ATS/ESS 452: Synoptic Meteorology
Review #3
• Continue Review Material
• Potential Temperature
• Stability
• Isentropic Analysis
Thermodynamic Review
• What is an adiabatic process?
One in which there is no heat transfer; for example, no heat exchanged between a
parcel and its environment would be considered an adiabatic process
dq = 0
• Lapse rate – the rate of decrease with height for an atmospheric variable, typically
temperature
• The dry adiabatic lapse rate (DALR) is the rate of temperature decrease with height
for a parcel of dry or unsaturated air rising under adiabatic conditions
• No heat exchanged between the parcel and the environment
• Pressure decreases as the parcel rises, causing it to expand
• As the parcel expands, it pushes on the air around it, causing it to lose internal
energy, so the temperature of the parcel decreases
≈ 10 K km-1
Potential Temperature, θ
• Poisson’s Equation:
where p0 is some reference pressure level,
usually 1000 mb
• What is the physical interpretation of potential temperature?
The temperature that an air parcel would have if it were dry adiabatically
compressed (or expanded) to a reference pressure level
• What are some potential uses of potential temperature?
Used to determine static stability
An examination of the horizontal θ distribution is useful in locating frontal
boundaries -- especially over uneven terrain where surface temperature
differences may be caused by the terrain.
When diabatic processes are neglible, air parcels are typically constrained to
travel along surfaces of constant θ, known as isentropic surfaces. This can
help identify areas of lift. (Chapter 3).
Static Stability
The link between potential temperature and stability…
Below is an idealized vertical cross section through a well-mixed planetary boundary
layer (PBL) beneath a capping inversion layer. On the right, the corresponding profile of
potential temperature as a function of height is given.
In this layer, air parcels undergo quasi-random ascent and descent – turbulent mixing
The potential temperature profile is a result of the “mixing” of the random motions of the
air parcels.
Surface based mixed-layers usually = PBL
Notice the potential profile slightly decreases with height near the surface – this is an
absolutely unstable situation and is due to strong solar heating at the ground. This is
known as a superadiabatic situation
Being able to identify/forecast the mixed-layer has important considerations for surface
temperature and wind conditions
Surface Temperature & Mixed Layer:
The development of a deep mixed layer often means warmer surface temperatures
because compression of air with warmer potential temperature from aloft.
Skew-T diagram, 00Z 21
August 2007 – Greensboro, NC
What were the surface
conditions like in central NC
based on this sounding?
Warm and dry
T ~ 35°C = 95°F
What do you see on this
sounding that supports the
extremely warm & dry weather?
Unusually deep mixed layer
extending to near 700-mb
What’s happening, synoptically,
to cause the deep mixed layer?
High pressure is causing the air
to sink and warm adiabatically
at the DALR… note how the
temperature parallels the dry
adiabat
An Aside - Inversions
An inversion is a departure from the usual decrease or increase with altitude of the value
of an atmospheric property. Usually in meteorology, an inversion refers to an increase in
temperature with height.
Temperature inversions occur when air above a certain level is warmer than the air
below.
3 Main Types:
Subsidence Inversion – an increase in temperature with height that develops aloft as a
result of air gradually sinking over a wide area and being warmed by adiabatic
compression. Often enhanced by vertical mixing in the air below the inversion.
Radiation Inversion – formed when the lowest levels of the atmosphere are cooled by
contact with the earth’s surface, which cools by emitting radiation
Factors that help form a radiation inversion include:
calm winds, dry air, clear skies, long nights, surface wetness,
surface type, vegetation
Frontal Inversion– an increase in temperature and dewpoint with height that develops
aloft as a result of the sounding passing through a frontal interface.
