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.
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.
Each cylone is quantified using three fundamental measures of its phase:
where h = hemisphere (1 = NH, -1 = SH).
DeltaZ_{R} = mean 900-600hPa thickness in semicircle right of motion
DeltaZ_{L} = 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.
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:
The cyclone's full 3-D structure is given by the vertical derivative of phi', which is the thermal wind (V_{T}):
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, -V_{T}^{L}, and -V_{T}^{U} 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:
Additional information provided on the actual diagrams include:
Phase Diagram #1 Example:
Phase Diagram #2 Example:
Interpretation of these example diagrams:
Understanding the phase of cyclones gives a broader, yet insightful, perspective on the overwhelming distribution of all cyclones. Cyclones aren't simply tropical or extratropical; there is a great continuum of cyclone types, with a significant fraction of them having characteristics of both tropical and extratropical cyclones.
The phase of a cyclone (warm vs cold-core, in particular) is related to its intensity, size, forecast uncertainty, and ultimately the threat it poses to us. Generally, a warm core cyclone has more forecast uncertainty associated with it than a cold-core cyclone. For example, the development and intensification of tropical cyclones is one of the most difficult forecasts to make. The diagnostics shown here give an indiciation of whether a cyclone that is developing within the models is a pure warm-core development, or hybrid development.
Forecasting the phase conversion or transition of cyclones (e.g. tropical into extratropical and subtropical into tropical) is a difficult but important task, and the phase diagnoses provided here provide considerable insight into the essence of model cyclone development. In concert with real-time surface, ship, and satellite observations, a more accurate diagnosis and forecast of cyclone evolution is possible when the phase diagrams are used.
During the 2001 hurricane season, the phase diagnoses provided here accurately forecast the conversion of two subtropical cyclones into Hurricanes Karen and Olga. The extratropical transition of Hurricanes Gabrielle and Michelle were both well-forecast by the phase diagrams as well.
These diagrams also provide for forecast track and intensity comparison.
NCEP AVN, Eta and NGM; U.S. Navy NOGAPS; U.S. Air Force MM5; Canadian Meteorological Center Global CMC; UK Met Office UKMET. The ECMWF model data available to us is (unfortunately) far too coarse to allow reliable phase analysis.
A cyclone will not be analyzed on the phase diagrams if: 1) the cyclone is currently too close to the edge of the model domain, 2) the cyclone does not last for 24h, or 3) the cyclone has a minimum SLP > 1018mb.
The black ones are existing cyclones, the red ones are forecast cyclones that have not yet developed.
Either the cyclone has just formed, has not yet formed, or the database has a large gap in it from missing data that has prevented continuous tracking of the cyclone from one model analysis to the next.
The detection of cyclones and tracking them is a very complex and difficult process. Cyclones may form downstream from an already existing cyclone. In this case, it can become difficult to determine where the path of one cyclone ends and another begins. As a result, the track and phase evolution of a cyclone can appear to jump, if another, stronger, cyclone forms relatively close to an existing cyclone. Also, if a cyclone moves extremely quickly (> 30 ms^{-1}), the tracking algorithm may interpret the new cyclone position as the formation of a new cyclone. This is most likely in the NOGAPS, UKMET, and CMC analyses which are available only every 12h -- and, consequently, large changes in cyclone position can result.
Thumbnail versions of the phase diagrams for a given cyclone are compared, allowing the user to compare the current and forecast phase evolution for a single cyclone among several models. Note that not all models will necessary resolve the same cyclones, especially in the tropics. The consensus forecast (unweighted mean of all available models) for the given cyclone is also presented. The 1 standard deviation variability from this mean is given as the shading either side of the mean phase analysis.
The two parameters used to describe the cyclones (B, -V_{T}) are calculated over the pressure ranges of 900-600hPa. If high terrain enters this pressure range, then the field values for thickness and geopotential height are contaminated by interpolation below ground. As a result, the diagnostics are not representative of the atmosphere or the cyclone's true phase, and it will not be analyzed.
Existing computer resources do not allow us to examine the entire globe.
The phase analyses shown here are derived completely from model analyses and forecasts. They will not and cannot represent the complete or true structure of the observed cyclone. The phase diagrams are only as accurate as the model analyses and forecasts from which they are derived. As model analyses become increasingly more accurate, so will the phase diagrams.
5. Relevant/helpful references
Last Updated: 18 February 2003