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Prognos av clear air turbulence i DMI-HIRLAM : situationsstudier

Mannerberg, Christina LU (2012) FYSK01 20121
Department of Physics
Abstract (Swedish)
På Danmarks Meteorologiska Institut har två index, TI3 och TI4, utvecklats i syfte att lokalisera riskområden för clear air turbulence (CAT). Indexen är ännu inte i operationellt bruk. Deras samband med andra parametrar i DMI-HIRLAM är inte känt och studeras här genom tre situationsstudier. CAT utgör en fara för flygtrafiken och bra prognoser av detta fenomen är nödvändigt. Att två olika index används motiveras av att de indikerar CAT som har utlösts av olika mekanismer. CAT förekommer framför allt i samband med jetströmmen, där starka vindshear genom Kelvin-Helmholtz-instabilitet och inflexionspunktsinstabilitet ger upphov till turbulens. TI3 är en vidareutveckling av ett annat CAT-index, Turbulence index, vilket bygger på produkten... (More)
På Danmarks Meteorologiska Institut har två index, TI3 och TI4, utvecklats i syfte att lokalisera riskområden för clear air turbulence (CAT). Indexen är ännu inte i operationellt bruk. Deras samband med andra parametrar i DMI-HIRLAM är inte känt och studeras här genom tre situationsstudier. CAT utgör en fara för flygtrafiken och bra prognoser av detta fenomen är nödvändigt. Att två olika index används motiveras av att de indikerar CAT som har utlösts av olika mekanismer. CAT förekommer framför allt i samband med jetströmmen, där starka vindshear genom Kelvin-Helmholtz-instabilitet och inflexionspunktsinstabilitet ger upphov till turbulens. TI3 är en vidareutveckling av ett annat CAT-index, Turbulence index, vilket bygger på produkten mellan vertikalt vindshear och horisontell deformation. I TI3 har ett beroende på den statiska stabiliteten lagts till. TI4 ska indikera riskområden för CAT då deformationen är liten, där värdet för TI3 blir litet. Detta kan ske främst på den anticyklonala sidan av jetströmmen, där det finns risk för att flödet blir obalanserat och CAT kan utvecklas från direkt eller indirekt shear instabilitet. Tre tillfällen i februari 2012 har valts ut och studerats. Modellens beräkning av indexen har jämförts med dess beräkning av turbulent kinetisk energi (TKE), en parameter som beskriver turbulensens intensitet. För samma situationer har också indexens utseende i två modeller,T15 och SKA, med olika upplösning jämförts. Studien visar att det inte går att se ett klart systematiskt samband mellan indexen och TKE, men ett visst samband ses mellan TI3 och TKE. Detta beror troligtvis på parametriseringen av TKE i DMI-HIRLAM. Undersökningen av hur indexen beror på modellupplösning ger ett ganska väntat resultat med fler detaljer för indexen i SKA. Dock finns tveksamheter kring den stora skillnaden för TI4s utseende i de två modellerna. Innan indexen kan tas i bruk behövs verifiering, vilket försvåras av att observationerna är få och kan vara oprecisa. (Less)
Abstract
Two indices, TI3 and TI4, have been presented at the Danish Meteorological Institute with the purpose to indicate clear air turbulence (CAT). The indices are not yet in operative use. The correlation between the indices and other parameters in DMI-HIRLAM is not known and is investigated in three case studies. CAT is a hazard to aviation and accurate prediction of CAT is important. The use of two indices is motivated by the fact that they indicate CAT developed from different mechanisms. CAT occurs mainly in association with the jet stream, where strong wind shears give rise to turbulence through Kelvin-Helmholtz and inflection point instability. TI3 is a further development of an other CAT-index, the Turbulence Index, which is the product... (More)
Two indices, TI3 and TI4, have been presented at the Danish Meteorological Institute with the purpose to indicate clear air turbulence (CAT). The indices are not yet in operative use. The correlation between the indices and other parameters in DMI-HIRLAM is not known and is investigated in three case studies. CAT is a hazard to aviation and accurate prediction of CAT is important. The use of two indices is motivated by the fact that they indicate CAT developed from different mechanisms. CAT occurs mainly in association with the jet stream, where strong wind shears give rise to turbulence through Kelvin-Helmholtz and inflection point instability. TI3 is a further development of an other CAT-index, the Turbulence Index, which is the product of vertical wind shear and horizontal deformation. In TI3 a dependence on the static stability is included. TI4 is made to indicate areas with risk of CAT where the deformation is small, where TI3 has a low value. This happens mainly on the anticyclonic shear side of the jet, where the flow can be unbalanced and CAT can develop from direct or indirect shear instability. Three cases in February 2012 have been selected and analyzed. The indices are compared to the model output of turbulent kinetic energy (TKE), a parameter describing the intensity of the turbulence. Also the structure of the indices in the two numerical models, T15 and SKA, with different resolution is compared for the same cases. The case studies show that a clear systematic correlation between the indices and TKE is not to be seen, but some correlation is observed between TI3 and TKE. This probably has its reason in the parametrization of TKE in DMI-HIRLAM. The result of the investigation of the dependence on model resolution is rather expected, the details in SKA were greater. The difference of TI4 in the two numerical models give rise to questions. Before the indices can be taken into operational use verification is needed, a process that is constrained by the lack of observations and of accuracy in the observations available. (Less)
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author
Mannerberg, Christina LU
supervisor
organization
course
FYSK01 20121
year
type
M2 - Bachelor Degree
subject
language
Swedish
id
2861325
date added to LUP
2012-07-10 11:59:19
date last changed
2012-07-10 11:59:19
@misc{2861325,
  abstract     = {Two indices, TI3 and TI4, have been presented at the Danish Meteorological Institute with the purpose to indicate clear air turbulence (CAT). The indices are not yet in operative use. The correlation between the indices and other parameters in DMI-HIRLAM is not known and is investigated in three case studies. CAT is a hazard to aviation and accurate prediction of CAT is important. The use of two indices is motivated by the fact that they indicate CAT developed from different mechanisms. CAT occurs mainly in association with the jet stream, where strong wind shears give rise to turbulence through Kelvin-Helmholtz and inflection point instability. TI3 is a further development of an other CAT-index, the Turbulence Index, which is the product of vertical wind shear and horizontal deformation. In TI3 a dependence on the static stability is included. TI4 is made to indicate areas with risk of CAT where the deformation is small, where TI3 has a low value. This happens mainly on the anticyclonic shear side of the jet, where the flow can be unbalanced and CAT can develop from direct or indirect shear instability. Three cases in February 2012 have been selected and analyzed. The indices are compared to the model output of turbulent kinetic energy (TKE), a parameter describing the intensity of the turbulence. Also the structure of the indices in the two numerical models, T15 and SKA, with different resolution is compared for the same cases. The case studies show that a clear systematic correlation between the indices and TKE is not to be seen, but some correlation is observed between TI3 and TKE. This probably has its reason in the parametrization of TKE in DMI-HIRLAM. The result of the investigation of the dependence on model resolution is rather expected, the details in SKA were greater. The difference of TI4 in the two numerical models give rise to questions. Before the indices can be taken into operational use verification is needed, a process that is constrained by the lack of observations and of accuracy in the observations available.},
  author       = {Mannerberg, Christina},
  language     = {swe},
  note         = {Student Paper},
  title        = {Prognos av clear air turbulence i DMI-HIRLAM : situationsstudier},
  year         = {2012},
}