Transcript Document

Discussion: lecture course examination possible dates:
Thursday, 14 December (last day of lecture)
Another date in January (09 or 10 January)?
Examination will last 1 hr 30 minutes (but you may not need
all of that time). Format: primarily multiple choice, a few
short-answer “essay” (two or three sentences)
** Must be concluded before 15 January 2007**
Front: a narrow zone of transition between air masses of contrasting
density, that is, air masses of different temperatures or different water
vapor concentrations or both.
** Named by the airmass that is advancing
• Fronts are actually zones of transition, but sometimes the transition
zone, called a frontal zone, can be quite sharp.
• The type of front depends on both the direction in which the air
mass is moving and the characteristics of the air mass.
• There are four types of fronts that will be described: stationary
front, cold front, warm front, and occluded front.
When 2 different
air masses come
together,
interesting things
can happen
Indentifying a front on a surface weather map or by your
own weather observations
Look for:
1. Sharp temperature changes over a relatively short
distance
2. Change in moisture content
3. Rapid shifts in wind direction
4. Pressure changes
5. Clouds and precipitation patterns
Stationary front: a nearly stationary narrow zone of
transition between contrasting air masses;
 winds blow parallel to the front but in opposite directions
on the two sides of the front
 in the mid-latitudes, typically separates cold dense cP air
from milder mP air
 often associated with a wide region of clouds and rain or
snow on the cold side of the front.
 clouds and precipitation result from overrunning, as warm humid air
flows upward over the cooler air mass, more or less along the frontal
surface, cools through adiabatic expansion which triggers
condensation and precipitation.
Cold front: a narrow zone of transition between advancing
relatively cold (dense) air and retreating relatively warm (less
dense) air.
• over Europe, temperature contrast across a cold front is typically
greater than that across stationary or warm front.
• cold frontal passage is associated with a sharp temperature drop in
winter and a noticeable humidity drop in summer.
Some of the characteristics of cold fronts include the
following:
• steep slope
• faster movement / propogation than other fronts
• most violent weather among types of fronts
• move farthest while maintaining intensity
• tend to be associated with cirrus well ahead of the front,
strong thunderstorms along and ahead of the front, and a
broad area of clouds immediately behind the front (although
fast moving fronts may be mostly clear behind the front).
• can be associated with squall lines (a line of strong
thunderstorms parallel to and ahead of the front).
• usually bring cooler weather, clearing skies, and a sharp
change in wind direction.
The slope of a cold front is steeper (1:50 to 1:100) than the
slope of a warm front (1:150)
General weather characteristics of a cold front
Weather
Feature
Before frontal
passage
Region of front
After frontal
passage
Winds
SE to SW
gusty
W to NW
Temperature
warm
sudden decrease
steady cooling
Dew point
high
steady
decreases
steadily
Pressure
falling steadily
minimum; rapid rise
steady rise
Visibility
fair to poor
poor then improving
good
Clouds
Ci, Cs, Cb
Cb
Cu
Precip
showers
heavy precip
clearing
Warm front: a narrow zone of transition between advancing
relatively warm (less dense) air and retreating relatively cold
(dense) air.
 warm front is associated with a broad cloud and precipitation
shield that may extent hundred of kilometers ahead of the surface
front
Some of the characteristics of warm fronts include the
following:
• slope of a typical warm front is more gentle than cold fronts
• tend to move slowly.
• are typically less violent than cold fronts.
• although they can trigger thunderstorms, warm fronts are
more likely to be associated with large regions of gentle
ascent (stratiform clouds and light to moderate continuous
rain).
• are usually preceded by cirrus first (1000 km ahead), then
altostratus or altocumulus (500 km ahead), then stratus and
possibly fog.
• behind the warm front, skies are relatively clear (but
change gradually)
The type of frontal weather depends on the stability of the warmer air:
 when warm air is stable, a frontal inversion may exist in the upper
frontal region, a steady light-to-moderate rainfall or frontal fog is
observed in the presence of nimbostratus or stratus clouds,
respectively.
 when the warm air is unstable, brief periods of heavy rainfall are
observed in the presence of cumulonimbus clouds.
General weather characteristics of a warm front
Weather
Feature
Before frontal
passage
Region of front
After frontal
passage
Winds
NE to E
variable
S to SE
Temperature
cool, slowly
warming
steady rise
warmer
Dew point
steady rise
steady
increases, then
steady
Pressure
usually falling
levels off
slight rise,
followed by fall
Visibility
poor
improving
fair
Clouds
Ci, Cs, As, Ns, St, stratus
fog
Clearing with
scattered Sc
Precip
light to moderate,
can be SN or RA
usually none
drizzle or nothing
Occluded front (occlusion): a narrow zone of transition formed
when a cold front overtakes a warm front.
Dry Line:
• boundary that separates moist air mass from a dry air mass
• also called “Dew Point Front”
• most commonly found just east of the Rocky Mountains; rare
east of the Mississippi River
• common in TX, NM, OK, KS, and NE in spring and summer
Hot, dry air
Warm, moist air
Gusty southwest
winds
Southeast winds
Rocky Mountains
Dry Line
States like
Texas, New
Mexico,
Oklahoma,
Kansas, and
Nebraska
frequently
experience
dry lines in
the spring
and summer.
Dry lines are
extremely
rare east of
the
Mississippi
River.
How do fronts form?
1
5
9
2
4
3
6
7
8
10
11
12
Terms 1, 5, 9: Diabatic Terms
Terms 2, 3, 6, 7: Horizontal Deformation
Terms
Terms 10 and 11: Vertical Deformation Terms
Three-Dimensional
Frontogenesis Equation
Terms 4 and 8: Tilting Terms
Term 12: Vertical Divergence Terms
Bluestein (Synoptic-Dynamic Met. In Mid-Latitudes, vol. II, 1993)
Assumptions to Simplify the Three-Dimensional Frontogenesis Equation
y’
θ
x’
θ+1
θ+2
• y’ axis is set normal to the frontal zone, with y’
increasing towards the cold air (note: y’ might not always
be normal to the isentropes)
• x’ axis is parallel to the frontal zone
• Neglect vertical and horizontal diffusion effects
Simplified Form of the Frontogenesis Equation
d     u  v  w    d 
F 



