Upper air and soundings
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Upper Air and Soundings. MSC 243 Lecture #6, 9/26/13. Pressure Levels. Pressure is the force exerted on an object by all air molecules that impinge on a surface area – in general, the weight of a column of air per unit area Pressure decreases with height.

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Upper air and soundings

Upper Air and Soundings

MSC 243 Lecture #6, 9/26/13


Pressure levels

Pressure Levels

  • Pressure is the force exerted on an object by all air molecules that impinge on a surface area – in general, the weight of a column of air per unit area

  • Pressure decreases with height.

  • Meteorologists concentrate on a few standard pressure levels, plus the surface

  • Each of these levels are important in weather forecasting for different reasons


Upper level weather maps

Upper Level Weather Maps

  • Upper level weather maps are plotted on a constant pressure surface

  • Contours of equal geopotential height are plotted (e.g. height in meters of the 500 mb pressure surface)

  • Thickness= difference in height between 2 pressure surfaces. It is directly proportional to the mean temperature of the layer (e.g. 1000 - 500 mb). Thickness is useful in determining precipitation type.


Ridges and troughs aloft

Increasing Height

Ridge

Ridge

500 mb

500 mb

Trough

Trough

700 mb

850 mb

Surface

Very warm

column

Cool

column

Warm

column

Very cool

column

Ridges and Troughs Aloft

  • Mountains and valleys of warm and cool air

  • The height of the pressure level depends on the temperature of the column of air below it


Height of pressure surfaces

Height of Pressure Surfaces

Pressure SurfaceTypical Height

850 mb1500 m / 5000 feet

700 mb3000 m / 10000 feet

500 mb5500 m / 18000 feet

300 mb9000 m / 30000 feet

200 mb12000 m / 39000 feet


Pressure obs on surface maps

Pressure obs. on surface maps

  • All stations are not at the same elevation. Pressure decreases with height.

  • Hence, all pressure readings at the surface are adjusted to sea level

  • This allows accurate comparison of horizontal differences in pressure and eliminates vertical differences in pressures due to elevation.

  • Isobar - line of equal pressure

  • Units: millibars or Pascals (Pa) (mb = 100 Pa)

  • Standard atmospheric pressure = 1013.25 mb


Height on a pressure surface is analogous to pressure on a height surface

Height on a pressure surface is analogous to pressure on a height surface!


Surface chart

Surface Chart

Typically look at mean sea level pressure (isobars), precipitation, and winds and temperatures to identify surface weather such as fronts. These are the conditions that affect us directly. The behavior of these systems is largely governed by what is going on in the upper troposphere.


850 mb chart

850 mb Chart

The 850 mb chart is good for estimating surface temperatures, low level moisture, and determining precipitation type (rain, snow, sleet).


850 mb chart1

850 mb Chart

The 850 mb chart is good for estimating surface temperatures, low level moisture, and determining precipitation type (rain, snow, sleet).


700 mb chart

700 mb chart

The 700 mb chart is used to determine cloud cover or rainfall, using the relative humidity field and the vertical motion field.


700 mb chart1

700 mb chart

The 700 mb chart is used to determine cloud cover or rainfall, using the relative humidity field and the vertical motion field.


700 mb chart2

700 mb chart

The 700 mb chart is also used to determine short-wave disturbances via the geopotential height field.


500 mb chart

500 mb chart

The 500 mb chart is the forecasters’ favorite for depicting the motion of weather systems. It shows the large-scale flow (long waves) and jet streams, and also the small-scale flow (short-waves, low level storm systems)

RIDGE

TROUGH

TROUGH


500 mb chart1

500 mb chart

The 500 mb chart is the forecasters’ favorite for depicting the motion of weather systems. It shows the large-scale flow (long waves) and jet streams, and also the small-scale flow (short-waves, low level storm systems)

RIDGE

TROUGH

TROUGH


250 mb chart

250 mb Chart

The 250 mb chart is used to locate the jet stream.

Strong upper- level winds help develop surface low pressure in mid-latitudes.


Soundings

SOUNDINGS


Heavy rainfall in oct 2011

Heavy rainfall in Oct 2011


Upper air and soundings

NWS Miami: “Strongest tornado to hit Broward or Miami-Dade Counties since the March 27, 2003 Brownsville / Liberty City tornado”

http://www.srh.noaa.gov/media/mfl/SunrisePlantationTornado.pdf


Upper air and soundings

Something is wrong here.


