Fully Coupled Hydrometeorological Modeling of Extreme Weather Events over Complex Orography Areas: Applications of WRF and WRF-Hydro Model Configurations over the Tiber River Basin.

Historically, the atmosphere and the terrestrial hydrologic cycles have not been well integrated in terms of coupled modeling systems. In the past, the precipitation and runoff relationship has been maintained separately in a one-way cause and effect relationship. Recently, the need for improving hydro-meteorological predictions for flood, droughts and water resources has become more critical and has promoted a full two-way coupled atmospheric-hydrologic approach. Fully coupled high-resolution models, such as the WRF/WRF-Hydro system, are new-generation tools designed to link multi scale processes of the atmosphere and terrestrial hydrology and to perform coupled and uncoupled multi-physics simulation at different spatial and temporal scales. The improved representation of lateral redistribution and infiltration runoff and exfiltration processes provide a more complete depiction of terrestrial hydrologic states and fluxes which influence land-atmosphere energy exchanges.

In this study, the use of WRF-Hydro fully coupled model is compared to the classical WRF stand alone meteorological approach to investigate situations in which the fully coupled configuration may provide tangible improvement in the study of precipitation events. The experiment has been performed over the Tiber river basin, one of the most important catchments in central Italy. The Tiber river basin is characterized by the presence of the Appennine mountain range for most of its basin area extension and by elevations reaching 2500 m above sea level. The comparison of the WRF stand alone approach with the WRF-Hydro fully coupled application has been carried at the seasonal scale, using a one year simulation period for 2012.

The two model approaches (stand-alone WRF vs. fully coupled WRF/WRF-Hydro) are explored for different time scales (from single event to seasonal scales) in order to investigate: 1) the capability of each model suite to describe the physical processes leading to extreme rainfall in complex topography 2) the sensitivity of each model configuration to the choice of different parameterizations and the importance of a correct model calibration for both atmospheric and hydrological needs 3) the possible improvement in predictability by comparing WRF stand alone and WRF-Hydro model applications 4) the relative importance of improved dynamics (higher resolutions, correct choice of parameterizations etc.) or a better representation of the physical processes (meteorological-only approach Vs. fully coupled hydro-meteorological WRF-Hydro simulations) in the representation of selected hydrometeorological events.

The simulations of the fully coupled hydrometeorological experiment reveal more reactive soil moisture dynamics and a higher contribution of evapotranspiration to the water balance with consequent minor production of surface runoff and higher underground runoff. Despite an overall modest feedback effect on the occurrence of extreme precipitation events, the overall simulations show a good agreement with the available observations and represent an added value in the study of the water cycle.