Topic: convergence lines

[Mike]

Morning all.

I have some questions requiring some experienced feedback. 

(1) What level are convergence or boundaries usually held at which initiate CB formation?  I've been looking at the models for the NT of late and see that on Sat there is a convergence of wind profiles at the 300mb area to the SW of Darwin. 

Now I know that is in the uppers, but is it better to locate convergence of winds at the lower levels, say at 850 or 500hpa?  The profiles I viewed were roughly...500 easterlies, 850 nor'easterlies and at 300 NW,NE and N which would give some moisture to it all.  Temps in the uppers are between -31, -32.  Wind speeds are not strong and only around 5-10kts but speed up in the uppers.  CAPE levels will be around 1400-1600 and LI's only at -2 to -3.

There is a low pressure area (heat low) in the Kimberley region again, but no trough line attached to it that will reach us and there is an eddy in the Gulf of Carpentaria which I don't like as they are 'tropical' high pressure systems usually associated with fine weather - albeit inflow of moisture from the NE.

My dilemma is trying to forecast if this convergence will create some storms in that area.  There have been storms frequently in that area around Pine Creek (roughly SW of Darwin at 150km away) in the past with similar wind profiles - I just wanted some opinions on what hpa levels convergence or boundaries are most often found.  Should I ignore boundaries that high up?

Hope you can help as it would assist in my working out models during the storm season....I viewed the stormtrack GFS that is available on WZ for Sat this week coming and went through the models for 7pm'ish

[Michael Thomas]

Hi Mike,

I really don't have much knowledge on thunderstorms in the tropics but I will say a few things.

1) What level are convergence or boundaries usually held at which initiate CB formation?
You should be looking for these at the surface (or 1000 hPa). Possible these may also show up around 850 hPa but certainly not around 500 hPa or above. By the way, the 500 hPa level is pretty much the middle of the troposphere. Convergence at the lower levels helps initiate convection by forcing air at the surface to rise. Once the level of free convection is reached you can get deep convection.

Michael

[nmoir]

That Nth Westerly sea breeze seemed quite reliable when i was up there for that week , it was often pushing deep into the territory and made it quite easy to pick target areas. I would think any development from convergence that high would not get far ,

[Mike]

most helpful guys and thanks.  It's a learning curve for me and your info has helped.  I was not sure about the 300hpa level also but had to ask.  I do remember now that one of the met officers here wrote an article on storms and she did mention the 850mb area as the 'starting point' for our convection.  I'll keep an eye on this mb zone given your comments and as Nick mentioned I know that he chased a fair bit whenever there were good indicators down the track with these set ups.

It's all about learning

[Michael Bath]

Hi Mike,

You need convergence in the surface layer and preferably diverging winds in the upper levels. Any convergence in the mid and upper levels will lead to sinking air, whereas convergence near the surface can only result in air rising - which can be a thunderstorm trigger provided other conditions are right.

So if plotting winds at various levels:

- on the 1000-925 hPa charts look for winds coming together - this may be on a relatively small scale
- on the 300-200 hPa charts look for a broad spreading out of the wind fields

Two examples are attached.

[Mike]

Thanks MB for the info and examples.  Back to the charts!

[Mike]

Questions for the experienced:

With the convergence winds...is it much better to have stronger convergence winds and if so or if not, why?

With diverging winds does this also apply for them to have stronger winds diverging - say anything 10kts upward to help clear any anvil cloud/other left over? 

I do understand re convergence and divergence being either on the sfc or uppers and reversed, but am just understanding the importance of wind speeds of both that either assist or no assist storm formation and motion.

Tks

[Michael Bath]

Might be something for you to research and post here Mike.

Outside the tropics you want strong divergence aloft, ideally the jetstream. Anything less than about 40 knots in the 300 to 200 hPa level is a bit light for thunderstorms and will inhibit strong storms developing or persisting.

As for low-level convergence, the speed may not be as important as to whether the winds are bringing lower/higher dew points, or increasing/decreasing the temps.