Using nowcasting technique and data assimilation in a meteorological model to improve very short range hydrological forecasts

Forecasting flash floods some hours in advance is still a challenge, especially in environments made up of many small catchments. Hydrometeorological forecasting systems generally allow for predicting the possibility of having very intense rainfall events on quite large areas with good perfor- mances, even with 12–24 h of anticipation. However, they are not able to predict the exact rainfall location if we consider portions of a territory of 10 to 1000 km2 as the order of mag- nitude. The scope of this work is to exploit both observations and modelling sources to improve the discharge prediction in small catchments with a lead time of 2–8 h. The models used to achieve the goal are essentially (i) a probabilistic rainfall nowcasting model able to ex- trapolate the rainfall evolution from observations, (ii) a non-hydrostatic high-resolution numerical weather predic- tion (NWP) model and (iii) a distributed hydrological model able to provide a streamflow prediction in each pixel of the studied domain. These tools are used, together with radar ob- servations, in a synergistic way, exploiting the information of each element in order to complement each other. For this pur- pose observations are used in a frequently updated data as- similation framework to drive the NWP system, whose out- put is in turn used to improve the information as input to the nowcasting technique in terms of a predicted rainfall vol- ume trend; finally nowcasting and NWP outputs are blended, generating an ensemble of rainfall scenarios used to feed the hydrological model and produce a prediction in terms of streamflow. The flood prediction system is applied to three major events that occurred in the Liguria region (Italy) first to pro- duce a standard analysis on predefined basin control sections and then using a distributed approach that exploits the ca- pabilities of the employed hydrological model. The results obtained for these three analysed events show that the use of the present approach is promising. Even if not in all the cases, the blending technique clearly enhances the prediction capacity of the hydrological nowcasting chain with respect to the use of input coming only from the nowcasting technique; moreover, a worsening of the performance is observed less, and it is nevertheless ascribable to the critical transition be- tween the nowcasting and the NWP model rainfall field.