Transcript Radar-Derived Precipitation

Radar-Derived Precipitation
•Deriving Precipitation Rates
•Radar Sampling Issues
•Validating: Comparing Radar
Estimates with Gauge Reports
COMAP Symposium 00-4
(Heavy Precip/Flash Flood)
Matt Kelsch
Wednesday, 27 September 2000
[email protected]
Z-R Relationships
WSR-88D, Marshall-Palmer (general), and
Tropical
Z300 R1.4
200 R1.6
250 R1.2
20 dBZ
25
30
35
40
45
50
51
52
53
54
55
56
57
58
59
60
65
70
0.5 mm/h (0.02 in/h)
1.0 (0.04)
2.4 (0.09)
5.4 (0.21)
12.3 (0.48)
27.9 (1.10)
63.4 (2.50)
74.7 (2.94)
88.1 (3.47)
103.8 (4.09)
122.4 (4.82)
144.2 (5.68)
170.1 (6.70)
200.5 (7.89)
236.4 (9.31)
278.6 (10.97)
328.4 (12.92)
748.2 (29.46)
1702.2 (67.01)
0.7 mm/h (0.03 in/h)
1.3 (0.05)
2.7 (0.11)
5.6 (0.22)
11.5 (0.45)
23.7 (0.93)
48.6 (1.91)
56.1 (2.21)
64.6 (2.55)
74.8 (2.94)
86.5 (3.41)
99.8 (3.93)
114.8 (4.52)
133.0 (5.24)
153.8 (6.05)
177.4 (6.98)
205.1 (8.07)
420.7 (16.56)
865.0 (34.05)
0.5 mm/h (0.02 in/h)
1.2 (0.05)
3.2 (0.12)
8.3 (0.33)
21.6 (0.85)
56.5 (2.22)
147.2 (5.80)
178.6 (7.03)
216.3 (8.52)
261.8 (10.31)
317.7 (12.51)
384.6 (15.14)
465.6 (18.33)
564.9 (22.24)
683.9 (26.92)
827.9 (32.59)
1004.6 (39.55)
2618.2 (103.07)
6839.1 (269.26)
Sampling Issues
Radar domain cannot be sampled at
consistent elevations, with consistent bin
volumes, or for precipitation with similar
stage of development or phase.
• Range degradation
• Low-level beam blocking
• Changes in precip phase have inconsistent effects-bright band, hail contamination
These are not effectively corrected by changing Z-R
coefficients
16 sep99: Storm Total Radar-derived
Accumulation from KRAX (Raleigh
NC)
16 sep99: Storm Total Radar-derived
Accumulation from KAKQ (Wakefield
VA)
Bright Band 
Radar-Rain Gauge Comparisons
• Radar samples a volume of the atmosphere
– At discrete intervals
– Up to several thousands feet AGL
– Over a surface area which may exceed 1 mi2
• Rain gauges sample
– Continuously
– At the surface
– Over an area less than 1 ft2
• Accumulations are measurements with the error
factors associated with the gauge type
0500 UTC 7 Aug 1999
0700 UTC 7 Aug 1999
1215 UTC 27 June 1995
1815 UTC 27 June 1995
1402 UTC 27 June 1995
1658 UTC 27 June 1995
Virginia Topography
Radar-derived accumulation
27 June 1995
Changing Z-R
Will help when:
• Consistently different average DSD (climate)
– Tropical versus mid-latitude (warm vs. cold process)
– Maritime versus continental
• Consistently different average DSD (season)
– Convective versus stratiform
Is not the solution when:
• Range degradation, overshooting low-levels
• Phase change: hail, melting snow
• Snowfall
KRAX Storm Total 1159
UTC 6 Sep 96: Z=300R1.4
KRAX Storm Total 1159
UTC 6 Sep 96: Z=250R1.2
Radar-derived Precipitation:
A Summary Of Major Points
• Radar provides excellent storm-scale information about the
spatial and temporal evolution of precipitation systems.
• Radar provides very valuable input as part of a
comprehensive, multi-sensor precipitation system.
• Quantitative reliability issues are related to the fact that
radar samples some volume at some elevation to estimate
precipitation at the ground.
