Radar-based Accumulated Precipitation 80th Percentile

Please note that this product is experimental and is not operationally supported, meaning that there may be some gaps in the availability on the EFAS web interface due to outages.

The radar-based accumulated precipitation 80th percentile layer is an animation that shows the 80th percentile of the ensemble forecasted precipitation accumulated in each hour (for the first 6 hours of the forecast lead time) and every 6-hours for lead times from 6 to 120 hours.

The colorscale of the layer (Figure 1) relates to the rainfall accumulation within the timestep that is being visualised. For the first 6 hours of the forecast the layer shows the hourly rainfall accumulation, thereafter the accumulation is 6 hourly. The length of the timestep that is currently being visualised is displayed in a box just above the animation slider. Note that the colourscale does not change to accomodate for the change in the forecast timestep, therefore users may notice what looks like an increase in rainfall accumulation when moving from the 1-hour to the 6-hour timestep.

Figure 1. The Radar-based Accumulated Precipitation 80th percentile product for the forecast of the 25th February 2025 at 11:00 UTC. a) Shows the product when zoomed out at pan-European scale, b) more spatial detail appears when zoomed-in

Visualising within the EFAS Webviewer

The layer can be viewed after selecting it from the Flash Flood layers tab within the EFAS webviewer. An animation slider box will appear in the bottom left of the screen and by default the data from the most recent forecast will be loaded. A drop down menu just above the animation slider box can be used to select a different forecast date and time within the past 5 days.

The buttons on the left of the animation slider can be used to step forwards and backwards in time for each timestep of the forecast. The play button will play the animation in a continuous loop.   When animating each timestep, it can take a few seconds to load the next timestep, this is shown by a loading icon which appears within the animation slider which will disappear once the next timestep has been loaded. 

Methodology

Blended Precipitation for Lead Times 0-6 hours

For the first 6-hours of the forecast lead time, the precipitation accumulations have been obtained by the blending of the:

• 20-member ensemble nowcast of 1-h accumulated precipitation, generated using SBMcast (Berenguer et al., 2011) using the gauge-adjusted rainfall estimates from the pan-European OPERA radar composite (Park et al. 2019). These have a spatial resolution of 2 km.
• 51-member ensemble forecast of 1-h accumulated precipitation generated by Numerical Weather Prediction (NWP) from the ECMWF Ensemble Prediction System (EPS). These data have a spatial resolution of 9 km, they are interpolated onto the same 2 km resolution as the nowcast data using a nearest neighbour method.

The blending technique (Wong et al. 2009) is applied for the first 6 hours and involves 6 steps:

• Phase shifting the NWP forecasted precipitation to ensure a closer between the locations of rain in the NWP and radar nowcasts
  • For each member of the NWP forecast, an optical flow method is used to compute the phase shift in the first 1-hour time step between the forecasted precipitation in the NWP ensemble member and the first ensemble member of the nowcast 
  • The computed phase shift is applied to the accumulated precipitation forecasted by the NWP for each hourly time step up to a lead time of 6 hours
• Bias correcting the NWP forecasted precipitation:
  • For each member of the NWP forecast, a scale factor is computed in the first 1-hour time step in each grid cell between the accumulated precipitation from the first member of the ensemble nowcast and the NWP forecast and the first member of the ensemble nowcast at the first time step is computed
    • A Gaussian filter is applied to the scale factor grid which was computed above
    • The phase shift corrected accumulated precipitation forecasted by the NWP in each time step, up to a maximum lead time of 6 hours, is multiplied by the scale factor
• A spatial weighting factor, w_s ,  is computed by applying a hyperbolic tangent function to the Radar Coverage product to give greater weight to the radar nowcast data in areas which are closer to the locations of radar stations
• A temporal weighting factor, w_t , is computed for each 1-hour time step of the 6-hour forecast lead time using a hyperbolic tangent function. This gives more weight to the accumulated precipitation from the NWP forecast at longer lead times
• Each member of the phase shifted and bias corrected NWP ensemble forecast (see the first two steps) is matched to the member of the radar-based nowcast which has the smallest difference - computed as the sum of the square differences divided by the sum of the normalised square differences
• A blended accumulated precipitation forecast, Tp_Blend , is produced by adding each ensemble member, m, of the phase shifted and bias corrected NWP precipitation forecast, Tp_NWP ,  to the precipitation from the closest matching member of the radar-based nowcast, Tp_Nowcast . Spatial and temporal weighting factors (w_s and w_t respectively) are multiplied to both the NWP and nowcast precipitation forecasts. This produces a blended forecast of accumulated precipitation at each 1-hour time step, t, for the first 6 hours lead time:
  • Tp_{Blend t,m} = Tp_{Nowcast t,m} * w_s*w_t + Tp_{NWP t,m} *(1-w_s*w_t)

NWP Precipitation for Lead Times 6-120 hours

For lead times from 6-120 hours, the accumulated precipitation is only from the 51-members ECMWF ensemble NWP forecast. This dataset has a spatial resolution of 9 km, it is regridded onto the same 2 km grid as the radar-based nowcasts using a nearest neighbour method. The forecasts at this lead time range have a time step of 6-hours. 

Calculating the 80th Percentile

At each 1-hour time step for the first 6-hours lead time, the accumulated precipitation associated with the 80th percentile is computed from the 51-member blended forecast. For each 6-hour time step for lead times from 6 to 120-hours, the accumulated precipitation associated with the 80th percentile is computed from the 51-member ECMWF NWP forecasts.

References

Berenguer, M., Sempere-Torres, D. and Pegram, G, 2011: SBMcast - An ensemble nowcasting technique to assess the uncertainty in rainfall forecasts by Langrangian extrapolation. Journal of Hydrology, 404(3), 226-240

Park, S., Berenguer, M. and Sempere-Torres, D, 2019: Long-term analysis of gauge-adjusted radar rainfall accumulations at European Scale. Journal of Hydrology, 573, 768-777

Wong, W., Yeung, L., Wang, Y. and Chen, M, 2009: Towards the Blending of NWP with Nowcast - Operation Experience in B08FDP. WMO Symposium on Nowcasting, 30 Aug- 4 Sep, Whistler, Canada.