Atmospheric motion relating to high pressure systems re: Sc clouds and fronts?

[Rich West]

Hi all

It was suggested that my original question might be being missed, due to the thread title, so I have reposted here.

The link to the discussion so far is:

http://www.stormtrack.org/forum/showthread.php?28649-Greetings-and-Introductory-Post

When I got into meteorology last year it seemed straightforward that high pressure areas involved sinking air. Then I saw a sky, under the high pressure system we had at the time, filled to the horizon with Stratocumulus. If the air is subsiding I wanted to know how clouds were forming as this required lift or stationary air at the least. At another time I noticed frontal systems spreading through the high pressure systems on the MET surface charts. I wanted to know how these "low pressure system phenomena" were spreading deep into (and sometimes through) the high.

Odd patches of cloud (like the other day's alto cumulus) seem to be small features and are probably related to orographic lifting. However, the Sc, being 8/8 at points, seems to be a different phenomena in my newbie eyes. I eventually found (after a huge amount of search time) a diagram (without detail) that showed HP sometimes subsiding from below cirrus level to just above Sc level (where it diverges/diffluences(?)). This made sense of the clouds' existences. However, if the air was outwardly horizontal above Sc level then why the hell was the ground showing a high barometric reading? This was my initial question that was never cleared up.

My own stab at a hypothesis after thinking I might never get to the bottom of this was:

"The air below Cirrus and above Stratocumulus is descending as it should. The air from ground up to the top of Sc is stationary, however, I think the subsiding atmosphere might be compressing the stationary layer and thus causing it to be high pressure too. Obviously I can't say for sure as the experts are silent but that's my newbie guess."

Any data on this general question or the frontal impingements woudl be gratefully received.

Cheers

Rich

 

[MClarkson]

Their are many possible reasons, but it boils down to the atmosphere is not simple and only occasionally follows textbook cases. A region of high pressure does not necessarily have subsidence at all points, especially if it is not strong or if you are on its periphery. The sinking drying motion associated with the high could be less important than the surface heating/boundary layer effects... this would be especially true in the afternoon when the boundary layer is at its most energetic.

 

[Rich West]

Thanks for the reply. Agreed, but for the surface barometer to keep reading high pressure we need subsiding or (as I propose now) compressing air. If the barometer had changed downwards when these phenomena manifest then I could agree fully that this would be the case. Does that make sense, or am I missing something still?
Rich

 

[MClarkson]

The surface pressure will increase in a region so long as there is more air flowing in (from any direction) than air flowing out. That does not require that all air is flowing down. Even ignoring the horizontal directions entirely, you could still have areas of ascent scattered amongst a larger amount of descent. So long as the mass rate of descent is higher than the mass rate of ascent (convergence>divergence), then the pressure will increase by mass continuity. Also temperature changes result in pressure changes, and small scale motions can cause minor fluctuations totally unrelated to the large scale feature (in this case, the region of high pressure).

Hope that helps.

 

[Rich West]

Cheers M Clarkson. I appreciate your replies. So given that the Sc I am referring to was 8/8 cover, would you say that this still counts as local ascent/non-movement or is my hypothesis a possible solution? The other option is that both ideas could be right in different circumstances. Would you agree or disagree with this?
Have a great day
Rich

 

[Patrick Marsh]

Hi, Rich,

The key here is that you need to think in three dimensions. Just because a region is under the influence of a surface high pressure does not necessarily mean the entire troposphere is under the influence of subsidence. (Also, not that in the definition of a high pressure, subsidence is not mentioned; subsidence is not a requirement for defining a region of high pressure.)

The best way to illustrate this is to think about what happens immediately after a strong cold frontal passage. As cold, dense surface air filters in, the atmospheric pressure measured at the surface begins to rise. At this point one begins to be under the influence of the surface high pressure. However, aloft, the mid and upper-level troughs are most likely still to your west. At these levels, you wouldn't claim to be "in a region of a high", instead you'd still be considered in a trough.

Another way to think about this is the class "over-running" setup, where warm, moist air is advected up, and over, a strong surface stable layer. (I use quotes around over-running as the correct term for this is isentropic ascent, not over-running.) In this scenario, you are typically north of a warm front, in the cold air associated with a surface high pressure area to your north or northeast. The ascent (and associated cloud-cover and precipitation) are taking place above the surface stable (high pressure) layer. So, even though the barometric pressure, and human analysis, would suggest a surface high pressure, you will still be getting ascent --- just at a level/layer above the high pressure.