• Radar-derived precipitation is most reliably modeled for
liquid hydrometeors; hail and snow add complexity.
• The above two points are not effectively corrected by
changing Z-R coefficients; Z-R changes should be related
to Drop Size Distribution knowledge.
• Radars and rain gauges do not measure equal samples
• Rain gauges do not provide a good representation of
Heavy/Intense Precipitation
The precipitation
part to the flash
flood problem
COMET
COMAP Symposium 00-4
Heavy Precip/Flash Flood
Matt Kelsch
Wednesday, 27 September 2000
[email protected]
7 May 2000 radar accumulation
near St Louis,MO
Heavy/Intense Precipitation
Convective
Enhanced Intensity
• General Increase in
Precipitation Efficiency
• Storm-specific
Enhancement of
Precipitation Efficiency
Extended Duration
•
•
•
•
Regeneration
Propagation
Axis of Development
Movement
Merger of supercell and squall line over
Dallas-Ft Worth Metroplex, 5 May
1995
Precipitation Intensity
• Enhanced Precipitation Efficiency
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–
–
–
–
Deep above freezing cloud layer: >3 km (>4 km SE)
Well-defined high-e inflow (low-level jet)
High Precipitable Water values (25-50 mm)
Low-centroid storms, warm rain process
Enhanced low-level lift (topo or metr boundary)
• Minimal Condensate Loss
– Moist ambient environment with relatively weak shear
(little entrainment of dry air)
– Weak to moderate updrafts (minimal detrainment)
From Funk et al, 1991 Weather & Forecasting, vol 6, 548-564
Be aware that atypical moisture is relative, it will take less
moisture in an atypically “cool” pattern for efficient precip
production
Precipitation Duration
• Synoptic Pattern (timing and potential duration)
– Initiation “trigger”
– What will shut it off?
• Evolution of Meso- and Storm-scale Features
– Common regeneration area for new storms
– Instability/ Rapid Moisture Replenishment
– Will new cell development balance movement away from
area?
• Presence of Orographic Features
0600 UTC
0500 UTC 7 Aug 1999
0700 UTC 7 Aug 1999
1800 UTC 23 July
1987
Moisture Convergence (shaded) and
Theta-E at 2100 UTC 23 July 1987.
0000 UTC 24 July
1987
8 July 1999 18z: 500 mb hght (ETA) and water vapor imagery
8 July 1999 12z: ETA 850 mb wind & dewpoint with
precipitable water image
8 July 1999 23z: radar accumulation
Excessive Rainfall from Tropical
Cyclones
• In recent decades the greatest number of
hurricane-related fatalities have been from
inland flooding. Three situations (or
combinations of) to watch for are:
1. Slow movement (Alberto 1994, Mitch 1998),
2. Interaction with topography (Camille 1969, Fran 1996,
Mitch 1998),
3. Interaction with midlatitude system (Camille 1969,
Tico 1983, Floyd 1999).
Excessive Rainfall from Tropical
Cyclones
• South of about 35o latitude
– typically from slow movement/terrain influence
– typically associated with eyewall convection
(especially where wind is perpendicular to coastline)
and feeder bands.
• North of about 35o latitude (western US too)
– often associated with midlatitude interaction (most
intense rainfall north & west of track).
– Terrain focus
ETA 1200 UTC 16Sep99 analysis:
925 mb Wind, Omega, and
Frontogenesis Image
Frontal positions at various times,
24-h accumulation (>5” dashed
red, ~7” dashed orange).
16 sep99: Storm Total Radar-derived
Accumulation from KAKQ (Wakefield VA)
KAKQ Radar-derived rainfall 6 Sep 1996
Total Accumulation
Heavy/Intense Precipitation
• Enhanced Intensity, Greater Efficiency
– Tropical Maritime Connection
– Atypical Moisture Availabilty, Warm Rain Process
– Low-level Jet, Rapid Moisture Replenishment
– Low level Focus (terrain and metr. Boundary)
• Extended Duration
– Common Regeration Area
– Balance Between Movement and Regeneration
– Slow Movement
– Quasi-stationary Axis of Disturbances