1.
Ruben Imhoff
Thanks to: Aart Overeem, Albrecht Weerts, Claudia Brauer, Hidde Leijnse,
Klaas-Jan van Heeringen, Lesley de Cruz en Remko Uijlenhoet,
Delft-FEWS International User Days
9 November 2022
Rainfall nowcasting for flood
early warning

2.
Contents
• What is nowcasting?
• The value of nowcasting for rainfall and discharge forecasting
• Seamless forecasting with blending
• Operational water management applications
2
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

3.
Numerical weather prediction models not sufficient on
short lead times for flood early warning
3
Discharge
Time
Obs
Forecast
Effect on discharge
forecast
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

4.
Possible solution for flood early warning: nowcasting
4
Observation-based
forecasts
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

5.
What is nowcasting?
5
From most recent
weather radar
observations
To rainfall forecast up
to 6 hr ahead
Statistical extrapolation Fast! But no physics involved..
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

6.
What is nowcasting?
6
Determine
movement of
rainfall fields
Statistical extrapolation Fast! But no physics involved..
Source: Pulkkinen et al., Geosci. Model. Dev., 2019
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

7.
What is nowcasting?
7
Determine
movement of
rainfall fields
Development
of rainfall
field
Statistical extrapolation Fast! But no physics involved..
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

8.
Determine
movement of
rainfall fields
Development
of rainfall
field
Extrapolate
to future
Map on 2D
grid
What is nowcasting?
8
Statistical extrapolation Fast! But no physics involved..
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

9.
The skill of nowcasting
9
Added value up to 2 hours
into the future
Depends on:
• Rainfall type (season)
• Duration of event
• Location of catchment
• Catchment size
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

10.
Resulting discharge forecasts
Geul catchment, July 14, 2021 – issue time 07:00 UTC
10
UTC
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

11.
Resulting discharge forecasts
Geul catchment, July 14, 2021 – issue time 07:00 UTC
11
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

12.
Resulting discharge forecasts
Geul catchment, July 14, 2021 – issue time 07:00 UTC
12
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

13.
Can we even go a step further?
13
Blending: optimal
combination between
nowcasting and NWP
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

14.
Simple linear blending?
14
How to combine
a case like this?
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

15.
An advanced blending method (STEPS) – included in the
open-source pysteps framework
15
Blending per spatial scale:
Level 1
Nowcast
NWP
Stochastic
perturbation
W1
e x
W1
n x
W1
ε x
+
+
+
W2
e x
W2
n x
W2
ε x
+
+
Level 2
+ +
[…]
[…]
[…]
W7
e x
W7
n x
W7
ε x
+
+
Weights depend on initial skill and
expected future skill
Per ensemble member:
Level 7

16.
An advanced blending method
16
Project together with Royal
Meteorological Institute,
Belgium
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

17.
Resulting discharge forecasts
Geul catchment, July 14, 2021 – issue time 07:00 UTC
17
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

18.
Resulting discharge forecasts
Geul catchment, July 14, 2021 – issue time 07:00 UTC
18
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

19.
Open-source pysteps nowcast runs through Delft-FEWS
19
March 15, 2021 00:45 UTC Observation
Nowcast
+15 hours ahead
Dwarsdiep (Water board Noorderzijlvest)
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

20.
Source weather radar image: https://en.wikipedia.org/wiki/ARMOR_Doppler_Weather_Radar
A glance at the forecasting chain
20
Discharge
Time
Radar QPE
• Availability of
weather radar
observations
• Bias correction
of QPE
Rainfall
forecasting
• Nowcasting with
(corrected) QPE
product
• Blending
method to
optimally
combine
nowcasting and
NWP
Flood
forecasting
• Improvement
with nowcasting
for first hours
• Blending often
best of both
worlds
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

21.
Requirements to apply nowcasting
21
Nowcasting Seamless forecasting
Blending between nowcasts and (multi-model
/ensemble) NWP forecast
• Same as for nowcasting
• High-resolution NWP forecasts in time (ideally
same frequency as radar)
• Even better: high update frequency of NWP
forecast
• QPE from radar or other source on 2D (/3D)
grid
• Rain gauge observations to bias adjust QPE
• Nowcasting algorithm, for instance pysteps
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

22.
Focus areas that could profit from a nowcasting system
22
Local rain
showers
(missed by
NWP)
Still challenging to
forecast severe
thunderstorms well..
Urban
catchments
The smaller the spatial
and temporal scale, the
bigger the advantage of
nowcasting
Quickly
responding
and
mountainous
catchments
Real-time control
opportunities
Delft-FEWS
User
Days
2022
|
Rainfall
nowcasting
for
flood
early
warning

23.
Thanks!
Ruben.Imhoff@deltares.nl