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TROPICAL CYCLONE KELVIN INTENSIFIES OVER THE AUSTRALIAN 'BROWN OCEAN'

​Ah the beautiful Pilbara and south Kimberley, a place where the hot desert soil meets the hot waters of the Indian Ocean and just occasionally a place where Tropical Cyclones like the hot desert soil just as much as the hot Indian Ocean. This was the case of the recent meteorologically incredible Tropical Cyclone Kelvin. Tropical Cyclone Kelvin hit the Sth Kimberley coastline on the 18th February 2018 at about 5AM WA time as a rapidly intensifying borderline Category One/Two system. Its improvement in structure and internal dynamics was remarkable in the 3 hours leading up to its landfall and that improvement and intensification didn't end once it hit the coast either. Check out the following radar and satellite animations of this warm cored midget cyclone after it hit the coast. 

At its peak, this small Tropical Cyclone's satellite and microwave signatures resembled those of a high end category Three system s it left the Kimberley and tracked SE cross the Eastern Pilbara, but officially due to the frictional effects of the Pilbara landmass the maximum winds the system was estimated to pack were in the vicinity of 150km/hr (a higher end Category 2 system). So why did this amazing cyclone intensify so dramatically after it hit the coastline? Well the answer is very likely due to the 'Brown Ocean' effect. Let's explore what the Brown Ocean Effect is and why it applies to this scenario. 


WHAT IS THE BROWN OCEAN EFFECT?

Brown Ocean TC's between 1979 and 2008 around the world - Andersen and Shepherd (2013)

​It has long been known that Tropical Cyclones require warm oceans (above 26 degrees celsius) to fuel their massive heat engines, but in Australia in 2001 a Tropical Cyclone named Abigail maintained its intensity and even intensified over land in the Northern Territory. Another American system did something similar in 2007. This prompted a study and theory by renowned professor of Meteorology Kerry Emanuel and BoM Meteorologists Jeff Callaghan and Peter Otto about why and how this could happen (you can read their full study here in the American Meteorological Society Journal) The study was published in 2008 and it concluded that "warm-core cyclones can indeed intensify when the underlying soil is sufficiently warm and wet and are maintained by heat transfer from the soil." 

This finding was then verified by further research just a few years ago from NASA scientists Andersen and Shepherd (you can read their full study here in the International Journal of Climatology)  following a spate of strange Tropical Cyclone intensifications over land in the USA. They studied Tropical Cyclones all around the world and in fact came to the conclusion that the area this is most likely to happen in is North-West Australia.

The studies concluded that for a Tropical Cyclone to intensify over land you needed three factors to be met over and above the usual things required for Tropical Cyclones (e.g. low wind shear, Coriolis force, deep environmental moisture, pre-existing disturbances) You needed:

  1. The ground must be very warm and have a similar temperature to the sea from where the tropical storm or cyclone has come from.
  2. The ground must be very wet or saturated and so this land 'mimics' the sea - and hence the name 'brown ocean effect'. The high amount of moisture in the soil, then allows a high rate of evaporation, which then acts as a source of heat energy for the storm or cyclone, technically known as latent heat - similar to the process over the sea.
  3. And the amount of this latent heat must be at certain level - at least 70 watts per square metre. Over the sea it would normally be around 200 watts per square metre.

DID KELVIN HAVE A BROWN OCEAN TO PLAY WITH?

The short answer is a resounding YES, but let's explore Kelvin's environment first to see why we have come to this conclusion.


VERTICAL WIND SHEAR

 We can see that TC Kelvin was in a perfect low vertical wind shear environment with excellent outflow to the north and south of the system. Had the cyclone been located over water it would be intensifying rapidly as well. 


ENVIRONMENTAL DEEP MOISTURE

​Tropical Cyclones that hit the Pilbara often quickly weaken as they entrain dry mid level air from the desert regions of Western Australia. You can see in this HWRF analysis a few hours after landfall that Tropical Cyclone Kelvin had an almost saturated mid level atmosphere to play with. 


So with low shear and high amounts of deep atmospheric moisture levels, Kelvin was in an ideal environment except for one thing - he was over land.


