Convergence

Discussion may follow.

Article written by Milt Hare and posted to the Hollister Yahoo Groups site on 17 July 2013.

Republished here – without permission- by P. Kelly.

Wed Jul 17, 2013 2:32 am (PDT) .

Convergence
by Milton Hare

I’ve written up a few thoughts about convergence for those who can’t stay
away from the subject. Much more interesting than my current to do
list… This is based on perhaps a hundred convergence flights over 5
years or so, often while flight testing the first implementation of BLIPMAP
by Dr. Jack while flying from Williams.

Convergence is a complex subject – I don’t pretend to understand it but I
have learned a few things over the years that I believe to be true. Any
actual experts out there please correct anything that seems vague,
misleading or incorrect. This didn’t come from a book, it’s just what I’ve
come to believe after trying and failing many times to figure out how to
tame this evil but fascinating and sometimes awesomely strong and
continuous source of lift.

Keep in mind that experts have been wrong about convergence too, including
the fact that BLIPMAP can predict them. I believe I was the first one to
actually test the accuracy of BL UP/DOWN MAX, and Dr. Jack even wrote it up
and sent it to the NWS model developers to tell them that we had proved
that it was possible to predict the convergence here in Northern California
at least. They thought the model resolution of 20km at that time was too
large to resolve convergence here).

So the fundamentals:

If surface winds are blowing towards each other (convergence), they meet at
some point and go up. Once they are high enough they may diverge (go away
from each other) or do some other confusing thing.

This means that if you are above the surface winds, the wind direction may
be completely different from that on the ground. It may be in the opposing
direction, or something else – I have seen a large variety of wind
directions above the marine layer. If there is a strong enough wind at
altitude, the lower convergence isn’t going to change the direction of the
upper winds. The converging winds meet, are forced up and then follow the
path of least resistance. Please note that an upper air convergence
actually goes DOWN until it hits the ground and then DIVERGES. I believe I
have seen that on BLIPMAP – not good!

Our common coastal convergence is the 500 to 2000 or even 3000 foot deep
marine layer blowing from the ocean through the low points in the coastal
hills. 3000 feet means deep morning stratus in Sacramento, Red Bluff and
other normally hot places in the Central Valley. Some of the best
convergence days bring light rain and deep stratus in the SF Bay Area –
after almost turning back a few times I realized that it’s actually a good
sign, assuming the airport is clear enough fly.

Marine layers act kind of like water, flowing through the gaps in the
ridges that are lower than the top of the marine layer, being sucked to the
east by the giant vacuum of light, hot air over the inland valleys. So
this means generally speaking the Eureka area (west of Red Bluff), Sonoma
Coast/Carquinez Straights, Golden Gate south to Half Moon Bay, Monterey,
San Luis Obispo and other low points along the coast. If the marine layer
is over 2,500 feet it spills over many of the ridges. Below 500 and it’s
not going very far.

1) Marine air blows generally from the west towards the convergence with
the warm valley air to the east. If you see wind on the ground from the
west, the convergence will be somewhere to the east of that point. Looking
inland will hopefully show signs of wind from the east with an area of calm
wind on the ground. Calm does not mean the wind ‘stopped’ – it just
changed direction to UP since it had no where else to go. Terrain, wind
and other things affect the location and timing of the convergence. It is
in essence the line where two dissimilar bodies of air touch each other and
interact, generally with the colder one forcing the warmer one upward.
This line can be stretched out, snake around and pushed this way and that
over time, especially over flat land. It’s a little like the tide coming
in over a 3 to 8 hour period, generally starting around 1pm or so. If you
look at enough BLIPMAPS you can see that there are often many broken up
lines of convergence as the terrain does it’s thing to break up the marine
air as it pushes inland.

2) The convergence line may or may not be moving over the ground.
Generally speaking, it can move pretty fast over flat ground, and tend to
‘stick’ to terrain for longer. So as the marine layer moves inland, you
can find the convergence near the low passes for a while, moving slowly or
not at all, and then suddenly it moves out over flat ground and speeds up
tremendously. That’s why you need to look at the hourly or fifteen minute
BLIPMAPS to get some idea about the progress of the convergence zone during
the day. Just looking at the 1400 run isn’t going to tell you everything,
although it’s often good enough since the patterns are often similar from
day to day.

3) The incoming wedge of marine layer air is going to trigger any buoyant
warmer air as it flows inland. The surface behind that line will be deader
than a doornail and will not generate useable thermals (Hollister after the
marine incursion arrives). These thermals may be MUCH higher than the
marine layer, depending on the lapse rate and surface temp. 16,500 is the
highest I’ve reached in convergence – it can be pretty low however. 2000
or less? Keep in mind that there is huge difference between thermals
triggered by the marine layer incursion and the convergence zone itself,
which may be very narrow and meandering around like some drunken Louisiana
river, not to mention changing in altitude. The convergence may be below
you as you fly between areas of higher altitude lift.

