Analysis of regional budgets of sulfur species modeled for the COSAM exercise

The COSAM intercomparison exercise (comparison of large‐scale sulfur models) was organized to compare and evaluate the performance of global sulfur cycle models. Eleven models participated, and from these models the simulated surface concentrations, vertical profiles and budget terms were submitted. This study focuses on simulated budget terms for the sources and sinks of SO₂ and sulfate in three polluted regions in the Northern Hemisphere, i.e., eastern North America, Europe, and Southeast Asia. Qualitatively, features of the sulfur cycle are modeled quite consistently between models, such as the relative importance of dry deposition as a removal mechanism for SO₂, the importance of aqueous phase oxidation over gas phase oxidation for SO₂, and the importance of wet over dry deposition for removal of sulfate aerosol. Quantitatively, however, models may show large differences, especially for cloud‐related processes, i.e., aqueous phase oxidation of SO₂ and sulfate wet deposition. In some cases a specific behavior can be related to the treatment of oxidants for aqueous phase SO₂ oxidation, or the vertical resolution applied in models. Generally, however, the differences between models appear to be related to simulated cloud (micro‐)physics and distributions, whereas differences in vertical transport efficiencies related to convection play an additional rôle. The estimated sulfur column burdens, lifetimes and export budgets vary between models by about a factor of 2 or 3. It can be expected that uncertainties in related effects which are derived from global sulfur model calculations, such as direct and indirect climate forcing estimates by sulfate aerosol, are at least of similar magnitude.     

On the selection of prognostic moments in parametrization schemes for drop sedimentation

Common parametrization models for cloud microphysical processes use condensate mass density and/or particle number density as prognostic properties. However, other moments of the particle size distribution can likewise be chosen for prediction. This study deals with parametrization models with one and two, respectively, prognostic moments for the sedimentation of drop ensembles. The spectral resolving model defines the reference solution.
The evolution of the vertical profiles of liquid water content, drop number density and rain rate strongly depend on the choice of the prognostic moments in the parametrization models. In models with a single prognostic moment, its vertical profile is copied by all other moments. The moment of most physical pertinence is recommended for prediction. In models with two prognostic moments, the vertical profiles of all moments differ. The orders of the prognostic moments should be chosen close to the order of moments of highest relevance. Otherwise large errors occur. For example, comparison of modelled versus observed radar reflectivity (6th moment with respect to diameter) does not tell much about the quality of other properties if reflectivity is diagnosed from for example, number density and mass density. Furthermore, mass conservation is fulfilled only if mass density is forecasted.     

Fundamental and derived modes of climate variability: concept and application to interannual time-scales

The notion of mode interaction is proposed as a deterministic concept for understanding climatic modes at various time-scales. This concept is based on the distinction between fundamental modes relying on their own physical mechanisms and derived modes that emerge from the interaction of two other modes. The notion is introduced and applied to interannual climate variability. Observational evidence is presented for the tropospheric biennial variability to be the result of the interaction between the annual cycle and a quasi-decadal mode originating in the Atlantic basin. Within the same framework, Pacific interannual variability at time-scales of about 4 and 6 yr is interpreted as the result of interactions between the biennial and quasi-decadal modes of climate variability. We show that the negative feedback of the interannual modes is linked to the annual cycle and the quasi-decadal mode, both originating outside the Pacific basin, whereas the strong amplitudes of interannual modes result from resonance and local positive feedback. It is argued that such a distinction between fundamental and derived modes of variability is important for understanding the underlying physics of climatic modes, with strong implications for climate predictability.