Department of Meteorology

Forecast of 94L based on GFS operational data

Back to 2008 archive

The following is a description of the quantities plotted:

ZETA

Relative vertical vorticity. Unit: 10^-5 s^-1.

OW

Okubo-Weiss parameter, defined as relative vorticity squared minus strain rate squared:
OW = zeta² - S1² - S2² = (Vx-Uy)² - (Ux-Vy)² - (Vx+Uy)²

Positive OW highlights the region where curvature vorticity dominates shear vorticity. Note that both cyclonic and anticyclonic vorticity contribute to positive OW, but in the products, only the area with cyclonic vorticity is highlighted. Unit: 10^-9 s^-2.

TPW

Total precipitable water in the column from GFS. Unit: kg/m².

UT

Vector wind difference between 500hP and 850hPa. (In some previous storms/invests the deep shear between 850 hPa and 200 hPa was calculated, as indicated in the bottom of the figures). Unit: m/s

SF

Atmospheric column relative humidity from GFS, which is equivalent to Dave Raymond’s "Saturation Fraction" (the ratio of total precipitable water in the column to the total saturated precipitable water in the column). It usually has the same pattern/variations as TPW. Unit: %(percent).

Cp

Phase speed of the wave pouch (the translation speed). For simplicity, we assume that the wave propagates zonally at a constant phase speed. The phase speed is updated for each model run. Unit: m/s. [A generalization of this quanitity will be presented soon for oblique wave propagation.]

X

Vertical cross sections of meteorological variables within the pouch. Click here [PDF] for a detailed description of this product.

The streamlines in each figure are shown in a frame of reference which moves at the same (zonal) speed as the wave pouch, and the (zonal) translation speed is indicated at the top right label in each figure. The purple curve represents the local critical surface, where the zonal flow equals the phase speed of the wave pouch. The black curve is the wave trough axis, where the meridional flow is zero and the relative vorticity is cyclonic. The black circle indicates the center of the pouch.

The 5-day track of the pouch is shown in the top panel of the figure in the last column of the table.

If an invest eventually develops into a named storm, the location of the genesis (from NOAA's National Hurricane Center) is marked on the map as a red circle, and the location of the pouch at the actually genesis time is highlighted in red. The other four panels in the figure show the time evolution of several variables averaged in a 3-degree by 3-degree box following the pouch. Missing values (-999) indicate an open wave (the flow is open in the moving frame of reference).