Atmospheric Soundings – Approaching Frontal System

21 Jun

In a recent blog I discussed the usefulness of atmospheric soundings in terms of aiding to predict thunderstorm and convective activity. Well, forecast soundings have many uses and one other primary one is using them to view an approaching frontal system or to see the atmosphere through an active frontal system. A great example of an approaching frontal system is in evidence today (Fri 21st June) across Ireland. So, lets take a look at some of the synoptics;

12Z synoptics18Z synoptics

The low pressure and associated frontal systems can be seen clearly to the west of Ireland on the two synoptic weather charts. At midday you’ll notice that across Ireland there is a minor ridge of high pressure in evidence, this is represented by the isobars ‘ridging’ northwards. At midday the atmosphere across Ireland is forecast to be relatively stable beneath this ridge of high pressure, but you can clearly see by 1800Z the frontal systems associated with the surface low pressure are across Ireland, the ridge has been removed and the weather by the end of the day is essentially wet and windy in association with the passage of an active set of frontal systems.

The approach and passage of a frontal system (warm front) has certain characteristics and events that take place within the atmosphere.  These include anything from increasing wind speeds at high altitudes, the backing of surface wind speeds at surface level to generally a south-westerly direction and also the introduction of cloud and moisture, initially at high altitudes and then progressively at lower altitudes. The approach of a frontal system is represented well on the following graphic;

warmfront

Now, taking a look at some forecast soundings through the course of the afternoon across Ireland;

sounding13.curr.1200lst.d2sounding13.curr.1300lst.d2sounding13.curr.1400lst.d2sounding13.curr.1500lst.d2sounding13.curr.1600lst.d2sounding13.curr.1700lst.d2sounding13.curr.1800lst.d2sounding13.curr.1900lst.d2

1200BST and 1300BST: On these two forecast soundings you’ll notice that the red line (temperature) and the blue line (dew point) are particularly separated, especially between 700mb and 500mb. On a forecast sounding this represents a particularly dry atmosphere and hence little cloud. Also notice that the wind direction is still westerly or west-southwesterly and the wind speeds haven’t really increased very much through the atmosphere. This profile is representative of being beneath a ridge of high pressure and this tallies in well with the weather chart earlier in the blog which highlights a ridge of high pressure across Ireland.

1400BST and 1500BST: As the afternoon progresses the primary importance and change here is that the space/distance between the temperature and dew point line is decreasing within the upper levels of the atmosphere in particular. This is indicative of cloud increasing within the upper levels of the atmosphere as the frontal zone approaches. If you refer back to the earlier image showing the profile of an approaching frontal system you will see that cirrus and cirrostratus clouds are first to be expected and that is what these forecast soundings are indicating, an increase in upper level frontal cloud. Also note how the wind direction at lower levels has backed significantly and is now south-westerly

1600BST onwards: From around 1600BST onwards you’ll notice that the temperature and dew point lines get progressively closer and closer and by 1900BST are practically the same around the 700mb level. This is a clear example of how the frontal system has continued to approach the region introducing thicker cloud at increasingly lower altitudes and by the end of the afternoon a large portion of the atmosphere is moist and cloud laden in association with the frontal systems. You’ll also note that the wind speeds have increased within the mid and upper levels of the atmosphere in association with the jet stream and low pressure system.

The difference between the first image at 1200BST compared with the image at 1900BST is significant. The atmosphere goes from being stable and particularly dry indeed around the middle of the atmosphere (500mb) to being almost completely saturated by 1900BST in association with extensive frontal cloud which will clearly be producing frontal rain at surface levels.

Regards to all,

M.

Acknowledgements:

http://www.southamptonweather.co.uk/fronts.php

http://www.avbrief.com

http://rasp.inn.leedsmet.ac.uk

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