A PERFECT STORM
Early on the morning of Apr 16, the chat network of my school class which is replete with global insights after 40 years of dispersion, lit up with reports of unprecedented rain from Brendan in Bahrain and Ant in Dubai.
Ant is a pilot and was flying out of Dubai that morning. He duly relayed photographs of his flight over the saturated desert.
Parts of the Arabian Peninsula received 18 months of rainfall in 24 hours that day. The airport looked more like a harbour. Being the weather-man in the chat group, I looked at the satellite and the forecast model data. What I saw were the ingredients of a perfect storm.
What normally keeps the old deserts, such as those of the Arabian Peninsula, so very dry is persistent and intense sinking of air – the very opposite of what is required for rain. The sinking air is bone dry, having come from the cold, top of the atmosphere, and is compressed and warmed as it descends. It arrives near the surface like a hairdryer.
Below this layer, especially in deserts close to warm oceans, evaporation is plentiful. But that humidity is kept captive by the sinking air above. It is a cauldron with the lid firmly on.
What took that lid off the cauldron on Apr 16 was a high-altitude jet stream unusually far south. In fact two jet streams, the subtropical jet and the polar jet that had joined forces and left behind a cut-off circulation of imported, cooler air. The sinking air, along with the cauldron lid, was gone.
Meanwhile a feed of moisture laden air was accelerating in from the northern tropical Indian Ocean and converging over the desert. Dew point temperatures over the UAE were similar to those normally found in the rainforests of the Congo basin.
Under these conditions, thunderstorms develop very readily and in this case a special kind of storm, a mesoscale convective system, built and sustained itself for many hours. Infrared satellite data showed it to be about the size of France.
