Multiscale water budget in the upper troposphere and lower stratosphere in the TROpics – TRO-Pico

Water vapour (WV) is a key species of stratospheric climate and photochemistry. In the upper troposphere- lower stratosphere (UTLS), its abundance is controlled by two competing mechanisms: hydration by local injection of ice crystals by convective overshooting up to 19-20 km over land which after sublimation at sub-saturated levels are moistening the stratosphere and dehydration by water vapour condensation in the form of thin cirrus in the region of coldest tropopause temperature followed by the sedimentation of the crystals. However, if the existence of the hydration process over continents is now accepted as well as successfully captured by cloud resolving models, its impact at the global scale on stratospheric water vapour, is still largely unknown. TRO-pico is an attempt to better quantify the global flux of water vapour resulting from the overshoot process and its role in the known seasonal modulation of the species.
The project is to make use of a combination of balloon, ground-based and satellite observations and model simulations at various scales. Field measurements consist in a balloon sonde campaign in the most convectively active part of South America, the state of Sao Paulo in Brazil in the summer season, extending on two time periods:
- the first, called SMOP for « Six Month Observing Period » from October 2011 to April 2012, of regular water vapour soundings two to three times per month using the Pico-SDLA diode laser hygrometer and the mini-SAOZ NIR spectrometer for studying the change of water vapour during the full convective season, and,
- a shorter period called IOP for « Intense Observing Period » during the most intense convective period in January-February 2012, dedicated to the study of troposphere-to-stratosphere transport and stratospheric moistening and their impact at the regional scale by a number of soundings of water vapour, methane, NOx, ozone, aerosols, ice particles and electric field using adequate light-weight instruments.
Associated to these, are ground-based C-band radar observations allowing to identified the overshoots within a 250 km range, and satellites observations of water vapour by MLS-AURA, IASI-Metop and Saphir Megha-Tropiques, aerosols and cirrus clouds by CALIPSO, and temperature profiles by the GPS COSMIC constellation.
On the modelling side, the proposal is to carry a rang of simulations spanning from the local scale using BAMS and Meso-NH simulations in cloud resolving mode, to continental and global scale with the Meteo-France MOCAGE-PALM model assimilating MLS water vapour measurements. The idea is to investigate how well each could reproduce the observations and possible changes for better capturing these. Additionally, the experimental data should allow tackling two other open questions: the possible impact of the electric field associated to thunderstorm on the ice particles, that of NOx formed by lightning on the composition of the sub-cirrus crystals (NAT?), and finally a thorough evaluation of water vapour retrievals of the French satellites IASI-Metop and Saphir-Mega-Tropiques instruments.
If both TRO-Pico and CONVECTRO were selected, the strong interaction planned between the two projects might allow a real breakthrough in the understanding of vertical transport and humidity in the lower stratosphere, which remains a region still badly represented in meteorological and climate models.