CHARACTERISTICS OF RAINFALL AND PRECIPITABLE WATER IN THE ANNUAL AND SEMIANNUAL RAINFALL IN THE AREA WITH VARIOUS INTENSITY OF EL NINO (SUMATERA CASE STUDY)

Indonesia has various islands lying in the maritime continent surrounded by oceans with strong cloud convection. El Nino events, whether at weak, medium or, strong intensity, have the potential to cause different convection effects. Based on these conditions, it is necessary to analyze the characteristics of rainfall when El Nino occurred in Sumatra. The analysis was carried out by looking at the percentage of dominant frequency events of Below Normal (BN), Normal (N), and Above Normal (AN) rainfall in El Nino years. The analysis was also carried for the relationship between the content of precipitable water and sea surface temperatures in the Central Pacific (Nino of 3.4 indexes). The analysis results show that the effect of El Nino (in the annual rain cycle) is stronger than the region with the semi-annual rain cycle. The annual rain cycle area in the El Nino intensity category is weak, medium, strong (predominantly BN-N rain characteristics). In the semi-annual rainfall cycle (rainfall between N-AN) it is medium (BN-AN), and strong (N-BN). The average composite percentage in the years of El Nino shows a weak intensity with the average of rain BN between 37-50%, medium (42-53), and strong (42-48%). Whereas in the annual cycle region, the average rainfall characteristics of the BN category are weak (50-58%), moderate (36-56%), and strong (3260%). Characteristics of different rain characteristics are due to differences in the atmospheric ability to capture moisture. Correlation in the region with the annual rain cycle (June-July-August) between the Nino 3.4 Index and the precipitable water content shows a strong (r) correlation (-0.4 to 0.6). This is in contrast with the region semi-annual rain cycle, which shows a weaker correlation (0.2 to 0.3).


INTRODUCTION
Sumatra region is vulnerable to forest and land fires. Knowledge of El Nino Southern Oscillation (ENSO) that causes rainfall anomalies is needed by local governments and other stakeholders to mitigate natural disasters. El Nino is a climate anomaly event that causes the characteristics of rainfall in Indonesia decreased [1]. Actions are needed when El Nino intensity is different; therefore, the right mitigation in a region can be undertaken. The results of previous studies indicate that there is a difference in rainfall cycles between the two regions of northern Sumatra (the annual rain cycle) and the southern (the annual rain cycle) [2,3]. El Nino pattern has a stronger intensity in the dry season compared to the rainy season [4]. Based on these conditions, a study of the impact of El Nino is needed in both regions El Nino with strong intensity influences the lead and lag of the dry season period in Sumatra. Dry season shifts generally lead two to three decades of the El Nino Strong period [5]. Rainfall also affects the northern Sumatra region due to El Nino [6]. The influence of ENSO shows that sea surface temperature affects rainfall anomalies in Indonesia [7]. Under these conditions, it is also necessary to study the variability of rainfall when El Nino occurs under strong, moderate, and weak intensity.
To illustrate the monsoon pattern, several parameters can be used, including zonal wind and Precipitable Water. Zonal winds at an altitude of 850 millibars are also a parameter of the onset of the South China Sea monsoon system [8]. The zonal wind is also to identify anomalous rainfall due to ENSO [9]. Precipitable water (the potential for bulk water) further illustrates the physical characteristics of rainfall that is influenced by the potential for rainfall in the predicted region (maritime continent). The potential of this water vapor becomes a study to find out the characteristics in the annual and annual rain areas. Some PW characteristics to explore atmospheric physical conditions in Sumatra include the PW parameter that can capture the monsoon rainfall system [10] and has a strong relationship with the intensity of the monsoon [11], the monsoon variability that can be explained by the variation of precipitable water (PW) [12] and temporary increase in PW is indicated by the arrival of the monsoon [13]. Besides, PW is also spatially able to explain monsoon variability in spatial distribution as an effect of broad-scale atmospheric circulation [14]. PW analysis can use with reanalysis data (integration of model data and observation) to find out the beginning and variability of the monsoon [15]. The evolution of air humidity in the monsoon system can also be explained by PW [16], and PW can also explain synoptic-scale disturbance in the monsoon system [17]. Precipitable water can also describe climate change [18]. Based on this reason, it is necessary to study the characteristics of water vapor in the Sumatera monsoon system (semi-annual and annual rainfall) when a climate anomaly occurs.

