# 10.1: What does turbulent drag do to horizontal boundary layer flow?

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The turbulent drag force on the horizontal air flow within the atmospheric boundary layer can approach the size of the other terms in the horizontal equation of motion. Note that this turbulent drag force acts to reduce the air velocity and therefore is opposite the air flow velocity vector.

Let’s look at the force balance when the turbulent drag force is included. We will call this turbulent drag "friction" because it is commonly called that, but it is really quite different. Horizontal map showing the geostrophic wind with friction in the Northern Hemisphere.

Credit: W. Brune

Note that the turbulent drag (friction) force is parallel to the velocity vector and is opposite in direction.

In the x-direction (along the isobars) the balance of forces is:

friction (x-component) = Coriolis (x-component).

In the y-direction (perpendicular to the isobars), the balance of opposing forces is:

friction (y-component) + Coriolis (y-component) = PGF

Because of the turbulent drag force and the velocity dependence of the Coriolis force, the parcel velocity points toward the lower pressure and the air parcel will tend to move across isobars toward the low pressure. Typical cross-isobaric flow in the boundary layer makes an angle of $$30^{\circ}$$ with the isobars. Thus, surface air moves toward low pressure and away from high pressure.

Above the atmospheric boundary layer, however, turbulent drag is not generally important and geostrophic and gradient flows are good approximations.

The effects of turbulent drag are very important for weather. When divergence aloft causes upward-moving air below, it becomes associated with a low-pressure region near Earth's surface. A pressure gradient force is created, but the air moving toward the low pressure is turned to travel counterclockwise around the low by the Coriolis force. However, turbulent drag slows the wind and turns it to cross isobars toward the low pressure, creating convergence, which causes uplift, clouds, and perhaps precipitation. The opposite is true for surface high-pressure regions occurring under regions of convergence aloft, which causes descent. The anticyclonic winds around the surface high are slowed and turn outward, causing divergence near the surface, leading to descending air and clear skies. The following video (1:43) provides further discussion of friction:

Friction