Tropical Meteorology Lecture Summaries

1. Introductory Lecture (Wednesday, 29 August)

1. Homepage address http://www.essc.psu/ evans/T97
2. Syllabus and reference texts (see homepage)
3. Why should we care about tropical cyclones? (most destructive form of natural disasters; vulnerability of some nations, such as Bangladesh)
   • USA ``top 10 most deadly'' storms
   • USA ``top 10 most costly'' storms
   • USA and Australian coastline vulnerability
4. What is a tropical cyclone?
   • WMO definition
   • west-east cross-section of rotational wind (Hurricane Inez)
   • west-east cross-section of temperature (Hurricane Inez)
5. What factors influence tropical cyclogenesis and intensity?
   • Gray Seasonal Genesis Parameter (SGP)
   • Summarize into four factors
   • Controversy about the role of SST - dominance versus modulation
6. Where are cyclones and how variable is their occurrence?
   • Global coverage (Gray, 1975)
   • Basin-by-basin variability plot 1970-1989
   • Global total variability plot 1970-1989

2. Barotropic motion I:QUIESCENT ENVIRONMENTS (Friday, 31 August)

1. How reasonable is a barotropic approximation for TC motion forecasts?
2. Deep layer-mean (DLM) winds

   

3. The simplest initial state
   • Symmetric, balanced vortex
   • Barotropic, nondivergent system the nondivergent, barotropic vorticity equation as the governing equation for this system

     

   • No background flow
   • plane ( ), where y=latitude (also use )
4. Adem (1956) analytic solution - evolution of vortex asymmetries (known as gyres)
5. Comparison with Chan and Williams (1987) and Evans' (1990) nonlinear solutions
6. Thinking of the vortex as a Rossby wave packet

3. Barotropic motion II: SYMMETRIC AND ASYMMETRIC CONTRIBUTIONS (Wednesday, 3 September)

1. Review justification for the barotropic assumption (deep convection, vertical stacking, self-similar vortex, outflow broader and much weaker)
2. Review of nonlinear asymmetries evolving for vortex on quiescent plane
3. Consider the vortex system in terms of symmetric and asymmetric components ( , )
4. Now add a non-zero background flow
5. Do we see these types of asymmetries in the real data?
6. How can we think of the storm/ gyres/environment system? ,
7. Examples of barotropic dynamics and DLM flow used for tropical cyclone motion forecasts:
   • Chan, J. C.-L., and R. T. Williams, 1987: Analytical and numerical studies of the beta-effect in tropical cyclone motion. Part I: Zero mean flow. J. Atmos. Sci., 44, 1257-1265.
   • Evans, J. L., G. J. Holland and R. L. Elsberry, 1991: Interactions between a barotropic vortex and an idealized subtropical ridge. Part I: Vortex motion. J. Atmos. Sci., 48, 301-314.
   • Fiorino, M., and R. L. Elsberry, 1989: Some aspects of vortex structure related to tropical cyclone motion. J. Atmos. Sci., 46, 975-990.
   • Holland, G. J., 1983: Tropical cyclone motion: Environmental interaction plus a beta-effect. J. Atmos. Sci., 40, 328-342.
   • Kasahara, A., and G. W. Platzman, 1963: Interaction of a hurricane with the steering field and its affect upon the hurricane trajectory. Tellus, 15, 321-335.
   • Velden, C. S., and L. M. Leslie, 1991: The basic relationship between tropical cyclone intensity and the depth of the environmental steering layer in the Australian region. Wea. and Forc., 6, 244-253.

