By the remote sensing of clouds, the atmospheric winds at several pressures levels can be derived. These atmospheric winds are the steering mechanisms by which tropical cyclones propagate. These derived winds can also yield other important information about the atmospheric winds, namely upper-level divergence and wind shear, that are crucial to the development and maintenance of a hurricane.
Numerical weather prediction models are the best way in which to forecast
hurricane movements. Numerical weather prediction models are entirely
dependent on observations at many different locations (grid points) and
at different pressure levels. However, over the tropical oceans observations
are very scarce, except for some scattered aircraft and ship reports.
In order to make more accurate forecasts the "hurricane hunters"
use dropwindsondes to retrieve vertical profiles from within the hurricane
and its surrounding environment to be assimilated into the forecast models.
By adding satellite-derived winds to the hurricane models, a tremendous
number of observations can be added to the model's initial conditions.
With more accurate initial conditions the model's accuracy will increase
tremendously.
Hurricane Motion
Hurricanes are steered by the trade winds in the
tropics, which means that hurricanes generally move from east to west.
Their movement is affected by many different factors, such as the large-scale
flow pattern, subtropical highs located in the centers of the Atlantic
and Pacific, their intensity, and as they move farther north the mid-latitude
westerlies. For an accurate hurricane track forecast all of these
variables must be taken into account. Remote sensing is the primary
instrument to measure these factors (Chaston 1996).
Low Level Winds
Low level winds are the winds from the mid-troposphere to the surface. They are the primary steering currents by which tropical cyclones move.
Upper Level Winds
Upper level winds are the winds from the mid-troposphere
to the stratosphere. They important because they can affect the development
of the storm. Hurricanes require light upper level winds into order
to maintain their outflow structure. Below is an upper level wind
map for Hurricane Mitch. Notice the anti-cyclonic structure to the
winds around the storm. This is crucial for hurricane development
because it allows the winds from below to diverge out from the hurricane's
interior.
Upper Level Divergence
Upper level divergence is another crucial ingredient to maintain a strengthening hurricane. As air converges at the surface it is forced upward to maintain continuity. As this air reaches the upper-troposphere it is blocked from going into the stratosphere so it forced to diverge between the 300hPa and 150 hPa pressure levels. Shown below is an upper level divergence map for Hurricane Mitch. Around the hurricane one can see positive divergence meaning that the storm has sufficient outflow to maintain itself.
A schematic of a hurricane's structure is shown below.
Source
Wind Shear
Wind shear is another important part of determining the intensity of a tropical cyclone. Wind shear is the difference between upper level winds and lower level winds. In regions of significant wind shear hurricanes can not properly maintain their fragile structure. In the figure below the brown streamlines show the direction of the shear and the yellow contours show the magnitude of the shear.
All satellite-derived winds images on this page can be found at: CIMSS Western North Atlantic Satellite Derived Winds
General
Information on Derivation and Image Displays of GOES-8/9 High Density Winds