Topic: Drylines In Australia and around the world

[Jimmy Deguara]

Michael Thomas wrote:

I'm not sure if it is technical a dry line but certainly west of the coast line in NW Western Australia there is often a very sharp moisture gradient. Take Broome, it currently has a dew point of 25.6C while further inland at Fitzroy Crossing the dew point is -1.3C. The Broome sounding often has massive elevated mixed layers that cap down the moist boundary layer. It's not rare there to have 3000+ CAPE and no storms. Form my understanding I would call that a dryline. I'm not sure if that is a feature in NT storm setups or not though.

You may wish to elaborate on other dryline situations or regions that experience drylines or perhaps sharp moisture gradients. Someone may wish to indicate a proper definition of the dryline before people begin to hammer in with perhaps not so accurate assessments.

There are special dynamics that are in place with daytime heating and also nocturnal cooling that drive the dryline. Quite interesting actually. And of course, given the moisture profile and capping that can often take place, storms erupting from the dryline can do so explosively!

Regards,

Jimmy Deguara

[Michael Thomas]

Okay I'll expand on this point a little bit more. From my understanding, elevated mixed layers (EMLs) and drylines go hand in hand. In sounding EMLs are easy identified by a layer air with dry adibatic lapse rate, also though this layer the mixing ratio should be the same. Here is an example from the US, the EML can be seen from 800 to 600 hPa-

http://bangladeshtornadoes.org/EML/10049400zounsnd.gif

Compare this to sounding from Broome, the elevated mixed layer can be seen from 900 to 600 hPa-

http://soundings.bsch.au.com/skew-t.html?source=wyoming&lat=-17.9492&lon=122.2336&gribdate=&month=12&day=10&year=2006&hour=00&window=on

This site has good section EMLs- http://bangladeshtornadoes.org/EML/emlpage.html

"EMLs develop when arid regions heat up and deep, dry adiabatic lapse rates extend from the surface to between 450mb and 600mb...... In general, the surface dryline marks the southwestern edge of the EML in the Great Plains. To the east of the surface dryline, the moist layer is capped by the EML. To the west of the dryline, the mixed layer extends all the way to the ground and is not "elevated"."

In NW Western Australia there is quite a large evelated arid region. This region would certainly get dry adiabatic lapse rates from the surface to between 450mb and 600mb. Unlike the great plains though, NW Western Australia is in the tropics and therefore easterlies are usually present between 900 and 500 hPa. Steep lapse rate air from the interior can therefore be blown over the moist westerlies present along the coast forming the dryline.

Elsewhere in Australia, drylines are far less common, I thing the only place that gets them with any regularity would be the east coast of Australia, particularly from say Sydney to Rockhampton. There is no elevated arid region to consistently form the steep lapse rates needed to cap down a moist boundary layer. From memory the last clear dryline event was the 15/11/07. Here is the Brisbane sounding from that evening-

http://soundings.bsch.au.com/skew-t.html?source=wyoming&lat=-27.4178&lon=153.1142&gribdate=&month=11&day=15&year=2006&hour=12&window=on

Though the region from 900 to 600 hPa is not strictly dry adiabatic I think it is close enough to call an EML, certainly on that evening there was a very sharp moisture gradient marking the dryline.

I'm by no means an expert so correct me if I'm wrong.

Michael