First airborne water vapor lidar measurements in the tropical upper troposphere and mid-latitudes lower stratosphere: accuracy evaluation and intercomparisons with other instruments

[摘要] In the tropics, deep convection is the major source of uncertainty in watervapor transport to the upper troposphere and into the stratosphere. Althoughaccurate measurements in this region would be of first order importance tobetter understand the processes that govern stratospheric water vaporconcentrations and trends in the context of a changing climate, they aresparse because of instrumental shortcomings and observational challenges.Therefore, the Falcon research aircraft of the Deutsches Zentrum fürLuft- und Raumfahrt (DLR) flew a zenith-viewing water vapor differentialabsorption lidar (DIAL) during the Tropical Convection, Cirrus and NitrogenOxides Experiment (TROCCINOX) in 2004 and 2005 in Brazil. The measurementswere performed alternatively on three water vapor absorption lines ofdifferent strength around 940 nm. These are the first aircraft DIALmeasurements in the tropical upper troposphere and in the mid-latitudeslower stratosphere. Sensitivity analyses reveal an accuracy of 5% betweenaltitudes of 8 and 16 km. This is confirmed by intercomparisons with theFast In-situ Stratospheric Hygrometer (FISH) and the Fluorescent AdvancedStratospheric Hygrometer (FLASH) onboard the Russian M-55 Geophysicaresearch aircraft during five coordinated flights. The average relativedifferences between FISH and DIAL amount to −3%±8% and betweenFLASH and DIAL to −8%±14%, negative meaning DIAL is more humid.The average distance between the probed air masses was 129 km. The DIAL isfound to have no altitude- or latitude-dependent bias. A comparison with theballoon ascent of a laser absorption spectrometer gives an averagedifference of 0%±19% at a distance of 75 km. Six tropical DIALunder-flights of the Michelson Interferometer for Passive AtmosphericSounding (MIPAS) on board ENVISAT reveal a mean difference of −8%±49%at an average distance of 315 km. While the comparison with MIPAS issomewhat less significant due to poorer comparison conditions, the agreementwith the in-situ hygrometers provides evidence of the excellent quality ofFISH, FLASH and DIAL. Most DIAL profiles exhibit a smooth exponentialdecrease of water vapor mixing ratio in the tropical upper troposphere tolower stratosphere transition. The hygropause with a minimum mixing ratio of2.5 µmol/mol is found between 15 and 17 km. A high-resolution (2 kmhorizontal, 0.2 km vertical) DIAL cross section through the anvil outflow oftropical convection shows that the ambient humidity is increased by a factorof three across 100 km.

[学科分类] 大气科学