Horizontal Convective Rolls - Development and Required Environmental Conditions

Development and Required Environmental Conditions

The exact process that leads to the formation of horizontal convective rolls is not well understood, but most formation theories involve a combination of thermal or convective instability and dynamic instability. In thermal instabilities, turbulent energy that forms the rolls is produced from buoyancy. For this condition to be satisfied, a moderate surface heat flux is necessary to produce a slightly unstable environment; however, too much heat flux will cause the environment to be too unstable, and will lead to cellular, instead of roll convection.

The most favorable conditions for their formation occur when the lowermost layer of air is unstable, but is capped by an inversion-by a stable layer of air. This often occurs when upper air is subsiding, such as under anticyclonic conditions, and is also frequently found when radiation fog has formed overnight. Convection occurs below the inversion, with air rising in thermals below the clouds and sinking in the air between the streets.

Turbulent energy derived from dynamic instabilities is produced from wind shear energy. Dynamic conditions favorable for roll development generally have high speed shear, and very little directional shear. Low speed shear environments generally produce cellular convection. It has been suggested that dynamic instability requires an inflection point in the cross roll wind speed profile—that is, the wind speed profile is curved, reaching a maxima within the boundary layer. Although inflection points are common in many wind profiles of convective roll events, some events have been observed lacking inflection points, suggesting that this is not a requirement for roll convection.

The combination of convective and dynamic instability can be quantified in the ratio, where is the depth of the boundary layer and is the Monin-Obukhov length. In the case of roll convection, the Monin-Obukhov length is the height at which turbulent energy production from buoyancy dominates that produced from wind shear. Roll convection is typically observed for values that are approximately greater than -5 and approximately less than -25. These conditions are most frequently observed in cold air outbreaks, and produce the combination of necessary dynamic and convective instabilities that produce roll convection. Roll convection is more frequently observed over water surfaces than land surfaces, due to the interactions of orography making the convection less distinct.