 
 
dt  y   y  x  y  y  y  z y   dt 
A
B
Term A: Shear term
Term B: Confluence term
Term C: Tilting term
Term D: Diabatic Heating/Cooling term
C
D
Frontogenesis: Shear Term
Shearing Advection changes
orientation of isotherms
Carlson, 1991 Mid-Latitude Weather Systems
Frontogenesis: Confluence Term
Cold advection to
the north
Warm advection
to the south
Carlson, 1991 Mid-Latitude Weather Systems
Why are cold fronts typically stronger than warm fronts?
Look at the shear and confluence terms near cold and
warm fronts
Shear and confluence
terms oppose one
another near warm
fronts
Shear and confluence
terms tend to work together
near cold fronts
Carlson (Mid-latitude Weather Systems, 1991)
Frontogenesis: Tilting Term
Adiabatic cooling to north and warming to
south increases horizontal thermal gradient
Carlson, 1991 Mid-Latitude Weather Systems
Frontogenesis: Diabatic Heating/Cooling Term
frontogenesis
T constant
T increases
frontolysis
T increases
T constant
Carlson, 1991 Mid-Latitude Weather Systems
Frontogenesis/Frontolysis with Deformation with
No Diabatic Effects or Tilting Effects
d
1
F
     Def R cos 2  Div
dt
2
where:
  v u  2  u v  2 
Def R   
  
  
 x y  
  x y 
and
ß= angle between the isentropes
and the axis of dilatation
Petterssen (1968)
1
2
MID-LATITUDE CYCLONES
 the cause of most
of the stormy
weather in the
northern
hemisphere,
especially during the
winter season
Understanding
their structure and
evolution is crucial
for predicting
significant weather
phenomena such as
blizzards, flooding
rains, and severe
weather.
Mid-latitude (or frontal)
cyclones
 large traveling
atmospheric cyclonic
storms up to 2000
kilometers in diameter
with centers of low
atmospheric pressure
 located between 30
degrees and 60 degrees
latitude (since the
continental United States
is located in this latitude
belt, these cyclones
impact the weather in the
U.S.)
 form along the polar front
 an intense system may have a surface pressure as low as 970 millibars
 normally, individual frontal cyclones exist for about 3 to 10 days moving in a
generally west to east direction
 precise movement of this weather system is controlled by the orientation of the
polar jet stream in the upper troposphere
 commonly travels about 1200 kilometers in one day
How many mid-latitude cyclones can you identify
from this satellite image?
How many mid-latitude cyclones can you identify
from this satellite image?
What causes mid-latitude cyclones to form?
Upper-air
surface
Surface extratropical (i.e., nonhurricane)
cyclones are
directly coupled
with the upperlevels.
Typically an
upper-level
trough, and its
associated supergeostrophic wind
maximum, move
over a surface
temperature
gradient.
Remember our equation for relative vorticity (spin)
generation? Notice the 2nd term: “baroclinic term”. The
stationary front / temperature boundary provides the
necessary baroclinic energy for the surface cyclone to
develop.
Stages of mid-latitude cyclone development:
Two models of development
Mid-latitude cyclone model application: 6 June 1944
(Petterssen’s forecast)
• Mid-latitude cyclones are "deep" pressure systems extending from the
surface to the tropopause
• A surface low-pressure system grows if there is vertical wind shear
(winds increasing with height) and thermal instability (convection).
• The factors that lead to lowering of the pressure at the surface are:
• Diverging airflow at
high altitudes
• Inflow of warm, moist
air at low and mid
levels.
• Latent heat release
caused by convection in
the warm air mass sector
of the growing storm
system.