Transcript ATMO 251 - Texas A&M University

ATMO 251
Fronts and Frontogenesis
Special Thanks to Dr. Russ Schumacher
who originally developed these slides
for a guest lecture.
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
12
What’s going on here?
16 20
Three hours later (0900Z)
College Station meteogram: 16 November 2009
Temp and dew point (deg C)
Pressure
Wind
Precipitation