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Hadley Cells in the Atmosphere
To get a better picture of what the Hadley cell looks like and is doing, we will do some investigation. We know that there is a net north-south heat flux, so we'll look at meridional i.e. north-south winds.
After having developed the idea of an overturning convective cell at low latitudes to transport heat away from the equator and analyzed tank data to develop intuition for the pattern of motion, we will now look at what physical evidence we have for its existence in the atmosphere. The Hadley cell transports heat from the equator to the poles, so we will be looking at wind velocities to see where the net movement is. We know that there is a net north-to-south heat flux, so a good first guess is that this happens via meridional winds i.e. north-south winds. Looking at a plot of meridional wind speed vs. latitude for January, which corresponds to winter in the northern hemisphere, we can see that there are poleward winds at high altitudes and equatorward winds at low altitudes. The plot is shown below with warmer colors meaning a northward velocity direction and cooler colors meaning a southward velocity direction.
Air is This data is taken for the month of January and is averaged from the years 1948-2016. The positive winds are northward, and the more purple colors are southward. We see that at high altitudes, there is poleward winds, and at low altitudes there are winds heading to the equator. We see air being pushed to the pole up high and returning to the equator at the surface, so our initial guess needs to be modified to include how air travels between high altitudes to low altitudes. As seen in our last project, convection is the process that the atmosphere uses to transport heat from the surface , so we're missing the part that's pushing the air to higher altitudes where the heat is then radiated off into space. It makes sense then that a convective plume would exist at the equator, so we must look at a plot of vertical velocity to verify that winds bring air back to the surface at higher latitudes and the convection plume at the equator.
The above graph shows vertical wind velocity. The cooler colors are rising columns, and the warmer colors are sinking columns. While there are adjacent rising and sinking columns spanning the whole surface of the Earththe whole surface of the Earth, the strongest winds occur between the equator and 30N. The strongest winds appear in the northern hemisphere because that is where it is winter in January. When it is winter in the southern hemisphere, the strongest columns occur will be between the equator and 30S. The equatorial column corresponds to convection, and the 30N . The strongest columns appear in the North Hemisphere because it is wintertime there. When it is winter in the southern hemisphere, the strongest columns will be between the equator and 30S. The equatorial column corresponds to convection, and the 30N column corresponds to the point on the Earth at which the warm air has lost sufficient heat and sinks back to the surface. Putting the two pictures together, we can see the full cycle of the Hadley cell.
column corresponds to the point on the Earth at which the warm air has lost sufficient heat and sinks back to the surface. If 30N is the place where warm air becomes cold again, then this is where heat transport stops with the Hadley cell, and as a result, there should be a strong temperature gradient. From our polar front project, we know that horizontal temperature gradients produce a vertical wind shear. Therefore, the place where we observe the strongest temperature gradient should be where we see the fastest increasing winds and the greatest overall wind speed. Looking at the following plot of zonal (i.e. west-east) winds, the greatest wind shear and resulting wind speed occurs at 30N, where the greatest horizontal temperature gradient is and where the air in the Hadley cell cools off again.
Eddy Heat Transport in the Tank
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