ATMO 251

Special Thanks to Dr. Russ Schumacher who originally developed these slides

for a guest lecture.

Fronts and FrontogenesisFronts and Frontogenesis

What is a front?

• We see fronts shown on weather maps all the time – but what really are they?

• Basically, “a narrow, elongated zone with a locally strong temperature gradient”

• But how narrow is narrow, how elongated is elongated, and how strong is strong?

Frontal positions analyzed by expert meteorologists

What is a front?• More precise definition (N-G, chapter 13):

A synoptic-scale front is an air mass boundary that extends up into the troposphere. It includes at least a locally enhanced temperature gradient and a vector wind shift.

• Other things often seen with fronts:– Pressure rise behind cold front– Pressure trough (“kink” in isobars)– Dewpoint gradient– Clouds and precipitation

• But these are secondary to the temperature gradient and wind shift when deciding where to analyze the front

Wind shifts

• With fronts, we need a wind shift that is either convergent or cyclonic, or both

Convergent wind shifts

Wind shifts

• With fronts, we need a wind shift that is either convergent or cyclonic, or both

Cyclonic wind shifts

Wind shifts

• With fronts, we need a wind shift that is either convergent or cyclonic, or both

Both convergent and cyclonic

Wind shifts

• With fronts, we need a wind shift that is either convergent or cyclonic, or both

Anticyclonic wind shifts: NOT FRONTS

Five types of fronts

• Cold• Warm• Stationary• Occluded• Upper-level

*Now more commonly:

*

Surface fronts• As the name suggests, surface fronts are

strongest at the surface• They generally get weaker with increasing

height• Usually, the region of the wind shift is very

narrow, but the region of temperature gradient (the “frontal zone”) is wider

• Often accompanied by a pressure trough and dewpoint gradient

Is it a cold or warm front?• This is really only determined by what happens after

the front passes: if there are colder temperatures after the front passes, it’s a cold front!

• Can use the geostrophic wind: if it blows from warm air toward cold air, it’s a warm front (and vice versa)

Stationary fronts

• Fronts are rarely truly stationary• If you’re doing the analysis, you can’t just look

at the present time: need to look at earlier analyses to see how much the front has moved!

• Rule of thumb: if it moves less than 2 degrees of latitude (~200 km) per day, you can think about calling it a stationary front

Occluded fronts• Usually with a strong cyclone, when air behind the cold

front meets up with air ahead of the warm front• Usually, the cold front rises up over the warm front– Therefore, analyze it as an extension of the warm front

Upper-level fronts• Upper-level fronts don’t receive as much attention

because:– We don’t experience their effects (unless in an airplane!)– Sounding stations are too far apart to resolve them very well– In a strong upper-level front, air from the stratosphere is

brought down into the troposphere: a “tropopause fold”

Fronts and wind shear• The thermal wind relationship says that when

there are surface temperature gradients, there is vertical wind shear

• When it’s colder toward the pole, the westerlies get stronger with height

• This is one of the most fundamental concepts in meteorology

Thermal wind and shear

Frontal analysis• Contouring temperature (every 4°C or so) will reveal the

temperature gradients (and the locations of possible fronts)

• However, near mountains, this doesn’t work: temperature changes just because the elevation changes

• A proposed method is to contour potential temperature (θ) at the surface. (If you’re using GEMPAK or GARP, you can plot this as STHA).

• You can also look at θe, or the 850 mb T when the surface is confusing.

• Still need to be careful near mountains, coastlines, etc. – there may be a temperature gradient but this doesn’t necessarily mean there is a front (for example, the sea breeze)

http://www.atmos.albany.edu/products/thetaq_gif/latest_us.theta.gif

Example

CLL

2016

12

What’s going on here?

Three hours later (0900Z)

Temp and dew point (deg C)

Pressure

Wind

Precipitation

College Station meteogram: 16 November 2009