the role of stability in forecasting

8/14/2019 The Role of Stability in Forecasting

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EVSC 493-Weather Forecasting

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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the role of stability in forecasting

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