Cyclone Phase Analysis and Forecast: Help Page

The cyclone phase analysis page presents historical, analyzed (current), and model-forecast cyclone phase diagrams for northwestern hemisphere cyclones with the goals of improved cyclone phase forecasting and providing measures of phase predictability. The main intended goals of this page are increased forecast accuracy of tropical cyclone development (in particular from extratropical or subtropical cyclones), and the development of warm-core or hybrid structure within extratropical cyclones.

[ Back to main page | Cyclone Parameters | Phase Diagram Construction | Example phase diagram | Frequently Asked Questions | References ]

This help page will aid explanation of the phase diagrams shown.

A cyclone is defined here as:

For each model analysis and forecast time, every cyclone is labeled on the MLSP analysis with an 'L' (see example on main page).

For each cyclone analyzed or forecast, two phase diagrams of its evolution are produced. A description of each phase diagram follows.

  1. Cyclone Parameters:

    Each cylone is quantified using three fundamental measures of its phase:

    1. "B" - The storm-motion-relative 900-600hPa thickness gradient across the cyclone.

      where h = hemisphere (1 = NH, -1 = SH).

      DeltaZR = mean 900-600hPa thickness in semicircle right of motion

      DeltaZL = mean 900-600hPa thickness in semicircle left of motion

      The mean thicknesses are evaluated in semicircles of radius 500km.

      This parameter measures the strength of the frontal nature of the cyclone. Three examples of varying magnitudes of B are shown below.

      B = 0 The example to the left illustrates a near-zero value of B. The magnitude of B is calculated as the difference in thickness (shaded) between the semicircle right of motion from that left of motion. A value near zero for B indicates a nonfrontal cyclone. The example to the left is a schematic for a conventional tropical cyclone, which has a maximum of thickness in the center of the cyclone that decreases almost uniformly outward in all directions.




      B = 0 The example to the left also illustrates a near-zero value of B. This schematic example is for an occluded extratropical cyclone.





      B >> 0 The example to the left illustrates a positive value of B. The semicircle right of motion has a substantially larger thickness (warmer) than that left of motion. Thus, this schematic represents a frontal cyclone with strong temperature gradients perpendicular to the storm motion. This schematic is for a conventional intensifying or mature extratropical cyclone.





    2. "-VTL" - The magnitude of the cyclone lower troposphere thermal wind. This is measured as the vertical profile of cyclone geopotential height gradient (cyclone strength) between 900 and 600hPa.

      This second parameter measures the fundamental cold, neutral, or warm core structure of the cyclone in the lower-middle troposphere.

      Thermal wind dictates that cyclone phase is related to cyclone strength profile:

      • A warm core cyclone has a profile of cyclone strength that decreases with height.
      • A cold core cyclone has a profile of cyclone strength that increases with height.

      • Cyclone strength is measured as the height perturbation, phi'.

      • phi' = phiMAX - phiMIN

      • This difference approximates the height gradient on an isobaric surface and the magnitude of the geostrophic wind (Vg).

      • phi' = df|Vg|/g

      • where d = distance between extrema, g = gravity, and f = coriolis parameter.



      The cyclone's full 3-D structure is given by the vertical derivative of phi', which is the thermal wind (VT):

    3. "-VTU" - This is the same as (2), except is calculated for the middle/upper troposphere -- between 600 and 300hPa. This gives the measure of cyclone phase (cold vs warm core) for the upper troposphere. This third measures helps to distinguish full-troposphere warm-core cyclones (e.g. tropical cyclones) from shallow warm core cyclones (warm-seclusions or subtropical cyclones).

      Note that although cold-core cyclones typically tilt (westward) with height, thermal wind is still calculated in a vertical column and the calculations here are still valid for tilting cyclones.

    These three parameters, B, -VTL, and -VTU represent each of three dimensions of the experimental cyclone phase space. In reality, there are many more dimensions of cyclone phase (larger scale flow, stratospheric interaction, surface fluxes, etc); however, the three chosen here represent the majority of the variability among known synoptic-scale cyclones.

    Since a 3-D cube of cyclone phase space is difficult to visualize and interpret in real-time, the cyclone phase is plotted using two cross sections through the cube:

    • B vs. -VTL
    • -VTL vs. -VTU



2. Phase Diagram #1 Construction: B Vs. -VT






3. Phase Diagram #2 Construction: -VTL Vs. -VTU





4. Actual phase diagrams

Additional information provided on the actual diagrams include:

Phase Diagram #1 Example:

Phase Diagram #2 Example:

Interpretation of these example diagrams:

4. Frequently Asked Questions

5. Relevant/helpful references

Hart, R.E., 2003: A cyclone phase space derived from thermal wind and thermal asymmetry. Mon. Wea. Rev., 131, 585-616.

Evans, J.L. and R. Hart, 2003: Objective indicators of the extratropical transition lifecycle of Atlantic tropical cyclones. Mon. Wea. Rev., 131, 909-925.

Beven, J.L. II, 1997: A Study of Three "Hybrid" Storms. Proc. 22nd Conf. on Hurricanes and Tropical Meteorology, Fort Collins, CO. Amer. Meteor. Soc., 645-646.

Miner, T., P. J. Sousounis, J. Wallman, and G. Mann, 2000: Hurricane Huron. Bulletin of the American Meteorological Society. 81, 223-236.

Last Updated: 18 February 2003