4. Barotropic motion III: ENVIRONMENTAL MODULATION (Friday, 5 September)

1. Hurricane Erika has just been upgraded in the North Atlantic. Let's see what's going on:
   • Latest tropical storm advisory - look at information included and note for class forecasts
   • HRD reconnaissance flight plans - what are they looking for?
   • Track to present time - where will Erika go? (everyone opted for persistence!)
   • More information: (a) 500hPa heights and surface winds; (b) 500hPa flow and relative vorticity
   • Given this information, what is your track forecast for the next 36h? (9 straight movers; 2 recurvers)
   • NHC hurricane strike probability chart: opting for straight mover here
2. Review contributions to motion:
   1. Environmental advection (cork in stream)
   2. gyres ( )
   3. “Environmental gyres” )
   4. Upper-level flow vertical shear of the environmental winds
   5. Other environmental factors: SST, decreasing height of tropopause, horizontal wind shear, conservation of absolute vorticity of the TC system
   The last two items moderate storm structure and hence motion. We'll discuss structure in the upcoming classes.
3. Idealized environments:
   1. Tropical cyclone south of ridge, recurving or not
   2. TC/TC interaction: the Fujiwhara effect
      • Centroid-relative tracks: approach, capture, rotation, escape
      • Point vortex theory has possibility of vortex merger for equal-strength vortices
      • In the atmosphere, no vortex merger unless have strong/weak pair
      • 3-vortex system, or equivalently, 2 vortices on a plane, is the minimum required for chaotic motion
   3. Examples of real storm tracks from the western North Pacific 1989-1991 for storms south of the subtropical ridge
   4. Observational analysis of the absolute vorticity gradient ( ) for the Pacific basin ( E, S N) - NH August and SH January (peaks of storm seasons)
   5. Map of implied tropical storm propagation resulting from this field - see that the propagation vector may go to zero in the region of the subtropical ridge
4. Real case study of multiple-vortex interaction: western North Pacific typhoons Owen and Nancy (11-16 August 1989)
   1. Actual and centroid-relative tracks
   2. 500hPa streamlines for (11-16 August 1989), examining storm-storm and storm-environment interactions in the context of the storms' motion

5. Baroclinic Modulation of Motion (Monday, 8 September)

1. VALIDATION: Did Erika recurve? Yes, but after 36h from our forecast, so we were successful with our gross forecast!
2. FORECAST: Where will Erika go next? All vote for continued recurvature into the ``hurricane graveyard''.
3. Introduce effects of vertical wind shear in the environment (dry)
   1. Vortex tilt due to differential advection in the vertical
   2. Vortex-vortex advection due to relative displacement in the vertical
   3. effect propagation
   4. Propagation of vortex due to tilt
   5. ``Environmental effect'' propagation from the sheared environment

6. Baroclinic Modulation of Motion (Wednesday, 10 September)

1. VALIDATION: Did Erika continue to recurve and weaken? Yes, so Bermuda started to worry and the Canadian Hurricane Centre started to watch.
2. Recap of last lecture
   1. How does storm tilt in westerly shear?
   2. Look at dry model runs. Consider effects of:
      1. Flow along isentropes no thermal advection
      2. Displacement of 2 PV centers in vertical induced vertical motion
      3. Relative placement (quadrature) of the vertical motion and theta anomaly maxima
   3. What is Potential Vorticity (PV)?
   4. PV changes due to convection
   5. CISK - Introduce ``Conditional Instability of the Second Kind''
   6. Response of moist processes to asymmetries, via intensity change

7. Objective TC Track Forecast Aids (Friday, 12 September)

1. What characteristics of the storm will make it sensitive to baroclinity?
   1. Size affects propagation and differential advection
   2. Strength also affects propagation and differential advection
2. Classes of Hurricane Track Forecast Guidance:
   1. Extrapolation/Persistence
   2. Climatology
   3. Analogues
   4. Dynamical (introduced)

8. Motion Objective Forecast Aids and their Biases (Monday, 15 September)

1. Recap last lecture: extrapolation, climatology and analogue techniques
2. Dynamical (numerical) model aids
3. Statistical
4. Hybrid
5. Biases of these
6. Forecast error statistics

9. 3D Tropical Storm Structure and Factors Affecting Structure (Wednesday, 17 September)

