D. Fläschner

Fast and Slow Precipitation Responses to Individual Climate Forcers: A PDRMIP Multimodel Study

Precipitation is expected to respond differently to various drivers of anthropogenic climate change. We present the first results from the Precipitation Driver and Response Model Intercomparison Project (PDRMIP), where nine global climate models have perturbed CO2, CH4, black carbon, sulfate, and solar insolation. We divide the resulting changes to global mean and regional precipitation into fast responses that scale with changes in atmospheric absorption and slow responses scaling with surface temperature change. While the overall features are broadly similar between models, we find significant regional intermodel variability, especially over land. Black carbon stands out as a component that may cause significant model diversity in predicted precipitation change. Processes linked to atmospheric absorption are less consistently modeled than those linked to top-of-atmosphere radiative forcing. We identify a number of land regions where the model ensemble consistently predicts that fast precipitation responses to climate perturbations dominate over the slow, temperature-driven responses.

Understanding the Intermodel Spread in Global-Mean Hydrological Sensitivity*

AbstractThis paper assesses intermodel spread in the slope of global-mean precipitation change ΔP with respect to surface temperature change. The ambiguous estimates in the literature for this slope are reconciled by analyzing four experiments from phase 5 of CMIP (CMIP5) and considering different definitions of the slope. The smallest intermodel spread (a factor of 1.5 between the highest and lowest estimate) is found when using a definition that disentangles temperature-independent precipitation changes (the adjustments) from the slope of the temperature-dependent precipitation response; here this slope is referred to as the hydrological sensitivity parameter η. The estimates herein show that η is more robust than stated in most previous work. The authors demonstrate that adjustments and η estimated from a steplike quadrupling CO2 experiment serve well to predict ΔP in a transient CO2 experiment. The magnitude of η is smaller in the coupled ocean–atmosphere quadrupling CO2 experiment than in the noncoup...

PDRMIP: A Precipitation Driver and Response Model Intercomparison Project—Protocol and Preliminary Results

As the global temperature increases with changing climate, precipitation rates and patterns are affected through a wide range of physical mechanisms. The globally averaged intensity of extreme precipitation also changes more rapidly than the globally averaged precipitation rate. While some aspects of the regional variation in precipitation predicted by climate models appear robust, there is still a large degree of inter-model differences unaccounted for. Individual drivers of climate change initially alter the energy budget of the atmosphere leading to distinct rapid adjustments involving changes in precipitation. Differences in how these rapid adjustment processes manifest themselves within models are likely to explain a large fraction of the present model spread and needs better quantifications to improve precipitation predictions. Here, we introduce the Precipitation Driver and Response Model Intercomparison Project (PDRMIP), where a set of idealized experiments designed to understand the role of different climate forcing mechanisms were performed by a large set of climate models. PDRMIP focuses on understanding how precipitation changes relating to rapid adjustments and slower responses to climate forcings are represented across models. Initial results show that rapid adjustments account for large regional differences in hydrological sensitivity across multiple drivers. The PDRMIP results are expected to dramatically improve our understanding of the causes of the present diversity in future climate projections.

Weak hydrological sensitivity to temperature change over land, independent of climate forcing

We present the global and regional hydrological sensitivity (HS) to surface temperature changes, for perturbations to CO2, CH4, sulfate and black carbon concentrations, and solar irradiance. Based on results from ten climate models, we show how modeled global mean precipitation increases by 2–3% per kelvin of global mean surface warming, independent of driver, when the effects of rapid adjustments are removed. Previously reported differences in response between drivers are therefore mainly ascribable to rapid atmospheric adjustment processes. All models show a sharp contrast in behavior over land and over ocean, with a strong surface temperature-driven (slow) ocean HS of 3–5%/K, while the slow land HS is only 0–2%/K. Separating the response into convective and large-scale cloud processes, we find larger inter-model differences, in particular over land regions. Large-scale precipitation changes are most relevant at high latitudes, while the equatorial HS is dominated by convective precipitation changes. Black carbon stands out as the driver with the largest inter-model slow HS variability, and also the strongest contrast between a weak land and strong sea response. We identify a particular need for model investigations and observational constraints on convective precipitation in the Arctic, and large-scale precipitation around the Equator.Climate change: Global warming increases rainfall most over oceansGlobal warming leads to more rain – but little of the change occurs over land. An international team of researchers, led by Bjørn H. Samset at the Norwegian CICERO Center for Climate Research, used ten global climate models to study how precipitation changes when just one factor in the climate system was allowed to change at a time. While models tend to give very different predictions of future rainfall for realistic scenarios, changes due solely to greenhouse gases, aerosols, or the amount of incoming sunlight, give clearer results. Overall, the amount of rain over oceans increases by 4% per degree Celsius, no matter what caused the surface warming. Over land, the increase is only 1–2%. This difference helps explain why observed rainfall changes over land have so far been modest.

