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 μ m and 1.65 μ m in combination with direct-differential absorption spectroscopy are efficient tools to monitor in situ stratospheric H^sub 2^O andCH^sub 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 H^sub 2^Oand CH^sub 4^ simultaneous measurements obtained on October 2001 atmidlatitudes (43° N). The H^sub 2^O vertical profile achieved with the lasersensor in the lower stratosphere is compared with the H^sub 2^O data yielded by a balloonborne frost-point hygrometer. The total hydrogen mixing ratio in the lower stratosphere, 2[CH^sub 4^] + [H^sub 2^O], appears to beconstant at 7.5 ± 0.1 ppmv. Nevertheless, an unexpected largedehydration of 0.5 ppmv was detected by both the laser sensor and thehygrometer between 16 km and 23 km. We suspect the occurrence of a tropicalair intrusion into mid-latitudes. We support this interpretation using a high-resolution advection model for potential vorticity.[PUBLICATION ABSTRACT]