Boundary layer clouds in the global climate model (GCM) ECHAM5-HAM

The planetary boundary layer (PBL) is the layer near the surface where the turbulence generated by the interaction with the surface is dominant. In terms of aerosols the PBL is very important because the emissions from surface are transported by turbulence or convection. Two projects are focused on improving the representation of PBL clouds (stratocumuli and shallow cumuli) in the ECHAM5 general circulation model (GCM).

STRATOCUMULUS

Marine stratocumulus-capped boundary layers show a strong net cooling on the Earth-Atmosphere system. Indeed, their high albedo reflects more incoming solar radiation than the underlying ocean, while their low altitude causes the longwave emissions to space to be similar to those of the Earth's surface (Figure 1). Moreover, they are highly persistent over subtropical oceans. They are therefore very important in GCMs in order to make reliable projections of future climates. However, the various state-of-the-art GCMs do not agree on the response of low cloud amount to a doubling of CO2. This contributes strongly to the uncertainty in the global climate projections.

Fig. 1: Left: Mean net cloud radiative forcing at the top of the atmosphere obtained from data of the NASA Earth Radiation Budget Experiment (ERBE) scanner instruments on board the ERBS and NOAA 9 satellites during January 1985 through December 1986. Right: Cloud cover of low- level clouds from ISCCP averaged over the period from 1983 to2005 (Rossow and Schiffer, 1999).

Unfortunately, marine stratocumuli are difficult to simulate in a GCM, due to its coarse resolution. This work focuses on three particular aspects, which contribute to their rather poor representation in the ECHAM5-HAM GCM. First, the vertical turbulent diffusion in the standard ECHAM5-HAM is performed on variables, which are not conserved during adiabatic motion, such as the liquid water mixing ratio. This tends to smooth the vertical profile of the latter, destroying the cloud. Second, most GCMs tend to overestimate the second aerosol indirect effect with respect to satellite based studies. The latter aerosol effect is based on the assumption that an increase in the aerosol loading in a cloud results in more but smaller cloud droplets (for constant liquid water content). The smaller cloud droplets might be less efficient to precipitate, increasing the liquid water path (LWP) and the lifetime of the cloud. On the other hand, large eddy simulation models showed that smaller cloud droplets induce an enhanced entrainment of warm dry air at cloud top, which potentially leads to a decrease of LWP. Third, marine stratocumuli lie under a sharp inversion, which is difficult to represent in a coarse resolution model. This project addresses these three problems.

SHALLOW CONVECTION

Shallow cumuli are one of the most common cloud types on Earth. They are common over the oceans, but also over continents during fair weather periods. Shallow cumuli are important in mid-latitudes, but the influence on large-scale atmospheric dynamics is most evident in the trade wind areas in the subtropical belts above the oceans in both hemispheres, where they are called ``trade wind cumuli''. In these regions, they occur in the transition between stratocumuli and deep convective clouds.

Cumuli have a short lifetime from minutes to one hour. Differently from stratocumulus they are more intermittent and scattered. They transport humid air from the surface mixed layer to the free atmosphere, influencing temperature, humidity, winds, cloud cover and depth of the boundary layer. They contribute fundamentally to the moisture and energy balance in the lower troposphere.

Shallow convection actively mix with the environment leading to strong dilution. The strong interaction with the vertical structure and turbulence of the PBL requires an accurate representation of shallow convection as PBL clouds. Its parameterization in global circulation models (GCMs) is one of the largest sources of uncertainty. A recently developed shallow convection scheme by von Salzen and McFarlane (2009) is implemented into the general circulation model ECHAM5-HAM. The scheme of von Salzen and McFarlane (2002) is a bulk parameterization for an ensemble of transient shallow cumuli. A life cycle is considered, as well as inhomogeneities in the horizontal distribution of in-cloud properties due to mixing. The scheme is further developed in this thesis to take the ice phase into account. Additionally, a detailed double moment microphysics approach for cloud droplets and ice crystals has been added. In this approach, the freezing processes and precipitation formation are dependent on aerosols. Furthermore, tracers are transported in the scheme and scavenged consistently as in the rest of the model. The ice phase permits to alter the criterion to distinguish between shallow and the other two types of convection, namely deep and mid-level, which are still calculated by the Tiedtke (1989) scheme.

von Salzen, K. and McFarlane, N. A. (2002). Parameterization of the bulk effects of lateral and cloud-top entrainment in transient shallow cumulus clouds. J. Atmos. Sci., 59, 1405–1430.

Tiedtke, M. (1989). A comprehensive mass flux scheme for cumulus parameterization in largescale models. Mon. Wea. Rev., 117, 1779–1800.