Cumulonimbus Cloud- Ill/well Defined

[Thundery wintry showers]

I think I might have an idea of the answers but I'm not completely sure and maybe someone better versed in the ins and outs of cumulonimbus clouds might know more than me.

Often on showery days in the UK, particularly when the weather is windy, and particularly near windward coasts, cumulonimbus clouds are quite poorly defined- in a sense "smudged". Here's a few pictures to illustrate (note, not my pictures- taken from the site Weatherpictures.com):

http://www.weatherpictures.50g.com/pictures/2005/01/21012005020.jpg

http://www.weatherpictures.50g.com/pictures/2003/04/02042003001.jpg

http://www.weatherpictures.50g.com/pictures/2005/01/21012005031.jpg

http://www.weatherpictures.50g.com/pictures/2008/07/08072008025.jpg

...in comparison to the very well-defined ones that we sometimes see on relatively calm days, and which are more common on the continent:

http://www.weatherpictures.50g.com/pictures/2006/05/25052006006.jpg

http://www.weatherpictures.50g.com/pictures/2006/05/21052006003.jpg

http://www.weatherpictures.50g.com/pictures/2007/07/10072007015.jpg

I remember Phoenix from Morecambe Bay was the first person to bring up this issue with me via email (where he argued that convective clouds in the Lancaster/Morecambe area tend to be poorly defined due to the moist airmasses blowing from Morecambe Bay, and that it is less of an issue in East Anglia, which is consistent with my own experiences in Norwich). I'm guessing that the high moisture associated with winds off the sea, and strong winds creating unfavourable amounts of shear within the cumulonimbus cells, might be the main reasons, but does anyone else have any clearer ideas?

[knocker]

I think I might have an idea of the answers but I'm not completely sure and maybe someone better versed in the ins and outs of cumulonimbus clouds might know more than me.

Often on showery days in the UK, particularly when the weather is windy, and particularly near windward coasts, cumulonimbus clouds are quite poorly defined- in a sense "smudged". Here's a few pictures to illustrate (note, not my pictures- taken from the site Weatherpictures.com):

http://www.weatherpi...21012005020.jpg

http://www.weatherpi...02042003001.jpg

http://www.weatherpi...21012005031.jpg

http://www.weatherpi...08072008025.jpg

...in comparison to the very well-defined ones that we sometimes see on relatively calm days, and which are more common on the continent:

http://www.weatherpi...25052006006.jpg

http://www.weatherpi...21052006003.jpg

http://www.weatherpi...10072007015.jpg

I remember Phoenix from Morecambe Bay was the first person to bring up this issue with me via email (where he argued that convective clouds in the Lancaster/Morecambe area tend to be poorly defined due to the moist airmasses blowing from Morecambe Bay, and that it is less of an issue in East Anglia, which is consistent with my own experiences in Norwich). I'm guessing that the high moisture associated with winds off the sea, and strong winds creating unfavourable amounts of shear within the cumulonimbus cells, might be the main reasons, but does anyone else have any clearer ideas?

Do they not take on a fibrous appearance when they hit the freezing level? Would not a mature (well defined) Cb have an anvil. In other words the "glaciated cloud" consists of ice crystals that are pulled out horizontally by upper level winds.The last photo seems to be a variation on the first three. I.E. a type 3 Cb. I take it you are saying the strong winds are creating wind shear in the Cb and thus inhibiting growth to full maturity? Not sure sure about that as I've seen fully blown Cbs in Cornwall although they don't as a rule tend to be generated locally. Could it just be not strong enough convection on the coast with the influx of cooler air.

[Harry]

I'll have a punt. I think, amongst other things, CAPE may play a rather large part.

The Cb cloud is effectively a bubble of moist air rising rapidly into the sky. If the 'explosion' (for the want of a better word) is stronger, the cloud will hold greater structure due to the force billowing upwards, therefore lesser the effect wind will have on it. If however, CAPE is relatively weak, the energy powering the storm up will be less pronounced allowing the winds aloft to have a greater effect on it's structure.

