Sabitul Hidayati* and Richard Mahendra Putra 2. *corresponding author:

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1 DETERMINATION OF RELATIONSHIP BETWEEN CLOUD TOP BRIGHTNESS TEMPERATURE OF INFRARED CHANNEL HIMAWARI-8 SATELLITE AND RAINFALL EVENTS ON FEBRUARY 2016 AT PERAK I SURABAYA METEOROLOGICAL STATION Sabitul Hidayati* and Richard Mahendra Putra 2 1,2 State Collage of Meteorology Climatology and Geophysic Jakarta, Indonesia *corresponding author: sabitul.hidayati@gmail.com Abstract For the convective cloud situation, there is an assumption that lower cloud top temperature is associated with heavier rainfall. Research of relationship between rainfall and satellite data can be done through the analysis of cloud top brightness temperature which showed on 10.4µm channel furthermore can be utilized to estimate the amount of precipitation in the future. This paper aims to determine and analyze the relationship between the average of cloud top brightness temperature from Himawari-8 satellite IR1 data with the rainfall per hour data were observed on February 2016 at Perak I Surabaya Meteorological Station. The results of the analysis can be used to determine the distribution and the character of cloud top brightness temperature values during rain events at the sites. Analysis of the relationship is done by using warmer or colder average cloud top brightness temperature than -50 o C treshold and divide it into several groups events. The result show that from 154 rainfall events data there are 60 (38.96%) events of the total rain events are accompanied by colder average cloud top brightness temperature than -50 C, and there are 94 (61.04%) events of the total rain events are accompanied by the warmer average cloud top brightness temperature than -50 C. In general can be said that with the warmer average cloud top brightness temperature than -50 o C, rain in the region of Perak I Surabaya Meteorological Station still can occur. The condition occurs in two circumstance, those are when in the same period time of rain there are colder or warmer cloud top brightness temperature than -50 C. The result of the the analysis for precipitation cloud characteristic shows that for the rain event which not accompanied by colder cloud top temperature than -50 o C can occurs becuase an parallax error of cloud observation angle. This parallax error causes the cloud with top cloud temperature value colder than -50 o C not exactly detected above the interest area. Keywords: brightness temperature, precipitation, rainfall, satellite, and top cloud Introduction Rain as a liquid form of precipitation occur with the condensation process as the beginning. The amount of rainwater that fall from the rain cloud on an area within a given time referred as rainfall (Winarso, 2011). Rainfall is one important meteorology parameter which its data is required by almost all sectors of life such as agriculture, transportation, plantations, the early warning of natural disasters, floods, landslides, and droughts (Parwati et al., 2009). Research of rainfall from satellite data especially in subtropical region has been done by Hong et al. (2006, 2007), Naranjo (2007), and several more but has not done much in tropical region, especially in Indonesia which is also an archipelago (Parwati et al., 2009). One of those research is about rainfall estimation. Rainfall estimation can be done based on the cloud top temperature which the formation of rain clouds occur in clouds with top lower temperature (Handoko, 1994). High or low cloud top temperature which shown in satellite imagery mainly infrared channel imagery is frequently 283

