building a weather-ready nation increasingly common heavy rainfall events in iowa jeff zogg – nws...
TRANSCRIPT
Building a Weather-Ready Nation
Increasingly Common Heavy Rainfall Events
in IowaJeff Zogg – NWS Des Moines, IA
Doug Kluck – NOAA Central Region Climate Services
1
Building a Weather-Ready Nation
Agenda
The ProblemResearch & MethodologySummary & Impacts
2
Building a Weather-Ready Nation
Flooding is Iowa’s #1 weather-related hazard.43 of 55 (~80%) Presidential Disaster
declarations.Iowa ranks #2 in the U.S. for flood-related
losses.Flooding—happening more frequently?
A major driver of flooding—especially flash flooding—in Iowa is heavy rainfall.Heavy rainfall events seem to
be occurring more frequently.Perception or reality?
The Problem
3
Ames/Iowa State University, Aug 2010 –Des Moines Register
Building a Weather-Ready Nation
Mallakpour & Villarini (2015)Flood frequency increasing, not severity
Support of Flooding Trends
4
Changes in flood magnitude,1962-2011
Changes in flood frequency,1962-2011
Northern U.S.
Building a Weather-Ready Nation
Used two frequency publications for IowaRainfall Frequency Atlas of the Midwest
(1992)Huff & Angel – MRCC & Illinois SWS
NOAA Precipitation-Frequency Atlas 14, Volume 8 (2013)
Research & Methodology
5
Overview
Building a Weather-Ready Nation
Base: 275 stations (IA: 41) – NWScoop sites w/ POR > 50 yrs; supplemented w/ other data
Max precip conversion factors≥ 1 day: NWS empirical factors used
(& verified)< 1 day: factors from 1948-1983
data for 55 recording rain gages in IL & surrounding states – compared & verified with other studies
Partial Duration Series (PDS) extracted
MRCC Rainfall Frequency Atlas
6
Data & Analytical Approach
Building a Weather-Ready Nation
3 techniques evaluatedLog-log graphical (Huff-Angel)Maximum likelihoodL-moments
No significant differencesBy design, L-moments tends to give more
conservative (i.e., lower) precip values for same recurrence interval
Huff-Angel technique selectedAllows analyst to incorporate professional
knowledgeCutoff near 100-yr event
MRCC Rainfall Frequency Atlas
7
Statistical Methods
Building a Weather-Ready Nation
Point-basedIsohyetal maps derivedMore susceptible to subjectivity
& inherent variabilityShows small-scale variability
Areal-basedTabular data derivedNWS climate divisions used (& verified
appropriate)Average frequency distributions usedMitigates impacts of sampling errors
MRCC Rainfall Frequency Atlas
8
Output
Iowa NWS climate divisions
Building a Weather-Ready Nation
Base: IA – 276Most were NWS coop sites
Max precip conversion factors≥ 1 day: similar to Bull71< 1 day: hourly data used then
correction factors applied; resultsbelieved similar to Bull71
Annual Maximum Series (AMS) extracted, then PDS obtained from AMS
NOAA Atlas 14, Volume 8
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Data & Analytical Approach
Iowa daily stations used
Building a Weather-Ready Nation
L-moments techniqueLess impacts of outliers
Upper & lower 90%confidence intervalscalculatedMonte-Carlo simulation approach
Longer recurrence intervalsRegion of influence approach
NOAA Atlas 14, Volume 8
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Statistical Methods
Interactively removing stations from aregion—Minnesota
Building a Weather-Ready Nation
MRCC Bulletin 71
Both NOAA Atlas 14,
Vol 8
2-month
3-month
4-month
6-month
9-month
1-year 1-year 1-year
2-year 2-year 2-year
5-year 5-year 5-year
10-year 10-year 10-year
25-year 25-year 25-year
50-year 50-year 50-year
100-year 100-year 100-year
200-year
500-year
1000-year
Overlapping Return Pds & Durations
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MRCC Bulletin 71 Both NOAA Atlas 14, Vol 8
5-minute
6-minute
10-minute 10-minute 10-minute
15-minute 15-minute 15-minute
30-minute 30-minute 30-minute
1-hour 1-hour 1-hour
2-hour 2-hour 2-hour
3-hour 3-hour 3-hour
6-hour 6-hour 6-hour