1. Observing network in the western Pacific - 500 km spacing
2. 5 parts of the TC: frictional inflow, core, transition, outer and upper anticyclone
3. General storm structure
4. Core features: sloping eyewall, wind-temperature balance, cyclostrophy
5. Rainbands: contracting eyewalls one mechanism for intensification
6. Outflow layer: anticyclonic, asymmetric (shallower and higher equatorward); 2+ outflow channels favors intensification
7. Coherence of the storm in the vertical

10. Storm Structure, Maintenance and Genesis (Friday, 19 September)

1. Differences between intensifying and non-developing storms
2. Maintenance of storm structure
3. Budgets of kinetic energy, dry and moist static energy - TC as net KE source, dry static energy source and moisture sink (as measured by fluxes through the 12 latitude circle
4. Amount of angular momentum import required for (i) growth; (ii) to counter poleward motion spin-down (through absolute vorticity conservation); (iii) overcoming frictional dissipation; and (iv) intensification/strengthening - see that these are listed in decreasing order of required angular momentum import (Merrill 1984)
5. ``Symptoms'' of an atmosphere favorable for tropical cyclogenesis (from Gray)
6. Discussion of the physical implications of these
7. Basin and global annual distribution of tropical cyclones
8. Global seasonality of tropical cyclones
9. Summary of Gray's tropical cyclogenesis symptoms in terms of physical requirements for (i) support of deep convection; (ii) dynamic-thermodynamic interactions as a mechanism for instability growth
10. Theories for tropical cyclogenesis:
    1. Barotropic instability of the monsoon/ITCZ - proposed by Wayne Schubert and colleagues using a shallow water model, supported by 3D modeling work of Houjun Wang, further support from climate analyses by myself and Tim Kempisty
    2. 2-stage process of Zehr (based on observations, largely from satellites)
    3. Support for Zehr's idea from Lisa Breigel's Masters results

11. Tropical Storm Maximum Intensity and Intensification I (Monday, 22 September)

1. The axisymmetric momentum and thermodynamic equations
2. Early studies on tropical cyclone observed intensity
3. Banner Miller's model for Maximum Potential Intensity

12. Tropical Storm Maximum Intensity and Intensification II (Wednesday, 24 September)

1. Summarizing Miller (1958)
2. A quick survey of Malkus and Riehl (1960)
3. Recent observational studies of intensity (Merrill 1987)
4. Set up Emanuel's (1986) Maximum Potential Intensity

13. Emanuel's (1986) Maximum Potential Intensity (Friday, 26 September)

1. Underlying assumptions
2. Implied structure of the tropical cyclone
3. Equation derivation and closure
4. Implications and recent improvements (incl. Holland 1997)

END OF TROPICAL CYCLONES

14. The General Circulation: Contrasting the Tropics and Midlatitudes (Monday, 29 September)

1. Differential radiation
2. Zonally-averaged temperature and zonal wind structures
3. Zonally-averaged Hadley circulations - historical progression
4. Meridional variation of the Hadley cell - observations
5. Hadley cell seasonal variation - observations

15. Thermal Structure of the Tropics (Wednesday, 1 October)

1. Vertical Profiles of , and
2. Comparison of , tropics and midlatitude
3. Vertical velocity magnitude comparison
4. Distribution of water vapor

16. The Axisymmetric Hadley Cell I (Held and Hou) (Friday, 3 October)

1. Geometry
2. Assumptions
3. Thermal Wind Balance

REF: Held, I. M., and A. Y. Hou, 1980: Nonlinear axially symmetric circulations in a nearly inviscid atmosphere. J. Atmos. Sci., 37, 515-533.

