Application of quasigeostrophic Application of quasigeostrophic arguments in interpretation of arguments in interpretation of

extratropical cyclogenesisextratropical cyclogenesis

Niels Woetmann NielsenNiels Woetmann Nielsen

Danish Meteorological InstituteDanish Meteorological Institute

● Extratropical cyclones in a global perspective

● Energy conversions

● Simplified quasigeostrophic equations

● Typical flow patterns in extratropical cyclogenesis

● Example

● Exercise

Why bother about extratropical Why bother about extratropical cyclones and anticyclones?cyclones and anticyclones?

because they dominate the weather because they dominate the weather at middle and high latitudes.at middle and high latitudes.

Why use QG-theory to explain their Why use QG-theory to explain their development?development?

because it is the simplest way to because it is the simplest way to explain how they developexplain how they develop

because QG-theory, when combined because QG-theory, when combined with a few carefully selected NWP with a few carefully selected NWP fields, can explain the dynamics fields, can explain the dynamics behind the major cloud patterns seen behind the major cloud patterns seen in satellite images.in satellite images.

Extratropical cyclones and anticyclones Extratropical cyclones and anticyclones develop as result of what is called develop as result of what is called baroclinic instabilitybaroclinic instability. .

Baroclinic instabilityBaroclinic instability cancan not occur not occur everywhere on the globe.everywhere on the globe.

In a global perspective In a global perspective baroclinicitybaroclinicity is is generated where the northward relatively generated where the northward relatively warm and moist branch of the warm and moist branch of the Ferrell Ferrell circulationcirculation meets the southward relatively meets the southward relatively cold and dry branch of the cold and dry branch of the polar polar circulationcirculation

– – and and baroclinic instabilitybaroclinic instability can only can only occur in this zone.occur in this zone.

Global meridional circulation and surface winds

During extratropical cyclogenesis During extratropical cyclogenesis mean available potential energymean available potential energy is converted to is converted to eddy available eddy available potential energypotential energy on the cyclone on the cyclone scale and part of the eddy available scale and part of the eddy available potential energy is converted to potential energy is converted to eddy kinetic energyeddy kinetic energy in the in the developing extra-tropical cyclone.developing extra-tropical cyclone.

Conversion from Conversion from mean available mean available potential energypotential energy to to eddy eddy available potential energyavailable potential energy occurs occurs in a process where at the same in a process where at the same latitude and height cold air is latitude and height cold air is advected southward and warm air is advected southward and warm air is advected northward.advected northward.

Conversion from Conversion from eddy available eddy available potential energypotential energy to to eddy kinetic eddy kinetic energyenergy occurs in a process where at occurs in a process where at the same latitude and height cold air the same latitude and height cold air sinks and warm air rises.sinks and warm air rises.

Energy cycle for Northern Hemisphere winter

“Kinematical” south-north eddy transport of momentum

Upper: trough and ridge axes with positive tiltMiddle: trough and ridge axes without tiltBottom: trough and ridge axes with negative tilt

A developing extratropical cyclone A developing extratropical cyclone have have sinking cold airsinking cold air penetrating penetrating into a warmer air mass and into a warmer air mass and rising rising warm airwarm air penetrating into a colder penetrating into a colder air mass.air mass.

How can this be explained by QG-How can this be explained by QG-theory?theory?

Typical flow structures in extratropical cyclogenesis

Baroclinic instabilityBaroclinic instability Upper-tropospheric Upper-tropospheric wave amplificationwave amplification by by

differential temperature advectiondifferential temperature advection (cold (cold (warm) advection below upper trough (warm) advection below upper trough (ridge) – this process converts (ridge) – this process converts meanmean available potential energy (APE) into available potential energy (APE) into eddyeddy APE. APE.

Upper-tropospheric wave amplification Upper-tropospheric wave amplification intensifiesintensifies the the secondary circulationsecondary circulation (ageostrophic winds and vertical velocity) (ageostrophic winds and vertical velocity) in such a way that in such a way that ascendingascending ( (descendingdescending) ) motion intensifies in motion intensifies in warmwarm ( (coldcold) air ) air downstream of the upper-level downstream of the upper-level troughtrough ((ridgeridge) )

- this process converts - this process converts eddyeddy APE into APE into eddyeddy kinetic energykinetic energy (KE) (KE)

A A positivepositive feed-back occurs since the feed-back occurs since the intensified low level circulation (increase in intensified low level circulation (increase in eddy KE) leads to an eddy KE) leads to an increaseincrease in the in the conversionconversion from from meanmean to to eddyeddy APE, the APE, the latter promoting latter promoting increasedincreased conversion from conversion from eddyeddy APE to APE to eddyeddy KE and so on. KE and so on.

The feedback occurs through the The feedback occurs through the secondary circulationsecondary circulation (vertical velocity and (vertical velocity and ageostrophic wind).ageostrophic wind).

Relative vorticity advectionRelative vorticity advection is the most is the most importantimportant process in the process in the upperupper troposphere.troposphere.

