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Convergence covers a lot. Pretty much any thermal lift can be described as convergence. What I mean when I'm talking about convergence is thermal lift that is not primarily caused by differential heating from the ground.
Convergence lift is thermal lift, but the thermals are triggered and/or organised by factors other than heat transfer from the ground. This may be by airmasses with different properties or moving in different directions.
A cold front has a convergence on the leading edge. The cool dense air pushing into the warmer air ahead forms a wedge that lifts the warm air from the ground. If the warm air is unstable, once it starts lifting it will keep going up until it reaches an altitude where it is at equilibrium with the surrounding air. Effectively we get a line of thermals where the two differing airmasses meet. This will often be marked by a step in the cloudbase as the cooler air has a lower cloudbase and the warmer air has a higher cloudbase.
This split in the cloudbase is a common and significant sign that there is a convergence. It can be misleading though.
Because the convergence is thermal lift, and thermals come from the ground, the wind on the ground is the most reliable indicator of where the convergence is. The wind blows towards the convergence, and where the winds meet (converge) is where the thermal energy is being triggered.
We often see the sea breeze when flying from Omarama. There is the sea breeze near the east coast that often comes inland until stopped by a prevailing westerly, and there is the sea breeze that comes through the valleys and passes into the Mackenzie Basin.
The coastal breeze often comes across the southern Canterbury to the first line of hills and stops there. From the Waitaki, north along the Hunter hills, the Grampions, the Two Thumbs. If the Hakataramea valley fills with sea air, the edge of the convergence comes to the Kiklistons. When this happens you can run from the head of the Macauley to the Waitaki in strong lift the whole way. Because the coastal airmass is significantly different to the inland airmass the convergence is often marked with a dramatic change in cloudbase.
Sometimes the dramatic wall of cloud is false indicator. The cool dense sea air has overtopped the mountains and is flowing across the Mackenzie cutting off the thermal energy. The convergence is now at the leading edge of this flow. The most reliable indicator of where this is happenning is the wind on the ground.
Because of the formation of the Southern Alps with the main divide running at a 45 degree angle from the south west to the north east with large mountain ranges on the east side running north to south, the prevailing westerly flow comes over the divide into the large valleys formed by these mountain ranges and flows down them to the south. Anywhere these valley flows meet has the potential to form convergences. These down valley flows can also interact with the sea breeze to push the seabreeze around or form convergences at an angle to the seabreeze.
Often at the end of a nice thermal day inland, we get a strong easterly up the Waitaki Valley. This is more pronounced with a south or southwest flow. The sea breeze on the east coast between Oamaru and Waimate is funneled up the Waitaki Valley. As the valley narrows, the wind accelerates. The flow is cool dense marine air, so it comes up the valley in a thin wedge.
At the leading edge of the sea breeze it pushes under the warmer inland air, lifting it and creating a line of thermal energy across the valley. If there is a westerly inland this frontal convergence moves slowly and may become stationary.
The good lift is where the wind change on the ground is happenning.
In the afternoon at Omarama you can often look down the Waitaki Valley and see a wall of low cloud across the valley near Otematata, spilling over the top of the hills east of the Benmore Dam. When you see this you know the easterly is on its way.
When the easterly arrives at the airfield it is common to get the wind on the ground going from the westerly that has been blowing all day to a strong easterly. If you are trying to land while this is happenning it can get interesting. Best option is to climb away in the convergence, wait for the easterly to get established, and land when it is more settled.
As this layer is often only 500 - 1500ft thick you do descend through a pronounced windshear between the upper westerly and the lower easterly as you approach the airfield. Be aware of this. It's just a bit of turbulence if you know what's coming.
Briefing prepared by Phil Plane.
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