10.3.2, 10.3.3, 10.3.4 Lower Tropospheric fronts and air masses

The two key issues for frontal analysis

How loose the criteria for analyzed fronts should be and the balance between the use of feature symbols (types of fronts, contours, and station data)

What has improved the flexibility of chart analyses

Multicolor displays and new computer overlay options in recent decades

Some points of difficulty for frontal analysis

Consideration of vertical structure, the role of moisture, relation to surface features, and primary genesis/maintenance mechanisms

Three general categories of boundaries at or near the surface

Horizontal density discontinuities, air mass boundaries, and airstream boundaries

Nonfrontal baroclinic boundaries

Quasi-stationary gradients resulting from surface diabatic and topographic forcing

Nonbaroclinic kinematic boundaries

Common within quasi-homogeneous air masses, especially at lower latitudes

Air masses are conceptually defined in terms of … directly observed?

Measures of heat and moisture

Temperature and dew point

Potential temperature is used because…

the front is a three-dimensional feature

The first two terms on the right

kinematic frontogenesis

calculated diagnostically from gridded datasets

The first term by itself is due to

horizontal thermal advection by a confluent wind pattern

The second term

the shearing component of horizontal advection

If the angle is less than 45 deg, the advection will contribute positively to

frontogenesis by moving the isotherms closer together

At an angle of 45 deg, the rate of frontogenesis is

zero

The final term

differential diabatic heating

Net radiative heating is strongly dependent on

the presence or absence of cloud cover

Most visible impacts of midlatitude fronts are associated with the…

formation of clouds by ascending air/