the role of stability in forecasting
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The Role of Stability in Forecasting
It has been said that without vertical motions in the atmosphere,
there is no weather. Stability provides a crucial control on the ability
of air to rise or sink. For that reason, an assessment of stability is a
critical part of the forecast process. (MetEd Module: Skew-TMastery)
Assessing atmospheric stability is crucial to understanding if and
where clouds will form, if afternoon convection will occur, if the
convection is likely to be severe, etc.
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Thermodynamic Diagrams
Use of Sounding Data is very important!
You mustknow how to get the following information from a sounding:
Temperature Temperature at a specified pressure.
At pressure find T curve
Read value of isotherm Label in C
Potential Temperature Temperature a parcel of air would have it brought adiabatically to the reference pressure
At pressure find T curve Follow dry-adiabat down to 1000mb (1000hPa) Read value of isotherm
Dew Point Temperature Temperature at which a parcel of air will become saturated if it is cooled.
At pressure find Td curve Read value of isotherm Label in C
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Reading the Skew-T Log-P
Saturation (moist) adiabats slightly curved lines sloping from the lower right to upper left (orange lines on the Java-Plot
software, or curved blue lines on the UWy Skew-T
These lines represent paths that saturated air follows and represents the rate of
temperature change in a parcel of saturated air rising pseudo-adiabatically.
Pseudo-adiabatically means that all the condensed water vapor is assumed to fall outimmediately as the parcel rises.
Condensation at all temperatures is assumed to be liquid water and, therefore, no
latent heat of fusion is included.
Saturation mixing-ratio lines straight lines sloping from the lower left to upper right (light grey lines on the Java-Plot
software, or purple lines at UWy
These lines are labeled in grams per kilogram; grams of water vapor per 1000 grams of dry
air.
The amount of water vapor in the atmosphere depends on the temperature (the scale goes
from about 0.1 gm/kg at a Td =-40C to ~38gm/kg for a Td=35C)
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Forecasting Clouds
Mixing Ratio w The ratio of the mass of water vapor (in grams) to the mass of dry air (in kilograms).
At pressure find Td
Read value of mixing ratio line
Label in g/kg
Saturation Mixing Ratio ws The water vapor content of the air if it were saturated.
At pressure find T
Read value of saturation mixing ratio line
Label in g/kg
Relative humidity = 100 (w/ws) Why not just read these values from the table?
We can learn additional information about the state of the atmosphere, its stability, and how
air parcels might respond to forces when we use the full sounding
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Lowest Level Where Clouds
will form Lifting Condensation Level (LCL)
The pressure (or height) at which a parcel of air would becomesaturated if lifted dry adiabatically from the surface (the reason forparcels lift may be orographic, or frontal, for example)
At pressure find Td
Draw a line up parallel to mixing ratio line
At pressure find T
Draw a line up parallel to dry adiabat
The intersection of the 2 lines is the LCL
Read pressure and label in mb
LCL is the level at which the cloud baseexists
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Cloud Effects
Thick Cloud cover reduces diurnal temperature range:
When forecasting maximum temperatures, you need to remember that thick
clouds reduce solar heating at the surface, keeping daytime highs lower.
When forecasting minimum temperatures, you need to consider that thick
clouds reduce radiative cooling, keeping overnight lows higher.
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High Clouds
Cirrus clouds have little impact on the maximum T, but can still keep theminimum temperature from falling as low:
The solar insolation can penetrate these high level ice clouds during the
day, so daytime surface heating continues
However, the outgoing (longwave) terrestrial radiation is absorbed and re-radiated, helping to maintain higher overnight lows than would occur undera clear sky.
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Role of Surface Heating
Convective Condensation Level (CCL) The height to which a parcel of air (heated from below) will rise if lifted
adiabatically until saturation occurs (potential cloud bases of cumuliform clouds if
caused by surface heating).
At surface find Td
Draw a line up parallel to mixing ratio line until it intersects the environmentaltemperature curve
Level of this intersection is CCL
Works best with surface T/Td spread of 6C or greater
Convective Temperature (Tc) The temperature the surface must reach for this convection to begin.
Find convective condensation level (CCL).
Extend a line down dry adiabatically to surface.
Read temperature.
Label in C
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Cumulus humilis (fair weather)
Clouds develop from surface heating
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Adding Water = Adding Heat
The heating from
below may be real
heating, or it may
be virtual heating,where evaporation
of moisture (eg,
from a canopy, or a
lake or ocean) adds
buoyancy to an air
parcel
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Vertical Development
Level of Free Convection (LFC) The level beyond (above) which the air parcel becomes buoyant. An air parcel
above this level can be carried upward even in the absence of further lifting(surface heating etc)
Start at Lifting Condensation Level (LCL) and follow up the moist adiabatuntil you reach the temperature line from the sounding AND the moist
adiabat is to the right (warmer) than the sounding.
Equilibrium Level (EL) The height at which a parcel of air would become unsaturated if lifted moist
adiabatically from the LFC.
From the point where the LFC was found, follow a moist adiabat up untilcrossing the temperature line again.
That level is the equilibrium level, at which a parcel of air no longeraccelerates upward.
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Cumulus Congestus (or castellanus)
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Measures of Potential Instability
LIFTED INDEXa commonly utilized measure of stability which compares the differencebetween a lifted parcel's temperature at 500 mb and the environmentaltemperature at 500 mb. It incorporates moisture and lapse rate (staticstability) into one number, which is less vulnerable to observations atindividual pressure levels.
LI = T(500 mb envir) - T(500 mb parcel)in degrees C, where T (500 mb envir) represents the 500 mb environmentaltemperature and T (500 mb parcel) is the rising air parcel's 500 mbtemperature.
LI over 0: Stable but weak convection possible for LI = 1-3 if strong lifting ispresent.
LI = 0 to -3: Marginally unstable. LI = -3 to -6: Moderately unstable. LI = -6 to -9: Very unstable. LI below -9: Extremely unstable.
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Cumulonimbus
incus
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Convective Available Potential
Energy (CAPE)
CAPE represents the amount of buoyant energy available toaccelerate a parcel vertically, or the amount of work a parcel doeson the environment. CAPE is the positive area on a soundingbetween the parcel's assumed ascent along a moist adiabat and theenvironmental temperature curve from the level of free convection(LFC) to the equilibrium level (EL).
ELCAPE = g [(Tparcel - Tenvir) / Tenvir] dz (in J/kg)
LFC
- CAPE below 0: Stable.- CAPE = 0 to 1000: Marginally unstable.- CAPE = 1000 to 2500: Moderately unstable.- CAPE = 2500 to 3500: Very unstable.- CAPE above 3500: Extremely unstable.
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Convective Activity?
Where do we expect to see convective activity today?
Check out the recent balloon sounding from Oklahoma (OUN).
Is convective precipitation likely in Oklahoma based on this mornings
sounding?
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