02 definitions and structure

8/12/2019 02 Definitions and Structure

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ENVI 1400 : Meteorology and Forecasting : lecture 2 2

Course Website & Contact http://www.env.leeds.ac.uk/~ibrooks/envi1400

Notes, links, and data required for forecast exerciseswill be made available via this site throughout thecourse

Met. charts and satellite imagery are collectedautomatically and updated every 6, 12, or 24 hours

Email : [email protected]

Office : room 3.25 School of Earth &Environment (Environment Building)


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Units

The units used in meteorology are a mixture of S.I.(Systeme International) units (used throughoutscientific meteorology) and older systems of non -

SI units retained in use because of historicalreasons, for convenience, or for communicatingwith the general public.

It is important ALWAYS to give the units in whicha value is quoted.


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Kelvin (K) : (SI unit) necessary for manycalculations

Degrees Celcius ( C) : (non-SI) usually used to

quote temperature in general use more readilyunderstood and values in convenient range

0 K = -273.15 C

conversion:TKelvin = T Celcius -273.15

Temperature


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Degrees Fahrenheit ( F) : (nonSI) widelyused in America.

TFahrenheit = T Celcius + 3295


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Pressure

SI unit of pressure is the Pascal (Pa),atmospheric pressure is quoted inhectopascal (hPa) = hundreds of Pascals.

1 hPa = 100 Pa Pressure is frequently quoted in millibars

(mb) (nonSI) 1 mb = 1 hPa

Mean sealevel pressure = 1013.25 mb.


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Wind Speed

Metres per second (m s -1)(SI unit) allscientific use, and frequent common use

Knots (kt) = nauticalmiles per hour= 0.514 m s -1 0.5 m s -1

Kilometres per hour (kph) = 0.278 m s -1 Miles per hour (mph) = 0.447 m s -1

Wind speeds are quoted in a variety ofdifferent units



Wind Direction

It is meteorological convention to give thedirection that the wind is coming FROM Bearing in degrees from north ie a compass

bearing taken when you are facing directlyinto wind

Because of the high degree of variability in

the wind gustiness often only the generaldirection is quoted: northerly, south-westerly,etc



N

S

EW

Mean wind vector

Wind direction = 50



Relative Humidity : quotedas a percentage (%) (non-SI)

= water vapour content of theair as a percentage of themaximum possible, saturationor equilibrium vapour contentat that temperature.

Relative humidity is the measure mostclosely related to our comfort howhumid it feels . It is also useful indetermining where cloud or fog willform: condensation of vapour to formdroplets occurs when RH increases to100%.

Humidity

P vP s

RH = 100RH = 64%

RH = 100%

Saturation vapour pressure



Dew PointThe temperature to whicha parcel of air with

constant water vapourcontent must be cooled, atconstant pressure, inorder to becomesaturated.

Dew Point DepressionThe difference betweenthe temperature of a

parcel of air, and its dewpoint temperature.



Mixing RatioRatio of the mass of watervapour to mass of dry air.

Specific HumidityThe ratio of the mass of

water vapour to the massof moist air.

Absolute Humidity orVapour Density

The mass of water vapour

per unit volume of moistair.

q =Mv

Mv + M a

Mixing ratio =Mv

Ma


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Time

Time is usually quoted in 24-hour form andin UTC (coordinated universal time). Thisis (almost) the same as GMT

e.g. 1800 UTC Analysis of meteorological conditions to make

a forecast requires measurements made atthe same time over a very wide area -

including multiple time zones (possibly thewhole world). Using UTC simplifies theprocess of keeping track of when eachmeasurement was made


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TROPOSPHEREcumulusclouds

cirrostratusclouds

10km

20km

30km

40km

50km

70km

80km

90km

60km

110km

120km

130km

100km

MESOSPHERE

STRATOSPHERE

THERMOSPHERE

Mt Everest(8km)

ozone layer

meteorite

aurora

noctilucentclouds

tropopause

stratopause

mesopause

20

60

40

100

80

0-100 -80 -60 -40 -20 0 4020

100

10

1

0.1

0.01

0.001

0.0001

0.00001

TROPOSPHERE

MESOSPHERE

THERMOSPHERE

STRATOSPHERE

A l t i t u

d e ( k m

)

