight from Frankfurt (Germany) to Vancouver (Canada) in October 2012