EXERCISE 3:

The Impact of Indirect Effects of Aerosols on Meteorology

Model used: Enviro-HIRLAM and SILAM
        (Enviro-HIRLAM is based on the DMI version of the HIRLAM - HIgh Resolution Limited Area Model)

Teachers: Sergey Smyshlyaev (RSHU, Russia) and Allan Gross (DMI, Denmark)

Group 3.1:
Andy Delcloo (Belgium), Joana Soares (Finland), Marko Zirk (Estonia), Angelina Todorova (Bulgaria), Iakov Gontsov (Russia)

Group 3.2:
Svetlana Lazareva (Russia), Julia Palamarchuk (Ukraine), Artur Kertov (Russia), Marie Prank (Finland), Joakim Refslund Nielsen (Denmark)

Introduction Background:
Although there has been a large amount of progress in the development of the study of aerosol effects on the climate, uncertainty in the estimation of the indirect aerosol forcing remains on of the highest in the climate studies today. This is partly due the high temporal and spatial inhomogeneity in aerosol concentrations and partly due to the complex relationship between the aerosol chemical and physical properties and cloud microphysics.
Aerosols influence cloud radiative properties in several ways. The first and most widely studied is that aerosol particles increase the cloud droplet number concentration and decrease the droplet/crystal effective radius, therefore modifying the cloud optical properties. This effect is called the First Indirect Effect (or the Radius Effect) (FIE) (Lohmann and Feichter, 2005). The other effect is that the decreased cloud droplet effective radius inhibits the autoconversion rate and precipitation formation, causing longer cloud lifetime and higher cloud albedo. This effect is called the Second Indirect Effect (or the Lifetime Effect). The existence of absorbing particles (such as black carbon) may have other indirect effects related to the heating of air and cloud evaporation (the so-called Semi-direct Effect) which can change the vertical temperature profile and the dynamical structure of clouds, and can modify the cloud single scattering albedo when the absorbing aerosols are in the cloud droplet.
There are a number of estimates of the global indirect aerosol forcing in the literature. In most of these calculations, the prescribed or simulated aerosol field is related to cloud droplet number concentration through empirical or physically based parameterization. Many of these models also account for the influence of the change in cloud droplet effective radius on the autoconversion rate. The first indirect aerosol forcing can have both a negative or positive impact on the radiation in the atmosphere sign and its uncertainty is much larger than the uncertainty in the green house gas forcing, which is only about 10%.

Specific Objectives:  
All simulations of Enviro-HIRLAM shall be performed at a grid resolution of 40×40 km2 and time step of 10 min.
1)    Theoretical understanding of the impact of the First Indirect Effect. How the FIE can influence the radiation scheme in Enviro-HIRLAM (through changing the cloud droplet/crystal effective radius) is described in Wyser et al. (1999). Based on Wyser et al. (1999) explain how the effective radius is taken into account in the radiation scheme in Enviro-HIRLAM?
2)    Model simulation. Run Enviro-HIRLAM
a.    without any emissions of aerosols.
b.    with aerosols emissions only from sea/marine.
c.    with aerosols emissions only from urban.
d.    with aerosols emissions only from land minus urban.
e.    with aerosols emissions from sea/marine, urban and land.
3)    Data analysis. Answer the following questions:
a.    How does FIE influence the radiation balance?
b.    How does FIE affect the vertical temperature and atmospheric stability?
c.    How does FIE change the SW and LW radiation at the top of the atmosphere?
d.    How does FIE change the boundary layer height?
4)    Summaries the results form the simulations in form of an oral presentation (max. 15 min.)

Literature List:
The students shall read these papers before the Summer School.

REQUIRED READINGS:

Korsholm U.S., A. Baklanov, A. Gross, A. Mahura, B.H. Sass, E. Kaas, 2008: Online coupled chemical weather forecasting based on HIRLAM – overview and prospective of Enviro-HIRLAM. HIRLAM Newsletter, 54: 1-17.
Wyser K., L. Rontu, H. Savijärvi,  1999: Introducing the Effective Radius into a Fast Radiation Scheme of a Mesoscale Model. Contr. Atmos. Phys., 72(3): 205-218.

ADDITIONAL READINGS:

Boucher O. and U. Lohmann, 1995: The sulphate-CCN-cloud albedo effect: a sensitivity study with two general-circulation models. Tellus, 47(B): 281-300.
Lohmann U. and J. Feichter, 2005: Global indirect aerosol effects: a review. Atmos. Chem. Phys., 5: 715-737.