12-hour 12-hour 12-hour
18-hour
24-hour 24-hour 24-hour
2-day 2-day 2-day
3-day 3-day 3-day
4-day
5-day
7-day
10-day 10-day 10-day
20-day
30-day
45-day
60-day
Building a Weather-Ready Nation
Clip geographic area to IowaCompute zonal statistics
84 individual files, each with 228,347 gridsIndex each of the 84 rasters to
between 0 & 1Create average index value
GIS Analysis Procedure
12
NOAA Atlas 14, Volume 8
South Skunk River @ Colfax, Iowa(Aug 2010)
𝑥𝑖=𝑥−𝑟𝑚𝑖𝑛
𝑟 𝑚𝑎𝑥−𝑟𝑚𝑖𝑛
Building a Weather-Ready Nation
GIS Results
13
Building a Weather-Ready Nation 14
Building a Weather-Ready Nation
Documented Bull71 & Atlas14 results for all 9 Iowa climate sections
Calculated statewide averagesCompared all 12 common durations
of Bull71 vs. Atlas14 for all 7 common Tr s
Data Analysis Procedure
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Spreadsheet
Cedar River @ Cedar Rapids, Iowa (2008) – Scott Olson/Getty Images
Building a Weather-Ready Nation 16
ResultsData Analysis Procedure
Great Flood of 1993 – Des Moines –Des Moines Water Works
𝑦=(𝑐 ln 𝑥 )+𝑏x = Tr (yr); y = precip value (in)
𝑥=𝑒( 𝑦−𝑏 )
𝑐
Building a Weather-Ready Nation
All 7 common Tr s (↕) of Bull71 vs. Atlas14 for all 12 common durations (↔)Bull71 equation correlation: 0.9968Atlas14 equation correlation: 0.9934# Atlas14 elements > Bull71: 78 (93%)# Atlas14 elements < Bull71: 6 (7%)
Data Analysis Procedure
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Results
Iowa Hwy 92—Muchakinock Creek near Oskaloosa (Aug 2010) –
NWS Des Moines
Building a Weather-Ready Nation 18
ResultsData Analysis Procedure
Building a Weather-Ready Nation
BUT…Not simple relationship for width vs.
duration & Tr but (Atlas14 – Bull71) remains within 90% intervals15-min, 2-yr event: |∆| = 0.004 in6-hr, 100-yr event: |∆| = 0.834 in (Atlas14 ∆ ≈
1.54 in)
HOWEVER...
90% Confidence Intervals
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Remember Them?
Building a Weather-Ready Nation
Bull71 – YesExamine ratio of 2nd 40-yr period
to 1st 40-yr periodRatio > 1.1
“The increases appear to be greaterthan expected from natural climaticvariability”
“Findings suggest that the assumption of a stationary time series for fitting statistical distributions to historical precipitation data may be invalid.”
“An update on the order of every 20 years would be appropriate to capture any substantial changes.”
Rainfall Frequency Distribution
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Changing Over Time?
Building a Weather-Ready Nation
Studies suggest flood frequency—not severity—increasing over timeAlso suggest increases in heavy rainfall
days but not rainfall severityAtlas14 values > Bull71 values, but
Bull71 values fall within the Atlas14 90% confidence intervals
Bull71 suggests heavy rainfall frequency distribution is changing over time, Atlas 14 says precip AMS is not changingDoes not address precip PDS trend
Summary & Impacts
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Building a Weather-Ready Nation
Mallakpour & Villarini (2015)Findings similar to 3rd Nat’l Climate
Assessment (2014)
Support of Precip PDS Trends
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Changes in heavy rainfall magnitude,1948-2012
Changes in heavy rainfall frequency,1948-2012
Northern U.S.
Building a Weather-Ready Nation
Most likely explanationPrecip PDS trends are changing – heavy
rainfall becoming more frequent but not more severe
What if rainfall trends are to blame?Under-designed municipal storm water
systems?More frequent flash flooding
especially urbanIncreased soil erosion
Timing of rainfall important
Summary & Impacts
23
Central Iowa soil erosion –ISU Extension / Drake Larson
Continued
Building a Weather-Ready Nation
Thank You
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For questions & additional information:NWS Des Moines, IA
http://www.weather.gov/desmoinesEmail: [email protected]
Phone: 515-270-4501