There were some good transition season storms near Darwin, Wyndham and Broome on Monday and Tuesday.  I have put some satellite images and links to soundings on my web-page.

The positive area/CAPE on the soundings is something to behold, and seeems to be associated more with very warm low levels rather than with a lot of moisture.

I assume most of you already know this, but I'll say it anyway:  These high CAPE (presumably very electrical) transition seasons torms are quite differnt to the monsoonal-wet season storms that come later in the year, which tend to be associated with larger scale monsoon outbreaks, are "wet" and "soggy" with relatively small CAPE

Soundings:  Darwin: 04 1100 UTC 2300 UTC
                                05  1100 UTC 2300 UTC

                    Broome  04 1100 UTC 2300 UTC
                                05 1100 UTC  2300 UTC

Barry Hanstrum

John

These transition storms have been a topic of discussion in the WA region for the past week or so.  While we're very interested in the CAPE value of this environment we're equally interested in the DMAPE (Downdraft Available Potential
Energy) value since this value is closely related to downdraft strength potential. In the WA/NT region these storms are referred to locally at this time of the year as "knock-em-downs", named after their ability to generate very strong downdrafts and cause local damage.  It is rare for a year to go by when we don't receive a report from the Kununurra region of damage to crops and property there due to one of these storms.  They occur preferentially in Nov/Dec, the time of year where very hot moist low level air is surmounted by drier air in mid-levels.  When the wet sets in during mid-summer, the moisture deepens, lapse rates become closer to saturated adiabatic and reports of severe downdrafts are very rare in this region.

The environment in which these storms form combines the characteristic of both the wet and dry microburst model, we call them hybrids.  They have similarities with dry microbursts in that the temperature lapse rate is typically dry adiabatic to 10,000-14,000 feet, but unlike dry microbursts there is moisture available in low levels.    So with the large CAPE (implying high loading of liquid water and ice) the downdraft air can descend saturated to the surface, maximising the cooling through evaporation in the downdraft and maximising the temp difference between the core of the downdraft and the environment. All that adds up to massive gust potential of the order of 100 knots or so in the worse case.

This environment can be found in other parts of the country during the summer months (see for example:
http://www2.wa.bom.gov.au/sect_info/sevwx/whole_sum_studies.htm  and the attached powerpoint file that shows slides of a very destructive downburst environment at Richmond last December.  The full paper by Taylor and Webb from the NSW RO was presented at the Weather Services Scientific conference in June 2002 (see p97 of the proceedings).

Because theses storms typically occur in low shear environments and have low potential to become supercells these storms have traditionally not been given the level of respect they deserve.  Here in the west the occurence of DMAPE > 1000 J/kg and CAPE greater than 1000 J/kg is the criteria we use for issuing a severe thunderstorm advice for damaging wind gusts with storms.

Anyhow it all adds up to these events being very spectacular, being associated with massive updrafts and massive downdrafts. One day I'll get to see one first hand.

Baz