Indonesian Maritime Continent has the second longest coastline in the world, but the characteristics of offshore rainfall and its relation to coastline type are not clearly understood. As a region with eighty percent being an ocean, knowledge of offshore rainfall is important to support activity over oceans. This study investigates the climatology of offshore rainfall based on TRMM 3B42 composite during 1998-2015 and its dynamical atmosphere which induces high rainfall intensity using WRF-ARW. The result shows that concave coastline drives the increasing rainfall over ocean with Cenderawasih Bay (widest concave coastline) having the highest rainfall offshore intensity (16.5 mm per day) over Indonesian Maritime Continent. Monthly peak offshore rainfall over concave coastline is related to direction of concave coastline and peak of diurnal cycle influenced by the shifting of low level convergence. Concave coastline facing the north has peak during northwesterly monsoonal flow (March), while concave coastline facing the east has peak during easterly monsoonal flow (July). Low level convergence zone shifts from inland during daytime to ocean during nighttime. Due to shape of concave coastline, land breeze strengthens low level convergence and supports merging rainfall over ocean during nighttime. Rainfall propagating from the area around inland to ocean is approximately 5.4 m/s over Cenderawasih Bay and 4.1 m/s over Tolo Bay. Merger rainfall and low level convergence are playing role in increasing offshore rainfall over concave coastline.
Indonesian Maritime Continent (IMC) is an archipelago tropical country which has the second longest coastline in the world after Canada. The length of the coastline reaches 95,181 km and the numbers of islands are 16,056. Distribution of global and regional rainfall is affected by IMC’s coastline. The distribution of local rainfall amount (a rainfall point less than 300 km from coastline) is a gradually decreasing as function of coastal distance, while total regional rainfall (rainfall area more than 10,000 km2) is increasing as function of coastline density (coastal length/land area) [
The shape and size of the coastline over IMC are unique. Basically, there are three shapes of coastline which affect the rainfall amount, straight coastline, concave coastline, and convex coastline [
The rainfall system over IMC is known as the coastal regime. It has characteristic of heavy rainfall over coastal area and dominant movement phase which is divided into two types, seaside and landside regime. The seaside coastal regime peak occurs from evening to morning of the next day and propagates to offshore area. The landside coastal regime has landward phase propagation with the peak occurring from morning to evening [
Offshore rainfall is formed from the clouds which grow and develop over inland and move to the ocean. Offshore rainfalls over west Sumatera develop over inland and migrate to ocean until 400 km from coastlines with the average speed of migration being approximately 10 m/s [
Offshore rainfall over northern [
Several previous studies have described the characteristics and mechanism of the offshore rainfall over IMC without considering the shape and the effect of the coastline [
In order to detect the distribution and variability of offshore rainfall over concave coastline, we analyze Tropical Rainfall Measuring Mission (TRMM) 3B42 (version 7) rainfall product during 17-year observation from 1998 to 2015. The 3B42 product contains estimated rain rates (mm per hour) derived from a combination of infrared radiation (IR), passive microwave, and radar data from TRMM and IR data from geostationary satellites. The data is 3 hourly and covers the domain between 50°S and 50°N, with a horizontal resolution of 0.25° x 0.25°.
In order to examine the dynamical atmosphere inducing the offshore rainfall, we conduct the Weather Research and Forecast-Advanced Research Weather (WRF-ARW) v 3.3, based on the NCEP FNL (Final) Operational Global Analysis data used for initial and boundary conditions. NCEP FNL is a reanalysis product from the Global Data Assimilation System (GDAS) which continuously collects observational data from the Global Telecommunications System (GTS) and other sources.
In this experiment, we set up WRF-ARW with three nested configuration (see Figure
WRF-ARW nesting domain setting with (i) first domain resolution of 27 km, (ii) second domain resolution of 9 km representing Tolo Bay, and (iii) third domain resolution of 9 km representing Cenderawasih Bay.
The microphysics WSM 6 class single-moment scheme by Hong and Lim [
WRF-ARW configuration for this experiment.
