Introduction of prognostic equations for rain in the ECHAM5 GCM

Clouds play an important role in the energy budget of the earth. Aerosol particles and their precursors resulting from human activity are thought to change the physical and optical properties of clouds. The first indirect effect refers to decreasing cloud droplet sizes as the concentration of (anthropogenic) aerosols increase. For a constant liquid water content, the higher number of smaller cloud droplets leads to an increase in the cloud albedo and therefore, in the planetary albedo. Furthermore, it is harder for the smaller cloud droplets to grow into precipitation sized drops. This results in a prolonged lifetime of clouds within the atmosphere. This second aerosol indirect effect also causes an increase in the planetary albedo. However, the size of both of these effects is still very uncertain.

The investigation of aerosol effects on large scale precipitation is one of the major goals of this study. As a first step prognostic equations for rain mass mixing ratio and rain drop number concentration are introduced. In the standard version of the ECHAM5 the rain is treated diagnostically and the total rain water is removed from the model after one time step (as surface precipitation flux). This approach is only true for relativly large rain drops. Smaller drops (i.e., drizzle) also sediment but may not reach the surface within one time step.

To calculate the actual rain flux from one model level to the next an approach for the fall velocity of rain drops is introduced. In order to account for the larger fall speeds of larger rain drops different equations for the fall velocity of the rain mixing ratio and the rain drop number concentration are used. Starting from the equations for the fall velocity of a single rain drop by Rogers et al. (1993) the flux density approach used by Srivastava (1978) is applied to obtain the fall velocity for the two bulk parameters mass (LWC) and number. The moments of the rain drop distribution necessary to calculate the mass and number flux are obtained by assuming a Marshall-Palmer- Distribution for the rain drops with a slope parameter λ~LWC/N. The asymptotic solutions for small and large drops, respectively, are shown in the figure.

If using explicit fall speeds for the rain drops one has to pay attention that the criterion for numerical stability is not violated. This would be the case if relatively large rain drops fall too fast/too far down and, therefore, miss a model level. To prevent this (and the resulting chaotic behaviour of the model) a reduction of the time step is necessary. As this would be computational to expensive if applied for the whole model only the cloud microphysics routine is iterated and, thus, experiences a smaller timestep.

At this stage, results of single column simulation with the newly introduced prognostic rain will be presented. Comparisons with the standard ECHAM5 will be shown as well as sensitivity studies regarding the influence of aerosol concentration on precipitation formation.