3.1 Windshear frequency and intensity
The frequency of WS in different intensity such as light, moderate, strong, and severe at six station locations Abha, Jeddah, Madinah, Tabuk, Riyadh, and Dammam airport calculated using radiosonde data are discussed here.
The frequency of light WS is about 25 per year at the bottom most two levels 0–30 m and 31–60 m, which come down to about 15 at the next higher level 61–90 m for the station Abha (Fig. 2). The frequency of light WSA continues about 15–20 per year up to level 200 m and comes down to about 5–10 at 1200 m (not shown). The frequency of moderate and strong WSA are observed very few at this southwest hilly station location. The severe WSA observed about 5 per year which suddenly jumped to about 35 per year from 2015 at the bottom most level 0–30 m only. Such large number of severe WSA are not observed at other layers and before 2015 at any level.
The frequency of light WS is about 40 per year at the bottom most level 0–30 m for the western station Jeddah (Fig. 3). The frequency of light WS increases to 50 per year at higher level 31–60 m, which continues up to 900 m, then gradually comes down to about 10 per year at level 1200 m (not shown). In this station, the severe WS observed about 10 per year before 2015, which suddenly jumped to about 40 per year from 2015 at the bottom most level 0–30 m only. Such large number of severe WS not observed at other layers and before 2015.
The frequency of light WS is about 25 per year during the period 2006–2014 at the bottom most level 0–30 m for the station Madinah (Fig. 4). The frequency of light WS increased to about 45 per year at higher level 61–90 m, and continued up to 200 m, then gradually came down to about 10 per year at analyzed level 1200 m (not shown). In this station, the severe WS observed about 5 per year before 2015, which suddenly jumped to about 25 per year from 2015 at the bottom level 0–30 m only. Such large number of severe WS not observed at other levels and before 2015. When the frequency of severe WS increase after 2015, the light WS decrease to about 10 per year.
The frequency of light WS is about 25 per year at the bottom level 0–30 m for the station Tabuk for most of the year except for a few years (Fig. 5). The frequency of light WSA was almost the same 25 per year up to 800 m, then gradually came down to about 10 per year at analyzed level 1200 m (not shown). In this station, the severe WS observed about 5 per year before 2015, which suddenly jumped to about 25 per year from 2015 at the bottom level 0–30 m only. Such large number of severe WS not observed at other levels and before 2015.
The frequency of light WS is about 20 per year at the two bottom levels 0–30 m and 31–60 m for the station Riyadh (Fig. 6). The frequency of light WS increased to about 40 per year up to 900 m, then gradually came down to about 10 per year at analyzed level 1200 m (not shown). In this station, the severe WS observed about 5 per year without any jump after 2015. The behavior of severe WS at this station is different from the other stations analyzed in this study.
The Dammam station (OEDF) located in the eastern side of the country. In this station the frequency of light WS is about 15 per year at the bottom level 0–30 m (Fig. 7). Above this lower level, the frequency of light WS increase up to 35 per year up to 900 m, then gradually come down to about 10 per year at analyzed level 1200 m (not shown). In this station, the severe WS observed below 5 per year before 2015, which suddenly jumped to about 35 per year from 2015 at the bottom level 0–30 m only. Such large number of severe WS not observed at other levels and before 2015.
The annual cycle of the WS obtained in different intensities at six station locations is shown in Fig. 8. The highest WS frequency observed for light intensity followed by severe one almost at all stations. The light and strong WS occurred almost every month with little exception at station to station. For the station Abha, light WS observed highest (10.1) in October and lowest (4.9) in April. The severe WS is highest (6.5) in March and June while lowest (3.7) in November. The occurrence of the other two categories is below 2. Annually the light, moderate, strong, and severe WS occurred 93.3, 12.2, 6.8, and 65.5 respectively at Abha station. The light WS occurred highest (28.5) in autumn while severe WS occurred about 17.5 in almost all seasons except about 13 in autumn.
