Top End Storms, an overview for the meteorologically familiar.
The city of Darwin is located 12s and has a wet/dry tropical climate. The BOM refer to the climate as Equatorial officially, though many including myself would argue this isn’t the best classification for a region that receives a large portion of dry weather during winter. During this time, dewpoints can get to –14c and temperatures right on the edge of the coast can get down to 11c, with single figures less than 20km away.
The majority of Darwin’s rainfall occurs during January and February when ITCZ approaches the region. The monsoon trough is responsible for significantly heavy falls, and often strong blustery Northerly oriented winds. Another significant contributor to annual totals are tropical cyclones and depressions.
The Top End coast is usually affected by 4 or so a year, and the immediate area around Darwin city usually sees 1 to 2 cyclones come within 80km of the city, most of these are classified severe and are usually above category 4. This is a result of the extensive areas of well heated coastal waters, often reaching 34c on the surface, and favorable shear conditions offshore for cyclone longevity. Tropical cyclones within the region are usually fairly slow moving and their propensity to follow Westerly tracks across the top of the coast are historically noted.
Thunderstorms are the most common feature meteorologically speaking from late October to March. The region experiences the highest amount of thunderdays in the country, often peaking well above 100 per year.
The monthly distribution of thunderstorms in Darwin varies somewhat from year to year, but generally after the inland cells have started (September) and broad scale convergence initiates, Darwin begins to experience its first storms.
During the September to November period, vast areas of NW Western Australia begin to heat up, and create broad areas of low pressure known as the ‘Northern Heat Lows’. As the lows get established, they begin to have a very broad synoptic effect, drawing in large masses of warm moist air from Asia. As this moisture is ingested into the low pressure areas, it passes over Darwin and the Top End, this is the beginning of the build up.
With increased moisture levels and residual strong SE winds at 700hpa, the resultant convergence triggers huge thunderstorms inland, often isolated in the early season then becoming more established further into the period. To monitor the progress of buildup initiation, a keen eye can be kept on the pressure and temperature at places in the far NW of WA like Kununurra, Halls Creek and Fitzroy Crossing. A dramatic increase in temperature of a consistent nature, and a pressure drop is a reliable precursor to a solid moist NW to NE flow across the Top End.
Keeping an eye on the local atmosphere is even more important for monitoring the progression into the storm season. Darwin’s tropical climate is very unlike the rest of Australia’s tropical regions, it often sees quite moderate mid level shear, and combined with wider scale convergence, it is no surprise that the region experiences more than double most of Tropical Australia’s thunderdays annually.
The super dry inland areas of the Northern Territory and North Western Queensland feed the region with crisp dry mid level winds, often quite strong. 700hpa South Easterly surges often pass through the region with speeds varying from 15knots to 50 knots. During this time, if the temperature and moisture profile of the upper atmosphere is favorable, Darwin will receive its usual bout of severe thunderstorms.
Combinations of well over 4000 joules of CAPE (convective available potential energy) and shear of over 35 knots in the mid to lower levels are conducive to vigorous storm activity in any region, and thus Darwin receives a higher than average distribution of severe storm activity for a tropical region.
The Darwin regional forecasting centre uses an entirely different cape vs shear threshold to forecast severe weather.
Above are events that have been recorded for the purpose of correlating conducive factors with actual thunderstorm occurances. Note the most significant environmental shear is conducive to continental squall lines.
The Darwin severe storm warning area is very small, consisting of a 60km radius around the CBD. Only severe events within this predefined area are warned for, yet there is still on average around 8 to 10 warnings per year, and obviously many more severe storms than this that go unwarned for (mainly due to their isolated nature and/or likelihood of traversing populated areas).
The main phenomena in regard to local severe storms, are wind gusts. Although rainfall may classify in other regions, it is never warned for in the Top End, as being a tropical region, it is ill affected by falls that would otherwise flood other parts of Australia. A single storm just last season dropped 164mm in less than 30 mins. Wind gusts with severe storms in the warning region are relatively common, with 100km/hr + winds seen time after time per season. The Darwin BOM officially estimates that the warning area of Darwin and it’s immediate surrounds receives up to 50 severe storms per season, with most of them being relatively isolated in nature, and a portion a result of organized squall lines which frequent the Top End.
Here is a media release educating the Darwin public about the severe warning system during its implementation a few years ago.
The information pertaining to numbers of severe storms in the Darwin area might surprise many that have perceived the region to be a typically tropical regime like Cairns, Jakarta or Bangkok.
So how often does Darwin experience the conducive factors such as good shear in the mid levels, and high levels of CAPE?
The answers are best answered by historical sounding information, available at the university of wyoming’s sounding page. http://weather.uwyo.edu/upperair/sounding.html note that Darwin’s sounding identifier is ‘94120’
A typical sounding is the 1st of November for example, the first day of the storm season for the CBD as it is seen by many.
Note the 35 knot shear, but still significant dry regions of the atmosphere above 700mb.
The following sounding is a‘loaded gun’ for the Darwin region, a sounding like this usually precedes severe thunderstorm events.
Noteworthy is the presence of 30 knot shear in the mid troposphere, a super abundance of CAPE and a decent dry slot at 450mb. The result was 126km winds in the city, flattened trees, and lightning rates that did significant damage, ironically the Darwin Met Office weather station was blown up by a strike. Measured were 200 cloud to ground strikes per minute during its peak, significant for anywhere.
During monsoon time, 50 knot winds at around 700mb and strong upper level winds are fairly common, these are often brought to the surface with heavy precip with monsoon squall lines.
