This study investigates the future changes in the climate zones' distribution of the Earth's land area due to increasing atmospheric greenhouse gas concentrations in three IPCC SRES emissions scenarios (A1B, A2 and B1). The Köppen climate classif cation is applied to climate simulations of seven atmosphereocean general circulation models (AOGCMs) and their multi-model mean. The evaluation of the skill of the individual climate models compared to an observation-reanalysis-based climate classif cation provides a f rst order estimate of relevant model uncertainties and serves as assessment for the conf dence in the scenario projections. Uncertainties related to differences in simulation pathways of the future projections are estimated by both, the multi-model ensemble spread of the climate change signals for a given scenario and differences between different scenarios. For the recent climate the individual models fail to capture the exact Köppen climate types in about 24-39 % of the global land area excluding Antarctica due to temperature and precipitation biases, while the multi-model ensemble mean simulates the present day observation-reanalysisbased distribution of the climate types more accurately. For the end of the 21 st century compared to the present day climate the patterns of change are similar across the three scenarios, while the magnitude of change is largest for the highest emission scenario. Moreover, the temporal development of the climate shifts from the end of the 20st century and during the 21 st century show that changes of the multi-model ensemble mean for the A2 and B1 scenario are generally within the ensemble spread of the individual models for the A1B scenario, illustrating that for the given range of scenarios the model uncertainty is even larger than the spread given by the different GHG concentration pathways.The multi-model ensemble mean's projections show climate shifts to dryer climates in the subtropics (Australia, Mediterranean Basin, southern Africa). This is consistent with an increase of area classif ed as Tropical Savanna Climate as well as Dry Climates. Furthermore, there is a poleward extension of the warmer climate types in the northern hemisphere causing a retreat of regions with Cold Climate with Moist Winter and Tundra Climate. The European region shows largest changes comparing the shifts in the different continents (37.1 % of the European land area) as a result of a large extension of the Humid Temperate Climate across eastern and northeastern Europe at the cost of the Cold Climate with Moist Winter.

In simulations of the boreal summer Asian monsoon, generations of climate models show a persistent climatological wet bias over the tropical western Indian Ocean and a dry bias over South Asia. Here, focusing on the monsoon developing stages (May–June), process-based diagnostics are first applied to a suite of NCAR models and reanalysis products. Two primary factors are identified for the initiation and maintenance of the wet bias over the northwestern Indian Ocean (NWIO; 5°–15°N, 52°–67°E): (i) excessive tropospheric moisture and (ii) restrained horizontal advection of the 1000–800-hPa levels cold–dry air couplet that originates offshore of Somalia. Second, guided by the diagnostics, we hypothesized that insufficient dilution of convective updrafts is one possible candidate for model bias and performed a series of enhanced entrainment sensitivity experiments with NCAR CAM4. Over the NWIO, the results suggest that globally increasing the maximum entrainment rate εmax leads to a drier free troposphere, arrests the vertical extension of clouds, and weakens moisture–convection and cloud–radiation feedbacks; each factor contributes to a reduced wet bias. Moreover, a higher εmax leads to a reduced dry bias over South Asia through changes in the local circulation features. In CAM4, improved precipitation climatology due to increased εmax suggests that insufficient dilution is one factor, but not the only one, that contributes to systematic errors. Rather, realistic representation of boundary layer processes in climate models arising out of local ocean–atmosphere interaction processes off Somalia’s coast deserves attention in reducing the NWIO wet bias.

