Peinlich: Klimamodelle bekommen den Regen nicht in den Griff

Bartlein et al. 2017 machten eine eklatante Schwäche der Klimamodelle bekannt: Für die Zeit vor 5000 Jahren errechnen die Modelle im Vergleich zu den geologisch ermittelten Werten viel zu hohe Temperaturen und zu geringe Niederschläge. Abstract:

Underlying causes of Eurasian midcontinental aridity in simulations of mid‐Holocene climate
Climate model simulations uniformly show drier and warmer summers in the Eurasian midcontinent during the mid‐Holocene, which is not consistent with paleoenvironmental observations. The simulated climate results from a reduction in the zonal temperature gradient, which weakens westerly flow and reduces moisture flux and precipitation in the midcontinent. As a result, sensible heating is favored over evaporation and latent heating, resulting in substantial surface‐driven atmospheric warming. Thus, the discrepancy with the paleoenvironmental evidence arises initially from a problem in the simulated circulation and is exacerbated by feedback from the land surface. This region is also drier and warmer than indicated by observations in the preindustrial control simulations, and this bias arises in the same way: zonal flow and hence moisture flux into the midcontinent are too weak, and feedback from the land surface results in surface‐driven warming. These analyses suggest the need to improve those aspects of climate models that affect the strength of westerly circulation.

Yuan & Zhu 2018 weisen darauf hin, dass Effekte der Ozeanzyklen PDO und AMO unbedingt in Niederschlags-Simulationen mit einbezogen werden sollten, um die Prognoseleistung zu verbessern:

A First Look at Decadal Hydrological Predictability by Land Surface Ensemble Simulations
Abstract: The prediction of terrestrial hydrology at the decadal scale is critical for managing water resources in the face of climate change. Here we conducted an assessment by global land model simulations following the design of the fifth Coupled Model Intercomparison Project (CMIP5) decadal hindcast experiments, specifically testing for the sensitivity to perfect initial or boundary conditions. The memory for terrestrial water storage (TWS) is longer than 6 years over 11% of global land areas where the deep soil moisture and aquifer water have a long memory and a nonnegligible variability. Ensemble decadal predictions based on realistic initial conditions are skillful over 31%, 43%, and 59% of global land areas for TWS, deep soil moisture, and aquifer water, respectively. The fraction of skillful predictions for TWS increases by 10%–16% when conditioned on Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation indices. This study provides a first look at decadal hydrological predictability, with an improved skill when incorporating low‐frequency climate information.

Plain Language Summary: Decadal prediction, which was initially proposed to more accurately project near‐term (e.g., 10–30 years) climate change by using the experiences in weather and seasonal climate forecasting, has raised a wide concern since the IPCC CMIP5 project. The climate community is now trying to transition the decadal prediction from a pure research to a quasi‐real‐time operational effort. However, very limited information is known about the decadal hydrological predictability over land, which is more relevant to the livelihood and stakeholders. Here we combine an ensemble simulation method that is widely used to assess seasonal hydrological predictability, with the experimental design of the CMIP5 decadal climate hindcasts, to provide a first look at decadal hydrological predictability and skill by carrying out over 2,000 years global land model simulations. We found skillful decadal prediction for terrestrial water storage over one third land areas where deep soil moisture and aquifer have a nonnegligible variability. And the skill can be further enhanced by incorporating low‐frequency teleconnection information from Atlantic and Pacific Oceans. This study suggests that it is possible to provide water resources managers useful hydrological forecast information over arid and semiarid regions a few years or even a decade in advance.

Eine hochinteressante neue Arbeit stammt von Bothe et al. 2018. Darin stellen sie zunächst fest, dass die Niederschläge in Großbritannien während der vergangenen 350 Jahre stark von den Ozeanzyklen abhängen und externe Faktoren nahezu keine Rolle spielen. Zudem bemängeln sie eine starke Diskrepanz zwischen Modellsimulationen und echten Messdaten. Kurz gesagt: Die Modelle bekommen die Realität einfach nicht in den Griff. Abstract:

Inconsistencies between observed, reconstructed, and simulated precipitation over the British Isles during the last 350 years
The scarcity of long instrumental records, uncertainty in reconstructions, and insufficient skill in model simulations hamper assessing how regional precipitation changed over past centuries. Here, we use standardised precipitation data to compare global and regional climate simulations and reconstructions and long observational records of seasonal mean precipitation in England and Wales over the past 350 years. The effect of the external forcing on the precipitation records appears very weak. Internal variability dominates all records. Even the relatively strong exogenous forcing history of the late 18th and early 19th century shows only little effect in synchronizing the different records. Multi-model simulations do not agree on the changes over this period. Precipitation estimates are also not consistent among reconstructions, simulations, and instrumental observations regarding the probability distributions’ changes in the quantiles for severe and extreme dry or wet conditions and in the standard deviations. We have also investigated the possible link between precipitation and temperature variations in the various data sets. This relationship is also not consistent across the data sets. Thus, one cannot reach any clear conclusions about precipitation changes in warmer or colder background climates during the past centuries. Our results emphasize the complexity of changes in the hydroclimate during the most recent historical period and stress the necessity of a thorough understanding of the processes affecting forced and unforced precipitation variability.