Dynamic Pipe Effect

As spin-up of the mesocyclone develops, its rotating action begins to reorganize airflow in the updraft. The local pressure field and the strongly curved wind field move toward a dynamic equilibrium called cyclostrophic balance. In this state, the pressure-gradient force, which acts to move air inward in response to the lower pressure in the centre of the rotating column, is equaled by the outward-directed centrifugal force. When cyclostrophic balance is achieved, air readily flows in a circular path around the mesocyclone's axis, while flow toward or away from its centre is strongly suppressed. This state, in which airflow is constrained by its own rotation, is known as the dynamic pipe effect.

The middle level of the storm is usually the first area where cyclostrophic balance is achieved. Since air is constrained not to flow into the center of the mesocyclone, the pressures fall at the center of the mesocyclone where cyclostrophic balance is achieved. This leads to a crowding of the isobars on the lower bound of the mesocyclone and a greatly enhanced vertical pressure gradient force there. The resulting upward motion can be viewed as "suction" that brings more air into the bottom of the updraft, which, conserving its absolute angular momentum, develops tremendous tangential velocities.

There the air comes into cyclostrophic balance, and the above effects work their way downward. This is visible as a tornado vortex signature which develops aloft first and works its way downward. The vortex in cyclostrophic balance is viewed as a pipe with rigid lateral boundaries in which flow only occurs upward or downward at the center, while the pipe itself rotates.

Almost all the air flowing along the mesocyclone's axis is drawn in through the bottom of the pipe. This inflow further intensifies rotation at the pipe's lower end, causing it to extend rapidly downward as the more quickly rotating region comes into cyclostrophic balance.

Another important aspect of this is that while the VPGF is enhanced on the lower bound of the DP, it is suppressed (Isobars are further apart) in the pipe itself. Thus while the pipe has strongly rotating lateral boundary, its interior now has a downdraft, which works its way towards the ground. When it strikes the ground, it can be thought of as a part of the RFD, and may actually be a smaller scale downdraft appended to the RFD that is observed about the same time the outflow boundaries on either side of the updraft merge. Hence, this smaller scale downdraft may comprise at least part of the reasoning associated with the development of the term "occlusion downdraft."

Strong convergence of inflowing air at the lower end of the pipe causes air parcels to be accelerated upward and vertically “stretched.” Vertical stretching normally causes the mesocyclone to contract to a diameter of about 2 to 6 km (1 to 4 miles). As this happens, the mesocyclone rotates more quickly, which in turn strengthens the convergence of inflowing winds at its base. In this manner the mesocyclone grows in strength in a positive-feedback, or self-amplifying, process.

Development of the dynamic pipe effect can produce a mesocyclone that extends the full depth of the thunderstorm, from about 1 km (0.6 mile) above the ground to near the storm's top at about 15 km (9 miles). Frequently, the maturation of the mesocyclone is heralded at the bottom of the cloud by a lowering of a portion of the thunderstorm's base in the area of the updraft. This rotating* approximately cylindrical extension is known as a wall cloud. Surface winds with speeds as high as 33 metres per second, or 120 km per hour (110 feet per second, or 75 miles per hour) can be present beneath this swirling cloud, often producing damage even when no tornado forms.

*It is important that the wall cloud rotates in situations in which the DPE is in play. Many non-rotating or weakly rotating storms can have non-rotating wall clouds.