On 19 October 2018 a convective complex developed over open waters in the upper Persian/Arabian Gulf.
By HansPeter Roesli (Switzerland)
Starting off in Kuwait during the morning, it reached its climax around noon (local time) with cirrus beginning to stream eastwards off the highest towering cumulonimbus, as seen on the Meteosat-8 HRV image (Figure 1, top right, click to expand).
Later in the day an outflow boundary reached the coast, at the border between Kuwait and Saudi Arabia, spawning convection inland, that evolved further during the evening (Figure 2).
Apart from these later events, the convective complex remained the only noteworthy convective event in the region on 19 October.
Its life cycle can be followed on the Meteosat-8 Natural Colour RGB and IR10.8 channel animation, between 03:00 and 14:00 UTC (Figure 3).
Figure 3: Meteosat-8 Natural Colour RGB and IR10.8 channel animation, 19 Oct 03:00–14:00 UTC
In the mature stage the complex developed gravity waves on its western side, at the top of a mid-level cloud deck. Both VIIRS on Suomi-NPP, at 375m spatial resolution (false-colour IR11.45 and Natural Color RGB), and the HRV band of Meteosat-8 showed the waves well (Figure 4).
Gravity waves were already present earlier in the day, upwind of the cluster, which were visible in the upper left-hand corner at the beginning of the animation in Figure 3.
The reason why the development of the convective complex took place in isolation is unclear.
The nearest radiosounding station is almost 300 km to the north, at the Ahwaz Airport in Iran, and only 12:00 UTC profiles could be found.
The 12:00 UTC observation of 19 October (Figure 5) showed a humid profile above 700 hPa and, above the boundary layer, no significant wind shear in a generally moderate westerly flow.
The green line marks the profile of the equivalent potential temperature (θe, plotted as relative values). The curve shows neutral to slightly unstable conditions up to 600 hPa (decreasing or constant θe) and a generally stable atmosphere above (increasing θe).
According to the GFS model analysis at 06:00 UTC (Figure 6), similar atmospheric conditions (slightly unstable lower and stable upper troposphere) prevailed over a larger region.
The N-S cross section of θe, centred on the cloud cluster, showed an even stronger instability over its position above the high θe values (blue colour) in the maritime boundary layer.
Also, the sea surface temperature (SST) of the Gulf waters was quite high (normal for the season).
The SST product from SLSTR instrument on Sentinel-3 (Figure 7) gave values of slightly above 30 °C.
This, together with some convergence in the local wind/breeze systems, might have led to the initiation of the convection at this point in the Gulf.
It would be interesting to know whether this area is known for convective activity under similar conditions.