Transcript Verlagerung von RADAR-Daten mit LM

A new Method for the
Nowcasting of Precipitation
using Radar and NWP Data
Tanja Winterrath
[email protected]
WSN05 - Toulouse
Tanja Winterrath
05.09.2005
-1-
Overview
 Overview of Radar Nowcasting at DWD
 Method
 Clutter Filter
 Divergence of the Wind Field
 Examples
 Summary and Outlook
WSN05 - Toulouse
Tanja Winterrath
05.09.2005
-2-
Radar Nowcasting at DWD
- Convective: KONRAD = recognition and
linear extrapolation of cells (max. +1h)
- Stratiform: Rosenow = pattern recognition and
linear extrapolation of image structures (max. +2h)
- Blending of the results
- Latent Heat Nudging in NWP (LMK)
Forecast lead time
after Golding (1998)
WSN05 - Toulouse
Tanja Winterrath
05.09.2005
-3-
The new Approach...
 Aim
Extending the forecast beyond 2 hours
 Basic Approach
Displacement of radar precipitation data
with horizontal LM wind fields
 non-linear and temporally variable
displacement of precipitation fields
 external wind fields in contrast to directly
derived displacement vectors
WSN05 - Toulouse
Tanja Winterrath
05.09.2005
-4-
Input data
 Radar product
- Germany composite
- Precipitation amount, derived with improved
Z-R relationship (so-called RZ product)
 NWP product
- Horizontal wind components taken from
LM (Lokalmodell, 7 km) or LM-K (2.8 km)
WSN05 - Toulouse
Tanja Winterrath
05.09.2005
-5-
Method
 Horizontal LM wind field
- Choice of a suitable pressure level
- Transformation and spatial interpolation onto
radar-,grid‘ (1 km x 1 km)
- Temporal interpolation in each time step of the
advection scheme
 Displacement of the precipitation field
- 2d Eulerian advection scheme (Bott, 1993)
- Time step approx. 30-45 seconds (CFL criterion)
WSN05 - Toulouse
Tanja Winterrath
05.09.2005
-6-
Problems and Solutions
 Numerical diffusion leads to a smoothing of
clutter pixels
 Clutter filter
 Unrealistic stretching and compression of
precipitation patterns (i.e., creation of new extrema)
 Elimination of divergences of the wind field
WSN05 - Toulouse
Tanja Winterrath
05.09.2005
-7-
Clutter Filter
 Development of a procedure to completely
eliminate clutter pixels from input data
 Basic approach:
- Marking of all pixels, the 31x31surrounding square of which contains more
than 85% of pixels with a value of
1. less than 45% of the value of the centre pixel
2. < 0.01 mm/h.
- Replacement of the centre pixel‘s value by
1. the mean of the data pixels within
the surrounding square.
2. zero.
WSN05 - Toulouse
Tanja Winterrath
05.09.2005
-8-
Clutter Filter
Results
Original
Radar Data
Radar Data after
Clutter Elimination
WSN05 - Toulouse
Tanja Winterrath
05.09.2005
-9-
Divergence-free Wind Field
 Minimisation of the functional
 2
 u v 
0 2
2
0 2
E (u, v,  )    u  u   v  v     dxdy
 x y 
A


 Iterative Solution:


(after Sherman, 1978)
- Gauß-Seidel
- Successive Over-Relaxation (SOR)
- Chebyshev Acceleration
WSN05 - Toulouse
Tanja Winterrath
05.09.2005
-10-
Horizontal
LM wind field
original data
Horizontal
LM wind field
divergence-free
Difference
Shown are the absolute values of the wind velocities;
the mean relative change is approx. 10%.
WSN05 - Toulouse
Tanja Winterrath
05.09.2005
-11-
Example
 18.08.04, 500 hPa
 Start time = 20:00
 Hourly successive...
... radar measurements:
WSN05 - Toulouse
... model results:
Tanja Winterrath
05.09.2005
-12-
1. Row: Measurements; 2. Row: 500 hPa; 3. Row: 700 hPa
+1h
+2h
+3h
+4h
1
3
2
+1h, 700 hPa
WSN05 - Toulouse
+2h, 700 hPa
4
+3h, 700 hPa
Tanja Winterrath
+4h, 700 hPa
05.09.2005
-13-
Summary
 Aim: Extension of the precipitation nowcast
beyond 2 hours to close the gap
between linear extrapolation and model
nowcast
 Status:
- Effective clutter filter realised
- Area-preserving advection due to
divergence-free wind field
... Work in progress...
WSN05 - Toulouse
Tanja Winterrath
05.09.2005
-14-
Outlook
 Validation and Verification
 Determination of optimal pressure
level(s) for wind field extraction
 Determination of cross-over times to
linear extrapolation and NWP
 Introduction into operational process chain
Poster No. 2.39 on display now!
WSN05 - Toulouse
Tanja Winterrath
05.09.2005
-15-