External Influences on Cyclone Formation Working Group 2.1 W. M. Frank, G. J. Holland, P. Klotzbach, J. L. McBride, P. E. Roundy Contributions: J. Molinari.

Two Scales of Genesis Individual Storm Formation Climatological Variations e.g. – interannual variability, global change

Climatological Factors Warm ocean (SST of 26.5 C or greater) Persistent deep convection and upward motion Enhanced low-level cyclonic vorticity Weak vertical shear directly over the center Enhanced low-level moisture

Formation of Individual Storms Climatological conditions favorable for growth Mean flow may or may not be unstable (e.g. – ITCZ breakdown) Transient perturbations usually speed up the process

Recent Research Areas Individual Genesis Events Tropical waves Basin genesis parameters Synoptic-scale analyses Climatological Variations in Storm Numbers Interannual variability Climate change

Example Genesis Parameter – N. Atlantic DeMaria et al. (2001) Based on: vertical shear, convective instability, mid-level moisture

Waves and Genesis: 1. Tropical, zonally propagating waves provide local flow perturbations that can enhance cyclogenesis. 2. They do this by enhancing mean ascent, increasing low-level vorticity, /and/or changing the vertical shear. 3. As a result, many or most tropical cyclones form under the influence of wave circulations. 4. Since the waves can be forecast (with demonstrable skill), genesis can be forecast.

days 6 days 3 days Frequency (CPD) WestwardZonal WavenumberEastward MJO-ISOe TD-type MRG Kelvin ER Period (Days) Spectral Bands for Filtering *Adapted from Wheeler and Kiladis, 1999 h=50m h=12m

days 6 days 3 days Frequency (CPD) WestwardZonal WavenumberEastward MJO-ISOe TD-type MRG Kelvin ER Period (Days) Southern Hemisphere Spectrum h=50m h=12m

days 6 days 3 days Frequency (CPD) WestwardZonal WavenumberEastward MJO-ISOe TD-type MRG Kelvin ER Period (Days) Northern Hemisphere Spectrum h=50m h=12m

Storms Forming in Negative OLR Anomaly Region of Wave Filter Bands

Total Wind Fields, Westerly Phase of MJO, NWP (composite of 20 strong cases) Maloney and Hartman (2001)

Genesis and ER Waves in the South Indian Ocean Bessafi and Wheeler (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)

ISOe (MJO) – NE Pacific, lag = hPa Composite Analysis of Genesis, Frank and Roundy (2006)

ER – NE Pacific, lag = hPa

MRG/TD – NE Pacific, lag = hPa

General Results of the Composites: In each of the six basins most storms form in conjunction with a wave structure that strongly resembles the theoretical wave structure associated with that filter band. In each basin the storms form at almost exactly the same location within each type of composite wave. All of the composites show approximate flow reversal between the 850 hPa and 200 hPa levels, suggesting that the relevant wave structure has a baroclinic, wavenumber-one structure. All of the wave types seen in the composites are strongly asymmetric across the equator. Enhanced convection and low-level vorticity seem most important, anomalous vertical shear less so.

Implications for Forecasts Consistent phase relationship suggests waves are modulating genesis Anomalies can be detected in advance of genesis by at least: ISOe: days ER: 7-20 days ER: 7-20 days MRG/TD: 3-6 days Kelvin: 3-7 days

Locations of Data Used for an Atlantic Seasonal Forecast Klotzbach and Gray (2003)

Statistical forecast of genesis probability, 15-day lead time Paul Roundy -

Relationships Between Genesis and Basin-Scale Variations Several studies show apparent long-term trends in cyclone activity and/or intensity There are also long-term trends in some TC- related climatological variables (e.g. – SST) Relationships between large-scale/long-term variables and the occurrence of local, necessary conditions for genesis need to be explored.

Interannual Variability There are persistent patterns of correlation between annual storm numbers in different basins. These patterns are related to ENSO, and perhaps to other large-scale circulation modes. There is no discernable tendency for inter-basin compensation in terms of storm numbers or storm days. There is some suggestion that storm activity for Category 4-5 storms tends to be positively correlated between basins. Frank and Young (2006)

Summary, Storm-Scale Predicting genesis requires accurate synoptic- scale (mesoscale?) forecasts of the wind and moisture fields at the time and place of storm formation. Longer range genesis predictions are very difficult in the tropics using current deterministic models. There is strong evidence that tropical waves play a major role in triggering genesis in all cyclone basins.

Summary, cont. Tropical waves are predictable using statistical techniques. Other statistical genesis prediction techniques are also showing promise. Combined statistical/deterministic forecasts offer promise for forecasts of genesis with lead times of up to the time-scale of the MJO.

Summary, Longer-Term Genesis Variability Long-term variability of tropical cyclone formation depends on the long-term variability of the occurrence of local genesis conditions. The missing link is to determine how long-term variations in large-scale fields (SST, winds, etc.) are related to the frequency of occurrence of local necessary conditions for genesis.

Recommendations Experiment with statistical techniques for longer- range genesis forecasts. Improve understanding of the relationships between basin-scale variability and the incidence of genesis conditions.