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External Influences on External Influences on Cyclone FormationCyclone Formation
Working Group 2.1Working Group 2.1
W. M.W. M. Frank, G. J. Holland, P. Klotzbach, Frank, G. J. Holland, P. Klotzbach,
J. L. McBride, P. E. RoundyJ. L. McBride, P. E. Roundy
Contributions: J. Molinari.Contributions: J. Molinari.
Two Scales of GenesisTwo Scales of Genesis
Individual Storm Formation
Climatological Variations
e.g. – interannual variability, global change
Climatological FactorsClimatological Factors
Warm ocean (SST of 26.5 C or greater)Warm ocean (SST of 26.5 C or greater)
Persistent deep convection and upward Persistent deep convection and upward motionmotion
Enhanced low-level cyclonic vorticityEnhanced low-level cyclonic vorticity
Weak vertical shear directly over the Weak vertical shear directly over the centercenter
Enhanced low-level moistureEnhanced low-level moisture
Formation of Individual StormsFormation of Individual Storms
Climatological conditions favorable for Climatological conditions favorable for growthgrowth
Mean flow may or may not be unstable Mean flow may or may not be unstable (e.g. – ITCZ breakdown)(e.g. – ITCZ breakdown)
Transient perturbations usually speed up Transient perturbations usually speed up the processthe process
Recent Research AreasRecent Research Areas
Individual Genesis EventsIndividual Genesis EventsTropical wavesTropical waves
Basin genesis parametersBasin genesis parameters
Synoptic-scale analysesSynoptic-scale analyses
Climatological Variations in Storm Climatological Variations in Storm NumbersNumbers
Interannual variabilityInterannual variability
Climate changeClimate change
Example Genesis Parameter – N. AtlanticExample Genesis Parameter – N. Atlantic
DeMaria et al. (2001)
Based on: vertical shear, convective instability,
mid-level moisture
0.5
0.4
0.3
0.2
0.1
0-14 -10 -6 -2 2 6 10 14
30 days
6 days
3 days
Fre
quen
cy (
CP
D)
Westward Zonal Wavenumber Eastward
MJO-ISOe
TD-type
MRGKelvin
ER
0
20.0
10.0
6.67
5.0
4.0
3.33
2.86
2.50
2.22
2.0
Per
iod
(Day
s)
Spectral Bands for Filtering
*Adapted from Wheeler and Kiladis, 1999
h=50
m h=50
m
h=50m
h=12m
h=12m h=12m
Total Wind Fields, Westerly Phase of MJO, Total Wind Fields, Westerly Phase of MJO, NWP (composite of 20 strong cases)NWP (composite of 20 strong cases)
Maloney and Hartman (2001)
Genesis and ER Waves in theGenesis and ER Waves in the South Indian Ocean South Indian Ocean
Bessafi andWheeler (2006)
Series of TC formations in the NW Pacific that result from a train of MRG waves interacting with the large-scale flow.
Dickinson and Molinari (2002)
Continued
NW Pacific Composites
Frank and Roundy (2006)
Implications for ForecastsImplications for Forecasts
Consistent phase relationship suggests Consistent phase relationship suggests waves are modulating genesiswaves are modulating genesisAnomalies can be detected in advance of Anomalies can be detected in advance of genesis by at least:genesis by at least:
ISOe: ISOe: 13-20 days13-20 days ER: ER: 7-20 days 7-20 days
MRG/TD: MRG/TD: 3-6 days 3-6 daysKelvin: 3-7 daysKelvin: 3-7 days
Statistical forecast of genesis probability, Statistical forecast of genesis probability, 15-day lead time15-day lead time
Paul Roundy - http://www.atmos.albany.edu/facstaff/roundy/tcforecast/tcforecast.html
Relationships Between Genesis and Relationships Between Genesis and Basin-Scale VariationsBasin-Scale Variations
Several studies show apparent long-term trends Several studies show apparent long-term trends in cyclone activity and/or intensityin cyclone activity and/or intensity
There are also long-term trends in some TC-There are also long-term trends in some TC-related climatological variables (e.g. – SST)related climatological variables (e.g. – SST)
Relationships between large-scale/long-term Relationships between large-scale/long-term variables and the occurrence of local, necessary variables and the occurrence of local, necessary conditions for genesis need to be explored.conditions for genesis need to be explored.
Interannual VariabilityInterannual Variability
There are persistent patterns of correlation There are persistent patterns of correlation between annual storm numbers in different between annual storm numbers in different basins.basins.These patterns are related to ENSO, and These patterns are related to ENSO, and perhaps to other large-scale circulation modes.perhaps to other large-scale circulation modes.There is There is nono discernable tendency for inter-basin discernable tendency for inter-basin compensationcompensation in terms of storm numbers or in terms of storm numbers or storm days.storm days.There is some suggestion that storm activity for There is some suggestion that storm activity for Category 4-5 storms tends to be positively Category 4-5 storms tends to be positively correlated between basins.correlated between basins.
Frank and Young (2006)
Summary, Storm-ScaleSummary, Storm-Scale
Predicting genesis requires accurate synoptic-Predicting genesis requires accurate synoptic-scale (mesoscale?) forecasts of the wind and scale (mesoscale?) forecasts of the wind and moisture fields at the time and place of storm moisture fields at the time and place of storm formation.formation.
Longer range genesis predictions are very difficult Longer range genesis predictions are very difficult in the tropics using current deterministic models.in the tropics using current deterministic models.
There is strong evidence that tropical waves play a There is strong evidence that tropical waves play a major role in triggering genesis in all cyclone major role in triggering genesis in all cyclone basins.basins.
Summary, Summary, cont.cont.
Tropical waves are predictable using statistical Tropical waves are predictable using statistical techniques. Other statistical genesis prediction techniques. Other statistical genesis prediction techniques are also showing promise.techniques are also showing promise.
Combined statistical/deterministic forecasts offer Combined statistical/deterministic forecasts offer promise for forecasts of genesis with lead times promise for forecasts of genesis with lead times of up to the time-scale of the MJO.of up to the time-scale of the MJO.
Summary, Longer-TermSummary, Longer-Term Genesis Variability Genesis Variability
Long-term variability of tropical cyclone formation Long-term variability of tropical cyclone formation depends on the long-term variability of the depends on the long-term variability of the occurrence of local genesis conditions.occurrence of local genesis conditions.
The missing link is to determine how long-term The missing link is to determine how long-term variations in large-scale fields (SST, winds, etc.) variations in large-scale fields (SST, winds, etc.) are related to the frequency of occurrence of are related to the frequency of occurrence of local necessary conditions for genesis.local necessary conditions for genesis.
RecommendationsRecommendations
Experiment with statistical techniques for longer-Experiment with statistical techniques for longer-range genesis forecasts.range genesis forecasts.
Improve understanding of the relationships Improve understanding of the relationships between basin-scale variability and the between basin-scale variability and the incidence of genesis conditions.incidence of genesis conditions.