Subsidence Inversion
• Usually not surface-based,
occasionally can be
• Temp warms while the
Dewpoint cools (Dewpoint
Depression increases)
• Associated with subsidence
(sinking air) in which the
temperature warms at or
near the dry adiabatic lapse
rate
Radiation Inversion
• Surface-Based and Shallow
• Temp and Dewpoint both
typically increase through
the inversion layer
• Associated with calm to
light winds at the surface
• Usually found on a 12Z
sounding, but occasionally
found on a 00Z sounding
• Associated with significant
surface radiational cooling
Being able to identify/forecast the mixed-layer has important considerations for surface
temperature and wind conditions
Surface Temperature & Mixed Layer
The development of a deep mixed layer often means warmer surface temperatures
because compression of air with warmer potential temperature from aloft.
Surface Winds & Mixed Layer:
Strong winds aloft and the presence of a deep mixed layer often allows for efficient
vertical momentum transport  strong winds can be brought down to the surface
Skew-T diagram, 00Z 21
August 2007 – Greensboro,
NC
Imagine the winds were much
faster (~50 knots) just above
the subsidence inversion.
The air sinking and warming
along the lines of constant θ
can efficiently bring the higher
momentum (faster) air to the
surface.
Frontal Inversion
• Usually not surface based
• Temp and Dewpoint both
typically increase through
the inversion layer
• Typically a significant
wind shift across the
inversion layer
• Located on the “cool”
side of frontal boundaries
(ahead of a warm front,
behind a cold front)
Determine the Inversion
00Z - IAD
12Z - IAD
00Z - IAD
12Z-GSO
00Z - IAD
Back to Stability….
The slope of the temperature profile relative to the dry adiabats on an atmospheric
sounding can help you determine stability. Below is a sounding from Forth Worth, TX.
Where are the most and least stable layers? Where is the PBL?
Mixed layer of
neutral stability
extending to 875mb
Strong, stable
inversion layer that
extends to 825-mb
Atmosphere
exhibits weak
stability between
825 and 175-mb
Above 175-mb,
very strong
stability
corresponding to
the stratosphere.
• What is a diabatic process?
Process that involves heat transfer (addition or loss of heat to the surroundings)
dq ≠ 0
• What are some examples of a diabatic process?
Warming due to the sun
Evaporational cooling
Condensational warming
• What happens to a moist parcel of air as it rises?
As the parcel rises, it cools at the DALR (as described before) but now, moisture
begins to condense, which releases latent heat.
The latent heat release partially offsets some of the cooling; however, the DALR is
greater than the rate of warming due to latent heat release, so the parcel still cools
as it rises.
The saturated parcel cools at a lesser rate than an unsaturated parcel… moist
adiabatic lapse rate
Equivalent Potential Temperature, θe (theta-e)
• Similar to potential temperature, but now includes all potential heating due to
condensation
• What is the physical interpretation of theta-e?
The temperature that results after all latent heat is released in a parcel of air and
brought adiabatically to the 1000-mb level.
Theta-e increases as dewpoint (moisture) and/or temperature increases
• Theta-e has important operational significance:
Instability – a region with a relatively high theta-e is often the region with the most
instability. Warmer low-level temperatures and higher low-level dewpoints increase
instability.
Low-level Jet – a low-level jet from a moisture source (e.g. Gulf of Mexico) will
often bring in higher theta-e values and thus decrease stability
Theta-e Ridge – these are regions with higher theta-e. They are often the burst
point for convective activity
Similar to potential temperature, theta-e is related to atmospheric
stability.
What does it mean if theta-e decreases with increasing height?
The atmosphere is convectively (or potentially) unstable
Physically, this means that if the layer of air in which theta-e decreases
with height were lifted to saturation, then an unstable lapse rate would
occur.
The lifting is typically done dynamically (WAA, low-level convergence,
upper-level divergence).
Convective instability typically occurs when the mid-levels of the
troposphere are fairly dry and the PBL has high dewpoints.
When lifting takes place, the lower portion of the layer (moist) reaches
saturation first and cools at the lesser moist adiabatic lapse rate while
the upper layer (dry) still cools at the DALR…. This will increase the
environmental lapse rate, causing an instability
Assessing Convective Instability
This morning (12Z)
sounding displays
convective instability.
