Challenge question 6 – do we have a polar low?
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It is now 15 UTC on 5th February 2001. 9 hours have passed since our last assessment at 06 UTC.
Focussing on our target area in the region of the ‘Mike’ weather ship (66N, 02E), let’s look at some of the developments which have taken place.
It is important to recognise the signs of development and know when the criteria to classify the system as a polar low have been met. In the next part of the question, we will look at the low itself in more detail.
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Use the data viewer to look through the information which has arrived since 06 UTC. Then for each statement below, select the corresponding time of occurrence. If the event is not evident in the data, select the final column. Then decide which data gave you the earliest signs of significant development.
No – although the pressure fall at Mike is greater than at surrounding observation positions
No – although the pressure fall at Mike is greater than at surrounding observation positions
Yes – the observation indicates a 7.9 hPa fall in 3 hours, compared with 2.2hPa at the nearest observation
No – the pressure is stabilising, and has fallen only 1.7hPa since 12UTC
No – look again at the observations at Mike.
No – look again at the criteria for defining a polar low
No – look again at the criteria for defining a polar low
No – look again at the criteria for defining a polar low
Yes - the wind speed has increased to 35 kts. There may be stronger winds close by – definitely polar low strength now
No - look again at the observations at Mike
No – however, we can see that the wind has backed and strengthened due to the effect of the trough
No – the wind is still from the NE
No – this is not the largest change. However it is significant as the wind has backed again, and is now from the NW – it tells us that the low is approaching!
Yes – the direction has changed by nearly 180 degrees – the low must be passing over Mike
No - look again at the observations at Mike
No – snow showers are reported, but visibility at the ship remains good
No – snow showers are reported, but visibility at the ship remains good
No – snow showers are reported, but visibility at the ship remains good
No – snow showers are reported, but visibility at the ship remains good
Yes – the reported visibility suggests that the snow showers are close but not occurring directly over Mike
No – the sounding at Mike shows that T500 is -35C
No – we have no observations for this time
Yes – the profile has changed dramatically. The cooling effect since 00UTC at levels above 700hPa have eroded the inversion, and left a T500 of -43C
No – we have no observations for this time
No – look at the soundings from Mike
No – enhanced convection is evident, but a comma shape cannot be distinguished
No – there is a cluster of cold cloud tops, indicating enhanced convection in the trough, but there is no clear shape to it at this time
Possibly – the earliest AVHRR image after 0840 is one from 1240 UTC. It shows a coherent area of enhanced convection with a comma shape at its southern end, with centre just north of Mike
Yes – the enhanced convection is now clearly organised into a cyclonic system with diameter 300-400 km
No – look again at the AVHRR satellite imagery
No – the enhanced convection is not organised, and cirrus cover is not easily distinguishable
No – the enhanced convection is not organised, and cirrus cover is not easily distinguishable
No – the convection is still showing clear edges in the AVHRR imagery
Yes – there is a small area of cirrus cloud in the SE part of the system, but this is not extensive.
No – the strongest polar lows often show a cirrus sheet over a large part of the system – this one may not develop into a strong polar low
