Western Pacific Tropical Cyclogenesis Precursors

Ph.D. Prospectus

Carl J. Schreck, III

Tropical Cyclogenesis Precursors

Traditional Precursors

• Monsoon trough• Monsoon gyres• Easterly waves• Tropical cyclone

dispersion

Equatorial Waves

• The Madden–Julian oscillation (MJO)

• Equatorial Rossby (ER) waves

• Mixed Rossby–gravity (MRG) waves

• Kelvin waves

Tropical Cyclogenesis Precursors

Traditional Precursors

• Monsoon trough• Monsoon gyres• Easterly waves• Tropical cyclone

dispersion

Equatorial Waves

• The Madden–Julian oscillation (MJO)

• Equatorial Rossby (ER) waves

• Mixed Rossby–gravity (MRG) waves

• Kelvin waves

Monsoon Trough

• Roughly 70% of western Pacific TCs form in this climatologically favorable region (e.g., Gray 1968)

• Shear line provides cyclonic vorticity

• Confluence zone can induce wave growth through accumulation (Sobel & Bretherton 1999)

Briegel & Frank (1997)

Monsoon Gyre

• Monsoon trough is sometimes replaced with a large gyre (Lander 1994)

• Multiple TCs can form in the southeast quadrant of the gyre

• Gyre itself can become a large TC

Lander (1994)

Easterly waves

• Sometimes called “Tropical Depression (TD)-type disturbances”

• Westward or northwestward propagating synoptic scale disturbances

• Do not correspond to any linear equatorial wave solution

• Attributions range from 8% (Lee et al. 2008) to 71% (Fu et al. 2007) of TC formations

Ritchie & Holland (1999)

Rossby Dispersion from a Preexisting Tropical Cyclone

• TC-like vortex in a shallow-water model can radiate Rossby waves to the southeast

• Krouse et al. (2008) found that the TC must be moving westward relative to the background flow

• Stronger TCs may be more likely to generate wave trains (Fu et al. 2007)

Krouse et al. (2008)

Tropical Cyclogenesis Precursors

Traditional Precursors

• Monsoon trough• Monsoon gyres• Easterly waves• Tropical cyclone

dispersion

Equatorial Waves

• The Madden–Julian oscillation (MJO)

• Equatorial Rossby (ER) waves

• Mixed Rossby–gravity (MRG) waves

• Kelvin waves

TRMM Precipitation Spectrum

• Includes both symmetric and antisymmetric signals (Roundy & Frank 2004)

• Peaks in the spectrum tend to correspond with shallow-water equatorial wave solutions (Wheeler & Kiladis 1999)

• Boxes indicate filter bands (Frank & Roundy 2006)

• MRG waves and TD-type disturbance have similar periods, but wavelength is longer for MRG waves (Takayabu & Nitta 1993)Total Spectrum

No red background removed

Westward Wavenumber Eastward

The MJO

• TCs develop in region of enhanced convection, convergence, and cyclonic vorticity (e.g., Liebmann et al. 1994; Frank & Roundy 2006)

• May also provide favorable easterly vertical wind shear (Frank & Roundy 2006) and mid-level relative humidity (Camargo et al. 2009)

• Higher frequency modes also grow within the active MJO (Maloney & Hartmann 2001)

Frank & Roundy (2006)

ER waves

• TCs develop in region of cyclonic vorticity and enhanced convection (e.g., Frank & Roundy 2006; Molinari et al. 2007)

• May favorably influence vertical wind shear (Frank & Roundy 2006)

• Molinari et al. (2007) showed that a monsoon gyre was the first low in an ER wave packet

Kiladis et al. (2009) Molinari et al. (2007)

MRG waves

• MRG waves may turn to the northwest and develop TD-type structures (e.g., Liebmann & Hendon 1990; Takayabu & Nitta 1993)

• TCs develop in region of cyclonic vorticity and enhanced convection (e.g., Dickinson & Molinari 2002; Frank & Roundy 2006)

Kiladis et al. (2009)

Takayabu & Nitta (1993)

Dickinson & Molinari (2002)

Kelvin waves

• Less important than other waves for cyclogenesis (Frank & Roundy 2006)

• TCs may develop in association with cyclonic vorticity in observed Kelvin waves (Bessafi & Wheeler 2006)

Kiladis et al. (2009) Kiladis et al. (2009)

Primary Research Questions

• What percentage of western Pacific TC formations can be attributed to the MJO, equatorial waves, and TD-type disturbances?

