Andreas Roesch

Estimating the Decadal Predictability of a Coupled AOGCM

On seasonal time scales, ENSO prediction has become feasible in an operational framework in recent years. On decadal to multidecadal time scales, the variability of the oceanic circulation is assumed to provide a potential for climate prediction. To investigate the decadal predictability of the coupled atmosphere–ocean general circulation model (AOGCM) European Centre-Hamburg model version 5/Max Planck Institute Ocean Model (ECHAM5/MPI-OM), a 500-yr-long control integration and “perfect model” predictability experiments are analyzed. The results show that the sea surface temperatures (SSTs) of the North Atlantic, Nordic Seas, and Southern Ocean exhibit predictability on multidecadal time scales. Over the ocean, the predictability of surface air temperature (SAT) is very similar to that of SST. Over land, there is little evidence of decadal predictability of SAT except for some small maritime-influenced regions of Europe. The AOGCM produces predictable signals in lower-tropospheric temperature and precipitation over the North Atlantic, but not in sea level pressure.

Assessment of Snow Cover and Surface Albedo in the ECHAM5 General Circulation Model

Abstract Land surface albedo, snow cover fraction (SCF), and snow depth (SD) from two versions of the ECHAM climate model are compared to available ground-based and remote-sensed climatologies. ECHAM5 accurately reproduces the annual cycle of SD and correctly captures the timing of the snowmelt. ECHAM4, in contrast, simulates an excessive Eurasian snow mass in spring due to a delayed snowmelt. Annual cycles of continental snow cover area (SCA) are captured fairly well in both ECHAM4 and ECHAM5. The negative SCA trend observed during the last two decades of the twentieth century is evident also in the ECHAM5 simulation but less pronounced. ECHAM5 captures the interannual variability of SCA reasonably well, which is in contrast with results that were reported earlier for second-phase Atmospheric Model Intercomparison Project (AMIP II) models. An error analysis revealed that, for studies on SCA, it is essential to test the data records for their homogeneity and trends. The second part of the paper compares s...

Evaluations of Land–Ocean Skin Temperatures of the ISCCP Satellite Retrievals and the NCEP and ERA Reanalyses

Abstract This study evaluates the skin temperature (ST) datasets of the International Satellite Cloud Climatology Project (ISCCP) D satellite product, the ISCCP FD satellite product, the 40-yr ECMWF Re-Analysis (ERA-40), the NCEP–NCAR Reanalysis, and the NCEP–Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP)-II Reanalysis. The monthly anomalies of all the datasets are correlated to each other and to most of the ground-truth stations with correlation coefficients >0.50. To evaluate their qualities, the 5 ST datasets are used to calculate clear-sky (CS) outgoing longwave radiation (OLR) and upward surface longwave radiation (USLR); the results are compared with the Earth Radiation Budget Experiment (ERBE) satellite observation and 14 surface stations. The satellite-derived STs and ERA-40 ST tend to bias high on hot deserts (e.g., Sahara Desert), and the reanalyzed STs tend to bias low in mountain areas (e.g., Tibet). In Northern Hemisphere high-latitude regions (tundra, wetlands, d...

Global dimming and brightening - evidence and agricultural implications

There is growing observational evidence that the level of sunlight received at the Earth’s surface is not stable over the years but undergoes substantial decadal changes. These changes, known popularly as global dimming and brightening, are related to anthropogenic air pollution, as well as changes for cloudiness. Here, we summarize the evidence and discuss the implications of these variations in biospheric growth in general and agricultural applications in particular. Emphasis is placed on implications for forest canopies, grassland and agricultural crop production. Yet not only the total amount of sunlight that the biosphere receives is important but also the relative portion of the direct and diffuse light. Increasing air pollution and cloudiness can alter this partitioning towards a higher relative portion of diffuse light, in addition to a reduction in the total amount of sunlight. This diffuse light can enhance photosynthesis, particularly in tall and dense vegetation canopies, as it is distributed more effectively within the canopy compared with direct solar radiation, which is only accessible to the outermost (sunlit) layers. Here, we conclude that the growth of tall canopies such as forests or crop fields may be favoured under global dimming conditions owing to the enhanced diffuse light, despite the overall reduction in the total amount of sunlight. On the other hand, under the same dimming conditions, the yield of grassland, with its shallow open canopy, may be reduced, as it has little benefit from the enhanced diffuse fraction, while suffering from a loss in the total amount of sunlight for photosynthesis. Variations in solar radiation therefore may have largely varying effects on agricultural production depending on the structure of the crop canopy.

Snow Cover Fraction In A General Circulation Model

Snow cover fraction (SCF) has a significant influence on the surface albedo and thus on the radiation balance and surface climate. Long-term three dimensional simulations with General Circulation Models (GCMs) showed that the SCF greatly affects the climate in the Northern Hemisphere.