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According to http://www.universetoday.com/36816/winds-on-venus/, the high altitude winds on Venus travel at about 100 m/s.

I don't think frictional resistance is really what's limiting the speed of the winds. Here's a possible cause of resistance to the winds moving even faster.

  • Venus has a single convection cell per hemisphere.
  • The atmosphere cools by adiabatic expansion as it rises at the equator,
    • then because it's colder than the low altitude air, it emits radiation at a lower rate than the low altitude air
    • so it heat's by absorbing radiation from the low altitude air faster than it cools by emitting radiation
    • so the temperature drops less rapidly with altitude than that from adiabatic expansion.
  • As the air at the equator rises, it cools
    • but then it's colder than the rest of the air at that altitude
    • so it resists being accelerated upward by the high pressure of the air under it.
  • Also because of that resistance, it rises slowly enough that it heats up from absorption fast enough to not drop further in temperature as it goes higher preventing it from exerting a stronger force of resistance to the flow of the convection cells than the driving force for the convection.
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    $\begingroup$ Would you mind honing your question a bit and improving the formatting? I think this is a good question, but as it stands now, it reads like a stream of consciousness type thing which is hard to follow. $\endgroup$
    – zephyr
    Jan 10, 2017 at 20:34
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    $\begingroup$ It isn't clear what you're asking. I've always assumed that Venus high velocity upper atmosphere winds had to do with proximity to the sun, a hot side and a cold side, in the upper atmosphere. The lower atmosphere is very different and with minimal Coriolis effect due to slow rotation, it's as you said, one big Hadley cell with some variation around the poles. But your question on frictional resistance, I don't have a good sense on what you're asking. $\endgroup$
    – userLTK
    Jan 10, 2017 at 22:48
  • $\begingroup$ Timothy, I hope my reworded title is accurate. $\endgroup$
    – Carl Witthoft
    Jan 11, 2017 at 13:18

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I found some recent papers which, if nothing else, discuss apparent mechanisms. Quoting from Schubert, 2016,

The large-scale circulation of the upper atmosphere from ~90 to ~200 km altitude (upper mesosphere and thermosphere1 ) is a combination of two distinct flow patterns: (1) a relatively stable subsolar-to-antisolar (SS-AS) circulation cell driven by solar (EUV- UV) and IR heating, and (2) a highly variable retrograde superrotating zonal (RSZ) flow, in part a continuation of the lower-atmosphere RSZ flow discussed above

Then, ESA says, in part,

In 2006, average cloud-top wind speeds between latitudes 50° on either side of the equator were clocked at roughly 300 km/h. However, detailed cloud tracking studies revealed that these already remarkably rapid winds are becoming even faster, increasing to 400 km/h over the course of the mission. The reason for this dramatic increase is unknown. [emphasis mine]

Alternatively, these guys think they have a working model:

The atmospheric circulation in Venus is well known to exhibit strong super-rotation. However, the atmospheric mechanisms responsible for the formation of this super-rotation are still not fully understood. In this work, we developed a new Venus general circulation model to study the most likely mechanisms driving the atmosphere to the current observed circulation. Our model includes a new radiative transfer, convection and suitably adapted boundary layer schemes and a dynamical core that takes into account the dependence of the heat capacity at constant pressure with temperature. The new Venus model is able to simulate a super-rotation phenomenon in the cloud region quantitatively similar to the one observed. The mechanisms maintaining the strong winds in the cloud region were found in the model results to be a combination of zonal mean circulation, thermal tides and transient waves. In this process, the semi-diurnal tide excited in the upper clouds has a key contribution in transporting axial angular momentum mainly from the upper atmosphere towards the cloud region. The magnitude of the super-rotation in the cloud region is sensitive to various radiative parameters such as the amount of solar radiative energy absorbed by the surface, which controls the static stability near the surface. In this work, we also discuss the main difficulties in representing the flow below the cloud base in Venus atmospheric models

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