Marine Stratocumulus Clouds
Low level, marine stratocumulus clouds have a strong cooling effect as the underlying sea has a low albedo and their emitted thermal radiation is similar to the surface thermal radiation. A correlation between the cloud coverage by low level clouds and the net forcing (cooling) by clouds is observed. But representing stratocumulus clouds in global climate models (GCMs) is challenging (Figure 1) because of their coarse vertical resolution. Models forced by increased CO2 predict different increases in global average temperature. The large temperature range in these predictions corresponds to different responses in low cloud amount. Simulations of the anthropogenic aerosol effect can be expected to depend on the representation of stratocumulus clouds.
Improved representation in a GCM
A number of physical processes have to be accounted for when modeling stratocumulus clouds including cloud top radiative cooling which drives turbulent fluxes in the planetary boundary layer, absorption of shortwave fluxes in the cloud layer, entrainment of warm, dry air from the free atmosphere and microphysical processes. The representation of several of these processes will be investigated in the global climate model ECHAM6 coupled to the aerosol module HAM2 and a two-moment cloud microphysics scheme.
In the turbulent kinetic energy scheme used in ECHAM6, the turbulent diffusivities are the product of the turbulent mixing length, a stability function and the square root of turbulent kinetic energy. The stability function used in ECHAM6 is a so called ‘long-tails’ function which decays slowly with increasing Richardson number (Figure 2a). ‘long-tails’ function are also used in numerical weather prediction models but are known to result in excessive mixing at high stabilities. In a numerical weather prediction model a ‘sharp’ function was shown to give significant improvements in low-level winds. A ‘sharp’ function in ECHAM6-HAM2 increases stratocumulus cloud cover and liquid water path.
As entrainment at the cloud top interface as well as cloud top radiative cooling depend on vertical resolution an increased vertical resolution in the lower troposphere in ECHAM6-HAM2 (Figure 2b) may have an impact on stratocumulus clouds.
Aerosol processing in stratiform clouds by uptake into cloud particles, collision-coalescence, chemical processing inside the cloud particles and release back into the atmosphere changes the aerosol concentration, size distribution, chemical composition and mixing state. The change in the aerosol influences cloud droplet and ice crystal number concentrations and subsequently cloud liquid and ice water paths as well as cloud lifetime and cloud radiative forcing and therefore the indirect aerosol effect. HAM2 was extended by an explicit representation of aerosol particles in cloud droplets and ice crystals in stratiform clouds similar to Hoose et al., 2008a,b (Figure 2c).
The impacts of these changes on marine stratocumulus clouds and on the anthropogenic aerosol effects are studied in this project using ECHAM6-HAM2.