17. The Axisymmetric Hadley Cell II (Held and Hou) Plus Our Analyses (Monday, 6 October)

1. Deriving the Bounding Latitude
2. Sensitivity to Various Parameters
3. Comparison with Zonally-averaged Analyses

18. The Automated Tropical Cyclone Forecast System (ATCF) (Wednesday, 8 October)

1. Introduction
2. Using Remote Computers with Windowing
3. Familiarization with ATCF - TC Justin

19. Tropical Cyclogenesis: Visit by Dr Hugh Willoughby (Friday, 10 October)

20. 3D Structure of the Tropical Atmosphere: The ITCZ and the Walker Cells (Monday, 13 October)

1. MSLP maps from  N  S
2. Major weather systems around the Northern Hemisphere:
   • Pacific High
   • Aleutian Low (NH winter)
   • Bermuda High
   • Icelandic Low
   • Siberian High (NH winter)
   • East Asian Summer Monsoon (NH summer)
3. Similar, unnamed, features exist in the Southern Hemisphere
4. Seasonality in the tropics = monsoonal circulations
5. ITCZ located about from the equator in the summer hemisphere, except near continents
6. ITCZ stretches poleward to monsoons over summer continents
7. ITCZ and monsoons are surface low pressure centers, marked by deep, tropical convection

Reading on the monsoon to be completed by Wednesday

21. Webster's Elementary Monsoon (Wednesday, 15 October)

Builds up a picture of the monsoon as attributable to

1. Differential solar radiation
2. Land/sea temperature contrast and differing heat capacities
3. Coriolis effect
4. Surface evaporation
5. Moist processes latent heat release, leading to positive feedback on monsoon strength

22. Automated Tropical Cyclone Forecast System (ATCF) and Project One Forecasts (Friday, 17 October)

23. The Global Monsoons (Monday, 20 October)

• Cancelled.

24. Structure of the Asian Summer Monsoon (Wednesday, 22 October)

1. Geography of the Asian subcontinent and the Pacific nations
2. Characteristics of the Asian Summer Monsoon
3. Southeast to northwest progression of onset
4. Latitudinal structure of the Asian Summer Monsoon (south to north): (i) Australian high; (ii) cross-equatorial flow turning into the (iii) southwest monsoon winds; which flow into the (iv) monsoon trough/ITCZ. Trade wind flow on the northern extreme of the ITCZ wraps into the (v) western Pacific high. Further north is the (vi) Mei-Yu front (Baiu over Japan), an airmass boundary between the tropics and higher latitudes of the Northern Hemisphere.
5. Rôles/effects of the Tibetan plateau: (i) Thermal effect of an elevated heat source near 500 hPa; (ii) Deep ( km), well-mixed boundary layer in western regions of plateau; (iii) Observed rainfall gradient, with very dry ( mm in July) conditions to the north and west, tending to wet monsoon ( mm in July) over southeastern Tibet; (iv) Orography plays a rôle in enhancing upslope convection, but is deemed somewhat less important than the direct effect of the elevated heat source (see (i)); (v) Weather is modulated by the presence of the plateau, with early summer ``edge'' cyclones to the north and mid-summer ``Tibetan vortices'' that can bring heavy rain events to countries further east.
6. Tropical cyclogenesis occurs predominantly in the region of the monsoon trough over the oceans.
7. The Indian monsoon, and the crops that depend on it, have been shown to link strongly to El Niño.

25. Beginning of Project 2 talks (Friday, 24 October)

26. New day for Project 1 Forecasting Practicum (Monday, 27 October)

27. Eliassen's balanced vortex: Tropical cyclones and the Hadley cell (Wednesday, 29 October; Ms Sytske Drury)

1. Motivation and underlying assumptions: (i) axisymmetry; (ii) use of cylindrical and log-p coordinates to simplify the algebra; (iii) hydrostatic; (iv) first order gradient balance; (v) forcing of tangential motion and buoyancy through imposition of either a momentum source ( ) or a heat source ( ).
2. Derivation of the quasi-balanced equations
3. Importance of static stability ( ), baroclinicity (S (2v/r + f), where is the shear term) and inertial stability ( ).

28. Eliassen's Balanced Vortex II Plus Project 2 Talks (Friday, 31 October)

1. Review of terms in the predictive equation
2. Review of potential forcings
3. Consequences of a momentum source at varying locations in the vortex
4. Consequences of a heat source at varying locations in the vortex
5. Link to the general circulation

Also continuing with Project 2 talks.

