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.
Methodology
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)
- 51-member ensemble forecast of 1-h accumulated precipitation generated by Numerical Weather Prediction (NWP) from the ECMWF Ensemble Prediction System (EPS)
The blending technique (Wong et al. 2009) is applied for the first 6 hours and involves 4 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 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
- 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 spatial weighting factor 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 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 NWP ensemble forecast is matched to the closest member of the radar-based nowcast
- The match is computed by calculating the
and applies a phase shift and bias correction to blend radar nowcasts into NWP using a hyperbolic tangent weighting function. For lead times beyond 6 hours, the product relies only on the NWP forecasts. At each timestep this layer shows the 80th percentile of rainfall from the forecast ensemble except for the initial time step which shows the 1h accumulated precipitation derived from the latest observed radar composites.
The colorscale of the layer (Fig. 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.
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Figure 1. 80th percentile of the hourly blended precipitation ensemble forecast at 15:00 UTC on 2022-09-05
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.
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.