An extremely intense precipitation event hit the Venice Lagoon and the neighboring coastal zone in the early hours of September 26, 2007, with precipitation amounts exceeding 320 mm recorded in the Venetian mainland between 06 UTC and 12 UTC. This flash-flood was the result of a mesoscale convective system formed in a convergence area between a north-easterly wind coming from the Alps and a south-easterly flow over the Adriatic basin. In this study we describe the flash-flood event using the modeling suite COAWST (Couple Ocean Atmospheric Wave Sediment Transport system), exploring the importance of various oceanic components (Sea Surface Temperature, mixed layers depth, waves, etc.) on the atmospheric evolution. COAWST is a complex numerical system that couples the atmospheric model WRF (Weather Research ad Forecasting), the ocean model ROMS (Regional Oceanographic Model), and the wave model SWAN (Simulating Wave in Nearshore). The configuration of the WRF consists of 3 domains respectively at 25 km, 5 km, and 1 km of horizontal resolution. ROMS and SWAN models were run at a 1 km resolution with computational grid that covers all the Adriatic Sea. Using a standard configuration, the first phase of the study used WRF in "standalone model", using the SST obtained from the spectrum-radiometer RTG_SST at 8.3 km resolution. As a second step, the same WRF numerical configuration received the SST derived from an "uncoupled" ROMS simulation (produced after a long "spin-up" over the same grid used for the coupling at 1 km of horizontal resolution). In the third stage of the work, 2-way coupled numerical runs WRF-ROMS (run AO) and WRF-ROMS-SWAN (run AOW) were performed. The use of a low-resolution (in space and time) SST dataset has a significant impact on the timing of the phenomenon, since the satellite data have a generally poor performance in capturing thermal patterns in coastal regions. In contrast, a properly calibrated ocean model can provide a more realistic SST distribution. Lastly, a coupled description of atmosphere and ocean dynamics warrants the energetic consistency between the two systems and modifies the distribution of heat and humidity in the Planetary Boundary Layer (in particular when the coupling with the wave model is also activated). Results suggest that the use of high-resolution Sea Surface Temperature (SST) is crucial to realistically simulate this type of events, as the over/under estimation of the SST in the basin affects the wind intensity and consequently the spatial localization of the event, together with the precipitation intensity. Besides supporting a deeper insight in the physical processes underlying these intense phenomena, in the considered event the use of model coupling led to an unprecedented performance in reproducing recorded precipitation patterns and intensity.
On the use of a high-resolution atmosphere-ocean-wave coupled model to describe a flash-flood event over the North-East Italy
A Ricchi;D Bonaldo;S Carniel
2018
Abstract
An extremely intense precipitation event hit the Venice Lagoon and the neighboring coastal zone in the early hours of September 26, 2007, with precipitation amounts exceeding 320 mm recorded in the Venetian mainland between 06 UTC and 12 UTC. This flash-flood was the result of a mesoscale convective system formed in a convergence area between a north-easterly wind coming from the Alps and a south-easterly flow over the Adriatic basin. In this study we describe the flash-flood event using the modeling suite COAWST (Couple Ocean Atmospheric Wave Sediment Transport system), exploring the importance of various oceanic components (Sea Surface Temperature, mixed layers depth, waves, etc.) on the atmospheric evolution. COAWST is a complex numerical system that couples the atmospheric model WRF (Weather Research ad Forecasting), the ocean model ROMS (Regional Oceanographic Model), and the wave model SWAN (Simulating Wave in Nearshore). The configuration of the WRF consists of 3 domains respectively at 25 km, 5 km, and 1 km of horizontal resolution. ROMS and SWAN models were run at a 1 km resolution with computational grid that covers all the Adriatic Sea. Using a standard configuration, the first phase of the study used WRF in "standalone model", using the SST obtained from the spectrum-radiometer RTG_SST at 8.3 km resolution. As a second step, the same WRF numerical configuration received the SST derived from an "uncoupled" ROMS simulation (produced after a long "spin-up" over the same grid used for the coupling at 1 km of horizontal resolution). In the third stage of the work, 2-way coupled numerical runs WRF-ROMS (run AO) and WRF-ROMS-SWAN (run AOW) were performed. The use of a low-resolution (in space and time) SST dataset has a significant impact on the timing of the phenomenon, since the satellite data have a generally poor performance in capturing thermal patterns in coastal regions. In contrast, a properly calibrated ocean model can provide a more realistic SST distribution. Lastly, a coupled description of atmosphere and ocean dynamics warrants the energetic consistency between the two systems and modifies the distribution of heat and humidity in the Planetary Boundary Layer (in particular when the coupling with the wave model is also activated). Results suggest that the use of high-resolution Sea Surface Temperature (SST) is crucial to realistically simulate this type of events, as the over/under estimation of the SST in the basin affects the wind intensity and consequently the spatial localization of the event, together with the precipitation intensity. Besides supporting a deeper insight in the physical processes underlying these intense phenomena, in the considered event the use of model coupling led to an unprecedented performance in reproducing recorded precipitation patterns and intensity.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