No SBCAPE (surface
based convective
available potential
energy).
A forecaster should
expect daytime heating
to increase SBCAPE.
However, SBCAPE will
increase even more if
this location was in an
environment supportive
of dynamic lifting
The lifting will cool the
mid-levels at a rate
greater than the low
levels.
RAX base-level reflectivity, 902 UTC Sat 8/06/11
ExampleWhyy did storms form at this earlyy hour, before sunrise?
1
RAX (Raleigh, NC) base-level reflectivity, 902 UTC 8/6/2011
These storms formed very early, before sunrise. Why did this happen? – examine
nearby soundings for clues!
12 UTC 8/6/11 GSO (Greensboro, NC) sounding  upstream from RAX
What’s good about this sounding for heavy rain?
Very moist lowlevels
Wind profile veers
with height
 WAA!  lift!
Weakly unstable
Note the drier midlevels  convective
instability! This
likely caused the
environment to
become more
unstable.
Check upstream
sounding
(RUC)
Check further upstream
for moisture,
instability
for moisture, instab
Winston-Salem 4 UTC 8/6/11 – not pperfect location, b
RUC Analysis Sounding @ Winston-Salem, NC, 4 UTC 8/6/11
You can use BUFKIT to
plot these types of
model analysis
soundings.
http://wdtb.noaa.gov/to
ols/BUFKIT/
Winston-Salem
UTC 8/6/11
theta-e decreases with height,
Theta-e analysis
from the04
previous
RUC –
Sounding
in a layer nearly saturated in lower levels (see skew-T)…
Theta-e decreases with height in a layer that is nearly saturated in
the lower levels – convective instability
Theta-e as function of pressure
1
Below: 312K isentropic surface (theta surface). **Everywhere in the image, the
potential temperature is 312K
Contour lines represent pressure… so this map shows where the 312K surface is
located.
Since air tends to rise and fall at the DALR, or along surfaces of constant theta… this
type of analysis can show where upward motion is occurring.
In central NC, the air along the 312K surface is moving from 730mb to 710mb.
Remember that pressure decreases with height… so this is an area lift.
This is known as isentropic upglide
.DISCUSSION...
/ISSUED 307 AM CST WED FEB 1 2012/
A SPLIT UPPER FLOW PATTERN EXISTS ACROSS THE CENTRAL AND
EASTERN U.S. TODAY AS A SHORTWAVE APPROACHES THE
SOUTHERN/CENTRAL PLAINS. IN ADDITION, A WEAK CONFLUENCE
REGION OVER CENTRAL LA WITHIN A WAA REGIME IS BRINGING
THUNDERSTORM ACTIVITY OVER LA/MS. THE WEAK CONVERGENCE
AND ISENTROPIC LIFT ACROSS THE TN VALLEY IS BRINGING
SCATTERED SHOWER ACTIVITY THAT WILL BECOME MORE
WIDESPREAD THROUGHOUT THE DAY.
Example from last year
What’s going on here?
Key Concepts
• When theta-e decreases with height: if air lifted to saturation,
will become absolutely unstable
• BUT, only if saturation is reached
• If layer lifted, top cools more than bottom due to more latent
heat release at bottom of layer
• Parcels at bottom of layer rise along a warmer moist adiabat
than those higher up, denoting instability
Remember that theta-e includes both temperature and moisture!
This makes it an even better tool at helping to recognize surface
frontal locations!
Look for tight gradients in theta-e
Areas with low values of theta-e often are experiencing cold, dry
air at the surface
Areas with high values of theta-e are often experiencing warm,
humid conditions
But be aware of it’s limitations!
RUC Theta-E
Analysis for
0200 UTC
1/30/12
Where are the
gradients in
theta-e?
Corresponding surface analysis
Frontal locations generally analyzed in areas with theta-e gradients
Except the coastal locations… what’s going on here?