For Dammam station, severe WS occurred higher than the light WS from November to April except in March it is closer to light one. In this station, annually light, moderate, strong, and severe WS occurred 76.1, 15.4, 8.3, and 65.4 respectively. Seasonally, light WS occurred highest 22.9 in summer and severe WS occurred highest 18.8 in winter followed by 17.5 in spring. Annually Jeddah station shows light, moderate, strong, and severe WS respectively 166.5, 26.0, 11.9, and 94.9. The light WS is highest 81.9 in summer while for sever it is large in spring (27.1) and summer (26.0). In this station, severe WS occurred almost at the same frequency every month. Madinah station annually shows 92.5, 10.5, 8.0, and 54.5 for light, moderate, strong, and severe WS respectively. For both light and severe WS, almost similar number of WS occurred in every month. The light WS occurred highest 25.3 in spring followed by 24.9 in autumn while severe WS occurred highest 16.4 in summer followed by 15.2 in spring.
As mentioned earlier, Riyadh station shows most of the light WS activities. Annually, 104.3, 13.9, 6.0, and 18.4 light, moderate, strong, and severe WS observed in this station. Light WS occurred in all seasons with the highest 28.1 in autumn. Tabuk station annually shows 176.0, 31.9, 15.4, and 55.9 light, moderate, strong, and severe WS respectively. The light WS occurred highest 53.4 in autumn followed by about 46.5 in winter and spring while severe WS is highest 16.8 in spring season. Overall, the highest WS observed in autumn season for all stations except it is in summer for Jeddah.
The occurrence of WS in daytime (12:00) and nighttime (00:00) at different station locations over Saudi Arabia indicates that there are regional variations as well as depends on the category or strength (Table 1). In Abha station, frequency of light WS in the daytime is higher than the nighttime for all the months except lower in May, June, August, and September. However, severe WS frequency is higher in the daytime compared to nighttime round the year and difference is very large. In Jeddah station, frequency of light WS is higher in the nighttime as compared to the daytime in all months except lower in February. In this station, severe WS frequency is high in the nighttime as compared to daytime round the year. The frequency of both light and severe WS at Dammam, Madinah, Riyadh and Tabuk stations is almost opposite pattern of Jeddah station, in these four stations the occurrence of WS in the daytime is higher than the nighttime round the year. The frequency of moderate and strong WS are a few, hence the variation in day and night are closer.
