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As should have been clear from what I wrote, in the bottom third of the
thermal the air is still warmer, but steadily mixing in surrounding air. Once properly formed the momentum of the tons of air moving is presumed to keep the circulation active. Air close to the ground is also more humid which increases the buoyancy. We did not fully model the mechanism, just took measurements that illustrated the scale and characteristics of meso-scale air motion in a range of situations, including convective weather. At 13:33 03 November 2014, Andy Blackburn wrote: On Monday, November 3, 2014 3:15:06 AM UTC-8, pete purdie wrote: ...An important finding is that above about one-third of the distance to the inversion, there is no significant temperature difference between the thermal and surrounding air; near the inversion the temperature is actually lower since the warmer air above the inversion is being mixed down around the rising air. Humidity is a significant indicator, H2O molecules being lighter than O2 or N2. So thermals rise, not because of the sun heating the ground and making a bubble of warm air that breaks free, but because somehow a more humid bubble of air is created. What mechanism creates the bubble of humidity? 9B |
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On Monday, November 3, 2014 6:15:05 AM UTC-8, pete purdie wrote:
Once properly formed the momentum of the tons of air moving is presumed to keep the circulation active. Not entirely sure I buy momentum as a first-order effect since blue thermals don't typically overrun the top of the invasion level by very much, so I can't imagine the top 2/3 of the thermal is simply momentum. Also, if it's thoroughly mixed from a temperature perspective wouldn't it also be mixed from a humidity perspective as well so it wouldn't be rising from having higher humidity either? Curious. 9B |
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On Monday, 3 November 2014 16:15:05 UTC+2, pete purdie wrote:
As should have been clear from what I wrote, in the bottom third of the thermal the air is still warmer, but steadily mixing in surrounding air. Once properly formed the momentum of the tons of air moving is presumed to keep the circulation active. Air close to the ground is also more humid which increases the buoyancy. We did not fully model the mechanism, just took measurements that illustrated the scale and characteristics of meso-scale air motion in a range of situations, including convective weather. At 13:33 03 November 2014, Andy Blackburn wrote: On Monday, November 3, 2014 3:15:06 AM UTC-8, pete purdie wrote: ...An important finding is that above about one-third of the distance to the inversion, there is no significant temperature difference between the thermal and surrounding air; near the inversion the temperature is actually lower since the warmer air above the inversion is being mixed down around the rising air. Humidity is a significant indicator, H2O molecules being lighter than O2 or N2. So thermals rise, not because of the sun heating the ground and making a bubble of warm air that breaks free, but because somehow a more humid bubble of air is created. What mechanism creates the bubble of humidity? 9B Interesting study: http://journals.ametsoc.org/doi/abs/...B%3E2.0.CO%3B2 Figure 6 gives quite good overview of temperature and humidity variability during (weakish) convective activity. Temp. differences are appr. 0,5 deg and humidity 1 g/kg close to ground. Other Weckwerth papers are worth reading too, IMHO. |
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