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Old 16-02-2016, 04:16   #1
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Occluded fronts

Hope this is best place for this subject. Could not find meteorology.

I understand we can expect a Cold Occlusion to be more active than a Warm Occlusion. Thing is I am not aware how to identify whether an occlusion I am viewing on a surface pressure chart is a cold or warm occlusion?.

Thanks

Mike
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Old 16-02-2016, 04:37   #2
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pirate Re: Occluded fronts

Courtesy of Wiki for the uninitiated...


Occluded fronts usually form around mature low pressure areas. There are two types of occlusion, warm and cold:
In a cold occlusion, the air mass overtaking the warm front is cooler than the cool air ahead of the warm front, and plows under both air masses.
In a warm occlusion, the air mass overtaking the warm front is not as cool as the cold air ahead of the warm front, and rides over the colder air mass while lifting the warm air.

The occluded front symbol should thus be plotted at the position where the cold air is intersecting the surface, as on the image to the right. It thus varies between warm and a cold occlusions. The trowal on the other hand, being the projection of the warm air trough aloft, is at the same position in both cases. The position of the occluded front is often misplaced with the associated weather on a weather map but this is the position of the trowal.[3]
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Old 16-02-2016, 04:41   #3
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Re: Occluded fronts

Quote:
Originally Posted by Mike1956 View Post
Hope this is best place for this subject. Could not find meteorology.

I understand we can expect a Cold Occlusion to be more active than a Warm Occlusion. Thing is I am not aware how to identify whether an occlusion I am viewing on a surface pressure chart is a cold or warm occlusion?.

Thanks

Mike
You seem to have missed the 'Weather' thread
http://www.cruisersforum.com/forums/f147/

Here are the symbols for the three classical types of fronts (warm = drops of sweat )
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Old 16-02-2016, 04:46   #4
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Re: Occluded fronts

Excellent question! Keen to hear an answer myself.

If we get some real met expert, maybe they can also help me understand how Ana and Kata cold fronts tend to behave from a yachties (ie local surface wind) perspective and how best to identify them from synoptic charts and satpics.

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Old 16-02-2016, 05:16   #5
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Re: Occluded fronts

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Originally Posted by boatman61 View Post
Courtesy of Wiki for the uninitiated...


Occluded fronts usually form around mature low pressure areas. There are two types of occlusion, warm and cold:
In a cold occlusion, the air mass overtaking the warm front is cooler than the cool air ahead of the warm front, and plows under both air masses.
In a warm occlusion, the air mass overtaking the warm front is not as cool as the cold air ahead of the warm front, and rides over the colder air mass while lifting the warm air.

The occluded front symbol should thus be plotted at the position where the cold air is intersecting the surface, as on the image to the right. It thus varies between warm and a cold occlusions. The trowal on the other hand, being the projection of the warm air trough aloft, is at the same position in both cases. The position of the occluded front is often misplaced with the associated weather on a weather map but this is the position of the trowal.[3]
Thanks,

This may well answer my question perfectly but I am afraid the penny/cent has not dropped yet. I did imagine if there was a way of determining warm or cold occlusion it would be by displaying the occlusion sign a different way round. As I said the above description probably makes this clear but I confess I am a bit hard of thinking.

Mike
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Old 16-02-2016, 05:19   #6
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Re: Occluded fronts

Quote:
Originally Posted by Snowpetrel View Post
Excellent question! Keen to hear an answer myself.

If we get some real met expert, maybe they can also help me understand how Ana and Kata cold fronts tend to behave from a yachties (ie local surface wind) perspective and how best to identify them from synoptic charts and satpics.

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Does this reply suggest you do not feel the above reply has answered the question?

Mike
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Old 16-02-2016, 05:26   #7
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Re: Occluded fronts

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You seem to have missed the 'Weather' thread
Weather - Cruisers & Sailing Forums

Here are the symbols for the three classical types of fronts (warm = drops of sweat )
I am always mindful that this forum is international and different meteorologists in different countries may use different weather symbols but the symbols used in this post for an occluded front are different to the ones I generally encounter. I am used to seeing the triangles & semi circles joined up for an occluded front. Maybe this is the key to determine warm or cold occlusion?

Mike
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Old 16-02-2016, 05:39   #8
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pirate Re: Occluded fronts

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Originally Posted by Mike1956 View Post
I am always mindful that this forum is international and different meteorologists in different countries may use different weather symbols but the symbols used in this post for an occluded front are different to the ones I generally encounter. I am used to seeing the triangles & semi circles joined up for an occluded front. Maybe this is the key to determine warm or cold occlusion?