2 correlated with the rain event and the amount of precipitation. The average temperatures of rain cloud forming is 195 o K to 260 o K (Grifith et al., In Tahir, 2009). Griffith et al. (1978) have found that a convective system is more active and produces the greatest rainfall rates when the tops become colder and continue to expand (Tahir, 2009). Operationally, Perak I Surabaya Meteorological Station observing the value of rainfall using a conventional rain gauge that record rainfall data with three hour interval and Hillman rain gauge which can record data in one hour interval. In these daily activities, the rainfall events were not always accompanied by the cold (negative) temperature cloud tops. When the difference of rainfall intensity, the distribution of cloud top temperature is being random (not uniform) (Aryani et al., 2013). Aryani (2013) in her research found that the value of the cloud top brightness temperature for all the intensity of rain is in the range between -80 C to 10 C and there are two classes of top brightness temperature number, first -80 C to -30 C and the second is -30 C to 10 C. This paper aims to determine and analyze the relationship between the average of cloud top brightness temperature from Himawari-8 satellite IR1 data with the rainfall per hour data that were observed on February 2016 at Perak I Surabaya Meteorological Station. Analysis of the relationship was conducted by using warmer or colder average cloud top brightness temperature than -50 o C treshold and divided into several groups events. The treshold which used is based on the research that has been done before by Panjaitan (2013). It is intended to find out the distribution of cloud top brightness temperature values during rain events and to know the character of cloud top brightness temperature which can produce rain events on February 2016 at Perak I Surabaya Meteorological Station. Materials and Methods Materials This research used Himawari-8 weather satellite IR1 channel data (ten minutes interval) and rainfall observation data (one hour interval) which recorded by Hillman rain gauge from Perak I Surabaya Meteorological Station. The criteria of rainfall events which used is with various intensity that is from slight to heavy rain. The amount of precipitation with trace value (TTU) is regarded as 0 mm. a 284

3 b c Figure 1. Graphic of rain events and the amount of precipitation on February 2016 Perak I Surabaya Meteorological Station (per 10 days interval). a) 1st-10th of February 2016, b)11st-20th of February 2016, c) 21st-29th of February Methods After collecting data, the method was applied in data processing to determine and analyze the relationship between the average of cloud top brightness temperature from Himawari-8 satellite IR1 data with the rainfall per hour data is as follows: 1. The extraction of cloud top brightness temperature IR1 channel data The extraction of cloud top brightness temperature Himawari-8 satellite IR1 channel data (10 minutes interval) was conducted using SATAID software. The extraction was done within 7.28 LS E which are regarded as approximately coordinates of Perak I Surabaya Meteorological Station observation point. The cloud top brightness temperature data with 10- minutes intervals is averaged for one hour observation data and then compared with the amount of precipitation data at the same time. 2. Compiling the value of data series The data series of rainfall (per hour) during February 2016 as the time period of this research compiled with time series average cloud top brightness temperature IR1 channel data. The compilation of these two parameters was conducted to find out the relationship of the average cloud top brightness temperature for one hour with the incident and the amount of rainfall that occurred before the time of observation in the same time period. For example, for the amount of precipitation at 00 to 01 UTC, the analysis of the relationship of events and the amount of precipitation with the average cloud top brightness temperature for one hour is starting from 285

4 00 to 01 UTC and consist of cloud top brightness temperature data at 00:10, 00:20, 00:30, 00:40, 00:50, and 01:00 UTC. 3. Analysis a. Comparing the rainfall events data (per-hour) with the average cloud top brightness temperature IR1 channel data for one hour at the same time period. b. Grouping the comparing data into two : 1. Group A: there are rain at the observation time with colder average cloud top brightness temperatures than -50 C 2. Group B: there are rain at the observation time with warmer average cloud top brightness temperatures than -50 C The treshold which used is based on the previous research that has been done by Panjaitan (2013) to analyze the cloud brightness temperature with rainfall events in Sepinggan Balikpapan Meteorological Station.. c. Calculating the percentage of rainfall event two groups respectively to determine the relationship and distribution of cloud top brightness temperature with the rainfall events. d. Further analysis for Group B to find out the character of cloud top brightness temperature in every rainfall events throughout February 2016 at the observation location. Results and Discussion The observation and recording of precipitation result in Perak I Surabaya Meteorological Station shows that there are 154 events data throughout February 2016 with a variety of events ranging from low intensity to heavy precipitation. Rain events occur as much as 28 days and no rain event occur one time on 13 February The heavier rain event intensity occurred on 6 th of February 2016 with the amount of precipitation for 24 hours reached 65.8 mm. The comparison and relation between average cloud top brightness temperature and hourly precipitation data (per seven days interval) shows in Figure 2. The precipitation value as trace (TTU) was regarded as "0". a 286

