Comparing NEXRAD and Gauge Rainfall Data

Nate JohnsonCE 394K.2 Final Project

April 26, 2005

Motivation

• USGS performs hydrologic and water quality modeling using rainfall data from gauges.

9 km6 km

10 km

>20 km

• Sometimes there is greater than 20 km between gauges

• NEXRAD provides rainfall estimates on 4 km2 grid

• Could more acccurate hydrologic simulations could be performed with “accurate” distributed rainfall information?

How does NEXRAD precipitation data compare to gauges used for hydrologic modeling?

Outline

Project involved 2 major tasks:

1. Transferring data from NEXRAD files to something more accessible

2. Comparing NEXRAD rainfall estimates to rain gauge measurements

a)Cumulative Difference

b)Storm Bias

Background – File Structure

Space – Time – Variable dimensions

Gridded File – One Time, all space

TimeSeries File – One Location, all time

• To create a TimeSeries file (for a single location) from a collection of Gridded files (for a single time) must access thousands of gridded files!

24 hours * 365 days = 8,760 files/year

Background – Rainfall Estimates

• Compare NEXRAD to gauge based on…

• Point Comparisons (Grid Cell immediately above)

• Within cell spatial variability?

• Spatial Averages

• Interpolate between gauges?

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This project only considers point comparisons…

Stay tuned for Jessica Watt’s presentation containing spatial

comparisons

Methods – Extract NEXRAD Data• Extracted 2 months of hourly NEXRAD data for about

1,800 cells near San Antonio.

• August and September of 2001 were very rainy

• ~100,000 non-zero timeseries records

Methods – Compare NEXRAD/Gauge

First method – Cumulative difference over two months

• For each hour, calculate difference between Radar and Gauge and add to pervious total

• Removes biases from timing discrepancies

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iiiCum GRDiff

1

Results – Cumulative Differences

August – September 2001

> 11 inches of rain!

Results – Cumulative DifferencesS.A. Airport Gauge – 11.9 inches total

NEXRAD – Rain Gauge

Results – Cumulative DifferencesGovernment Canyon Gauge – 10.3 inches total

NEXRAD – Rain Gauge

Results – Cumulative DifferencesBulverde Rain Gauge – 10.8 inches Total

NEXRAD – Rain Gauge

Methods – Compare NEXRAD/Gauge

Second method – Storm-to-Storm Comparison

• Based on an analysis by NCAR and NOAA in 1979

• Considers each event independently and looks for systematic biases

• Uses Ratio ‘G/R’ of Gauge to Radar for total storm-event precipitation

Methods – Compare NEXRAD/Gauge

Average G/R

gauges i

iAve R

GRG /

Coefficient of Variation

Ave

ii

RGRG

VOC/

/..

N

i i

iiAve N

GRG

Diff1

/Average Difference

N

i i

AveiiAve N

GRGRG

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2Average Difference (Storm Bias Removed)

G/R < 1

G/R > 1 G/R > 1

G/R > 1

G/R < 1

Results – Storm Bias Comparison

Original DifferenceImprovement?

Date#

Gages

Storm Duration

[h]G [in]

Average G/R

Relative dispersion

about Average G/R

[%]

Average Difference

[%]

Average Difference

(Storm Bias Corrected)

[%]

Average Difference assuming Log

Distribution for G/R (Storm Bias

Corrected) [%]

8/19/2001 3 4 0.29 1.45 0.27 0.28 0.20 0.198/27/2001 8 6 2.26 1.12 0.32 0.23 0.24 0.238/28/2001 7 4 0.64 1.06 0.47 0.40 0.43 0.398/28/2001 6 2 0.19 1.16 0.27 0.14 0.17 0.168/29/2001 8 10 1.95 2.74 0.64 0.48 0.71 0.598/30/2001 8 10 1.86 1.06 0.42 0.47 0.50 0.458/31/2001 6 4 0.83 1.13 0.35 0.27 0.28 0.27

9/1/2001 8 4 0.82 1.14 0.47 0.20 0.29 0.249/5/2001 3 4 0.42 0.96 0.04 0.04 0.03 0.039/6/2001 8 6 1.79 1.09 0.23 0.16 0.17 0.17

9/23/2001 7 4 0.52 0.91 0.35 0.37 0.35 0.35

1.26 0.35 0.28 0.31 0.28

Not enough gauges? Too much scatter?

Conclusions / Future Work

• Some significant differences between NEXRAD and Gauge measurements

• May be too much scatter in G/R to remove “Storm Bias” from NEXRAD

• Still need to consider effects of spatial averaging

• Investigate the effects on hydrologic modeling results

Background – Rainfall Estimates

• Hydrologic modeling traditionally relies upon estimates for “Average Drainage – Area precipitation

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• Some question yet as to whether or not it is appropriate to use NEXRAD for hydrologic modeling

• Studies have shown some disturbing differences between NEXRAD and gauge measurements

Methods – Extract NEXRAD Data• First – Get NEXRAD data into Arc Hydro Geodatabase

• Second – Get Arc Hydro into Timeseries format

Arc Hydro GDB

Background – File Structure• NEXRAD data are stored in a gridded file structure

• Hydrologic models require data in a TimeSeries Structure

Gridded File – One Time TimeSeries File – One Location