Tropical High Altitude Clouds and their Impact on Stratospheric Humidity
Final Report Abstract
This project has investigated tropical high altitude clouds and their impact on stratospheric humidity in the time-frame of two years. In a first step the life cycle of a deep convective storm has been studied looking at in situ cloud particle measurements. The change of the cloud particle properties with increasing age of the clouds in the storm has been characterised and indications for changing freezing mechanisms revealed. Additionally, hints for in situ nucleation of cloud particles at the cloud edge in the dissipating stage were found. Though often neglected by observational and modelling studies, in the dissipating stage Hector consists of a persistent and vertically extensive cloud layer that is optically thin and has further characteristics similar to those of subvisible cirrus. Thus, the anvils of these high-reaching deep convective clouds have a high potential for affecting the upper tropical troposphere and lower stratosphere by modifying the humidity and radiative budget, as well as for providing favourable conditions for subvisible cirrus formation. The involved processes may also influence the amount of water vapour that ultimately reaches the stratosphere in the tropics. Another focus were the transport processes related to deep convection. For this a cloud resolving simulation was performed with the Advanced Research Weather and Research Forecasting (WRF-ARW) model. In situ trace gas observations and model simulation both showed a downward transport and mixing of stratospheric air into the upper tropical troposphere. Also the upward transport of boundary layer air into the high altitude clouds is demonstrated by the simulation. However, discrepancies in the amount of tracers transported from the stratosphere and boundary layer, respectively, indicate an erroneous model representation of the transport processes in the tropical upper troposphere and lower stratosphere. Furthermore, it could point to insufficient entrainment and detrainment in the model. Simulated in-cloud mass fluxes showed a net downward mass flux at cloud top. A statistical analysis of windprofiler data from Darwin also exhibited the increasing importance of downdrafts above 10 km, mostly peaking at cloud top. This convective-scale downward transport may have important implications for the chemistry in the upper troposphere (for example in-cloud ozone destruction). The simulation showed a general moistening of air in the lower stratosphere after the convective event. Surprisingly, the simulation also revealed local dehydration features, even in initially subsaturated air, caused by transport processes. Thus, the question of whether overshooting convection leads to hydration or dehydration is not only dependent on the initial water vapour profile, but also on the altitude of the layer and time since the overshooting. Overshooting convection in a subsaturated environment can actually lead to both hydration and dehydration.
Publications
- Modelling of Hector overshooting convection and its implications on water vapour and ozone distribution in the TTL and lower stratosphere, SPARC General Assembly, Queenstown, New Zealand, 12 - 17 Jan 2014
Frey, W., Lane, T., Borrmann, S., Schofield, R., Ravegnani, F.
- The impact of deep overshooting convection on the water vapour and trace gas distribution in the TTL and lower stratosphere, AGU Fall Meeting, San Francisco, USA, 15 - 19 Dec 2014
Frey, W., Schofield, R., Hoor, P., Ravegnani, F., Ulanovsky, A., Viciani, S., D’Amato, F., Lane, T.
- Tropical deep convective life cycle: Cb-anvil cloud microphysics from high-altitude aircraft observations, Atmos. Chem. Phys., 14, 13223–13240
Frey, W., Borrmann, S., Fierli, F., Weigel, R., Mitev, V., Matthey, R., Ravegnani, F., Sitnikov, N.M., Ulanovsky, A., and Cairo, F.
(See online at https://doi.org/10.5194/acp-14-13223-2014) - The impact of overshooting deep convection on local transport and mixing in the tropical upper troposphere/lower stratosphere (UTLS), Atmos. Chem. Phys. Discuss., 15, 1041–1091
Frey, W., Schofield, R., Hoor, P., Kunkel, D., Ravegnani, F., Ulanovsky, A., Viciani, S., D’Amato, F., and Lane, T.P.
(See online at https://doi.org/10.5194/acp-15-6467-2015)
