1

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

2

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)

3

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)

4

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

15

16

ETA 500 mb height, WV imagery 1 January 1997

17

Precipitable Water, 850 mb ht (green lines) and surface plot, 2 January 1997

18

19

20

21

22

23

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

29

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).

31

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

32

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

33

Flash Floods Investigated

34

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

35

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

40

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