Cloud Inversions In the Drakensberg

31 May 2021 11:46 #76919 by Serious tribe
@JonWells  thanks for the ongoing snow info.  I Have been meaning to ask you.  

What causes the low level cloud that sometimes sits below the escapement while the top is clear.  I am assuming that this in caused by what is termed an inversion, or at least the is what I have always called them.  
I came across this definition which while I understand the scenario, doesn't seem to talk to what happens in the berg -
'A cloud inversion, or temperature inversion is when the normal temperature distribution of air – warm at the bottom, colder as you go up – becomes inverted or flipped upside down. This means you have a cold layer of air trapped at ground level, overlain by warm air'
Whenever I seem to see them below me, the air is rushing in from Lesotho, down into KZN and is dam cold.  So I assume then that what happens in the Berg is the classic inversion.  Any thoughts on this?

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31 May 2021 13:25 #76920 by JonWells
Replied by JonWells on topic Snow Watch 2021
Hey ST, while I understand basic temperature inversions fairly well I dont claim to know much about cloud inversions specifically, but from some brief reading it seems it is related to the same phenomenon.  Requirements seem to be stable air where the coolest air sinks below the peaks and settles in the valleys, where it reaches the dew point and forms clouds.  You mention rushing wind though in your experience, which is a bit puzzling, but I guess it could just mean that its windy near the peaks, but could be calm lower down in the valleys allowing the cold air to concentrate and form a cold layer below the warmer air higher up.  

I've noticed a similar thing with frost, which cant form in windy conditions.  I've been on the escarpment when its windy and above freezing, while the valleys below are completely white with frost, indicating calm conditions.

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31 May 2021 17:39 - 31 May 2021 17:40 #76921 by tiska
Replied by tiska on topic Snow Watch 2021
> What causes the low level cloud that sometimes sits below the escapement while the top is clear.  I am assuming that this in caused by what is termed an inversion, or at least the is what I have always called them.  

The low, stratus cloud over KZN usually results from moist winds blowing up the topography from the Indian Ocean. As the wind hits the various steps in the topography, it rises, cools through expansion as it moves into high altitude and lower pressure, condenses and the clouds form. The wind is usually caused by a high pressure system that breaks off from the South Atlantic High and 'ridges' south of South Africa (because of the topography of the interior plateau) eastwards to join the Indian Ocean or Mascarene High. Sometime that ridging anticyclone is strong and deep and well south of the country in which case the moist air will be > 3000 m deep and the Berg will be covered in cloud. Other times that ridging anticyclone is weaker, shallow and not so far south of the country in which case the moist air will be < 3000 m deep and the top of the Berg will be out of the cloud. Other times that ridging anticyclone is even weaker and more shallow in which case the moist air will be <1000 m deep and only the escarpment at Pmb will be in the mist.

That is what goes on at lower levels.

On nearly every day in winter and many days in summer, there is a high pressure system over South Africa at altitudes of 5 km or more. South Africa is in the subtropics and the subtropics are the dumping ground for sinking air that has risen to around 12 km in the tropics. 

When air sinks it is compressed and it heats (1st Law of Thermodynamics). The rate of heating is 1 deg C for every 100 m that the air sinks. The heating creates a warm layer which is the inversion layer.
That inversion layer acts as a lid on the moist air which makes the cloud described above.

The inversion layer bottom is evident as top of the dusty and hazy lower layer of cloud free air on almost all days in winter.  

For snow, that anticyclone or high pressure system has to be displaced because for snow cold, moist air up to around 3500 m at least or more like 5000 m at least is needed.

 
Last edit: 31 May 2021 17:40 by tiska.
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31 May 2021 17:44 #76922 by tiska
Replied by tiska on topic Snow Watch 2021
 

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31 May 2021 17:47 #76923 by tiska
Replied by tiska on topic Snow Watch 2021
You can see the inversion layer in the photo above as the hazy layer around the same height as the nek between Cathkin and Sterkhorn.
The winds have blown moist air against the Berg to make the clouds. Above the clouds the air is sinking, heating through compression and making that nice inversion layer.
Photo from the first escarpment hike with my son. He got lucky. It was only on around my 10th hike that I was treated to a view above the clouds.
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01 Jun 2021 10:57 #76926 by Serious tribe
Replied by Serious tribe on topic Snow Watch 2021
@tiska  Thanks for the explanation, don't suppose you have this info in image or schematic form?  Always easier to crystallise the concepts from an image.

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01 Jun 2021 13:49 - 01 Jun 2021 13:49 #76928 by tiska
Replied by tiska on topic Snow Watch 2021

@tiska  Thanks for the explanation, don't suppose you have this info in image or schematic form?  Always easier to crystallise the concepts from an image.

This is a sequence of slides for different weather cases.
For each case, there is a visible satellite image, a near surface wind plot and a wind plot at higher levels (around 4 to 5 km).

Here are 3 images from relatively undisturbed conditions.
The satellite image for this day shows cloud free conditions over the Berg.


 



The wind field shows the two key high pressure systems surrounding SA near their average position 





Over the interior of southern Africa the continental high is evident at higher levels. This feature causes the sinking air and clear skies.

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Last edit: 01 Jun 2021 13:49 by tiska.

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01 Jun 2021 13:54 #76929 by tiska
Replied by tiska on topic Snow Watch 2021
This sequence shows conditions when clouds are below the level of the escarpment top. Note how the cloud also follows the Free State KZN border. 



Note that near the surface, the Indian Ocean high is not in its usual position. It has been responsible for pushing moist, cool air northwards from the ocean south of SA.



Meanwhile at upper levels the continental high is strong and firmly in place. This will cause widespread sinking air and stop the moist flow from overtopping the escarpment into Lesotho or Free State.

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01 Jun 2021 13:57 - 01 Jun 2021 16:33 #76930 by tiska
Replied by tiska on topic Snow Watch 2021
In this case there is deep cloud covering the escarpment



There is a ridging high south of SA




And, crucially, the interior high is displaced to the east by a big trough of low pressure which is busy advancing west to east across SA.
The usual sinking air from upper to mid levels is gone and the cloud layer is deep.









 

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Last edit: 01 Jun 2021 16:33 by tiska.

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01 Jun 2021 14:13 - 01 Jun 2021 15:11 #76931 by tiska
Replied by tiska on topic Snow Watch 2021
And a schematic of the typical 6 day cycle as shown by near surface weather systems.
The crucial bit of the sequence is the progression of the high pressure system from west to east south of SA.

 

The cycle was discovered by P.D Tyson and R.A. Preston-Whyte in the 1970s. They ran a Fourier analysis on surface pressure data for a bunch of weather stations around SA and found a notable 6 day peak.

Because 6 days is nearly as long as a week, the sequence can bring bad weather over the weekend for a few weeks on the trot.

Original paper:
Note on Pressure Oscillations Over South Africa
R. Preston-Whyte P. Tyson
Published 1973
Monthly Weather Review

Abstract Surface and 500-mb pressure series based on 12- and 24-hourly observations for 1958, 1966, 1965–66, and 1965–69 are analyzed for several South African stations. Preferential pressure fluctuations with periods of about 6 days are best-developed over southern and eastern coastal regions and are shown to advance from Cape Town to areas northwest of Durban at rates in good agreement with synoptic experience. 

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Last edit: 01 Jun 2021 15:11 by tiska.
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