29. The Asian Winter/Australian Summer Monsoon and the Madden-Julian Oscillation (MJO) (Monday, 3 November)

1. Geographic structure of the Asian Winter Monsoon: importance of the Australian Summer Monsoon!
2. Tropical cyclogenesis in the Australian region
3. Variability of the Australian Summer Monsoon: (i) modulation by the Madden-Julian Oscillation (MJO) and ENSO
4. The Madden-Julian Oscillation (MJO)

30. The El Niño-Southern Oscillation (ENSO) (Wednesday, 5 November)

1. What is the El Niño?
2. Other names for it.
3. What is the Southern Oscillation?
4. Structures of El Niño and the Southern Oscillation
5. Why ``ENSO''?

31. The El Niño-Southern Oscillation (ENSO) - Visit with Dr Kevin Trenberth (Thursday, 6 November)

• Special class to discuss effects and implications of ENSO globally

32. The El Niño-Southern Oscillation (ENSO) II Plus Project 2 Talks (Friday, 7 November) - THIS MAY CHANGE

1. Long-term history of ENSO
2. Implications of ENSO in the local climates of the tropics
3. Far-field effects of ENSO: the USA

33. Class Midterm Exam (Monday, 10 November)

34. Equatorial Wave Theory (Wednesday, 12 November)

1. The equatorial plane approximation
2. Shallow water equations, linearized about a state of no motion
3. Equatorially-trapped Kelvin waves: v=0
4. Mathematical solution for horizontal structure of Kelvin waves

35. Forecasting Challenges for Tropical Cyclones - Visit with Dr Lixion Avila (Thursday, 13 November)

• Special class to discuss current forecast challenges facing hurricane forecasters and research goals to meet these challenges.
• You have all been forecasting storms for a couple of months now, so should all have something to contribute to this discussion!

36. Dr Young's Hurricane Game Plus Project 2 Talks (Friday, 14 November)

• Continuing with Project 2 talks
• Dr Young's Hurricane Game is designed to test your awareness of the public safety and property protection aspects of hurricane track forecasting. We will forecast the motion of an Atlantic tropical storm, including issuing watches and warnings on the storm.

37. Forecasting Practicum (Monday, 17 November)

• This is the last forecasting practicum we will do. Completed forecast logs are due to me for final grading by Thursday, 20 November - i.e. later this week.

38. Equatorial Wave Theory II (Wednesday, 12 November)

1. Complete the mathematical solution for horizontal Kelvin wave structure
2. Schematic of a Kelvin wave
3. Scalings for Equatorially-trapped Kelvin waves: Timing of El Niño
4. Vertical structure of Kelvin waves

39. Equatorial Wave Theory III Plus Last Project 2 Talks (Friday, 21 November)

• Final Project 2 talks
  1. Coastally-trapped Kelvin waves - motion out of the tropics
  2. Rossby and mixed Rossby-gravity waves in the tropics
  3. Schematic of a mixed Rossby-gravity wave

40. The Madden-Julian Oscillation (Monday, 24 November)

• Term papers 2 and 3 due in class
  1. Observed structure of the Madden-Julian Oscillation
  2. Relationship to broader scales

Class Cancelled (By University) for Thanksgiving (Wednesday, 26 November)

• No class today

Class Cancelled (By University) for Thanksgiving (Friday, 28 November)

• No class today

41. Review of Projects 3 (Monday, 1 December)

• Review of Projects 3: All class members are expected to have read everyone's project 3 and to come ready to offer constructive criticisms of each other's work. You will each hand to me a grade sheet for the class. I will not give this information out to anyone.
• My aim here is to have you all understand the process of evaluation and what makes a successful project. If these projects are done well, you should all walk away with an improved understanding of the tropical climate and of the variations of the climate over one season.

42. Tropical Convection (Wednesday, 3 December)

• Final versions of Project 3 due today
  1. Timing of tropical convection over land, islands and water
  2. Effects of tropical convection on the atmosphere

43. Last Class!! (Friday, 5 December)

• This is our last class for the semester. Content will be based on needs.

• About this document ...