METHOD
The data used for research are observation data and reanalysis. Rainfall data consists of 9 stations, consisting of Sabang, Banda Aceh, Lhokseumawe, Medan, and Pekanbaru which represent semiannual rainfall patterns. Stations representing the annual rainfall cycle consist of Jambi, Taman Bogo, Pangkal Pinang and Palembang. Rain data were obtained from the Meteorology, Climatology, and Geophysics Agency. The data year period is between 20 years from 1986-2015. Rain data observation station location as in FIGURE 1 below. El Nino index data and PW reanalysis data from the National Centers for Environmental Prediction (NCEP). Preliminary analysis to determine the characteristics of rainfall by sampling a number of rainfall station points that represent semi-annual and annual rainfall patterns. Rainfall analysis was carried out in the El Nino years of strong, moderate, and weak intensity. The characteristic of rainfall is obtained from calculating the average in 1986-2015 (20 years) and then calculating the standard deviation (SD) every month. The Below Normal (BN) is defined if monthly rainfall < monthly average -0.5 SD, meanwhile Normal is defined if monthly rainfall average -0.5 SD ≤ Normal Rainfall ≤ monthly rainfall average + 0.5 SD, and Above Normal (AN) > monthly average + 0.5 SD. Furthermore, at each rain observation station, the frequency of rainfall below the BN, N and AN frequency was calculated. From these results obtained the dominant frequency (in the form of a percent) of the nature of rain.
In order to obtain information on the global influence on the nature of rainfall, an average (composite) percentage of rainfall in the years of El Nino occurred in various categories (strong, medium, and weak) was done at each of the rain observation station (TABLE 1). The results obtained are composite monthly averages from January to December. The rest are grouped into two annual and annual groups.  PW is a water vapor content that has the potential to become rain and is expressed in kg/m 2 . Precipitable water is obtained by integrating specific humidity at each pressure level in oneatmosphere column, according to EQUATION 1, with g is the acceleration due to gravity (m/s 2 ), q is the specific humidity (g/kg), and p is the atmospheric pressure (millibar) [19].

Dry season patterns in the Sumatra region
The dry season pattern in the northern region is marked by calculating the average amount of monthly rainfall. When monthly rainfall reaches 150 millimeters of rain [18] is called the rainy season. Dry season criteria occur if rainfall is less than 150 millimeters as shown in FIGURE 3. It shows an example of the start of the season at a dry station that starts in February and  and Pekanbaru) in all categories of El Nino intensity. The characteristics of the N to BN rain occur only in El Nino Strong. This pattern shows that the influence of El Nino for the northern Sumatra region is different for each region. Geographically, Sabang is a small island at the northern of Sumatra surrounded by the ocean of the northern Indian Ocean, Banda Aceh is in the lowlands, Lhokseumawe is surrounded by hills while Deli Serdang and Pekanbaru are lowlands. Different geographical characters cause different atmospheric convection processes from one region to another. Under these conditions, the global influence due to El Nino diminished when El Nino appeared in this region.   In the spatial analysis, the results of all stations for the southern Sumatra region in all El Nino categories (weak, medium, and strong) and stations (Kenten, Jambi, Lampung, and Tanjung Pinang) show the nature of rain between BN-N, as shown in

Water vapor pattern and El Nino index
The results of the analysis characteristic of the rainfall show a different characteristic of the El Nino effects in the two regions. Rainfall in the northern region varies between BN and AN, meanwhile, in the south, it is generally BN-N. Rainfall forming factors are identified from the moisture content in the atmosphere of Sumatra. Water vapor analysis was done to see the content characteristics in both regions. In a period of June-July-August, during the dry season period, the correlation between precipitable water and Nino. 3.4, the correlation is generally higher with a correlation of 0.5 compared with north (r < 0.3) as shown in FIGURE 4.a. This pattern shows that the water vapor content in Sumatra, in the southern part of the country, decreases when the surface temperature in the central Pacific region rises. Meanwhile in the northern shows a weaker association with sea surface temperatures in the Pacific region. This condition causes variability in rainfall in the northern and southern parts of Sumatra in the ENSO period.
Meanwhile, on the right side, in a period of December-January-February (DJF), during the rainy season, rising sea level temperatures in the Western Pacific region did not reduce rainfall in the Sumatra region but even increased rainfall especially in the West Coast region of Sumatera (    The results of previous studies indicate that there is a relationship between ENSO and rainfall in the Western Indonesia region, but it has not been detailed in the semi-annual and annual basis [20]. In general, the influence of semi-annual and annual was obtained in detail in this research. The influence of these two regions is different because the level of association with ENSO is different in the two regions.

CONCLUSION
Based on the character analysis of the rain nature and PW, the effect of El Nino (in the annual rain cycle) is stronger than in areas with a semi-annual rain cycle. The annual rain cycle area in the El Nino intensity category is weak, moderate, and strong, by showing the dominant rainfall properties of BN-N. Whereas, in the semi-annual rain cycle the El Nino intensity categories are weak (the characteristic of rainfall among N-AN), moderate (characteristic of BN-AN), and strong (N-BN). The average percentage of composites in El Nino in the two regions also shows the influence in Sumatra with an annual pattern is stronger than semi-annual. North Sumatra region has a weak intensity with average BN rain characteristic between 37-50%, moderate (42-53 (and strong (42-48%)), whereas in the annual cycle area the average rainfall characteristic of BN rain category is weak ( 50-58%), moderate (36-56%), and strong (32-60%). The characteristics of this rain are different in two regions because there are differences in the ability of the atmosphere to store water vapor (precipitable water). The correlation pattern in the area with the rain cycle (June-July-August) between precipitable water (Nino Index 3.4) and water vapor content showed a strong correlation (r) (-0.4 to -0.6). This shows that if the sea surface temperature at pacific is rising, the water vapor content in areas with annual rainfall cycles has fallen, unlike in the semi-annual rainfall cycle region which shows a weaker correlation (0.2 -0.3).