Extreme flood modeling and mechanism over Crisul Alb basin in Romania

Abstract

The mechanisms of regional climate extremes and their changes in a transient climate are a real concern in climate research. Moreover, mainly in the Northern Hemisphere these became increasingly threatening in the last years, presenting regionally, higher intensity, persistence and in some cases increased frequency.This paper investigates modeling ability to represent the occurrence and development mechanism of an extreme flooding event over Crisul Alb river basin (Romania) through sensitivity numerical simulations using a very high resolution regional climate model (from 30 km to 3 km) coupled to a hydrological model. We find that the main mechanism leading to the occurrence of severe flooding in the area is a dynamical modulation of the large-scale flow forced by large-regional scale interaction. Under northerly flow, a systematic formation of a secondary undulation of frontal lines, topographically forced, is responsible for the system steadiness and rotation with precipitation recurrence over the area and event amplification. The skill in representing these extreme events comes, as shown by the experiments, from an accurate representation of the large-scale information transmission that has an impact on the location of the event, from the representation of the diabatic large-scale sources into the inner domain with main impact on the timing of the development and from the inner domain resolution that has a strong impact in reproducing the dynamical modulation process and the amplitude of the event. This scale-interaction mechanism was validated on a 30 years data-basis of observed flooding events over the same domain, emphasizing that the key preconditioning found, remains valid. Further, the large-scale drivers of this mechanism were identified, providing potential operational flood prediction support, while their trends and variability's changes provide indications for near-future expectancy of extreme floods in the region.