Table 1
The monthly frequency of light, moderate, strong, and severe WS at six station locations in Saudi Arabian airports averaged over the period 1991–2020.
|
|
OEAB
|
OEDF
|
OEJN
|
OEMA
|
OERK
|
OETB
|
|
|
12
|
00
|
12
|
00
|
12
|
00
|
12
|
00
|
12
|
00
|
12
|
00
|
Light
|
JAN
|
3.7
|
2.4
|
2.7
|
0.8
|
2.5
|
3.3
|
3.6
|
2.7
|
4.1
|
3.0
|
7.7
|
7.0
|
FEB
|
4.1
|
2.3
|
2.4
|
1.5
|
3.5
|
3.0
|
3.9
|
2.1
|
4.7
|
3.4
|
7.0
|
5.6
|
MAR
|
3.6
|
2.0
|
3.5
|
2.2
|
3.6
|
4.2
|
5.4
|
3.3
|
5.2
|
3.3
|
8.0
|
7.0
|
APR
|
2.8
|
1.9
|
2.7
|
2.1
|
3.5
|
3.9
|
4.9
|
3.0
|
4.1
|
2.0
|
7.1
|
6.3
|
MAY
|
3.5
|
3.7
|
4.6
|
3.3
|
5.1
|
6.6
|
4.8
|
3.6
|
4.5
|
3.5
|
7.1
|
6.9
|
JUN
|
3.7
|
3.9
|
3.7
|
3.2
|
11.7
|
13.5
|
4.9
|
3.9
|
4.3
|
3.0
|
6.1
|
3.8
|
JUL
|
4.1
|
3.3
|
3.7
|
2.2
|
12.2
|
14.1
|
4.2
|
2.7
|
4.2
|
3.2
|
5.9
|
2.4
|
AUG
|
4.0
|
4.6
|
4.2
|
2.7
|
13.2
|
12.1
|
4.9
|
2.8
|
4.8
|
4.4
|
5.8
|
2.9
|
SEP
|
3.8
|
5.4
|
4.8
|
3.2
|
7.6
|
8.6
|
4.6
|
3.9
|
4.5
|
3.9
|
8.1
|
5.5
|
OCT
|
5.0
|
4.3
|
4.5
|
1.7
|
3.5
|
4.2
|
5.7
|
4.1
|
4.9
|
4.4
|
10.7
|
9.0
|
NOV
|
4.7
|
3.6
|
3.3
|
1.2
|
2.8
|
3.1
|
3.7
|
2.7
|
3.7
|
3.1
|
7.6
|
7.2
|
DEC
|
4.4
|
3.9
|
2.7
|
0.9
|
3.6
|
3.8
|
3.6
|
2.6
|
3.7
|
3.5
|
6.9
|
6.8
|
Moderate
|
JAN
|
0.5
|
0.3
|
0.3
|
0.8
|
0.8
|
0.5
|
0.4
|
0.3
|
0.5
|
0.5
|
1.4
|
1.8
|
FEB
|
0.7
|
0.1
|
0.2
|
0.6
|
0.7
|
0.3
|
0.4
|
0.2
|
0.6
|
0.5
|
1.2
|
1.6
|
MAR
|
1.1
|
0.3
|
0.2
|
0.5
|
1.0
|
0.4
|
0.6
|
0.1
|
0.5
|
0.4
|
1.3
|
1.7
|
APR
|
0.6
|
0.0
|
0.2
|
0.3
|
1.3
|
0.7
|
0.6
|
0.2
|
0.9
|
0.4
|
1.2
|
1.0
|
MAY
|
0.6
|
0.2
|
0.4
|
1.3
|
1.3
|
1.4
|
0.6
|
0.4
|
0.5
|
0.4
|
1.1
|
1.3
|
JUN
|
0.9
|
0.3
|
0.5
|
0.9
|
1.4
|
1.8
|
0.7
|
0.4
|
1.0
|
0.6
|
0.8
|
0.8
|
JUL
|
0.6
|
0.1
|
0.3
|
0.6
|
1.5
|
1.2
|
1.2
|
0.4
|
0.9
|
0.7
|
0.6
|
0.3
|
AUG
|
0.7
|
0.1
|
0.4
|
0.7
|
1.4
|
1.7
|
1.2
|
0.3
|
0.5
|
0.6
|
0.5
|
0.3
|
SEP
|
1.0
|
0.4
|
0.5
|
1.4
|
1.4
|
1.6
|
0.6
|
0.2
|
0.6
|
0.5
|
0.7
|
0.8
|
OCT
|
0.9
|
0.5
|
0.3
|
0.6
|
0.9
|
0.6
|
0.5
|
0.1
|
0.5
|
0.3
|
1.1
|
1.7
|
NOV
|
0.7
|
0.1
|
0.4
|
0.8
|
1.0
|
0.4
|
0.4
|
0.1
|
0.5
|
0.0
|
1.2
|
1.7
|
DEC
|
0.5
|
0.2
|
0.2
|
0.7
|
0.7
|
0.3
|
0.2
|
0.3
|
0.4
|
0.3
|
1.1
|
1.0
|
Strong
|
JAN
|
0.4
|
0.2
|
3.8
|
0.4
|
4.6
|
0.2
|
0.5
|
0.4
|
0.2
|
0.3
|
0.7
|
0.2
|
FEB
|
0.3
|
0.2
|
3.6
|
0.3
|
5.2
|
0.3
|
0.4
|
0.2
|
0.1
|
0.5
|
0.7
|
0.6
|
MAR
|
0.5
|
0.3
|
4.1
|
0.4
|
6.6
|
0.3
|
0.3
|
0.1
|
0.2
|
0.4
|
0.6
|
0.7
|
APR
|
0.5
|
0.1
|
4.1
|
0.2
|
7.0
|
0.3
|
0.5
|
0.2
|
0.3
|
0.8
|
0.6
|
0.5
|
MAY
|
0.5
|
0.1
|
3.2
|
0.6
|
6.8
|
0.3
|
0.4
|
0.2
|
0.1
|
0.7
|
0.6
|
0.7
|
JUN
|
0.3
|
0.1
|
3.1
|
0.5
|
6.4
|
0.7
|
0.5
|
0.2
|
0.3
|
0.6
|
0.5
|
0.3
|
JUL
|
0.5
|
0.0
|
3.5
|
