Niels Bohr Institutet > Kalender - det sker på NBI > 2009 > PhD defense: Ulrik Smi...

Integrated modeling of aerosol indirect effects

Development and application of an online coupled chemical weather model

PhD defense: Ulrik Smith Korsholm 

Abstract 

The atmosphere is heavily polluted with aerosols which affect cloud formation and precipitation development and continental clouds generally include two orders of magnitude more cloud droplets than marine clouds. An increased number of small droplets, in warm clouds, leads to an increase in cloud albedo and suppression of rain. Complex cloud and aerosol -microphysical and cloud dynamical feedbacks shape the response to albedo enhancement and suppression of rain and may feed back on the aerosol and trace gas distributions of the atmosphere on short time scales. The importance of such feedbacks is unknown and in this study an online coupled chemical weather model is developed, tested and employed in a case study, investigating the importance of such feedbacks on trace gas distributions. Enviro-HIRLAM is developed as an extension of the mesoscale short-range weather forecast model HIRLAM and includes emission, advection, turbulent diffusion, convection and deposition of trace gasses and aerosols as well as gas-phase chemistry, aerosol dynamics, gas-aerosol equilibration, aerosol activation. The activated aerosols are coupled to the cloud scheme leading to albedo enhancement and suppression of precipitation in warm convective and stratiform clouds. In a particular case study considering a convective summertime conditions with light rain it was shown that the prediction aerosol mass concentration was satisfactory and two-meter temperature predictions improved slightly when including the aerosol effects. The distribution of NO2 near the surface was greatly affected by the feedbacks over the 24 hour period. The feedbacks induced changes in cloud cover, temperature and in local circulations by inducing convective activity which lead to a dynamical redistribution of the species rather than to changes in chemical reactions. The suppression of rain was of greater importance than albedo enhancement and nonlinear effects acted to damp the influence of the feedbacks. Hence, in this case study the feedbacks were of great importance in determining the trace gas distributions.

Supervisors:Alexander Baklanov, Research Department DMI,  Eigil Kaas NBI