Diurnal Patterns of Surface Wind at Palabuhanratu, using ERA5 Global Reanalysis Data

Surface wind characteristics are one of the factors in the formation of weather in a place, such as coastal areas that are directly adjacent to the ocean. One of the areas located on the Southern coast of the Java island, is Palabuhanratu, Sukabumi. This study will provide an overview of the characteristics of the diurnal pattern of surface wind in the Palabuhanratu area in the form of bay. The data that will be used in this research is ERA5 global analysis data produced by The European Center for Medium-Range Weather Forecasts (ECMWF) and Observation data. Observational data and ERA5, show different diurnal patterns in each season. This can be seen especially during the day, from 10 to 16 local time. The diurnal pattern of ERA5 also shows different things at the summer (JJA) and during the rainy season (DJF) in Palabuhanratu.


Introduction
Wind is a continuous movement of air caused by a heating process at the ground surface (Nasir, 1990).The uniqueness of the arrangement of islands connected by the sea in the Indonesia maritime continent, has given rise to the characteristics of wind and sea gusts that reach 400 km from the coastline (Short, et.al., 2019).Local variations in land and sea levels as well as front phenomena can also affect the characteristics of surface wind patterns in coastal areas (Hughes and Veron, 2018).For tropical areas, local wind conditions on the surface affect the formation of weather (Handoko, 1995).
The existence of global data reanalysis, ERA5, can be used to determine the diurnal pattern of surface wind of an area.In addition, this pattern of surface wind distribution can also be used to determine the distribution of pollution caused by industrial combustion activities in the Palabuhanratu area.The strength of the surface wind that blows inland can cause waves that head towards the coast (Holthuijsen, 2007), so that it can also cause shoreline erosion or abrasion, as well as other damage in shore and coastal areas (Sugianto, 2010).The Palabuhanratu area, which is the location of this research, is a bay-shaped area in the Southern coast of Sukabumi regency, West Java province, which is directly adjacent to the Indian Ocean.In coastal areas, the process of weather formation is strongly influenced by the circulation of surface winds that blow towards the land and sea (Tjasyono, 2009).In addition, in a study it was said that the Southern region of the Java island has the potential to produce great energy from sea breeze (Purba, 2014).This research aims to explain the condition of the daily pattern of surface winds that exist in the Palabuhanratu area.

Observation Data
Observation data was obtained from the Palabuhanratu Geophysics observations post, Agency of Meteorological, Climatological and Geophysics.Surface wind data in the form of wind direction and speed at an altitude of 10 meters from the surface, from 07 to 19 local time, data used from 2019 to 2021.2.2.Global Reanalysis Data ERA5 ERA5 (ECMWF Reanalysis 5 th Generation), is atmospheric reanalysis data produced by the Copernicus Climate Change Service (C3S) at the European Center for Medium-Range Weather Forecast (ECMWF).ERA5 is capable of providing hourly forecast data of a number of climate elements of the atmosphere, land surface and oceans.Daily and monthly average data from a region can also be found in these dataset models.With its smaller spatial and time resolution, ERA5 is considered capable of capturing more subtle phenomena than previous generation datasets (Hersbach, et.al., 2020).Details of the data used in this study can be seen in the table above.The thing that must be done is to find the amount of wind blowing from each wind direction according to the predetermined interval class.
To determine the class of the wind interval, it is adjusted to the wind speed data.The formula used is : Explanation : F : relative distribution ∑dd : quantitative value of wind direction at each interval ∑ n : quantitative data value for each wind direction In using the Windrose diagram, the wind directions are divided into 8 (eight) directions : North, Northeast, East, Southeast, South, Southwest, West, and Northwest.While the wind speed classification is divided into 7 (seven), that is : 1 -4 knots, 4 -7 knots, 7 -11 knots, 11 -14 knots, 14 -17 knots, and more or equal to 22 knots (Fatkhuroyan, 2018).

Results and Discussion
Analysis is carried out every 3 months, namely : December to February (DJF), March to May (MAM), June to August (JJA) and September to November (SON).This is adjusted to the pattern of season in Indonesia, where DJF represents the wet season, JJA represents the dry season, while MMA and SON are considered transitional seasons (Hermawan, 2015).This applies to both observational data and ERA5 Reanalysis data.

Diurnal Pattern of Observation Data
During the DJF period, the dominant surface wind blows from East-Southeast in the morning, at 7 am LT.During the day, starting at 11 am LT, surface winds begin to predominantly blow from the Southwest -West.In the afternoon, starting at 4 pm LT, surface winds blow from various directions, but are still dominant from the Southwest -Northwest.In the evening, at 7 pm LT, the surface wind blew again from the East -Southeast.
In other periods, the same daily pattern is also shown during the wet season and transition periods.Where in the morning, the wind blows from the East-Southeast, then turns into a gust from the Northwest -Southwest.Then in the afternoon, surface winds blow very dominantly from the Northwest, but during the SON period, strong gusts also occur from the opposite direction, Southeast.At night, 7 pm LT, the dominant wind blows from the East-Southeast.Using observational data, the daily pattern of dominant surface winds blowing from the mainland in the morning is illustrated.But, throughout the day, surface winds blow from the sea, starting around 11 am LT.Then at 7 pm LT, surface winds have been blowing again from the mainland.

Diurnal Pattern of ERA5
The DJF period shows almost the same pattern of surface winds throughout the day.In the morning, at 7 am LT, the dominant surface wind blows from the Northwest -North.
Then during the day and evening, the wind blows from the West -Northwest.At night, the wind blows from the North -West.
During the MAM period, the dominant surface wind blows from the East -Southeast in the morning.During the day, wind gusts vary from the Southeast, South, Southwest, and West.In the afternoon until evening, the dominant surface wind blows again from the East -Southeast.The other periods, namely JJA and SON, have almost the same daily pattern.Surface winds throughout the day, from morning to evening, predominantly blow from the East -Southeast.Only during the day at SON period, there are dominant surface winds from the South.

Conclusion
The analysis carried out, illustrates that by using observational data, the daily pattern of surface winds in Palabuhanratu shows the dominance of local factors, namely the wind that blows from the sea during the day which are called sea breezes, and the wind blows from the land at night which is called with land breezes.Reanalysis of global data ERA5, shows a diurnal pattern that is different form the diurnal pattern shown from the observation data for surface wind at Palabuhanratu.Local factors mentioned earlier are not shown here.ERA5 shows a different diurnal pattern of surface wind, between the dry season (December -February) and the wet season (June -August).

Figure 1 .
Figure 1.Location of the research area (Red box)

Figure 2 .
Figure 2. Windrose diagram of surface winds from observational data

Figure 3 .
Figure 3. Windrose diagram of surface winds from ERA5 Reanalysis data The daily pattern of wind gusts shown using ERA5 Reanalysis data is more influenced by seasonal factors that occur in the Palabuhanratu area, namely the West and East monsoons.Where the West season occurs in the DJF period, while the East season occurs in the JJA period (Aldrian, 2011).

Table 1 .
Data type from ERA5 a method used to analyze the direction and speed of the wind in a place and at a certain time.The windrose diagram shows a comparison of the directions and speeds of the blowing winds.