Domain 1 | Domain 2 | Domain 3 | |
---|---|---|---|
Resolution | 27 km | 9 km | 9 km |
Parameterization | |||
Microphysics | WSM 6 class | WSM 6 class | WSM 6 class |
Cumulus | Kain–Fritsch | Kain–Fritsch | Kain–Fritsch |
Planetary Boundary Layer | YSU | YSU | YSU |
Radiation | Dudhia | Dudhia | Dudhia |
Land Surface | Noah LSM 4-layer | Noah LSM 4-layer | Noah LSM 4-layer |
Simulation of rainfall using WRF-ARW over IMC has been studied by Fujita [
The IMC has the greatest convective activity and the largest amount of rainfall in the world [
Total mean of offshore rainfall over Indonesian Maritime Continent based on composite TRMM during 1998 to 2015. The heavy offshore rainfall occurs over (i) Manado Bay, (ii) Tomini Bay, (iii) Tolo Bay, (iv) Bone Bay, (v), Berau Bay, (vi) Cenderawasih Bay, and (vii) Strait of Makassar. Red line cross section for Tolo Bay and white line cross section for Cenderawasih Bay.
This pattern of offshore rainfall shows the greatness of the local effect. The peak of rainfall over concave coastline is located near the focus point of curvature line. Focus point is the point where sea breeze meets inducing the convergence zone with assumption that curvature has smooth line and the land breeze direction has perpendicular direction to the coastline. Because real coastline is irregular and unsmooth lines, the peak of offshore rainfall is shifted and irregularly shaped. Peak of offshore rainfall over Manado Bay and Tomini Bay is approximately 100–200 km from coastline while that over Tolo Bay, Bone Bay, and Cenderawasih Bay is very close to coastline. Previous research by Love [
The strong or weak offshore rainfall intensity will be determined by the width of sea [
The effect of terrain height on land breeze was simulated by Qian [
Cenderawasih Bay and Tolo Bay are two of highest offshore rainfalls in Eastern Maritime Continent. For the next analysis, we use Cenderawasih Bay and Tolo Bay to reveal the characteristic of offshore rainfall over Eastern Indonesia Maritime Continent. Figure
Box and whisker plot for monthly offshore rainfall variability over Cenderawasih Bay (blue) and Tolo Bay (red). The top and bottom of the large box denote the 25th and 75th percentiles. The circle in the box denotes the 50th percentile or median of the distribution. The bars extend to the largest or smallest value within 1.5 times the interquartile range of the 75th or 25th percentiles, respectively. The interquartile range is defined as the distance between the 25th and 75th percentile values. The whiskers are outliers.
Cenderawasih Bay has peak of offshore rainfall in March while Tolo Bay has peak of rainfall in June (see Figure
The main factor of the different season of peak of offshore rainfall over Cenderawasih Bay and Tolo Bay is the direction of concave coastline faces. Because Cenderawasih Bay faces north, the northwesterly monsoonal flows on boreal winter will be blocked by the mountains and produce offshore rainfall on the north side of the mountains and over oceans. Conversely, because Tolo Bay faces east, the easterly monsoonal flows on boreal summer will be blocked by the mountains around Tolo Bay and produce offshore rainfall on Tolo water. The role of the mountain as a monsoonal wind block resulting in strong local rainfall around Cenderawasih Bay (Papua Island) is explained by Ichikawa [
The rainfall cycle over IMC is predominant by diurnal variability. Nikita and Sekine [
Different rainfall day times–nighttime around IMC. Minus value describes the rainfall mostly at nighttime over concave coastline, i.e., (i) Manado Bay, (ii) Tomini Bay, (iii) Tolo Bay, (iv) Bone Bay, (v) Berau Bay, and (vi) Cenderawasih Bay.
The regional variation of annual mean difference between morning rainfall and evening rainfall over IMC observed by TRMM 3G68 has been examined by Mori [
In order to examine the dynamical atmosphere which induces intensive offshore rainfall over the concave coastline, we use WRF-ARW to simulate the offshore rainfall and determine the wind variability. The study of comparing the rainfall over IMC was simulated by a 20 km grid Meteorological Research Institute General Circulation Model (MRI-GCM) and the near-surface rain data of TRMM 2A25 examined by Hara [
Figures
Diurnal rainfall over Maritime Continent based on TRMM 3B42 (left side) during March 2014 with (a) 12LT, (b) 15LT, (c) 18LT, (d) 21LT, (e) 00LT, (f) 03LT, and (g) 06LT. Diurnal rainfall over Maritime Continent based WRF-ARW (right side) during March 2014 with (h) 12LT, (i) 15LT, (j) 18LT, (k) 21LT, (l) 00LT, (m) 03LT, (n) 06LT, and (o) comparison of TRMM and WRF rainfall over Cenderawasih Bay and Tolo Bay.