• How do traditional and equatorial wave precursors interact to promote cyclogenesis?

Equatorial wave attributions• Spatially average filtered

anomalies around genesis location

• If this average exceeds a threshold, then attribute the genesis to that wave

• In this example, the MRG/TD band averaged over 5° × 5° produces a 30 mm day-1 anomaly

• How large of an area should anomalies be averaged over?

• What threshold should be used for attributions?

Sensitivity of attributions

• Percentage of storms attributed to each wave decreases as the threshold or averaging box increases

• MRG/TD and ER waves are consistently associated with the most TC formations

Composite TC

• All TCs 1998–2006, Eq – 25N

• Assumed to represent TC, but could indicate waves signal

• Anomalies averaged 5° × 5° around composite genesis:– MJO: 0.63– Kelvin: 0.44– ER: 1.59– MRG/TD: 2.54

Composite TC

• All TCs 1998–2006, Eq – 25N

• Assumed to represent TC, but could indicate waves signal

• Anomalies averaged 5° × 5° around composite genesis:– MJO: 1.89– Kelvin: 1.32– ER: 4.77– MRG/TD: 7.62

Selection of Threshold and Averaging Area

• Threshold: 7.62 mm day-1

– Maximum TC contribution to any filter band– Greater than 95% of filtered anomalies at all

grid points

• Box size: 5° × 5°– Smaller box would increase the TC

contribution– Larger box may overly smooth wave

anomalies

MRG/TD

57%

ER

34%

Kelvin

4%

MJO

2%

None

27%

• Hurricane symbols indicate genesis locations of TCs attributed to a given band

• Variance of filtered anomalies is contoured

May–November 1998–2006

Typhoon Lingling

• Example of genesis with an MRG wave

• Also attributable to ER wave, but this signal is less clear

• Other waves are inactive at genesis time

Typhoon Lingling

• Example of genesis with an MRG wave

• Also attributable to ER wave, but this signal is less clear

• Other waves are inactive at genesis time

Proposed improvements to equatorial wave attributions

• Separate MRG and TD-type disturbances using the idealized MRG dispersion relation

• Compare the TRMM results with OLR and other datasets• Reduce noise by filtering with Roundy and Schreck’s

(2009) extended empirical orthogonal function (EEOF) method

• Incorporate other influences on genesis, such as low-level vorticity, mid-level relative humidity, and vertical wind shear

• Examine how attributions vary in different parts of the basin or with ENSO

Primary Research Questions

• What percentage of western Pacific TC formations can be attributed to the MJO, equatorial waves, and TD-type disturbances?

• How do traditional and equatorial wave precursors interact to promote cyclogenesis?

Tropical Cyclogenesis Precursors

• Objective equatorial wave attributions – MJO– ER waves– Kelvin waves– MRG waves– TD-type disturbances

• Subjective monsoon attributions– Shear line– Confluence region– Monsoon gyres

• Fu et al. (2007)– Synoptic wave trains– Easterly waves– Tropical cyclone

energy dispersion

Issues to be addressed by comparing attributions

• How do MRG waves, TD-type disturbances, TC dispersion, and the monsoon trough interact to form northwestward propagating wave trains?

• How often are monsoon gyres and ER waves interrelated?

• How frequently does genesis result from equatorial waves growing within the monsoon confluence region via wave accumulation?

• What are the roles of the MJO and ER waves in modulating the monsoon trough?

• What fraction of TCs develop in situ with no wave precursor? Why do they form?