Two cases of severe convection along the Red Sea coast of Saudi Arabia.
31 May 2022
21 December 2009
By Humaid Al Badi, Juma Al Maskari (DGMAN Oman) and HansPeter Roesli (EUMETSAT)
Late autumn and winter bring relatively frequent convective events to the central part of the eastern Red Sea coast. This stretch of the coast, the Hejaz region, is characterised by shallow coastal waters, a very narrow coastal plain and the steep slope of the Sarawat mountains. Around Jeddah (Sarat al-Hejaz) the mountain ridge rarely rises above 2100m, while the middle and southern portions (Sarat 'Asir and Sarat al-Yemen, respectively) can reach heights of over 3300m.
The interplay of typical cold-season synoptic situations with orographic and thermal forcing may produce significant convection along the coast and the mountain range, like the cases that occurred on 25 November 2009 (case 2) and 22 December 2009 (case 1). The two events show obvious differences in their initial conditions, dynamics and rain patterns.
Case 1 (see image below) had a convective cluster following the coastline from its initiation close to the Gulf of Aqaba to its slow dissipation near the Yemen boarder, ie the cluster moved south-southeastward at an average speed of 25km/h over a period of 15 hours, and had a continuous outflow at the cloud top in a north-easterly direction.
The sequence of SEVIRI Airmass RGB images at 15-minute intervals depicts the evolution between 22 December 03:00 UTC and 23 December 06:00 UTC. The convection happened under a red-brown cloud band that moved with the outflow from a cloud top. The particular colouring in the Airmass RGB is indicative of the presence of a band of relatively dry mid/upper-level air. This band was linked to the mid/upper-level tropospheric flow as shown by the ECMWF analyses of 00, 06 and12 UTC (300, 500 and 950hPa, source: ECMWF) that are overlaid with the colder cloud tops (<240K) as seen in the IR10.8 channel.
A rapidly deepening Mediterranean trough (see 500hPa geopotential height, centre) dug down across the eastern coast of the Red Sea. The convection was aligned with the right entrance of a moderate upper-level jet stream (see 300hPa isotachs, left) and some low-level convergence (see 950hPa wind flow, right) over the Red Sea. Though rain intensity temporarily might have been high in places, because of the fast displacement of the convective system it probably accumulated nowhere near exceptional amounts. At Jeddah King Abdul Aziz International Airport a total of 7mm of rain was measured, 70km ESE at Makkah 12mm and another 70km further inland at Taif 6mm (source: www.ogimet.com). Also, the Cb tops hardly showed the signs of severe convection, as shown e.g by the combination of the b/w HRV image with the false-colour IR10.8 image from SEVIRI (see sequence of 15-minute scans).
Compared to case 1, case 2 (see image below) had completely different characteristics. Preceded by a couple of other convective outbreaks of relatively short duration, in the early morning of 25 November — at the end of the Muslim Haj — a major convective outbreak occurred just north of the harbour town of Jeddah. The system then moved right over the town and remained almost stationary during the following hours.
The storm top developed clear signs of severe convection, a very prominent cold U-shape in the IR10.8 channel and an elongated plume in the HRV channel meandering in an easterly direction towards Makkah. U-shapes and plumes are well shown by a combination of the two channels, HRV in semi-transparent b/w rendering and IR10.8 with false colouring below 230K as already used in case 1 (see Animation, 03:45–10:00 UTC). Interestingly, footage of the storm (Youtube video) was captured by a passenger of an airplane from an easterly direction, which shows the expansive cirrus anvil plume as well as the localised Cb cloud which produced the heavy rainfall.
MODIS on the NASA satellite Terra overflew the area in the late morning, giving an even more spectacular view of the plume than the Meteosat SEVIRI image closest to the Terra overflight. The following image composite compares MODIS channel 1 at 250m resolution (bottom) to SEVIRI HRV (top). The notable shift of the geographical position of the cloud formation between the two images is due to the very different viewing angles of the two satellites combined with the considerable height of the cloud tops.
In line with the clear signs of the severity of this convection event, case 2 produced larger rain amounts at the three stations listed above for case 1. While at Taif and Makkah, both with 12mm, the rain amount can already be considered as relatively high for the area, the 140mm measured at Jeddah was definitely much more than what usually might be expected in the area. The severity of the precipitation can also be appreciated on the radar image (source: Presidency of Meteorology and Environment, Saudi Arabia) that shows the one-hour accumulation of radar-measured rain rates. Coupled to the considerable runoff through the wadis that drain the higher ground east of Jeddah (see Google Earth image, source: Google Earth) the stationary storm created a significant flash-flood situation in the streets of Jeddah (Youtube video).
Looking at the synoptic situation on 25 November at 06 UTC (300, 500 and 950hPa, source: ECMWF) there are no marked signs that might have hinted at particularly severe storm development. The storm occurred away from the jet stream at 300hPa and beside some possible ripples the geopotential height at 500hPa shows no significant forcing over the area of interest. Only the low-level wind field at 950hPa points to an important convergence over the Red Sea off the Jeddah region.
Animation RGB Airmass (22 Dec 20:45–23 Dec 06:15 UTC)
Animation RGB Airmass (22 Dec 03:00–23 Dec 06:00 UTC)
Animation IR10.8 Channel (00:00–13:30 UTC, Animated source: CIMSS)
Animation RGB Natural Colours (03:45–08:30 UTC)