# The Dynamics of Frontogenesis

We often see fronts on weather maps on television or the internet. In this blog, we’ll discuss the formation and intensification of fronts, known as frontogenesis.

Strictly speaking, frontogenesis is defined as an increase in the magnitude of a horizontal density gradient. To explain the dynamics of frontogenesis, we begin by looking at the frontogenetical function.

Mathematically speaking, frontogenesis is defined by the time rate of change in the horizontal gradient of potential temperature.  The terms on the right hand side of the equation represent the effects of confluence, tilting, and diabatic heating on frontogenesis. We’ll begin by looking at each term individually.

The first term on the right hand side of the frontogenetical function represents the effect of confluence or diffluence on frontogenesis. A confluent flow will encourage frontogenesis, whereas a diffluent flow will promote frontolysis (the decay of a front). It can also be said that the first term of the frontogenetical function represents the thermodynamic effect of a gradient in horizontal temperature advection.

The second term of the frontogenetical function represents the tilting of the vertical potential temperature gradient onto the horizontal. Physically, we can say that this term represents a gradient in adiabatic temperature change due to vertical motion. In a stable atmosphere, ascent and adiabatic cooling on the cold side of a front and descent and adiabatic warming on the warm side of the front increase the temperature gradient. Near the ground where vertical motion is negligible, tilting has no effect on frontogenesis. However, where the slope of the ground varies, the horizontal temperature gradient varies as upslope and downslope motions vary. Tilting is usually most pronounced in the upper troposphere. Toward the middle troposphere where temperature advection is usually small, tilting usually dominates. Near the tropopause, the effects of deformation may play an important role if the tropopause slopes.

The third term represents the effects of diabatic heating on frontogenesis. If the warm side of a front is heated by the sum without heating on the opposite side, frontogenesis occurs. At night, cooling on the clear side and limited cooling on the cloudy side leads to a frontolytical process. If there is a boundary between snow cover and bare ground, differential heating can lead to frontogenesis. In cases when the surface front is shallow and there is some warmer air above the colder air mass, differential heating can lead to frontolysis.

Frontogenesis can play an important role in winter storms. If frontogenetical forcing is present, heavy bands of snow can develop, resulting in high snowfall rates. These bands are usually aligned along the mean shear vector. Shear that is strong and mainly unidirectional is often an indication of frontogenetical forcing. Precipitation banding can be influenced by the wind profile and stability. Deep unidirectional shear with low stability tend to enhance snow banding.