What is a sounding

What is a sounding ?

  • Weather balloon (“radiosonde”) rises and measures conditions of the atmosphere at different altitudes


Why do we need soundings

Why do we need soundings ?

  • We don’t know a lot about the atmosphere above the ground

     but we need to know the state of the atmosphere at many levels to make a forecast!

    Soundings provide meteorologists and the models with several atmospheric variables in the vertical, but only in isolated locations.

  • Weather balloons (rawinsondes) launched twice per day, at 00 Z and 12 Z.


Isobars isotherms dry adiabats

Isobars, Isotherms, Dry Adiabats

Skew-T chart is a graphical display of an upper air sounding for a particular ground station.

Isobars - straight horizontal solid lines at pressure levels → height information.

Isotherms - straight diagonal solid lines sloping from lower left to upper right. These lines are labeled in °C

The concept of Skew-T means that the temperature is not plotted vertically but is oriented to the right at a 45º angle.

Dry adiabats - slightly curved solid lines of constant potential temperature (in K), sloping from the lower right to upper left.


Isobars isotherms dry adiabats inversions

Isobars, Isotherms, Dry Adiabats, Inversions

Isobars are lines of constant pressure.

Isotherms are lines of constant temperature.

Dry adiabats are lines of constant potential temperature.

ISOBAR

ISOTHERM

DRY-ADIABAT


Temperature

Temperature

Temperature

Temperature at a specified pressure.

At pressure find T reading

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 reading

Follow dry-adiabat down to 1000mb

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 reading

Read value of isotherm

Label in °C


Moist adiabats mixing ratio

Moist Adiabats, Mixing Ratio

Moist adiabats - slightly curved dashed lines sloping from the lower right to upper left.

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 out immediately as the parcel rises.

Mixing-ratio lines - slightly curved dashed lines sloping from the lower left to upper right.

These lines are labeled in grams per kilogram; grams of water vapor per 1000 grams of dry air.


Moist adiabats mixing ratio1

Moist Adiabats, Mixing Ratio

Moist adiabats represent the lines of constant temperature change in a parcel of saturated air rising pseudo-adiabatically

(Sometimes known as saturation adiabats)

Mixing-ratio lines are lines of constant water content.

MOIST-ADIABAT

MIXING-RATIO


Moisture

Moisture

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 mixing ratio line

Label in g/kg


Temperature and moisture

Temperature and Moisture

here: at 700 mb

  • Data from a Skew-T available at each level are:

  • Temperature

  • Potential Temperature

  • Dew Point

  • Mixing Ratio

  • Saturation Mixing Ratio

Clouds are more likely to occur when T and Td are similar

313K

5 g/kg

11 g/kg

-1 C


Relative humidity

Relative Humidity

Relative Humidity

The ratio (in %) of the amount of water vapor in a given volume to the amount that the volume would hold if saturated.

At pressure find Td (w)

Note value of mixing ratio line

At pressure find T (ws)

Note value of saturation mixing ratio line

RH = w/ws x 100


Inversions

Inversions

Portion of a sounding where the atmospheric temperature stops increasing with height.

Types of inversions:

Radiation Inversion

A thermally produced surface based inversion formed by the rapid cooling of air in contact with the surface as compared to the upper layers.

Favorable conditions include:

Long nights or periods of low solar radiation.

Clear skies.

Dry air, as moist air absorbs infrared energy.

Little winds.


Inversions1

Inversions

Subsidence Inversion

A mechanically produced inversion formed by adiabatic warming of a layer of sinking air.

Favorable conditions include:

Strong anticyclones or stable air masses that force air to sink.

Frontal Inversion

The transition layer between a cold air mass and the warmer air mass above it.

Favorable conditions include:

Warm air overriding cold air.


Where is the inversion

Where is the inversion?

Rising parcels that encounter an inversion are cooler than their environment, and stop rising.

Inversion


Parcel motion

Parcel Motion

  • As a parcel moves up and down, so long as it is not saturated, its temperature will change at the dry adiabatic lapse rate.