In the In the lowerlower troposphere the most troposphere the most importantimportant process is process is temperature temperature advectionadvection and and diabatic heatingdiabatic heating..

Baroclinic instability can Baroclinic instability can notnot occur if occur if the wave length is too the wave length is too shortshort and and static stability too static stability too highhigh..

Under these conditions the Under these conditions the secondary circulation (secondary circulation (SCSC) induced ) induced by upper-tropospheric processes by upper-tropospheric processes (relative vorticity advection) does not (relative vorticity advection) does not reach the lower troposphere and the reach the lower troposphere and the SCSC induced by low-level temperature induced by low-level temperature advection does not reach the upper advection does not reach the upper troposphere. troposphere. NoNo interaction between interaction between the processes can take place. the processes can take place.

“Small-scale” extratropical cyclogenesis case 18 November 2009

““Small-scale cyclogenesis 18 November Small-scale cyclogenesis 18 November 20092009

DMI-HIRLAM analyses of mslp and 300 hPa pot. temp. (K)DMI-HIRLAM analyses of mslp and 300 hPa pot. temp. (K) Left: 00UTC and right: 12UTCr Left: 00UTC and right: 12UTCr

““Small-scale” cyclogenesis example Small-scale” cyclogenesis example (1)(1)

Incipient stage of cyclogenesis 00UTC 18 November 2009Incipient stage of cyclogenesis 00UTC 18 November 2009 Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and 300 hPa Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and 300 hPa

windwind

“Small-scale” cyclogenesis example (1)

Incipient stage of cyclogenesis 00UTC 18 November 2009Incipient stage of cyclogenesis 00UTC 18 November 2009 Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and rel. vorticity and Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and rel. vorticity and

wind at 300 hPawind at 300 hPa

““Small-scale” cyclogenesis Small-scale” cyclogenesis example (2)example (2)

Deepening phase of cyclone 06UTC 18 November 2009Deepening phase of cyclone 06UTC 18 November 2009 Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp

and 300 hPa wind and 300 hPa wind

““Small-scale” cyclogenesis example (2)Small-scale” cyclogenesis example (2)

Deepening phase of cyclone 06UTC 18 November 2009Deepening phase of cyclone 06UTC 18 November 2009 Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp

and rel. vorticity and wind at 300 hPa and rel. vorticity and wind at 300 hPa

““Small-scale” cyclogenesis Small-scale” cyclogenesis example (3) example (3)

Mature stage of cyclone 12UTC 18 November 2009Mature stage of cyclone 12UTC 18 November 2009 Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and 300 hPa windLeft: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and 300 hPa wind

““Small-scale” cyclogenesis example (3)Small-scale” cyclogenesis example (3)

Mature stage of cyclone 12UTC 18 November 2009Mature stage of cyclone 12UTC 18 November 2009 Left: MSG channel 5; Right: DMI-HIRLAM analysis of Left: MSG channel 5; Right: DMI-HIRLAM analysis of

mslp and rel. vorticity and wind at 300 hPamslp and rel. vorticity and wind at 300 hPa

““Small-scale” cyclogenesis Small-scale” cyclogenesis example (4)example (4)

Filling stage 18UTC 18 November 2009Filling stage 18UTC 18 November 2009 Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and 300 hPa windLeft: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and 300 hPa wind

““Small-scale” cyclogenesis example (4)Small-scale” cyclogenesis example (4)

Filling stage 18UTC 18 November 2009Filling stage 18UTC 18 November 2009 Left: MSG channel 5; Right: DMI-HIRLAM analysis of Left: MSG channel 5; Right: DMI-HIRLAM analysis of

mslp and rel. vorticity and wind at 300 hPamslp and rel. vorticity and wind at 300 hPa

ExerciseSuppose that a forecaster has only access to nwp-analyses of mean sea level pressure (mslp), wind and relative vorticity (rvor) at 300 hPa and a MSG channel 5 image valid at analysis time.

The forecaster is asked to consider the surface low (L) west of Spain (see s1) and is given the following tasks: In connection with L

● identify regions with significant positive and negative rvor advection at 300hPa and infer qualitatively their contribution to vertical velocity (w)

● identify regions with significant cold and warm advection (mean from surface to 300hPa) and infer qualitatively their contribution to w

● Identify a region with both warm advection and positive rvor advection and a region with both cold advection and negative rvor advection

● Which way will L most likely move? - and is it likely that L will intensify?

● What is the sign of the correlation between rvor at 300 hPa and bright(+) and dark (-) colours on the MSG image – is the correlation weak or strong?

DMI-HIRLAM analysis of mslp and wind at 300 hPa valid 12UTC 27 Feb. 2010 (s1)

DMI-HIRLAM analysis of mslp and rvor and T at 300 hPa valid 12UTC 27 Feb. 2010 (s2)

MSG channel-5 12UTC 27 February 2010

s3s3

s3

s4

s5

s6