Deep Convection in the Tropics: Airborne Water Vapour Lidar Observations

Kurzfassung

In the tropics, deep convection is the major source of uncertainty in water vapour transport to the upper troposphere and into the stratosphere. The DLR research aircraft Falcon carried a water vapour differential absorption lidar (DIAL) during the Tropical Convection, Cirrus and Nitrogen Oxides Experiment (TROCCINOX) in 2004 and 2005 in Brazil and the SCOUT-O3 campaign in 2005 in Darwin, Australia. Extended aerosol and water vapour measurements in the UT/LS were performed during the campaigns and along the intercontinental transfer flights to these remote locations. Sensitivity analyses and comparisons with in-situ hygrometers reveal that the DIAL water vapour profiles have an accuracy of 5-10% between altitudes of 8 and 16 km. In Northern Australia, a 90% cloud cover of optically thin (optical thickness <0.5) but geometrically thick cirrus (typically extending between 12 and 17 km) with embedded outflow from deep convection was observed during all nine local flights in November and December 2005. The persistent cirrus layer originated from the SE Asian warm pool region located to the north and vanished over the central Australian continent. A total of 30 flight hours allowed ample observations of the Hector thunderstorm, its anvil cirrus and its humidity outflow into the TTL. Of particular interest are the transfer flights between Europe and Australia where the subtropical jet stream was crossed several times and the Asian monsoon region was traversed. The DIAL observations show that the UT water vapour distribution is considerably modified by dynamical processes. The DIAL profiles are compared with ECMWF analyses to investigate the model’s skill in reproducing the observed water vapour distributions and to help interpret the observations. Over Brazil the DIAL sampled both the background UT humidity in the absence of convection and air masses affected by tropical deep convection. The results show the large variation range of TTL humidity. A high-resolution (2 km horizontal, 200 m vertical) DIAL cross section through the anvil outflow region of tropical convection on 3rd March 2004 shows that the background humidity is increased by a factor of three over an area of ~10,000 km² between 12 and 13 km altitude. From the volume affected and observed by the lidar, a rough estimation reveals that the convective system, probably composed of several thunderstorms, has lifted ~100,000 tons of water up to an altitude of 12 km. Future plans include the use of the recently newly developed four-wavelength DIAL “WALES” on the new German high-altitude and long-range research aircraft HALO and an extension of the DIAL to measure aerosol, water vapour and ozone profiles simultaneously on board of that aircraft, which is currently under development.