LET'S TAKE A LOOK AT THE PILBARA SOIL ON THE 18TH FEBRUARY 2018

​To get an understanding of the soil conditions across the Eastern Pilbara and Southern Kimberley let's take a look at how much rain those regions have had in the weeks leading up to Tropical Cyclone Kelvin's arrival. 

January rainfall deciles - BoM

​You can see that in January the month preceding Kelvin's arrival the section of coastline that Kelvin had hit (and the all important area inland of the coastline) had well above average rainfalls and in parts of that inland eastern Pilbara, the heaviest January rainfalls ever recorded. In fact there was a Tropical Cyclone in January (Joyce) that hit the coast near Bidyadanga and as it traveled just inland of Port Hedland actually seemed to improve its structure and organization (a case can be made that it too showed elements of Brown Ocean intensification, however the case isn't as strong as Kelvin's) This previous Tropical Cyclone had crossed the coast about a month before Kelvin and had dumped copious amounts of rain on this section of the coast and adjacent inland parts. 

Track of Tropical Cyclone Joyce January 2018 - Wikipedia

 Following Joyce, a monsoonal LOW tracked inland across the Kimberley and far Eastern Pilbara slowly once again dumping hundreds of mm of rainfall on the already wet desert soils of Western Australia. Tropical LOW 11U operated between the 20th January and the 1st February 2018

Track of Tropical LOW 11U - 20th Jan to Feb 1 2018 - Wikipedia

​Finally the week leading up to Tropical Cyclone Kelvin's arrival was characterized by sporadic showers and storms across Pilbara districts, and in the day preceding Kelvin's landfall, the Tropical Cyclone did not move far off the Western Australian coastline. The steering mechanisms governing its motion were finely balanced and negated each other, thus the system became quasi stationary and within 200kms of the coastline for much of Saturday the 17th February resulting in heavy rainfall due to its circulation and the feeder bands east of the system wetting the already saturated/near saturated soils of the Pilbara and Kimberley. 

Rainfall for the week up to and including the 18th Feb - the day of landfall for TC Kelvin - BoM

​Now knowing just how much rain this region has had, let's take a look at the soil profile of the area on the 18th February using the Bureau Of Meteorology's Root Zone Soil Moisture levels (the amount of moisture present in the top 1m of soil). You can see from the diagram below that the soil was near saturated across this region and this would have greatly facilitated the necessary heat flux across the surface boundary layer to continue to drive the heat engine of the small but strengthening Tropical Cyclone. 

Soil moisture levels in the top1m of soil - BoM

 Unfortunately at the time of writing, the final piece in the Brown Ocean puzzle - the soil temperature was unavailable. However if we extrapolate a saturated or near saturated soil profile along with the maximum temperature experienced for the region on the 18th February 2018 we can deduce that the soil was likely close to the same temperature as the environmental temperature which was around 30 degrees celsius and this closely corresponds to the water temperature off the East Pilbara which was between 29 and 30 degrees celsius thus likely confirming the first necessary condition of Brown Ocean intensification. 

Model sea surface temperatures off the Pilbara and Kimberley coastlines - soil temperatures need to be at or above this level to fulfill one of the necessary components of "Brown Ocean Effect" intensification

TROPICAL CYCLONE KELVIN'S SIZE

Although I haven't found the scientific study to help support this, I do believe the small size of Tropical Cyclone Kelvin also facilitated its overland intensification. The necessary heat flux requirements to support the development of a tropical cyclone are closely correlated to the size of the system moreso than its strength. thus a larger Tropical Cyclone may have had a more challenging time in gaining the necessary energy requirements (70 Watts/square metre) from the soil surface than a small system like Kelvin did. Tropical Cyclone Kelvin's gale radius overland measured approximately 150 kilometres in diameter (about 75kms from the centre in any direction). This puts it in the 'midget' class of tropical cyclone sizes. More research on the size of overland systems and their intensification potential would need to be done to verify this claim though.


In summary using the available evidence shown, there is a strong case to be argued for the intensification of Tropical Cyclone Kelvin while overland to be a clear and obvious case of Brown Ocean intensification.

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