General Information
The winds are derived using a sequence of three images. Features targeted in the first
image (cirrus cloud edges, gradients in water vapor, small cumulus clouds, etc.) are tracked
within the second and third images yielding two displacement vectors. These vectors
are averaged to give the final wind vector which is displayed. Vector heights are assigned in a two-step
process. The first utilizes the measured radiances of the target and is based on
the spectral response function of the individual satellite and channel being sampled.
The brightness temperature of the target is derived from this radiance measurement. Once
determined, the brightness temperature is compared with a collocated numerical model guess
temperature profile, from which an initial height is estimated. The final vector height is derived
in the post-processing of the vector field.
CIMSS runs the raw winds through a process called autoediting,
a two-stage, three-dimensional objective analysis of the wind field
(Hayden and Pursor, 1995).
This scheme utilizes conventional data assimilation,
neighboring wind "buddy" checks, and numerical model analyses for
wind vector editing and height adjustments. For more detailed information on
these processes, see Velden et al. (1997).
The satellite-derived wind products displayed within the CIMSS Tropical Cyclone page
are a result of McIDAS-based software developed at CIMSS over the past 10+ years.
The products are created by automated algorithms utilizing a combination of
UNIX shell scripts and McIDAS-X mcenv process sessions.
Wind Product Information
The satellite-derived wind products are created and displayed on the home page
for various regions of the globe. Winds are grouped into specified layers within each
product for display purposes.
Layers are given in pressure unit MILLIBARS. Wind speeds are given in KNOTS, with speed
and direction indicated by the individual wind barbs (explained below). 1 knot
(nautical mile/hour) is approximately 1.15 miles/hour or 0.51 meters/second.
|
|
As described above, the multispectral satellite derived winds are assigned a level (mb) height. However, for display on the CIMSS Tropical Cyclone Page, the winds are grouped into the layers shown below to allow for easy identification of atmospheric feature heights (e.g. hurricane outflow, jet streams, and upper level circulation centers).
150-250mb
251-350mb
351-600mb
400-599mb
600-799mb
800-999mb
600-800mb
800-950mb
Wind vectors are labeled using "wind barbs". Direction is given by the orientation of the post, with speed given by the barbs/flags at the end of the post. The head of the post is the location at which the wind was determined. Speeds are indicated using three symbols : a "flag" (triangle) indicating units of 50, a "full-barb" (long line) indicating units of 10, and a "half-barb" (shorter line) indicating units of 5.
 |
5 knot wind from the northwest. One half-barb. |
 |
10 knot wind from the southwest. One barb. |
 |
30 knot wind from the northeast. Three barbs. |
 |
35 knot wind from the southeast. Three barbs and one half-barb. |
 |
50 knot wind from the northwest. One flag. |
 |
75 knot wind from the south-southwest. One flag, two barbs, and one half-barb. |
 |
110 knot wind from the southwest. Two flags and one barb. |
Wind Analyses Information
Analyses are calculated utilizing gridded output from the autoediting process
of the wind fields. The wind data are fit to a one degree grid at the following heights
(150, 200, 250, 300, 350, 400, 500, 600, 700, 775, 850, and 925mb), with data
sparse regions at each height filled with numerical model output).
Divergence within the layer 150-300mb. Positive values are given by solid lines, with negative values (convergence) plotted by dashed lines.
These analyses depict vertical wind shear as calculated by subtracting the low-level layer-averaged flow (925-700mb) from the upper-level layer-averaged flow (150-300mb). The analyses used to create the layer mean flow fields are as descibed above. The brown streamline contours indicate the direction of the shear. The yellow contours show the vector magnitude of the shear (absolute value).
In the case when a tropical cyclone is present, the analyses are modified in the storm region by a procedure which removes the storm circulation at prescribed radii. This allows a more representative depiction of the environmental shear acting on the storm.