Mike
Where the two actually meet and upwards is when you get the alternating triangle/half pizza... which gradually moves down as the one overtakes the other..
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Old 16-02-2016, 06:06   #9
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Re: Occluded fronts

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Where the two actually meet and upwards is when you get the alternating triangle/half pizza... which gradually moves down as the one overtakes the other..
Thanks but I am not understanding how this answers the question.

I have just looked at a set of surface pressure charts for UK for next 5 days and the only thing I seem to have determined by observation alone is the "warm" symbol in the occluded front is always nearer the centre of the low.

Mike
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Old 16-02-2016, 06:18   #10
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pirate Re: Occluded fronts

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Originally Posted by Mike1956 View Post
Thanks but I am not understanding how this answers the question.

I have just looked at a set of surface pressure charts for UK for next 5 days and the only thing I seem to have determined by observation alone is the "warm" symbol in the occluded front is always nearer the centre of the low.

Mike
Not a weather expert.. as a matter of fact I rarely look at these things but to me it seems logical that the warm will be the central to the low symbol..
No warm air rising.. no vacuum for cold air to rush in and fill..
But.. others will definitely know better..
I'm a mossy rock man myself...
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Old 16-02-2016, 08:26   #11
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Re: Occluded fronts

Mike,
I think the question you have on determining whether an occluded front has warm or cold characteristics probably ought to be directed to the source of the weather charts you are referencing - in your case the UK Met Office. If you were pose the same question to NOAA you might get a different answer. I know from looking at NOAA and UK charts of the same area and period, the frontal analyses don't often agree. This might because the storm systems on those common chart areas are moving away from the NOAA area of responsibility and moving toward the Met Office area.
I don't remember being taught any convention for depicting occluded fronts when I was studying meteorology back in the 60's, and I did all of my forecasting in the tropics and sub-tropics, so I never had an opportunity to draw such a front on my analyses.
To venture a guess, I would say if a Trowal line is drawn on the chart it is a warm occluded front and if not it is a cold occluded front. The trowal line for a cold occluded front is normally very close to the depicted frontal line and therefore might be dropped for reasons of chart clarity. Of course it may also be because the weather office does not believe the differentiation of the two types of occluded fronts is of any significance to the general public.

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Old 16-02-2016, 08:45   #12
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Re: Occluded fronts

Thank you John,

Good reply, now I need to know what a trowal is, I will google it in a tick. Before I posted on here I did look on the UK met office site and looked at their tutorial on fronts. Very informative but not to the extent it answered my question.

I then emailed them and received their standard response, they should get back to me within 24h.

Mike
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Old 16-02-2016, 08:49   #13
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Re: Occluded fronts

Mike,
The definition of the trowal is in the Wikipedia reference provided by Boatman

John
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Old 16-02-2016, 10:54   #14
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Re: Occluded fronts

There is nothing special about it or to much to say, simply and short...

Occluded Front is when a cold front overtakes a warm front.

or bit longer....

A developing cyclone typically has a preceding warm front (the leading edge of a warm moist air mass) and a faster moving cold front (the leading edge of a colder drier air mass wrapping around the storm). North of the warm front is a mass of cooler air that was in place before the storm even entered the region.
As the storm intensifies, the cold front rotates around the storm and catches the warm front. This forms an occluded front, which is the boundary that separates the new cold air mass (to the west) from the older cool air mass already in place north of the warm front. Symbolically, an occluded front is represented by a solid line with alternating triangles and circles pointing the direction the front is moving. On colored weather maps, an occluded front is drawn with a solid purple line.
Changes in temperature, dew point temperature, and wind direction can occur with the passage of an occluded front. Let say temperatures ahead (east of) the front were reported in the low 40's while temperatures behind (west of) the front were in the 20's and 30's. The lower dew point temperatures behind the front indicate the presence of drier air. A noticeable wind shift also occurred across the occluded front. East of the front, winds were reported from the east-southeast while behind the front, winds were from the west-southwest. .
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Old 16-02-2016, 11:36   #15
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Re: Occluded fronts

Mike,
You got me interested the question of depicted the different types of occluded fronts so I checked the NOAA manual on weather analysis (http://www.wpc.ncep.noaa.gov/sfc/UAS...alVersion1.pdf), and it does not reference different symbols for the different occluded fronts, just one symbol.
Thinking about it, I would have great difficulty trying to determine whether an occluded front is warm or cold from a surface plot of weather data. I think it would require an upper air dataset in the vicinity of the front, which would not always be available. So my further guess is the types of occluded fronts are not depicted on the weather analysis charts because the types cannot be determined from surface data and it isn't all that important for the development of a forecast.
Of course that's just a guess on my part.

John
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