INVESTIGATION OF RAINFALL CHARACTERISTICS

ASSOCIATED WITH TROPICAL STORMS

BONNIE, FRANCES AND IVAN

IN UPSTATE SOUTH CAROLINA DURING 2004

C. W. ULBRICHClemson University

Clemson, SC

and

L. G. LEENWS, Greer, SC

RADAR MOSAIC IMAGES FOR

TROPICAL STORM BONNIE

12 AUGUST 2004

0600Z TO 1900Z

(0200 TO 1500 EDT)

RADAR MOSAIC IMAGES FOR

TROPICAL STORM FRANCES

7-8 SEPTEMBER 2004

0000Z,9/7/04 TO 0300Z,9/8/04

(2000 EDT,9/6/04 TO 2300 EDT,9/8/04)

RADAR MOSAIC IMAGES FOR

TROPICAL STORM IVAN

16-17 SEPTEMBER 2004

0800Z,9/16/04 TO 1300Z,9/17/04

(0400 EDT,9/16/04 TO 0900 EDT,9/17/04)

RAINFALL DATA COLLECTED FOR ALL THREE OF THESE STORMS AND ANALYSIS IS UNDER WAY.

TODAY, RESULTS OF ANALYSIS ONLY FOR TROPICAL STORM FRANCES WILL BE SHOWN.

CONCLUSIONS DRAWN ARE VALID FOR ALL THREE TROPICAL

STORMS.

PRINCIPAL INSTRUMENTS USED IN THIS WORK

FOR ANALYSIS OF BONNIE, FRANCES AND IVAN.

• LOCATED AT THE C.A.R.L.

STANDARD JOSS DISDROMETER

IMPORTANT PARAMETERS DETERMINED FROM DISDROMETER DATA:

1. RAINFALL RATE R (mm h-1)

2. RADAR REFLECTIVITY FACTOR (dBZ)

3. AVERAGE PARTICLE SIZE Dm

4. RAINDROP SIZE DISTRIBUTION N(D)

• OTHER INSTRUMENTS:

TIPPING BUCKET RAINGAGES

MISC. METEOR. INSTRUMENTS

• LOCATED AT THE NWS, GREER, SC

WSR-88D WEATHER RADAR

KGSP

GREER, SC

PRINCIPAL PARAMETER OF INTEREST:

RADAR REFLECTIVITY FACTOR Z (dBZ)

IN THE NWS, THE RAINFALL RATE R (mm h-1) FOUND FROM RELATION

Z = A Rb

POSSIBLE VALUES:

A=300, b=1.4 (Default)

A=250, b=1.2 (Tropical)

LOCATION OF THE C.A.R.L.

RESULTS FOUND FOR FRANCES FROM DISDROMETER DATA

09/07/04, FRANCES

R

Z

Do

EMPIRICAL ANALYSIS TO DETERMINE Z-R RELATION

DEFAULT

TROPICAL

ACTUAL

Z

R

EXPECTED ERRORS FROM USING Z-R RELATION

+35% ERROR IN R

-35% ERROR IN RZ

R

COMPARISON OF RAINFALL DEPTHS COMPUTED FROM

Z-R RELATIONS WITH THAT MEASURED BY DISDROMETER

9/07/04, FRANCES

MEASURED BY DISDROMETER AT C.A.R.L.

ACTUAL

TROPICAL

DEFAULT

COMPARISON OF RAINFALL AMOUNTS FOUND FROM WSR-88D DATA WITH DISDROMETER AMOUNTS

REFLECTIVITY FACTORS Z WERE DETERMINED FROM LEVEL II DATA FOR THE KGSP WSR-88D RADAR OVER THE LOCATION OF THE C.A.R.L. AT THE LOWEST ELEVATION ANGLE SCANS AND CONVERTED TO RAINFALL RATES R USING

Z = A Rb

VALUES OF A AND b USED IN THE ANALYSIS WERE

A=300, b=1.4 (default relation used by NWS)

A=250, b=1.2 (NWS relation for “distinctly tropical rain”)

A=271, b=1.41 (actual relation for Frances)

HEIGHT OF PULSE VOLUME ABOVE C.A.R.L. APPROX. 500 m

ACTUAL RAINFALL DEPTH MEASURED BY DISDROMETER

EMPIRICAL

TROPICAL

DEFAULT

09/07/04, FRANCES

RADAR UNDERESTIMATES RAINFALL DEPTH BY AT LEAST A FACTOR OF 2, PERHAPS MORE

POSSIBLE EXPLANATION OF RESULTS:DISDROMETER IS MISCALIBRATED AND IS OVERESTIMATING THE REFLECTIVITY FACTOR BY A LARGE AMOUNT.

TO ADDRESS THIS POSSIBILITY, DATA FROM NOAA WEIGHING RAINGAGES FROM SEVERAL LOCATIONS HAVE BEEN USED TOGETHER WITH WSR-88D RADAR DATA OVER THE GAGES.

RAINFALL AMOUNT MEASURED BY GAGE COMPARED WITH THAT COMPUTED FROM REFLECTIVITY FACTOR USING Z-R RELATION.

GAGE AMOUNTS SPECIFIED EVERY HOUR. RADAR DATA SPECIFIED APPROXIMATELY EVERY 5 MINUTES.

DATA FOR PICKENS RAINGAGE ARE REPRESENTATIVE:

COMPARISON OF RAINFALL AMOUNT MEASURED BY NOAA WEIGHING RAINGAGE WITH AMOUNT MEASURED BY RADAR USING Z-R RELATIONS

NOAA WEIGHING RAINGAGE (HOURLY)

RADAR UNDERESTIMATES RAINFALL AMOUNT BY AT LEAST A FACTOR OF 2

COMPARISON OF REFLECTIVITY FACTORS MEASURED BY DISDROMETER AND KGSP WSR-88D

DISDROMETER

KGSP WSR-88D

09/07/04, FRANCES

CONCLUSIONS AND SUGGESTED FUTURE

RESEARCH

1. KGSP WSR-88D NOT CALIBRATED CORRECTLY. SOURCE OF CALIBRATION PROBABLY LIES IN AN INCORRECT ASSUMPTION ABOUT VALUE OF ANTENNA GAIN (Maybe).

2. TO VERIFY THE CONCLUSIONS OF THIS WORK DISDROMETER SHOULD BE SITUATED NEAR ANOTHER WSR-88D, SAY, KCAE, AND MEASUREMENTS LIKE THOSE DESCRIBED HERE REPEATED.

3. DEMONSTRATION GIVEN IN THIS WORK IMPLIES THAT A PROPERLY CALIBRATED WSR-88D COULD PRODUCE ACCURATE RESULTS FOR RAINFALL DEPTH.