Unusual High Pressure Systems

[virtualsphere]

Hi there,

New-ish member here, been lurking for a while in a futile attempt to try and learn a bit before posting!

I've got two beginner's questions relating to the legendary Greenland high. Firstly, I noticed this chart posted earlier this evening on the Model Output Discussion which I thought looked quite unusual. As I understand it, the "T" stands for Trough and signals an area of low pressure, yet there are several "T"s marked over Greenland and one over Svalbard in areas showing some of the highest pressure on the chart, over 1035MB. If I'm right, and these are troughs, what weather would you expect to find there at the time? Normally I would associate a trough with precipitation, however surely pressure at this level would inhibit convection so you might just get a bit of drizzle (in snow grains)?

This leads me to my second question! Quite often I notice that when the charts show Greenland highs, the isobars seem really tight in comparison with the type of high you would normally see over the UK, Azores or Europe. At a glance and without the figures the 1048MB high in this GFS F.I. chart would look to me more like a low pressure system (which may sound silly, but bear in mind I'm a complete beginner). Is there a reason why these type of highs tend to form over Greenland and not the UK? I appreciate this isn't a great example as I'm sure I've seen charts in the past with much tighter isobars, but hopefully you get the point.

So... putting these questions together, my basic knowledge leads me to the contradiction that in Greenland you could still have snowy and windy weather with the pressure at 1040MB?

I'm probably the only person on here wondering what is happening in Greenland and not the UK when looking at these charts! And now, having shown my ignorance, I'm going to hide in the corner again!

John

[J07]

I don't know much about the weather of Greenland, so I don't know what a Norwegian model depression (which is what you're thinking of) would bring to them.

But bear in mind that low pressure can form in all sorts of different ways, and it doesn't necessarily have to have fronts associated with it. What may be showing up there is an orographic trough. That is an area of low pressure (which sometimes forms a closed circulation) on the lee side of a large land mass or mountain range. It's created by a deficit of air there due to the wind piling up behind a barrier of some sort and struggling to get around, or over it.

It can also form due to heating in the summertime (heat lows). These will often pop up if you look at North Africa during the warm months.

Likewise, there are "cold highs". Cold, sinking air near the surface increases air pressure at the surface. This has the effect of making winter time highs being more intense than summertime highs. "Intense" meaning how high is the central air pressure.

That is what happens over Greenland and Siberia in winter, when a high pressure area forms, it frequently will go over 1040hPa due to all that cold sinking air. There is a lot more to the Greenland high than this of course, for example whether it has good support from higher up in the atmosphere, but that's another question.

The issue that occurs with winter highs with big central pressures, is that at some point, the isobars have to be packed tightly together. These will be around the periphery of the high, but there will exist that strong gradient that you can see there, simply because there has to be a grading at some point from that very high pressure over Greenland to "normal" pressures elsewhere. That naturally makes for a tight gradient somewhere.

In contrast, when you see summertime highs that linger about 40 degrees latitude in the summer, their central air pressure may be little above 1020hPa. That may lead you to believe that it's a weak high and won't last long or won't bring amazing weather. But, in summertime those highs are referred to as the subtropical ridge, and they have very strong support in the upper atmosphere (essentially air is sinking from a very great height). This is what gives Mediterranean climates their dry summers.

Basically because there is no cold air near the surface, you don't get anomalously high central air pressures to kick it up, and so you won't usually see 1045hPa highs over Europe in summer. That means that when you move away from the high, the isobaric gradient is naturally slackened- there is no need for a tight gradient anywhere (though that doesn't mean it can't happen...). This is one reason why typically the "westerlies" (which are on the northern edge of the summertime highs) get weaker in summer.

[virtualsphere]

Thanks for this informative reply J07. I did wonder why even in the summer the high pressure never seemed to be as intense as in the winter, so that's answered another question for me too.

I guess that's why sometimes people refer to high pressure over Greenland as not being a 'real' Greenland high then, if it is just a surface pressure rise caused by the cold air and not supported by the upper atmosphere?

Funnily enough those tightly packed isobars I mentioned have shown up nicely on the 12Z GFS today, I see what you mean about the grading point going here from 996 - 1056MB!