Firstly, rainfall begins to form and grow near the mountains of the island (see Figures
Figures
At early evening (see Figure
During nighttime where the land breeze has intense activity, ocean region over Cenderawasih Bay and Tolo Bay has highest offshore rainfall intensity (see Figures
During early morning (see Figures
The migration of offshore rainfall over Cenderawasih Bay and Tolo Bay is revealed by Figures
Hovmoller diagram shows propagation of rainfall over (a) Cenderawasih Bay and (c) Tolo Bay. (b) is the terrain height around Cenderawasih Bay based on cross section in Figure
Low level convergence zone over Cenderawasih Bay during (a) daytime (12 am) and (b) nighttime and low level convergence zone over Tolo Bay during (c) daytime (12 am) and (d) nighttime.
The speed movement rainfall over concave coastline has a slower speed than the rainfall movement from the mountaintop to open ocean. Ichikawa and Yasunari [
Figure
Intense convergence over ocean occurs during the nighttime. Because of the strong land breeze flow, the convergence moves its position from the land to ocean. This convergence over ocean is formed between the land breeze and prevailing winds, facing inland direction. This mechanism also occurs in other regions such as Borneo [
Previous research has suggested schematic rainfall over the IMC without considering the shape of coastline [
The dynamical atmosphere which induces the offshore rainfall over concave coastline is not quite far different from the mechanism developing offshore rainfall over west coast of Sumatera or west of Borneo Island. The main difference is the direction of land breeze due to the shape of concave coastline which creates convergence zone just only located in the middle of ocean. The merger of rainfall and propagation of rainfall from inner to outer side of ocean also causes more complex system. The mechanism of developing offshore rainfall over concave coastline can be seen in Figure
Schematic of rainfall and migration during (a) daytime, (b) early nighttime, (c) nighttime, and (d) early morning.
Sea breeze plays a significant role in the formation of inland rainfall at daytime while land breeze plays a major role in developing offshore rainfall at night until morning time. The development of rainfall begins at inland when the sea breeze has intense wind speed (see Figure
At the early nighttime (see Figure
The highest offshore rainfall occurs during intense land breeze existence (see Figure
In the early morning (see Figure
Surface potential temperature over (a) Cenderawasih Bay and (b) Tolo Bay. Lower temperature (K) is indicated as cold pool area.
In this study, we found that the concave coastline is a significant factor increasing the nighttime convective activity over ocean. Concave coastline generates more intense land breeze convergence than adjacent ocean with a straight coastline. The highest mountaintop around Cenderawasih Bay and concave coastline shape compose the highest offshore rainfall over Indonesian Maritime Continent. Monthly peak offshore rainfall over concave coastline is related to direction of concave coastline and peak of diurnal cycle influenced by the shifting low level convergence. Concave coastline facing the north has peak during northwesterly monsoonal flow (March), while concave coastline facing the east has peak during easterly monsoonal flow (July). Low level convergence zone shifts from inland during daytime to ocean during nighttime. Strong land breeze induced more low level convergence intensity to produce more offshore rainfall.
The schematic offshore rainfall over IMC is a guidance weather forecaster operational in making weather forecasts especially maritime weather information. By considering the circulation of land breeze and convergence position, the weather forecaster can determine potential high offshore rainfall area.
The data supporting this article is provided within the article. The datasets generated and analyzed during the current study are available from the corresponding author upon reasonable request.
The authors declare that they have no conflicts of interest.
The authors would like to express their gratitude to Indonesian Agency for Meteorology Climatology and Geophysics (BMKG) and Bogor Agricultural University (IPB) for partial funding and opportunities to finish this research.