4) Our coastal convergence can be very complex, especially when you are
trying to climb at low altitude in the blue around terrain. The key point
here is to know when not to even try it. Up high they may behave like a
nice line of thermals, but down lower around the terrain it gets nasty –
more like white water rapids in slow motion. I think the first 7 of 10
blue convergence flights for me were terrifying failures. Make sure you
are high and NEVER trust convergence down low. The sink can be attention
getting and seemingly endless. You must be supernaturally aware of even
the smallest sign of wind direction on the surface or higher, along with
tiny scraps of condensing air, in addition to the movement of your wings as
the lift differential banks you one way or the other.

5) Use tractors and other dust or smoke generators as wind indicators –
NOT lift indicators. If you watch how the dust/smoke drifts you can see
where the air is going and hopefully figure out where the lift is. That
doesn’t mean the tractor or even a dust devil is where the lift is
centered. Although they can trigger lift, looking at the big picture from
a number of dust creators is the best bet. The dust devil on the surface
shows the center of the air spinning at the surface, which may not be under
the best lift. The thermal is going up while sucking in air from the
sides, and air is feeding in from various directions and speeds, which may
cause the dust devil to move around the bottom of the thermal or even off
the to side. So don’t assume the best lift is over the dust devil – if you
find lift nearby, take it. Your thermal is going to feed into any larger
one nearby anyway, so take the safe bet and stick with it. Diving directly
for the dust devil is will not always put you in the best lift.

6) Keep in mind that wind indications on flight computers may not be
accurate. If you haven’t circled for a while they can be completely
incorrect. Ask around to figure out if your computer gives good wind speed
and direction information, and how much circling you need to do to get it.
They can be shockingly incorrect – or even backwards! Flying west at 45
knots in a 60 knot wind will show you flying east at 15 knots (since you
are actually drifting backwards) That is head spinning when looking at
your moving map (that blew my mind over Hollister once).

7) Convergence does some weird things. Here is my theory for the square
miles of lift we used to fly near Lagoon Valley in the evenings when there
were very strong marine winds from the west. The strong marine air ‘river’
flowing through the Carquinez Straits blows east into the hot valley air.
This river starts a large eddy several miles across which turns
counterclockwise over Fairfield/Lagoon Valley. As this eddy turns, it
warms up, gathering hot air as it rotates back towards itself. When this
now warmed up air meets the incoming ‘river’ of cold air, it is forced over
it since it is now lighter. Perhaps this is what the hang glider pilots
call a ‘glass off’. Anyway it would be interesting if someone knows how
this actually works. It’s the only condition I’ve seen where the entire
sky seemed to be going up, so it would be nice to understand it.

8) Springtime and winter convergence is the best time to learn about
convergence since you can see the clouds. Remember that marine air makes
the clouds lie. Beautiful clouds with dark bottoms due to high humidity
are not the best – always go for the highest cloud fragments, even if they
appear for only a moment. They will generally be on the landward side of
the convergence, but I am pretty sure I’ve found lift on the ocean side in
some cases. I think sometimes really strong lines of thermals push up into
upper air winds which flow over the top of the cumulus cloud in a ridge
lift sort of situation. As Gimmey says, go where the lift is…

9) There are occasionally very large convergence lines that have nothing
to do with our coast. Do not ignore them! If you see something large and
unusual, cross-reference it with other sources and go for it! I towed from
Williams and glided to Chico, connected with convergence at 3,000 feet and
climbed to Lassen at 13,000 feet or so. That was a BLIPMAP convergence,
with strong winds from Shasta to the south meeting strong winds from the
east which met on a line between Chico and Lassen. Also I have seen many
times in the fall a giant triangle of convergence from Williams north to
the Trinity Alps, then east to Idaho, then back to Lassen area and a glide
home. If I had a crew or motorglider I would have tried it.

10) The BL UP/DOWN Max doesn’t just show ‘normal’ convergence – it may be
showing another type of lift, generally something big. Some mountain
ranges basically form their own convergence by creating huge lines of lift
which create their own winds from the sides to feed the giant thermals over
the high ground. BL UP/DOWN Max shows this because it is so big, and it is
actually a convergence (as every thermal is) but it will behave much
differently from a marine convergence.

11) BL UP/DOWN MAX doesn’t actually tell you at what altitude the ‘MAX’
lift rate occurs. You just know there is an area of strong ‘UP’ at that
point on the map. Dr. Jack is able to show the horizontal cross-section
but BL UP/DOWN MAX only shows the horizontal extent and strength of the
lift. The top of the convergence lift seems to match BL TOP which makes
sense, but I imagine there are circumstances where that won’t be the case.
If there is ‘pure’ convergence which has nothing to do with the lapse rate
but is simply winds converging and going up, you might be able to go higher
than the lapse rate would indicate. Add in a little CAPE (Convectively
Available Potential Energy or thunderstorms) and something interesting (or
scary) might happen.

Anyway… that’s my 2 cents on convergence for now. There is far more on
the subject, and this is very limited to our local coastal convergence but
hopefully useful to someone out there. Hopefully others can add to it or
correct as needed.

Obviously I need to get back into the sky soon…

Best,
Milt

————————————————————————————————-

 

Proudly powered by WordPress Theme: Adventure Journal by Contexture International.