5 b Figure 2. Graphic of brightness temperature and amount of precipitation (7 days interval). a) 1 st - 7 th February 2016, b) 8 th -15 th February 2016, c) 16 st -22 nd February 2016, d) 23 rd -29 th February

6 Based on Figure 2 there are variations in the average of cloud top brightness temperature for each rain events which observed. The results shows that from 154 rainfall events data, 60 (38.96%) events of the total rain events were accompanied by colder average cloud top brightness temperature than -50 C (Group A), and 94 (61.04%) events of the total rain events were accompanied by the warmer average cloud top brightness temperature than -50 C (Group B). Details of precipitation from rain events in group A are as follows: 1. Amount of Precipitatiton 0-5 mm = 51 events 2. Amount of Precipitatiton 5-10 mm = 4 events 3. Amount of Precipitatiton mm = 2 events 4. Amount of Precipitatiton > 20 mm = 3 events While the details for rain events in group B are as follows: 1. Amount of Precipitatiton 0-5 mm = 84 events 2. Amount of Precipitatiton 5-10 mm = 6 events 3. Amount of Precipitatiton of mm = 3 events 4. Amount of Precipitatiton > 20 mm = 1 event Rain events in group B are very interesting for further analysis. Based on data analysis of top cloud brightness temperature per 10 minutes it was known that within one hour (rain event period) there were some data which has brightness temperature of cloud top colder than -50 C, even though the average of top cloud brightness temperature is warmer than -50 C. Rain event with that state is classified as group C. There were 26 (27.66%) rain events of the total events in group B and there are 9 (34.61%) events of rain in group C with the amount of precipitation more than 1 mm for one hour. The data are presented in Table 1. Details of the rainfall amount that occurred as follows: 1. Amount of Precipitatiton 0-5 mm = 21 events 2. Amount of Precipitatiton 5-10 mm = 3 events 3. Amount of Precipitatiton mm = 2 events 4. Amount of Precipitatiton > 20 mm = 0 occurrences Table 2. The rainfall events from group C which have average cloud top brightness temperature < -50 o C Date Time (UTC) The average of cloud top brightness temperature ( 0 C) Amount of precipitation (mm) Brightness temperature < - 50 C Brightness temperature ( 0 C) , ,9-62, , , ,95 0,1-57, , ,7 1,7-60, , , , ,3-58, , ,4-53, ,1 0-51, , ,8-53, ,15 0,3-51, , , ,7 0-53, , , , , , ,1 288

7 , , , ,55 2, ,3 2, , , ,9 0, , , ,25 6, ,7 1,4-66,7-53,2-50,3-67,5-56,8-61,1-74,5-72,4-58,9-53,7 The further analysis was done to see the characteristic of precipitation cloud from all three groups. Rain events in group B were not accompanied with colder cloud top temperature than C within time period of rain, is hypothesized occur because the possibility of parallax error in Himawari- 8 satellite observation angle. Himawari -8 is at 128,2 0 E and the top cloud is at E 0 which with that posisition the possibility of parallax error in cloud top projection to observation point might be happen. It means that the cloud top brightness temperature which shown at the some point or interest area is not always reflects the real value of that point. It can be assumed because there are might be a change of observation point for cloud top projection in the imagery. The analysis for the possibility of parallax error is done for the rain event on 8 th of February 2016 at 7 to 8 UTC. The analysis was conducted using the cloud contour imagery which extracted with C as the maximum temperature. This analysis was done to see the characteristic of cloud around interest area. The imagery shown in Figure 3. Figure 3. Cloud contour imagery on 8 th of February 2016 at 7 to 8 UTC. Black circle is the interest area and red circle is cloud coverage. The Figure 3 shows that above the interest area there is no cold cloud detected as a trigger of the rainfall. The cold cloud with the cloud top temperature reach C was detected on the left side of interest area. This case is match with the hypothesis about parallax error of satellite observation angle for detect the presence of cloud which trigger the rain event at that time. Therefore the cloud top brightness temperature which showed at the time of rainfall event is not colder than C. The analysis for the possibility of parallax error is conducted too for the rain event which accompanied by cold cloud top temperature case. The analysis conducted using cloud contour imagery as suggest before which shown at Figure 4 for rain event on 6 th of February 2016 at 18 to 19 UTC. The analysis shows that there are cold cloud above the interest area with cloud top temperature reach - 289

