The Sumatra

Known as line squalls in other parts of the world, Sumatras take their name from the rugged Indonesian island that lies to the southwest of Peninsular Malaysia. They are most common during the transition periods when the predominant winds are unstable.

It would be interesting to know what causes such a strong storm with high winds, especially at Singapore near the equator. The banner photo is obviously taken out of context, but the regular pics seem to show a 20-30 mph wind. The lack of Coriolis effect and the abundance of moisture through the column suggest winds of over 20-30 mph would be rare.

Wasn't there a chaser who lived in Singapore for awhile (Cheryl Chang?)


Sumatras are line of thunderstorms which usually occur during the Southwest Monsoon season from May to October each year. These squalls develop at night over Sumatra or the Malacca Straits and move west towards Singapore and the west coast of Peninsular Malaysia during the pre-dawn and early morning. They are often characterised by sudden onset of strong gusty surface winds and heavy rain lasting from 1 to 2 hours as they move across the island. Maximum gusts of up to 50 knots have been recorded during the passage of a Sumatra squall.

Well as Tim pointed out, the tropics are much different than the mid-latitude. Geostrophic wind is invalid since Coriolis is basically 0 near the equator (they are at 1 degree I believe)... not to mention the pressure gradient force is usually much smaller (not much temperature gradient other than the monsoon).

So I'd throw out a cause by any sort of synoptic scale feature.

Instead, my thought is they may initate due to a low level jet that is induced by the thermal wind. I'm assuming that Sumatra is fairly mountainous... and of course at Singapore... you are at sea level, so I bet you can get some fairly strong temperature gradients at night do to radiation cooling etc etc.

Keep in mind that Singapore goes through a monsoon season... so you'll have thunderstorms because of that alone. Perhaps if you can get enough storms to develop along the mountains on Sumatra, they can form a squall as their cores collapse and develop coldpools that sweep down the mountainside (can you say density current!). Then gust front induced updrafts give appearence of a multicell cluster that moves off the mountains like a squall line?

Just two ideas that I could think up. BTW: Couldn't find any papers on the subject in AMS....