Tropical Cyclone Climatology and Genesis

1. Tropical Cyclone Climatology and Genesis

2. Tropical Cyclone Definition • Tropical cyclone is a generic term to describe what we call tropical depressions (sustained winds <34 kt), tropical storms (34-63 kt), and hurricanes (>63 kt). • Tropical cyclones exhibit a closed cyclonic surface circulation and a warm temperature anomaly greatest at mid and upper levels of the troposphere. • Maximum winds are usually within the lowest 0-2 km of the surface, and the radius of maximum winds is typically within a 100 mi of the center, though there are exceptions. Deep convection typically sustained by a relatively warm sea surface helps to lower SLP and convert potential into kinetic energy.

3. Tropical Cyclone (TC) Features • Common basic structure: PBL inflow due to AAM loss at sea sfc (f + 2V/R)*δv/δp = -C/p*δT/δn)p – δFn/δP + δ/δP*(δVn/δt)  As tangential winds increase, radius of maximum winds decrease, and friction and acceleration of wind increases but shear doesn’t have increase much in response, and storm strengthens. Rainbands Eyewall or eyewall-type maximum Cirrus shield UL outflow

5. Tropical Cyclone Basins • Tropical cyclones are commonly observed in all basins except the Eastern South Pacific and South Atlantic. • Why is this?

6. Tropical Cyclone Necessary Conditions • Six parameters generally conducive for TC genesis. Necessary but not sufficient conditions: • Thermodynamic i) High SST: 26.5°C for at least 50 m, generally ii) High low-mid tropospheric RH static stability tied to (i) and (ii)

7. Tropical Cyclone Basins Necessary but not sufficient conditions (cont): • Kinematic iii) Planetary vorticity, f: 500 km + from equator iv) Low vertical wind shear: < 20 kt Hact = Hp/ven v) UL Div Other: vi) Pre-existing disturbance- rather large scale and needs some organization, such as LL vort

8. Limiting Factors and Some Characteristics of Various Basins WPac: very few limiting factors; most active basin; strongest storms and long-lived EPac: warm pool is not large; second most active North Atlantic: low-mid level dry air, high VWS; strongest storms rival those of WPac and likewise long-lived North Indian: very high VWS, small warm pool; few storms, mostly weak

9. Limiting Factors in Various Basins South Indian, W South Pac: VWS SE Pac, South Atlantic: cool oceans and high VWS; ITCZ and tropical disturbances mainly in N Hemisphere in these regions Southern Hemisphere westerlies occur at low latitudes. Recurve earlier and storms are found earlier in the respective summer.

10. Global TC Numbers Northern Hemisphere dominance. Larger warm pools, esp WPac and N Atlantic Westerlies displaced further north: less VWS ITCZ and disturbances tend to be in N.H. on average At least part of the above reasoning is probably due to the Himalayas & Kuroshio as well as Rockies & Gulf Stream creating a general wavenumber 2 pattern in N.H.

12. Tropical North Atlantic • Official hurricane season June – November • June & July: W Carib, GOM, and W Atlantic developments, weak TCs; TUTT and SAL • August & September: All over Atlantic; VWS low, along with less SAL and weakening TUTT • October: W half of Atlantic; very little SAL and weak AEWs and TUTT; monsoon trof in W Carib • Nov: Caribbean and W Atlantic; VWS high elsewhere and SSTs cooler elsewhere

19. Disturbances which can initiate TC genesis. • n=1 Rossby waves and mixed Rossby gravity waves (generally, these are the generic “tropical waves”) • Westerly wind burst associated with a Kelvin wave can help spin up disturbances in both Hemispheres (usually in Pacific, also Indian) • ITCZ disturbances and ITCZ rollup • Monsoon Trough • Confluence Zone & Monsoon Gyre (WPac)

20. Disturbances which can initiate TC genesis. • Mesoscale Convective Systems • Easterly flow over mountains may generate lee Rossby waves (EPac) • Subtropical storms (N Atlantic and WPac). • Stalled out extratropical lows or baroclinic zones (N Atlantic and WPac). • Occasionally, old stalled out upper level lows or TUTTs (N Atlantic and WPac).

21. African Easterly Waves • Emerge off western Africa, particularly early and mid hurricane season. • Dry Saharan Air Layer dominant early in the season and inhibits further development then. • If the SAL is not too strong, AEWS may utilize left exit region of TEJ and cyclonic vorticity and moisture south of AEJ; also barotropic instability possibly. • Possible development in EPac if not in Atlantic.

22. African Easterly Waves • Only about 10% develop into TCs. • Need necessary conditions mentioned earlier. Remember, they also need some sufficient organization and cyclonic vorticity. • SAL and AEJ can inject low-mid level dry air and high VWS into a disturbance very quickly.

23. African Easterly Waves • Scale: a couple thousands of km • Movement: generally w around 5-10 ms-1 • Periodicity ~ 3-5 days • Cold low level, warm upper level anomalies • Often emerge off African coast with two apparent cyclonic vorticity centers. The one on the north side is typically rather dry and shallow. • Inverted trof signature.

24. Tropical Waves • On average in North Atlantic, African Easterly Waves have a positive slope with respect to latitude and transfer westerly angular momentum polewards. • They also tend to tilt eastwards with height. • Convection is mostly found on the east side of the wave axis, particularly in the western half of the basin.

25. End of Material • You are now ready for Test #1!