  • As a parcel moves up and down, so long as it is not saturated, it will conserve its ‘mixing ratio’, the ratio of water molecules to air molecules.


Parcel motion1

Parcel Motion

  • If a parcel becomes saturated, continued cooling will result in condensation into water vapor.

  • The latent heat released by condensation offsets the cooling from expansion, and the parcel will rise at the moist adiabatic lapse rate.

  • This moist adiabatic lapse rate is generally around 5-6 C / km, not as high as the dry adiabatic lapse rate.


Cloud formation

Cloud Formation

  • First, consider an air parcel at the surface.

  • When it is warmed at the surface, it is buoyant: it rises and its temperature falls.

  • If no heat is added or released, the parcel rises “dry adiabatically”.

  • Adiabatic cooling of rising air is the dominant cause of cloud formation…

    [On a Skew-T diagram, move up along the dry adiabat from the surface]


Cloud formation1

Cloud formation

  • The rising unsaturated air parcel reaches the lifting condensation level (LCL), where it begins to condense.

  • At the LCL, the air parcel becomes saturated as its temperature reaches the dew point (of the parcel). This is where the cloud base exists.

    • LCL = the height at which a parcel of air would become saturated if lifted dry adiabatically (point at which saturation would occur if parcel is lifted mechanically: orographic, frontal, etc.)


Lifting condensation level lcl

Lifting Condensation Level (LCL)

LCL

(~ 860 mb)

  • At surface pressure find T

  • Draw a line up parallel to dry adiabat

  • At surface pressure find Td

  • Draw a line up parallel to mixing ratio line

  • The intersection of the 2 lines is the LCL

  • Read pressure and label in mb


Above the lcl cloud formation

Above the LCL: Cloud Formation

  • Now that the air parcel is saturated at the LCL, it might be able to rise even higher under the right conditions

  • Condensation is a warming process: latent heat release may assist further rising.

  • But, this is partially offset by the cooling due to expansion.

  • As the parcel rises above the LCL, it cools at a slower rate: the moist adiabatic lapse rate.


Above the lcl cloud formation1

Above the LCL: Cloud Formation

  • Under favorable instability conditions, the air may rise to a level where it becomes warmer than its environment.

  • This is the Level of Free Convection (LFC)

    The level beyond (above) which the air parcel becomes buoyant. An air parcel above this level can rise upward even in the absence of any forced lifting

    • ON A SKEW-T: Start at Lifting Condensation Level (LCL) and follow up the moist adiabat until you reach the temperature line from the sounding


Key concepts buoyancy

Key Concepts: Buoyancy

Buoyancy: upward force that acts on a parcel of air due to density difference

Higher potential temperature and water vapor content increase buoyancy.

Precipitation acts to decrease buoyancy.

Main thing to look at: is the temperature of the air parcel warmerorcoolerthan the environment (i.e. the sounding temperature)?


Atmospheric convection

Atmospheric Convection

  • As an air parcel moves up from the LFC, its temperature is higher than that of the environment, and it is positively buoyant.

  • Q: What is the air parcel going to do?

  • A: It will rise further, with no extra help!

  • The air parcel will continue to cool at the moist adiabatic lapse rate.


Atmospheric convection1

Atmospheric Convection

  • The air parcel remains just saturated.

  • Hence, as it continues to rise, it condenses more water and forms more clouds and precipitation.

  • Therefore, convection is associated with clouds and rainfall.

  • The larger the difference in temperature between the parcel and environment, the faster the updraft, the faster the condensation, and the more severe storms!


Atmospheric convection2

Atmospheric Convection

  • This condensation continues until the air parcel finally reaches the Equilibrium Level (EL).

    • EL = The height at which the temperature of the buoyant parcel again becomes equal to the environmental temperature.

  • The EL is where the cloud anvil forms. Sometimes, air parcels with upward momentum may push up through the EL, but then they are heavier than their surroundings and sink back.

    • ON A SKEW-T: From the point where the LFC was found, follow a moist adiabat up until crossing the temperature line again.

    • That level is the equilibrium level, at which a parcel of air no longer accelerates upward.


Levels lcl lfc el in some cases the lcl and lfc are concurrent

Levels: LCL < LFC < EL(in some cases, the LCL and LFC are concurrent)

EL

LFC

LCL


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