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.