presentation overview
DESCRIPTION
Daily Mapping of 24 hr Rainfall at Pixel Scale over South Africa using Satellite, Radar and Raingauge Data. Geoff Pegram 1 , Izak Deyzel 2 , Pieter Visser 2 , Deon Terblanche 2 , Scott Sinclair 1 & George Green 3 Civil engineering, University of KwaZulu-Natal, DURBAN, RSA - PowerPoint PPT PresentationTRANSCRIPT
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Daily Mapping of 24 hr Rainfall at Pixel Scale over South Africa using
Satellite, Radar and Raingauge Data
Geoff Pegram1, Izak Deyzel2, Pieter Visser2, Deon Terblanche2, Scott Sinclair1 &
George Green3
1. Civil engineering, University of KwaZulu-Natal, DURBAN, RSA2. METSYS, South African Weather Services, BETHLEHEM, RSA3. Deputy Director, Water Research Commission, PRETORIA,
RSA
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Presentation Overview
• Objectives• Satellite information as a data source
– Producing a satellite rainfall map for South Africa– Validation of Satellite Rainfall Fields
• Use Gauge and Radar information to augment• Producing the Merged Rainfall Field• Verification of the Merged Rainfall Fields• How do we improve the product?• A spin-off is the ground-truthing of satellite data
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ObjectivesSouth Africa has limited resources:
– Sparse raingauge network– Patchy C-band radar coverage (non-Doppler)
SO … • Use Satellite data to derive a Daily Rainfall Field
over South Africa at highest spatial resolution• Combine rainfall estimates from METEOSAT,
Rain Gauges and Radar to produce a Spatially Interpolated Mapping of Rainfall (SIMAR) over South Africa
• Constantly seek ways to improve these estimates
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Producing a Satellite Rainfall Map for South Africa
• Overview of the Multi-Spectral Rain-Rate (MSRR) technique
• Flow diagram of the MSRR algorithm layout
• Components of the MSRR algorithm, particularly classification by texture
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METEOSAT data used in SIMAR
++ ++
==
VIS
WV IR
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Multi-Spectral Rain-Rate EstimationA: Mask out non-raining information
– Cirrus, sun, speckles– Separate topography – cold versus warm coastal rain
• When available, use VIS, WV & IR data to define mask
• Use texture analysis to identify potential rain• Use image processing techniques: median filtering
and edge detection to sharpen mask and clean up
B: Use IR to estimate cold, intermediate and warm (coastal) rain
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Infrared and Water Vapor Spectral Difference Cloud Mask
• Negative Infrared and Water Vapor spectral difference field• Spatial Correspondence to strong Radar echoes• Mask =1 for deep moist cold cloud areas
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Exploit Texture to Improve Estimate
• Compute the Grey-Level Co-occurrence matrix (GLCM) at every point in the field
• Thence compute the Angular Second Moment (ASM) at every point in the field
• Defines a Mask that yields improved Accumulated Rainfall Estimates – comparable to TRMM estimates
• Mask using WV when available, else IR
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Texture Analysis of Infrared & Visible images
• Grey Level Co-occurrence Matrix (GLCM) texture features• Correspondence between certain texture features of Infrared or Visible
cloud images to moderate Radar echoes
IR VIS
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Discriminant Function based on LDA delineates rainfall areas
• Linear Discriminant Analysis (LDA), trained on Radar data, delineates possible rainfall areas
Masked VIS Radar
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Flow diagram of the MSRR algorithm
Data available
IR IR & WV IR & VIS IR, WV & VIS
Initial Screen
- WV-IR<-3° - WV-IR<-3°
- Sun Angle > Th’ld
Texture
IR > 180°K VIS > 180°KASM > 1.50(115+IR) ASM > 6.08(VIS-42)
- QC on IR & VIS
Filter IR < 273°K
WR < 253°K
VIS > 42 (albedo)
WV & VIS warm
WAR Speckle Filter > 33%
IR mask: pass 1 – Go To Rain-Rate Estimation
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The IR → Rain-rate Relationship
Cool: Rc= 0.45(230-IR)
Medium: Rm= 0.00303(267-IR)1.85
Warm Stratiform: Rw=[{alog(73.32-0.173.IR)/2000] 0.625
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Rain-Rate Estimation algorithmCOLD CONVECTIVE < 218K MIDDLE LAYER 219-267K WARM CLOUDS 268-278K
Coastal NO
NODEEP
CONVECTIVE ACTIVITY
ADAPTED IR POWER LAW RAIN-
RATE
24-hr MSRR - Rs
Sufficient slope
Z-R derived
HALF-HOURLY MSRR FIELD - Rhs
Rhs ACCUMULATION
Recursive speckle filter
Image smoothing filter
Rhms 0RhwsRhcs
Rh*s = Half-hourly satellite rainfield
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Improvement of Daily Satellite Rainfall Fields
IR masked field & Final rainfield estimate• improve the vast spatial and quantitative overestimation of rainfall
fields due to Cirrus contamination• improve the estimated spatial structure of daily rainfall fields• improve the detection of warm rain conditions, using algorithms not
specifically designed for convective rain systems.
