28:00 I've always learned that Ekman induced vertical motions was basically a low-level effect, so the presentation here is a bit weird i think. To understand vertical motions in the free atmosphere, you have to look at something other than Ekman things.
At 26.30 for a surface low, partial dw/ partial dz is positive: but strictly speaking is this not just indicating that w increases with respect to z rather than indicating the w is positive (upward motion)? I guess the idea here is that the vertical change in w is large and positive and that w starts small/ zero at the surface?
If the surface is flat and air cannot flow into the surface, then at the surface we must have w = 0. With that boundary condition in hand we can draw conclusions about the sign of w even when only provided with information about dw/dz.
There are two aspects of your question: First, it's a bit of an idealization to say that the Hadley cell stops at 30 deg latitude, since it's more aptly described as a narrow region of strong convection and a broad region of subsidence that is roughly centered on the subtropics. See, for instance, the average vertical velocity found here upload.wikimedia.org/wikipedia/commons/6/6b/Omega-500-july-era40-1979.png. When air reaches the subtropics it is both cool enough to produce steady descent, and it has been affected sufficiently by Coriolis force for the flow to be roughly westerly. Second, the width of the Hadley circulation is a function of the strength of the temperature gradient and the rotation rate of the planet. More vigorous convection in the tropics is expected to enhance the strength of the Hadley circulation, driving up its meridional velocity and leading to a widening circulation. You can check out this paper for more info about the extent of the Hadley circulation: agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2008GL035847
28:00 I've always learned that Ekman induced vertical motions was basically a low-level effect, so the presentation here is a bit weird i think.
To understand vertical motions in the free atmosphere, you have to look at something other than Ekman things.
At 26.30 for a surface low, partial dw/ partial dz is positive: but strictly speaking is this not just indicating that w increases with respect to z rather than indicating the w is positive (upward motion)? I guess the idea here is that the vertical change in w is large and positive and that w starts small/ zero at the surface?
If the surface is flat and air cannot flow into the surface, then at the surface we must have w = 0. With that boundary condition in hand we can draw conclusions about the sign of w even when only provided with information about dw/dz.
@@introductiontoatmosphericd284 Thank you - your video lecture series are a gem!
Thanks for the video. Could you please explain why the Hadley circulation stops at 30 deg latitude?
There are two aspects of your question: First, it's a bit of an idealization to say that the Hadley cell stops at 30 deg latitude, since it's more aptly described as a narrow region of strong convection and a broad region of subsidence that is roughly centered on the subtropics. See, for instance, the average vertical velocity found here upload.wikimedia.org/wikipedia/commons/6/6b/Omega-500-july-era40-1979.png. When air reaches the subtropics it is both cool enough to produce steady descent, and it has been affected sufficiently by Coriolis force for the flow to be roughly westerly. Second, the width of the Hadley circulation is a function of the strength of the temperature gradient and the rotation rate of the planet. More vigorous convection in the tropics is expected to enhance the strength of the Hadley circulation, driving up its meridional velocity and leading to a widening circulation. You can check out this paper for more info about the extent of the Hadley circulation: agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2008GL035847