Severe convective storms produced unusually large hailstones over Oman in early May 2014.
28 September 2022
05 May 2014
By Khalid Al-Jahwari, Salma Al-Zadjali and Shima Alyazidi (DGMAN)
During the period from 5–6 May Oman witnessed two major hailstorms. Large hailstones were reported (about the size of tennis balls), causing damage to cars and properties.
Synoptically, there was a nicely curved 500hPa trough, unusual for the time of the year, which provided the uplifting mechanism combined with surface heating in the afternoon, leading to very strong updrafts.
There was also a dry line, as northwesterly dry winds collided with southeasterly moist winds in the eastern part of Oman.
According to the ECMWF analysis from 5 May, 12:00 UTC, the main synoptic situation was dominated by a trough of upper level pressure of 500hPa (300hPa extended to the surface). This is clearly shown on Figure 1.
This time of the year is critical (the so-called transition period) because the land is relatively warm. Add to that the role of Al-Hajar mountain (highest peak is 3075m) which provides an orographic uplifting mechanism.
A surface temperature of 41.2°C was recorded in Adam on 5 May, while the dew point was 19.9°C, resulting in an increasing buoyancy of the air.
Conditions at the surface and lower level of the atmosphere were not conducive to precipitation. The cold and dry northwesterly winds, with mean speed of 20kt, that started blowing from Iraq, were enough to cause the precipitation to evaporate before it reached the ground (virga). However, this did not stop hail falling.
During the two days many dry thunderstorms (without precipitation) were seen over Muscat and along the Al-Hajar Mountains.
The upper air stability indices gave a good indicator of the possibility of convective activity. However, the sounding showed a dryness in the atmosphere.
The satellite images from Meteosat-10 show the development of several thunder cells at about 09:00 UTC and their movement east toward northern coastlines.
Cloud top temperatures reached about -50 °C, as shown in the enhanced IR10.8.
On the following day, 6 May, the convergence line shifted eastward, as the trough advanced to the east with a deepening of the westerly wind. The same event was reported in Al-Kamil Wa Al-Wafi.
The Meteosat-9 rapid scan captured the thunder activity over Al-Kamil. Notice on the Airmass RGB image (Figure 4) that the clouds have shifted further east/northeast (compared to their true position) due to parallax effect.
The enhanced infrared image (Figure 6) indicates that the overshooting tops reached about -65 °C.
This storm produced large, almost opaque, and conglomerated (compacted) hail. This shape indicated that these stones had undergone dry growth. In dry growth the hailstone nucleus (a tiny piece of ice) is in a region where the cloud droplets are supercooled.
Upon colliding with a supercooled drop the water droplet immediately freezes as it collides with the nucleus. The air bubbles are 'frozen' in place, leaving cloudy ice.