3. Stability

Changing Stability » Penetrative Convection Introduction

Unlike the vertical motion of a layer in the atmosphere, penetrative convection consists of local vertical currents having cross-sections of the order of a few meters (feet) to a few kilometers (miles) across. Most of these vertical motions are due to thermal convection in an unstable layer, which leads to neighboring updrafts and downdrafts within the layer. These updrafts and downdrafts develop momentum that carries them beyond the unstable layer so that they penetrate into the adjacent stable layer. The effect is called penetrative convection and tends to destabilize the adjacent stable layer.

To see how penetrative convection destabilizes a stable layer, let's examine a fairly typical case of a saturated convective updraft rising through a conditionally unstable layer. At some height, a stable layer will cap the conditionally unstable layer. The rising updraft is warmer than the surrounding environment and it will remain warmer until it reaches the equilibrium level. The equilibrium level sits within the stable layer at some elevation above the unstable/stable boundary. Momentum carries the updraft past the equilibrium level. Now the updraft is cooler than the ambient air, so it slows and eventually falls back to the equilibrium level. So we can see that the rising convective updraft has penetrated a significant distance into the overlying stable layer. As it does, it warms the base of the stable layer through mixing, conduction, and radiation. This warming increases the lapse rate through the stable layer, effectively destabilizing it.