8 72 0 C. This cloud is regarded as the trigger of rainfall event. If it connected with the possibility of parallax error, actually this cloud was not exactly above the interest area. Altough there is parallax error, with its large coverage area the cloud still can be hypothesized occur above the interest area and as the trigger of the rain at that time. Figure 3. Cloud contour imagery on 6 th of February 2016 at 18 to 19 UTC. Black circle is the interest area and red circle is cloud coverage. Conclusion Based on a comparative analysis of the average cloud top brightness temperature within a period of one hour with hourly rainfall data, the result shows that the distribution of the average cloud top brightness temperature during the rainfall events were observed in the Perak I Surabaya Meteorological Station were highly variable. The rainfall event may occur in the colder average top cloud brightness temperature than C (group A) or warmer than C (Group B). There are 60 (38.96%) events in Group A of the total rainfall events and in group B occurs as much as 94 (61.04%) events with the various amount and intensity of precipitation. The results of futher analysis for the percentage of rain events in group B yields a new group named the Group C. Group C is a group of rainfall event that occurred with the warmer average cloud top brightness temperature than C in a condition which the brightness temperature of cloud top is colder than -50 C. The number of rain events in the group C is 26 (27.66%) times of group B total rainfall events. Group C analysis results shows that the rainfall events occurred in range from 0 to 20 mm and for amount of precipitation more than 1 mm is 9 (34.61%) events. In general can be said that with the warmer average cloud top brightness temperature than -50 o C, rain events in the region of Perak I Surabaya Meteorological Station still can occur. The condition occurs in two circumstance, those are when in the same period time of rain there are colder or warmer cloud top brightness temperature than -50 C. The result of the the analysis for precipitation cloud characteristic shows that for the rain event which not accompanied by colder cloud top temperature than -50 o C can occurs becuase an parallax error of cloud observation angle. This parallax error causes the cloud with top cloud temperature value colder than -50 o C not exactly detected above the interest area. 290

9 Acknowledgment The authors would like to say thank you for those who have helped in completing this paper especially for Mr. Andersen L Panjaitan and the data provider BMKG. Authors hope with this paper, we can all understand the relationship between brightness temperature of satellite data and the rainfall event. There are many flaws in this paper, to the critic and suggestion could be delivered to my . References Aryani. (2013). Pembuatan Peta Potensi Curah Hujan dengan Menggunakan Citra Satelit MTSAT di Pulau Jawa. E-Journal UNDIP. Handoko, (1994). Klimatologi Dasar. Pustaka Jaya. Himawari-8 Infrared Channel Data. Subbidang Pengelolaan Citra Satelit BMKG. ftp:// Panjaitan, B. (2013). Analisis Hubungan Suhu Kecerahan dari Satelit Kanal Inframerah dengan Hujan Pada Bulan Februari 2013 Di Stasiun Meteorologi Balikpapan. Parwati., Suwarsono., Ayu, K., Kartasamita, M., (2009). Penentuan Hubungan Antara Suhu Kecerahan Data Mtsat Dengan Curah Hujan Data Qmorph. Penginderaan Jauh Journal. Vol. 6, 2009, hal Tahir, W., Ibrahim, Z., Ramli, S. (2009). Geostasionary Meteorological Satellite-Based Quantitative Rainfall Estimation (GMS-Rain) For Flood Forecasting. Malaysian Journal of Civil Engineering. Vol.21 (1), Winarso, P. A. (2011). Analisa Cuaca I. Book. State Collage of Meteorology Climatology and Geophysic Jakarta. 291