This study assesses the ability of a high‐resolution downscaling simulation with the regional climate model (RCM) HIRHAM5 in capturing the monsoon basic state and boreal summer intraseasonal variability (BSISV) over South Asia with focus on moist and radiative processes during 1979–2012. A process‐based vertically integrated moist static energy (MSE) budget is performed to understand the model's fidelity in representing leading processes that govern the monsoon breaks over continental India. In the climatology (June–September) HIRHAM5 simulates a dry bias over central India in association with descent throughout the free troposphere. Sources of dry bias are interpreted as (i) near‐equatorial Rossby wave response forced by excess rainfall over the southern Bay of Bengal promotes anomalous descent to its northwest and (ii) excessive rainfall over near‐equatorial Arabian Sea and Bay of Bengal anchor a “local Hadley‐type” circulation with descent anomalies over continental India. Compared with observations HIRHAM5 captures the leading processes that account for breaks, although with generally reduced amplitudes over central India. In the model too, anomalous dry advection and net radiative cooling are responsible for the initiation and maintenance of breaks, respectively. However, weaker contributions of all adiabatic MSE budget terms, and an inconsistent relationship between negative rainfall anomalies and radiative cooling reveals shortcomings in HIRHAM5's moisture‐radiation interaction. Our study directly implies that process‐based budget diagnostics are necessary, apart from just checking the northward propagation feature to examine RCM's fidelity to simulate BSISV.

<p>„Klima, Klimawandel und Gesellschaft“ (CLICCS) ist ein DFG-Exzellenzcluster an der Universität Hamburg, in dem gemeinsam mit Partner-Institutionen erforscht wird, wie sich das Klima ändert und mit ihm die Gesellschaft, die somit auf das Klima zurückwirkt. CLICCS umfasst sowohl Grundlagenforschung zur Klima- und Sozialdynamik als auch die transdisziplinäre Untersuchung von Mensch-Umwelt-Wechselwirkungen. Dabei orientiert sich CLICCS an der übergeordneten Frage: „Welche Klimazukünfte sind möglich und welche sind plausibel?“.</p> <p>Drei CLICCS-Projekte konzentrieren sich auf die Entwicklung und Bewertung von Szenarien zur nachhaltigen Anpassung an den Klimawandel auf regionaler Ebene, wo der Klimawandel für den Menschen sichtbar wird und eine nachhaltige Anpassung durch lokale Akteure realisiert werden kann. Eines von ihnen, CLICCS-C1 („Wasser von 4 Seiten“; https://www.cliccs.uni-hamburg.de/de/research/theme-c/c1.html), untersucht die gekoppelte Mensch-Umwelt-Dynamik auf städtischer Ebene und konzentriert sich auf durch den Klimawandel induzierte wasserbedingte Stressfaktoren.</p> <p>Das CLICCS-„Wasser von 4 Seiten“ Projekt zielt darauf ab, einen komplexen integrierten Modellierungsansatz für das städtische System zu entwickeln und anzuwenden, der (i) die wissenschaftliche Bewertung mehrerer wasserinduzierter Auswirkungen auf das städtische System und dessen Rückkopplungen ermöglicht, (ii) für die Entwicklung nachhaltiger Anpassungsszenarien geeignet ist und (iii) bei Entscheidungen unterstützt, in denen die Auswirkungen von Anpassungsszenarien auf das Erreichen der UN-Nachhaltigkeitsziele bewertet werden. Ein wesentliches Merkmal des Modellierungsansatzes ist die Integration der wasserbedingten Stressfaktoren, wie Grundwasseranstieg, Sturmfluten, Flussüberschwemmungen und durch Starkniederschläge ausgelöste Sturzfluten. Die Wechselwirkungen der Stressfaktoren sowohl untereinander als auch mit gesellschaftlichen Komponenten des Stadtsystems spielen dabei eine wichtige Rolle. Im Vortrag werden der Forschungsansatz sowie erste Schritte der Modellentwicklung vorgestellt.</p> <p> </p> <p>Danksagung</p> <p>CLICCS wird gefördert durch die Deutsche Forschungsgemeinschaft (DFG) im Rahmen der Exzellenzstrategie des Bundes und der Länder – EXC 2037 „Klima, Klimawandel und Gesellschaft“ – Projektnummer: 390683824, Dies ist ein Beitrag zum Centrum für Erdsystemforschung und Nachhaltigkeit (CEN) der Universität Hamburg.</p>