In situ measurement of H2O and CH4 with telecommunication laser diodes in the lower stratosphere:: Dehydration and indication of a tropical air intrusion at mid-latitudes

Telecommunication laser diodes emitting near 1.39 mum and 1.65 mum in combination with direct-differential absorption spectroscopy are efficient tools to monitor in situ stratospheric H2O and CH 4 with a good precision error (a few percents), a high temporal resolution (ranging from 10 ms to 1 s), a large dynamic range in the concentration measurements (four orders of magnitude) and a high selectivity in the analyte species. To illustrate the capability of laser probing technique, we report balloonborne H2O and CH4 simultaneous measurements obtained on October 2001 at midlatitudes (43degreesN). The H2O vertical profile achieved with the laser sensor in the lower stratosphere is compared with the H2O data yielded by a balloonborne frost-point hygrometer. The total hydrogen mixing ratio in the lower stratosphere, 2[CH4] + [H2O], appears to be constant at 7.5 +/- 0.1 ppmv. Nevertheless, an unexpected large dehydration of similar to0.5 ppmv was detected by both the laser sensor and the hygrometer between 16 km and 23 km. We suspect the occurrence of a tropical air intrusion into mid-latitudes. We support this interpretation using a high-resolution advection model for potential vorticity.