Webinar, h. 12:30 pm - May 30, 2017

CMCC Division: OPA Division

Speaker
Giorgia Verri,
Fondazione Centro Euro-Mediterraneo sui Cambiamenti Climatici (CMCC – OPA Division)

Discussant
Edoardo Bucchignani
Fondazione Centro Euro-Mediterraneo sui Cambiamenti Climatici (CMCC – REMHI Division) 

Moderator
Silvia Torresan
Fondazione Centro Euro-Mediterraneo sui Cambiamenti Climatici (CMCC – RAAS Division) 

AGENDA:
12:30 - Welcome and intro – Silvia Torresan – RAAS Division
12:35 – Speaker’s talk, Giorgia Verri – OPA Division
13:05 – Discussant’s comments, Edoardo Bucchignani – REMHI Division
13:10 – Q&A

The regional ocean models generally treat the riverine freshwater release in an oversimplified way by means of climatological runoff, based on gauges located far from river outlets, and zero or at most constant salinity values corresponding to the runoff (MacCready and Geyer, 2010). Doing this the water exchange into the estuaries, owing to the ocean water entrainment, is not represented. Our goals are (i) a realistic representation of the riverine freshwater release into coarse resolution ocean models by taking into account the estuarine water exchange and (ii) a proper reconstruction of the resulting dynamics in the ROFIs (i.e. Regions Of Freshwater Influence), the shelf areas adjacent to the estuaries. Three approaches of estuarine dynamics are tested and compared: the simple Knudsen’s relation (Knudsen, 1900), an upgraded version of the Knudsen’s relation we developed with the addition of tidal effects, named CMCC Estuary Box Model (Verri et al., in prep.), and an estuary box model developed by the University of Connecticut, UCONN, jointly with the National Center for Atmospheric Research, NCAR (Qiang et al., in prep), hereafter named UCONN-NCAR EBM. In all the approaches, we assume that the estuary is a two-layer rectangular box and the estuarine dynamics is tidal-cycle averaged with riverine freshwater entering through the upper layer at the estuary head while at the estuary mouth the ocean water enters through the lower layer and estuarine water outflows through the upper layer. This is a rigorous representation for well stratified estuary (Geyer and Smith, 1987) while may be lacking for highly mixed estuaries. We consider the highly stratified estuary of the Ofanto River and the partially mixed delta of the Po river as our case studies. The estuary box is assumed to be coupled in 1-way mode at the estuary mouth with a regional ocean model, based on NEMO code (Madec, 2008) and covering the Central Mediterranean Sea. The river runoff entering the estuary head is based on gauged records for the Po river and on simulated river runoff by WRF-Hydro model (Gochis et al., 2013) for the Ofanto river. The sensitivity tests on the estuary dynamics pointed out that the tidal pumping strongly affects the salinity and volume flux of the resulting outflowing estuarine water. The explicit representation of the tidal and shear mixing into the estuary is crucial as well. Moreover a set of sensitivity experiments has been performed on the coastal dynamics off the estuaries by forcing the regional ocean model with “prescribed” runoff and salinity (based on observations along river network and/or climatologies), or with the “predicted” runoff and salinity provided by the different approaches for representing the estuarine dynamics. For the Ofanto river case, the modeling results in the ROFI show that the added value of  representing the estuarine dynamics and its effect on the coastal dynamics becomes particularly powerful when the upwelling wind regime characterizes the shelf area off the estuary with a resulting well defined river plume (Chao, S. Y., 1987; Kourafalou et al., 1996). For the Po river case, UCONN-NCAR EBM and CMCC EBM coupled with the ocean are both capable to reconstruct the permanent river plume and they outperform the “prescribed” approach. The validation carried out by comparing the model results with the recorded salinity at a buoy in the Po ROFI shows promising performance of the CMCC EBM approach.

Working language: English

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  • CMCC - Centro Euro-Mediterraneo sui Cambiamenti Climatici