0.3
|
6.8
|
0.9
|
0.8
|
0.4
|
0.5
|
0.4
|
0.3
|
0.3
|
AUG
|
0.5
|
0.1
|
3.6
|
0.5
|
6.0
|
0.7
|
0.5
|
0.1
|
0.4
|
0.3
|
0.6
|
0.3
|
SEP
|
0.4
|
0.2
|
3.1
|
0.3
|
6.6
|
0.6
|
0.5
|
0.1
|
0.2
|
0.4
|
0.5
|
0.4
|
OCT
|
0.2
|
0.1
|
3.0
|
0.2
|
5.8
|
0.1
|
0.3
|
0.1
|
0.2
|
0.2
|
1.0
|
0.8
|
NOV
|
0.2
|
0.0
|
3.2
|
0.5
|
5.3
|
0.2
|
0.2
|
0.1
|
0.3
|
0.2
|
0.8
|
0.5
|
DEC
|
0.5
|
0.2
|
3.8
|
0.5
|
5.1
|
0.2
|
0.4
|
0.2
|
0.2
|
0.3
|
0.8
|
0.5
|
Severe
|
JAN
|
4.9
|
1.0
|
2.7
|
0.4
|
0.6
|
1.8
|
2.5
|
0.8
|
0.8
|
0.3
|
2.2
|
1.5
|
FEB
|
4.2
|
1.3
|
1.6
|
0.1
|
0.3
|
1.6
|
2.7
|
1.1
|
0.9
|
0.1
|
2.2
|
1.4
|
MAR
|
5.0
|
1.1
|
1.4
|
0.3
|
0.5
|
1.9
|
3.9
|
1.1
|
1.2
|
0.2
|
3.2
|
2.3
|
APR
|
4.1
|
0.7
|
2.2
|
0.2
|
0.4
|
2.0
|
3.9
|
1.6
|
1.1
|
0.1
|
3.0
|
1.7
|
MAY
|
4.4
|
0.2
|
1.7
|
0.3
|
0.7
|
2.2
|
3.0
|
1.3
|
1.0
|
0.2
|
2.6
|
2.1
|
JUN
|
5.6
|
0.4
|
1.7
|
0.1
|
0.5
|
2.2
|
3.1
|
2.2
|
1.3
|
0.2
|
2.6
|
2.1
|
JUL
|
4.8
|
0.4
|
1.3
|
0.1
|
0.7
|
2.1
|
3.9
|
1.5
|
1.7
|
0.3
|
1.9
|
2.1
|
AUG
|
4.3
|
1.0
|
0.9
|
0.1
|
0.7
|
2.1
|
4.0
|
1.4
|
1.1
|
0.2
|
1.7
|
1.2
|
SEP
|
4.9
|
0.4
|
0.7
|
0.3
|
0.9
|
1.4
|
2.8
|
0.6
|
1.2
|
0.2
|
2.0
|
1.9
|
OCT
|
3.1
|
0.5
|
0.9
|
0.2
|
0.3
|
0.7
|
3.3
|
0.7
|
0.4
|
0.3
|
2.5
|
1.6
|
NOV
|
3.1
|
0.3
|
2.0
|
0.3
|
0.4
|
0.7
|
2.7
|
1.2
|
0.8
|
0.3
|
2.5
|
1.5
|
DEC
|
4.6
|
0.9
|
2.2
|
0.3
|
0.3
|
1.3
|
2.8
|
1.1
|
0.6
|
0.2
|
2.0
|
1.2
|
3.2 Windshear and synoptic conditions
The WS calculated from radiosonde data at different vertical height slices (each 30 m from 0 to 1200 m) indicates that about 25–30 per year light WS existed from bottom to up to 1200 m. The existence of moderate and strong WSA are not much (below ~ 5 per year) observed at different station locations in Saudi Arabia. However, the number of severe WS observed was about 30–35 per year at bottom level (0–30 m) from 2015 in all station locations except for Riyadh it is not observed. Before the year 2015, such a high number of severe WS is not recorded. Therefore, to understand the synoptic condition on the occurrence of such high number severe WS, the pressure, temperature and wind speed are discussed.
The surface pressure, temperature, and wind field of the continuous 10-year period for the period 2010–2019 indicate that the patterns of these variables are closer to each other year to year (Fig. 9). However, careful inspection indicates that there is a bit difference in magnitudes year to year at each station location. The monthly wind for Jan, July and Oct in two specific years 2014 and 2016, one before 2015 and another after 2015, are shown in Fig. 10. Monthly data show that wind field decrease, temperature increase and pressure may also increase in and around the station locations, which are not much clear from spatial distribution. In order to understand the variation of these meteorological parameters at each station location, the extraction of data and their anomaly with respect to the climatology is discussed next.
