International Symposium on the 15th Anniversary of the Equatorial Atmosphere Radar EAR
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Extreme Rainfall Analysis using Radar-based Rainfall Estimates, Ground Observation and Model Simulation in West Sumatra Case Study: Padang
Floods on June 16, 2016
Linda F Muzayanah
1
, Donaldi S Permana
1
, Alfan S Praja
1
, Eka S P Wulandari
2
, Wido Hanggoro
1
1
Research and Development Center, Indonesian Agency for Meteorology Climatology and Geophysics BMKG
2
Minangkabau Meteorological Station, Indonesian Agency for Meteorology Climatology and Geophysics BMKG E-mail:
fitrotullindagmail.com ;
donaldi.permanabmkg.go.id
ABSTRACT - This case study evaluates and compares the suitability of radar rainfall estimation, ground observed rainfall and model simulation during the extreme rainfall event on June 16, 2016 in West Sumatra which caused
flooding in Padang and closed of Minangkabau Airport due to minimum visibility. Radar-based rainfall estimation was produced from reflectivity dBZ that had been observed by C-band radar located in Minangkabau station
100.3°E;0.79°S;24 magl. Radar data processing and rainfall estimation were conducted using the open source library wradlib. Radar rainfall estimation were calculated using two reflectivity-rainfall rate Z-R relationships of Marshall-
Palmer MP and Rosenfeld RO. Three-hourly rainfall data from radar rainfall estimates and observed data were compared at two meteorological stations in Minangkabau and Teluk Bayur ~25 km from radar site during the day
event. Extreme rainfall were measured at these two stations with intensity of 384.1 mmday and 379 mmday, respectively, possibly due to local interaction with mountain ranges. The results show that both of radar rainfall
estimates generally underestimated the observation data with RO relationship was better than MP. Furthermore, radar rainfall estimations were better in Teluk bayur than in Minangkabau. The Mean Absolute Error MAE values for RO
relationship is 11- 14.2 mmhr and MP relationship is 18.3-19.3 mmhr and 21 - 22 mmhr for RO and MP relationships, respectively.
Keywords: extreme rainfall, floods, radar rainfall estimation, Z-R relationship, padang, WRF
1. INTRODUCTION
On June 16 2016, extreme weather and heavy rainfall have occurred in West Sumatra province which resulted in flooding in Padang city and its surrounding areas. Extreme rainfall were measured at two BMKG
stations in Minangkabau and Teluk Bayur with rainfall intensity of 384.1 mmday and 379 mmday, respectively Wulandari and Nugraha, 2016. There have been several factors that may cause this event,
which include a positive anomaly of sea surface temperature SST by 0.5-1.5°C and an active Madden Julian Oscillation MJO in the eastern Indian Ocean, a combination of low pressure area in the western part
of West Sumatra and an eddy in the Karimata strait, high relative humidity at surface up to 500 mb level, and also cooler top cloud temperatures over West Sumatra Figure 1; Wulandari and Nugraha, 2016. This
extreme event has inundated most of Padang city with about 30-60 cm of water level and impacted various sectors. For instance, during the event, Minangkabau airport has been closed 4 times at 09:57-11:00, 11:54 to
12:30, 13:53 to 14:15 and 14:48 to 15:35 UTC due to minimum visibility based on information issued by Airnav. As a results, there were five air planes had to be diverted to Pekanbaru and Medan airports.
Extreme events have been a big challenge for BMKG in providing fast, right and accurate information to the public. Therefore, BMKG has installed 40 weather radar across Indonesia http:radar.bmkg.go.id since
2006 including in Padang city. The advantages of weather radar are able to detect a very short-term weather conditions and have a high resolution. Weather radars measure the electromagnetic radiation backscattered
by cloud raindrops. They have the potential to estimate rainfall rates R by exploiting the reflectivity Z
International Symposium on the 15th Anniversary of the Equatorial Atmosphere Radar EAR
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values via empirical power-law Z-R relationships. This study presents the analyses and comparison of radar- based rainfall estimates from Padang weather radar data and ground rainfall observation at two BMKG
stations in Minangkabau and Teluk Bayur. Rainfall rates estimations were derived from two common Z-R relationships of Marshall-Palmer MP with Z = 200R
1.6
for general stratiform precipitation Marshall et al., 1947; Rinehart, 2010 and Rosenfeld RO with Z = 250R
1.2
for tropical convective rain Rosenfeld et al., 1993. In addition, WRF model simulation are also performed during this extreme event.
A B
C D
Figure 1. The global ocean and atmospheric conditions during the extreme rainfall on June 16, 2016 in West Sumatra,
Indonesia. A. SST anomaly B. Sea level pressure and streamline C. Relative humidity at 500 mb level and D. Infrared channel from HIMAWARI Satellite represent top cloud temperatures during the event.
2. DATA AND METHOD