Carbon dioxide physiological forcing dominates projected Eastern Amazonian drying

Future projections of east Amazonian precipitation indicate drying, but they are uncertain and poorly understood. In this study we analyse the Amazonian precipitation response to individual atmospheric forcings using a number of global climate models. Black carbon is found to drive reduced precipitation over the Amazon due to temperature-driven circulation changes, but the magnitude is uncertain. CO2 drives reductions in precipitation concentrated in the east, mainly due to a robustly negative, but highly variable in magnitude, fast response. We find that the physiological effect of CO2 on plant stomata is the dominant driver of the fast response due to reduced latent heating, and also contributes to the large model spread. Using a simple model we show that CO2 physiological effects dominate future multi-model mean precipitation projections over the Amazon. However, in individual models temperature-driven changes can be large, but due to little agreement, they largely cancel out in the model-mean.

Sensible heat has significantly affected the global hydrological cycle over the historical period

Globally, latent heating associated with a change in precipitation is balanced by changes to atmospheric radiative cooling and sensible heat fluxes. Both components can be altered by climate forcing mechanisms and through climate feedbacks, but the impacts of climate forcing and feedbacks on sensible heat fluxes have received much less attention. Here we show, using a range of climate modelling results, that changes in sensible heat are the dominant contributor to the present global-mean precipitation change since preindustrial time, because the radiative impact of forcings and feedbacks approximately compensate. The model results show a dissimilar influence on sensible heat and precipitation from various drivers of climate change. Due to its strong atmospheric absorption, black carbon is found to influence the sensible heat very differently compared to other aerosols and greenhouse gases. Our results indicate that this is likely caused by differences in the impact on the lower tropospheric stability.Precipitation changes are strongly linked to the Earth’s energy budget. Here the authors show that changes in sensible heat are the dominant contributor to the present global-mean precipitation change since pre-industrial time.

Intermodel spread in global and tropical precipitation changes

Precipitation remains among the most poorly represented climate variables in state-of-theart general circulation models. This thesis investigates reasons for intermodel spread in both global-mean precipitation as well as tropical precipitation patterns, and their change with warming. We examine the constraints on global-mean precipitation in experiments of different complexity provided by the Coupled Model Intercomparison Project phase 5 (CMIP5), and explore the intermodel spread in tropical precipitation patterns in idealized aquaplanet simulations from the Clouds On-Off Klimate Intercomparison Experiment (COOKIE). Literature estimates of the rate of global-mean precipitation increase with surface warming disagree about the intermodel spread, reporting either a large or a small spread. Our analysis of this rate in the CMIP5 ensemble corroborates the estimates of a small intermodel spread. The spread is small if the temperature-mediated rate of precipitation increase is explicitly separated from the direct precipitation response to a change in the atmospheric composition; we respectively refer to these quantities as hydrological sensitivity parameter (η) and adjustment (A). The intermodel spread in η arises from disagreement in lowertropospheric temperature and humidity changes in the tropics as well as diverse cloud radiative changes, as revealed by a radiative kernel analysis applied to the atmospheric heat budget. Three factors – η, A and the surface warming – determine the total precipitation change with time. The intermodel disagreement in these three factors affects the spread in the precipitation response (∆P ) on different time scales. Upon changes in the atmospheric composition, in the short term the spread in ∆P is dominated by A, while in the longterm the greatest spread arises from the surface temperature. From merely knowing A and η, the precipitation response in a transient forcing experiment can be replicated with a simple model. Contrary to the recent suggestion that cloud radiative feedbacks are the key reason for intermodel differences in the tropical precipitation response to warming in aquaplanet simulations, we explore the hypothesis that the seed for intermodel spread is present also when the cloud-radiation interaction is inhibited. Indeed, the model spread is greater in the absence of atmospheric cloud radiative effects (ACREs), e.g. in the location of the intertropical convergence zone (ITCZ), or the organization of the tropical circulation. The ITCZ shifts polewards in all models when ACREs are absent, and the tropical distribution of precipitation, which is governed by the vertical velocity, becomes more diverse. We develop a simple framework to diagnose the vertical velocity in the tropics. The framework is derived from the moist static energy (MSE) budget and assumes that the vertical velocity can be expressed by a specified vertical velocity structure associated with deep convection. Then the vertical mean vertical velocity is diagnosed as the ratio between column MSE heating terms and the gross moist stability, which is the vertical advection associated with the specified velocity structure. We find that the zonal-mean variation in the vertical velocity is governed by the column heating rather than the gross moist stability. Further, we employ the framework to understand the poleward shift of the ITCZ when ACREs are removed. The results suggest that the heterogeneity in cloud radiative heating with respect to the clear-sky column MSE heating terms is related to the shift in the ITCZ.