The anomaly of the pressure from ERA5 at six station locations for the period 1991–2020 indicates that it is above normal from 2015 and onwards except slightly below normal only for 2017 at OENJ and for 2018 at OEDF (Fig. 11a). It was above normal in 1992 and 1997 but did not continue for a few years. The increase of surface high pressure is likely a signal of the jump of severe WS at the station locations in Saudi Arabia. A similar signal of the increase of surface temperature is observed from 2015 and can be considered another indication of the jump of WS at the station locations (Fig. 11b). However, we do not find the signal of increase of surface wind after 2015 at station locations (Fig. 11c). This may be because the WS is the change of wind speed and direction within a short distance and related to cloudbursts, low-level jet, mountains, radiation inversions due to clear skies and calm winds. As mentioned in the introduction, there is mixing of increase and decreasing wind speed in the region. Khonker (2009) reported the increase of wind over the Arabian Gulf but Azorin-Molina et al. [20] showed decrease of wind speed over Saudi Arabia while Al-Douri and Waheeb [6] reported the increase of wind speed over Saudi Arabia.
To understand it, the observational pressure, temperature, and wind speeds for the six stations are analyzed (Fig. 12). The indication of rise of pressure and temperature from 2015 is clear along with the increase of wind speed from observed data, which was missing in ERA5. Hence, observations show that the three variables pressure, temperature, and wind speed provide a signal of increase in 2015. It is also not clear whether the severe WS found in this analysis below 30 m is due to microbursts or outflow from the airplane landing. If it is due to the airplane landing, then the large jump of severe WS frequency may be due to the increase of infrastructure in the airports. Therefore, further investigation is required to understand the sudden jump of the occurrence of severe WS at different station locations in Saudi Arabia after 2015 and its continuation, which may be related to the climate change condition of the region along with teleconnection to the largescale circulations. In the era of climate change, simulation of microbursts using mesoscale model is also interesting to understand WS of the region.