Make sense?

 

[Stephen Levine]

Greetings, Rich..I couldn't pull up the link for your original post, and have some questions for you:
What was your location relative to the center of the high pressure system?
What were your wind directions at your location?
It's possible to be under the influence of a high pressure system, yet have S.C. invade the sky from the backside of a strong low off to the East bringing humid C.P. air to the region.
When I lived in the Ohio Valley, a full 36 hours into the new high pressure system and under a barometer that exceeded 30.00", we sometimes had a sky filled with S.C. due to humid C.P. air mixing with breaks of sunshine and effects of moist earth due to previous rainfall. This especially occurred in the warmer months of the year, and when clearing ensued after dark, sometimes unseasonably cold lows would be recorded.

 

[Paul Knightley]

Remember that subsidence through an anticyclone does not often reach the surface, at least, away from mountains. Thus the lower part of the atmosphere is still subject, as always, to be warmed and cooled diabatically through the diurnal heating cycle, below a temperature inversion.

If the inversion is above the level of condensation for a buoyant parcel (the Normand Point), cumulus clouds will start to form if the air below can be warmed/lifted (e.g. by the sun). The cloud will then develop, but the buoyant parcel will reach the inversion and find itself cooler than the environmental air - it will then tend to sink. If lifting can occur over a reasonably large area, then numerous cumulus clouds will develop, and then spread out in a kind of 'pancake'. Once cloud forms in this way, it can be quite hard to shift, especially in the winter. Here in the UK, stratocumulus is very common, and can be very annoying!

 

[Rich West]

Hi Patrick. Yes thank you. It seems my learning curve from "Meteorology Today" will be steep. Between you and MClarkson I have some new learning so cheers.

Hi Stephen. It has occurred when the high has been centred in various places..bringing mT or mP or cP air. Your information on that mechanism is much apreciated. Cheers.

Hi Paul. Your first pragaraph was basically what I was saying. The diagram I found showed the diffluence above the Sc layer leaving the layer below untouched. This explained the anitcyclonic gloom but left me not knowing why the barometer was showing high pressure. Now the other good folks here have shown me ways in which the surface reading might be such but under the particular conditions you and I describe I still wonder can the geenral subsidence higher up compress the lower level to make the high reading? Thanks.

All the best to you all

Rich

 

[Paul Knightley]

Think about an area of high pressure as just one big process: convergence higher up in the troposphere, downward motion, and then slow diffluence at the surface. Generally, an area of high pressure means the troposphere is 'thicker' (warmer) through the column than an area of low pressure. As the column is deeper there is more air - hence it weighs more. Atmospheric pressure is simply a function of the weight of air in the column above.

 

[Rich West]

Hi Paul. thanks for the reply.

"Think about an area of high pressure as just one big process: convergence higher up in the troposphere, downward motion, and then slow diffluence at the surface."
Agreed, except in this case where diffluence is above the Sc layer.

"Generally, an area of high pressure means the troposphere is 'thicker' (warmer) through the column than an area of low pressure. As the column is deeper there is more air - hence it weighs more. Atmospheric pressure is simply a function of the weight of air in the column above. "
A warmer column will be a more expanded column and so (if we restrict width in a basic model) then a higher column can have the same amount of air weight as a lower pressure column that is cooler. So we would need downward movement (a natural result of more air confluencing at the top than diffluencing at the bottom) to guarantee a higher pressure, surely? This would then possibly compress the low stationary layer as I suggest?

Does my logic make sense?

Cheers and all the best

Rich

 

[Paul Knightley]

Yes, you do need the downward motion - hence my mention of convergence aloft and divergence near the surface.

 

[Rich West]

Hi Paul..yes I know this but what I was asking for confirmation of is whether that above Sc downward motion can compress the non descending level below it to increase the general pressure overall in a situation where the sky is 8/8 Sc?
I know that all the answers on here are possible causes for the high reading but still need confirmation or denial whether my proposed scenario is also a possible valid one too.
Cheers and all the best
Rich