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Preliminary Validation
• validated with 300 1x1 min gridded raingauge values from Radar interpolated raingauge fields.
Probability of Detection
0 0.2 0.4 0.6 0.8 1
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Skill Score
0 0.2 0.4 0.6 0.8 1
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Field Bias
-8 -7 -6 -5 -4 -3 -2 -1 0 1 2
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2002071820020719
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mm
False Alarm Ratio
0 0.2 0.4 0.6 0.8 1
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Verification of Satellite Daily Rainfall Fields
• Generally overestimated• Warm rain needs adjustment
Coastal < 1000m
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METEOSAT-7 Data availability
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Producing the SIMAR Merged Daily Rainfall Field
• Why merge the rainfall fields ?
• Characteristics of each data source
• Explain the merging techniques
• Discuss the operational implementation of the merging routines
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The steps in making a SIMAR map
• Collect 24-hour rainfall data (up to 8:00 am)• Clean 5-minute radar-rainfall images and
accumulate into a 24-hour mosaic• Process available satellite images of IR, WV &
VIS from METOSAT 7 to get 24-hour estimate of rainfall over RSA
• Combine: Gauge-Radar, Gauge-Satellite estimates
• Post the map on the web by 11:00 am
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Availability of Ground-based Rainfall Sensors
• Weather radars: 11 C-band – except one S-band
• Rain-gauges: 290 ± daily reporting climatological stations
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South African Radar Networksuperimposed on the Mean Annual Rainfall map
Radar range is an (ambitious!) 200km
1300 km N-S
1600 km E-W
Area 1.2 Mkm2
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Automatically reporting raingauges 290 ±some outside RSA via HYCOS
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Kriged Gauge Explained Variance Field - VG
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24-h Accum’d Kriged Gauge Field - GK
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Radar Explained Variance Field - VR
Over-ambitious estimation of radar accuracy with range
Needs revision
Note FFT wrapping
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24-h Accum’d Kriged Radar Field - RK
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Merged 24h Radar/Gauge Rainfall Field:R|G = (RK*VR+GK*VG)/(VR+VG)
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Best Estimate: 24h Satellite Rainfield - SR
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Mean Satellite Field smoothed from Satellite Estimates at Gauge locations by Splines - SZ
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Merged Satellite|Gauge Rainfall Field:S|G = SR – SZ + GK
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Satellite Bias Skill Score Field – SB: compare S|G with R|G in 9x9 blocks at gauges – interpolate
with Splines
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Final Merged Rainfall Field:R|G,R,S = {R|G*(VRorVG)+ S|G *SB}/{(VRorVG)+SB}
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SIMAR
Part of the introductory SIMAR web-page
Available daily by 11:00 am with previous day’s rainfall maps
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Verification of Merged Daily Rainfall Fields
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How do we improve this?
• Refine the merging of radar with gauge data to obtain better ground-truthing fields
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An alternative method to improve SIMAR?
• The explained variance method tried to be “fair” to gauges and radar
• If we believe the gauges, then we want to condition the radar field onto the gauge readings as we did with the satellite images to get the S|G fields
• We call it “Conditional Merging”
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Description of the Conditional Merging technique
(a) The rainfall field is sparsely observed on a regular grid at rain-gauge locations
(b) The rainfall field is also observed by radar on the regular grid - RR
Adapted from Ehret (2002)
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Adapted from Ehret (2002)
(c) The rain-gauge observation are Kriged to obtain the best linear unbiased estimate of rainfall on the radar grid - MG
(d) The radar pixel values at the rain-gauge positions are Kriged onto the remainder of the grid to give a mean field - MR
(e) A rainfall field that coincides with the rain-gauge readings, while preserving the mean field deviations of the radar field is obtained as RR-MR+MG
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Correlated Field Contaminated Field
Kriged Field Merged Field
Explained variance weighting
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Correlated Field Contaminated Field
Kriged Field Merged Field
Conditional merging
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And finally a real cross-validation field experiment
• Compare straight Kriging and Conditional Merging on 40 raingauges on a 4600 km2
catchment
• Use cross-validation – estimation of daily total at each gauge separately using the remaining data
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BETHLEHEM
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Layout of the Liebenbergsvlei gauge network
10 km
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Comparison of daily mean cross-validation errors
-10 0 10 20 30 40 50 60
96/01/24
96/01/25
96/01/27
96/02/01
96/02/05
96/02/11
Radar Error Mean Kriged Error Mean Merged Error Mean
mm/day
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Summary
• We have made a start
• Our Department of Water Affairs trusts the SIMAR fields enough to routinely use them in their Flood Forecasting Division
• Ongoing improvements are being made
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fin