Wednesday, 27 June 2007

Lightning and Hurricanes

I found this recent paper by a couple of Israelis that I have had the pleasure of meeting, and that our group has had some ties with. They have come up with an interesting correlation between lightning activity in the East African sector and Atlantic Basin hurricanes. Hopefully the abstract at least should be viewable to the general public at the link. But the crux of the idea is that:

In this paper we provide evidence showing the connection between lightning activity over eastern Africa, and the AEWs that leave the west coast of Africa, some of which develop into hurricanes. We have analyzed the 2005 and 2006 hurricane seasons, one a very active hurricane year (2005), and the other a very quiet year (2006). More than 90% of the tropical storms and hurricanes during these 2 years were preceded by periods of above average thunderstorm activity in eastern Africa.

Before we go to much further we need to define a few things. AEWs are African Easterly Waves, disturbances in the African Easterly Jet a (high altitude?) wind that forms a the region of strongest gradient of the temperature gradient between the relatively cool equatorial Africa (due to the Guinean coast) and the hot hot air above the Sahara - this is the reverse of the usual gradient where it is warmer at the equator than either side.

It appears that ~60 of these AEWs can form in a year mainly in the months of April - November, and they travel west across Africa (in a matter of a few days to a week) and then move out into the Atlantic ocean where they may or may not form Tropical Depressions, Storms or Hurricanes. There appears to be no correlation between number of AEWs and the number of Hurricanes although some 60% of minor Hurricanes and Tropical Storms and 85% of major Hurricanes can be traced back to AEWs.

Now the central African region is the global hot spot for lightning with a rate of over 50 flashes per square km per year. The lightning flash rate here is higher than any where else on the planet, though lightning is a very seasonal and time of day dependant. Lightning seems to prefer the late afternoon-early evening sector so at varying times of the day it will peak in different longitude sectors. Surprisingly enough lighting is more common in summer than winter. Which naturally fits with our increased AEW occurrence in this time period.
The AEWs are associated with deep convection and intense thunderstorms over tropical Africa... One of the key questions relating to these tropical waves is whether the waves trigger the convection, or whether the convection triggers the waves... We investigate whether the intensity of the convection, measured by lightning activity, is related to the AEW intensification.

So we have pressure instabilities that are related to strong lightning activity that originate in East Africa (in the paper they specifically look at 10–20 N and 30–40 E, in the region of the Ethiopian Highlands), and propagate across the continent and out into the atlantic where these drops in pressure can sometimes become hurricanes.

Now to get the local lightning rates for this region you can either stand there and count or you can make use of a network set up to detect lightning. That is all well and good if you are in a developed country which has one of these. Most lightning detection networks are extremely localised and need many stations to cover any decent sized area - mainly because of the frequency range that they use to detect the lightning (MF - 300kHz to 3MHz). Now if you use a much lower frequency range (VLF 3 - 30 kHz) it turns out that the absorption of the wave is less and hence you can see the waves at a much greater distance.

So using this long distance propagation and a few other nifty tricks, a former Professor here at Otago Prof Richard Dowden set up a company (LF*EM Research Ltd) and a World Wide Lightning Location Network (or WWLLN - pronounced "woolen" as wool from a sheep). Because the VLF part of the signal from the lightning travels much further then you need a much lesser density of stations. In fact there in the WWLLN there are currently about 30 stations, (we host one here in Dunedin) and this covers the whole globe since we are able to see the signals of lightning strikes happening many thousands of kilometers away.

This network is far from perfect, it requires at least four stations to register the signal from a lightning strike and then the timing of the reception must be with certain limits (other wise you cannot be sure that the stations are all looking at the same flash) or the network will not register the lightning. So this means you have a trade off between number of "legitimate" detections and the accuracy of these detections but then I imagine that is true of any similar system. So this tends to mean that the detection efficiency (number of "legitimate" detections versus total lightning) is lower than one would like, however those detections are favoured to more intense lightning strikes - since these are the ones more likely to be detected at more stations, as they radiate more power and the signals are likely to be observed further away.

So this is the technique that the authors used to look for lightning (they host a station in Tel Aviv and as such have acces to the full data set rather than just the summary on the web). But I think that the interesting issue here is the one that they clearly are not getting all the lightning happening in the region buit we are reasonably confident that they are getting the locations of all the storms and most of the stronger lightning. The authors certainly see a link between the strong lightning, and the AEWs and hurricanes, but I don't think that this could ever be a forecasting tool for hurricanes.