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Heavy/Intense Precipitation
Precipitation IntensityPrecipitation EfficiencyPrecipitation Duration
The precipitation part to the flood/flash flood problem
RFC/HPC Hydromet 01-2 Friday, 8 December 2000
Matt Kelschkelsch@comet.ucar.edu
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Precipitation Intensity
• Convective• Enhanced Precipitation Efficiency
– Deep, moist above freezing cloud layer: >3 km (>4 km SE)
– Well-defined high-e & high PW inflow (low-level jet)
– Low-centroid storms, warm rain process
– Enhanced low-level lift (can be storm-specific)
• Minimal Condensate Loss in MCCs– Moist ambient environment with relatively weak shear
– Weak to moderate updrafts (minimal detrainment)
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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– Quasi stationary axis of development – New cell development balances movement away from area
• Low-level anchoring features (terrain or metr boudnary)
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0600 UTC
0500 UTC 7 Aug 1999
0700 UTC 7 Aug 1999
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
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ETA 500 mb height, WV imagery 1 January 1997
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Precipitable Water, 850 mb ht (green lines) and surface plot, 2 January 1997
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Corresponding 0.5o reflectivity and 4-km IR
12aug00 1315 UTC
Storm Total from KDIX 12aug00 2004 UTC
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
Categories For Midwest ’93, # events, avg rainfall, and area (km2) of accumulation amount. (Junker et al, 1999)
Saturation Thickness (Funk et al 1991 WAF)
New Jersey event
Floyd
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Excessive Rainfall from Tropical Cyclones
• South of about 35o latitude– often slow system movement, increased duration– typically associated with eyewall convection and feeder
bands (and usually worst to the east of the track).– Terrain focus
• North of about 35o latitude (western US too)– often associated with the interaction with a mid-
latitude baroclinic system (most intense rainfall north & west of track).
– Terrain focus
Frontal positions at various times, 24-h accumulation (>5” dashed red, ~7” dashed orange).
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Heavy/Intense Precipitation
Summary
• 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, OR– Quasi-stationary Axis of Disturbances
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Defining the Flash Flood
A flash flood is a situation where the causative precipitation and the subsequent runoff are occurring on the same scales.
• Intense Precipitation• Small, Fast-response
basins
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Flash Floods Investigated
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Common Features of Flash Floods
Precipitation
• Accumulation: 50-500 mm over 1-12 h
• Duration of causative precipitation mostly <3 hours
• Intensity 75-300 mm/h
– Arid/semi-arid climates: avg 118 mm/h
– Humid climates: avg 192 mm/h
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Common Features of Flash Floods
Basin Characteristics• Small 10-125 km2 (avg 46 km2)• Majority are mountainous watersheds• >50% were altered
– Urbanization– Deforestation, fire scars
• Either thin/impermeable surface layer,– Clay, rocks (low infiltration)– Pavement (no infiltration)
• Or, thick, saturated soil layers
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The Fort Collins, Colorado (USA) Flash Flood of 28 July 1997
• Terrain-locked thunderstorm system increased rainfall duration
• Unusual maritime-tropical characteristics increased rainfall efficiency
• Urbanization increased hydrologic response (more rapid runoff)
• Flood control structures increased runoff complexity
October 1999 Hydromet class at Spring Creek
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