Future change in precipitation seasonality over the Horn of Africa in high-resolution simulation

<p>Extreme weather creating a widespread humanitarian crisis over East Africa in recent decades. The seasonal cycle of precipitation over the Horn of Africa (HOA) shows bimodality with long rain and short rain. Most of the models fail to capture biannual rainfall seasonal cycles, due to circulation response to unrealistically dominate the annual mean. The Community Earth System Model (CESM) high-resolution model simulation has been employed to study the sensitivity. Precipitation distribution over HOA shows regional variations where most of the region show the bimodal distribution and the intrinsically complex. This bimodality is nominally associated with tropical rain belt, but topography and SST-forcing also play an important role in influencing the timing and intensity of seasonal rainfall. The results show that overall rainfall seasonality is increased, with intensification over high elevation. Precise representation of rainfall seasonal cycle over HOA adds confidence for future projected changes in seasonality. An important question is whether and how the seasonal cycle over HOA responds to anthropogenic forcing. We show that the future change in precipitation seasonal cycle and accumulation over HOA can be explained by the surface ocean process which module SSTs along the coastline of Somalia. The moisture convergence over low elevation land is basically regulated through the north-south SST gradient. In conclusion, future global warming leads to the intensified seasonal cycle of precipitation with a projected increase in the short rain season over east Africa. Further analysis demonstrates how topography modulates the seasonality of HOA.</p>

Flood type classification and assessment of their past changes across Europe

This study was carried out to establish the characteristics of observed flood events across Europe in the past in terms of their spatial extent and the processes leading up to the events. Daily discharge data from more than 745 stations of the Global Runoff Data Centre were used to identify peak flows at each station for the period 1961-2010. The identified events at the different stations were further analysed to determine whether they form the same flood event, thereby delineating the spatial extent of the flood events. A pan-European hydrological model was employed to estimate a set of catchment hydrological and hydro-meteorological state variables that are relevant in the flood generation process for each of the identified spatially delineated flood events. A subsequent clustering of the events based on the simulated state variables was used to identify the flood generation mechanism of each flood event. Four general flood generation mechanisms were identified: long-rain flood, short-rain flood, snowmelt flood, and rain-on-snow flood. A trend analysis was performed to investigate how the frequency of each of the flood types has changed in time over the investigation period. In order to investigate whether there is a regional and seasonal pattern in the dominant flood generating mechanisms, this analysis was performed separately for winter and summer seasons and five different regions of Europe: Northern, Western, Eastern, Southern Europe, and the Alps. Continentally, the total number of flood events didn’t show a significant change. However, the frequency of winter long rain events increased significantly while that of summer rain-on-snow events decreased significantly over the investigation period. Regional differences were detected in the dominant flood generating mechanism and the corresponding trends. In Western Europe, the frequency of both winter and summer rainfall events increased significantly. In Northern and Eastern Europe, the frequency of summer rain-on-snow events decreased significantly. In addition, winter short rainfall events increased significantly in Eastern Europe. In the Alps, the frequency of summer short rain events increased significantly.