P e r c e n

t a g e o

f A t m o s p

h e r i c

M a s s

A b o v e

Temperature ( C)

Tropopause

Stratopause

Mesopause

ozonelayer

Mt Everes t

cumulonimbus

Vertical Structure:Temperature


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Vertical Structure

TROPOSPHERE Lowest layer of atmosphere ~8km deep at poles, ~16km deep at equator. Depth varies both

spatially & with time.

Region where virtually all weather occurs. Most of the watervapour in the atmosphere is concentrated in the lowertroposphere.

Temperature generally decreases with altitude (though withsignificant variability)

Capped by a region of increasing temperature (a temperatureinversion) or isothermal layer, the Tropopause .

The tropopause acts as a lid, preventing the exchange of airbetween troposphere and stratosphere.


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The Boundary Layer A sublayer of the troposphere In contact with the surface experiences direct effect of friction

at surface

Dominated by turbulence and surface exchange processes:heat, moisture, momentum Exhibits large diurnal changes in many properties: depth,

temperature, Depth varies from a few 10s of metres (in very stable conditions),

to ~2km over tropical oceans. A few 100 m to ~1 km is typical. Temperature decreases with altitude. Usually capped by a temperature inversion that inhibits mixing

with the air in the free troposphere above. N.B. A well defined boundary layer is not always present


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Temperature Profile

tropopause

free troposphere

boundary layer

temperatureinversion

stratosphere

April 24 2004


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Humidity Profile

tropopause

inversion


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STRATOSPHERE Extends from top of tropsphere to ~50 km. Temperature generally increases with altitude during summer

lowest temperature at the equatorial tropopause. Has a more

complex structure in winter. Contains the majority of atmospheric ozone (O 3). Absorption of

ultraviolet produce a maximum temperature at the stratopause(sometimes exceeding 0 C).

Interaction with the troposphere is limited, and poorly

understood.


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Vertical Structure: Pressure The pressure at any point

is the result of the weightof all the air in the columnabove it.

Upwards force ofpressure exactlybalances downward forceof weight of air above

Decreases approximatelylogarithmically withaltitude Departures from logarithmic

profile are due to changes inair density resulting fromchanges in temperature &moisture content.

Near the surface a 1mb change inpressure is equivalent to 7mchange in altitude.


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Length Scales


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Local (microscale ) Time: few hours to ~1 day Distance:


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There is a huge discrepancy between the length

scales associated with horizontal and verticalgradients of most quantities of interest. In generalvertical gradients are much larger than horizontalones.

Pressurevertical gradient:

~0.14 mb m -1 horizontal gradients: < 0.1 mb km -1

(typically ~0.01 mb km -1)


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500mb surface height (dm) : 60m contours : Analysis 0000-040927


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Temperature

vertical gradients:typically ~0.01 C m -1 can be larger locally, e.g. boundary layer temperatureinversion up to ~0.2 C m -1

horizontal gradients:On a large scale typically < 1 C per 100 km (0.01 Ckm -1), up to ~5 C per 100 km within frontal zones.Local effects (e.g. solar heating in sheltered spots) may

result in larger gradients on small scales.

N.B. How warm we feel is not a good indicator of the airtemperature.


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Surface temperature analysis 0600-040929 : 2 C contours


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850mb temperature (2 C contours), RH (%), wind (m s -1) : analysis 0000-040929


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Summary Atmosphere is divided

vertically into severaldistinct layers.

Only the lowest layer

the troposphere isclosely connected toweather.

A shallow sublayer of thetroposphere, theboundary layer, is directlyinfluenced by the surface& dominated by turbulentmixing.

Largescale horizontalgradients of pressure,temperature, etc aregenerally much smaller

than vertical gradients. A consequence of this isthat the forcing processesthat drive synopticweather systems are

almost horizontal. Large-scale vertical motions areslow.

02 definitions and structure

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