This makes sense when looking at coastal areas of the UK, where CAPE is often far less pronounced than say 20-30 miles inland. Bear in mind also, that many storms which affect UK coastal areas are either frontal induced, convergence zones or in the case of the south coast, french imports.

If you compare the clouds produced by atomic bombs and large oil/petrol explosions - the force is so great it causes a large mushroom cloud to form, irrespective of wind speed. This so is true of explosive volcanic eruptions. After the explosion, the cloud starts to lose structure and formation, unless the force driving it holds strong.

In the case of supercells (very high CAPE most often), the force of the updraught can last several hours, meaning the cloud holds its formation for a long time and produces robust anvils. Where there is less CAPE, storms tend to hold together less well.

[staplehurst]

The definition of such clouds will be closely related to how cold the cloud is. You get a stronger definition if the edges of the cloud are very cold and ice particles are present. In a warm cloud it is more, as you say, smudgy due to more in the way of vapour than crystals.

[Harry]

The definition of such clouds will be closely related to how cold the cloud is. You get a stronger definition if the edges of the cloud are very cold and ice particles are present. In a warm cloud it is more, as you say, smudgy due to more in the way of vapour than crystals.

I suspect we are arguing the same principles but using different meterological terms/ideas.

You're by far most likely to get larger crystals when cloud tops reach higher elevations. Clouds reach higher elevations with stronger, explosive convection, which correlates strongly with higher CAPE/lower LI values.

[knocker]

Not exactly relevant to this thread but a description of the complicated dynamics of Cb can be found here. Chap. 8 of a book, Cloud Dynamics By Robert A. Houze.

http://books.google.co.uk/books?id=5DKWGZwBBEYC&pg=PA269&lpg=PA269&dq=dynamics+of+cumulonimbus&source=bl&ots=HL6mSxXf2n&sig=oQg0_FZRBZFg2TfsXGpaIBz3BhY&hl=en&ei=6kSWTcCtLsKnhAfG4OXhCA&sa=X&oi=book_result&ct=result&resnum=7&ved=0CEwQ6AEwBg#v=onepage&q=dynamics%20of%20cumulonimbus&f=false

[Thundery wintry showers]

Just thought I'd say thanks for the responses- there's a strong consensus that "high sheared, low convective available potential energy" environments are the main culprit of unspectacular convective cloudscapes. I hadn't considered the CAPE angle but it does sound a lot more likely (and more strongly consistent with my personal experiences) than my "too much surface moisture" suggestion.

Maybe I need to do some brushing up of my knowledge of convective cloud processes!

[Evening thunder]

Personally I think it may be based on height of the freezing level as well as Cape like what people like Harry etc has mentioned above, I have noted that CB's tend to be more defined in warmer air masses and can tower higher into the atmosphere before they become glaciated or 'fuzzy', e.g in pictures from the US plains and from hot spells on the continent, or even in the UK. (but may also be more defined due to the higher CAPE) I have seen in cold winter and spring air masses where the whole shower cloud basically looked 'glaciated' or perhaps the frozen precipitation takes longer to fall and may sometimes drift a bit more from the cloud, giving it a more fuzzy appearance.

I think evidence for this may be that looking at the trees in the first 3 pics (ill-defined) it looks like it's winter as the trees do not have leaves on. It is also likely that the 'limit' for the height they can reach is lower, adding to them being less-defined.

The 4th picture to me looks more like a cumulus congestus or towering cumulus possibly in a weak CAPE environment. The top of the cloud still looks quite well defined and quite 'bubbly' looking.

The bottom 3 pictures (well defined) look like they are in summer, (can't actually tell for the first picture but the sun angle looks quite high on the clouds) and all show the CB cell reaching higher before it glaciates, (although this is probably also due to more CAPE in warmer air masses as well) The one over the sea seems to glaciate earlier so it may be a cooler air mass over the sea, but also less CAPE probably.

This may be mainly because when the freezing level is lower and the air mass cooler there tends to be less CAPE anyway, so I may be waffling about not much. But I think that the freezing level in cooler air masses may well play a large role in this as well, if I've made any sense.