Today’s Numerical Weather Prediction (NWP) models translate the physical laws by which the atmosphere operates into an interrelated web of mathematical equations. These equations describe the state of the atmosphere—everything from temperature, to humidity, to wind speed, to convection—at a given point in time for tens of millions of locations in the atmosphere. A limitation of this complexity is that the models require millions more observations of the state of the atmosphere than today’s direct observational systems—such as automated weather stations, aircraft reconnaissance, or weather balloons—can provide.
Satellite data from NASA’s Terra and Aqua satellites help fill in some of the needed information. The image above shows satellite-based estimates of wind speed, direction, and altitude over the Arctic on December 1, 2004, at 21:50 Coordinated Universal Time. Each arrow represents the wind's speed by the type of bars on the tail, the direction by the line between the tail and the tip, and the general altitude by color. The estimates were based on cloud and water vapor observations collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Aqua satellite.
The most noticeable pattern in the image is the whirlpool-like feature centered along 180 degrees longtitude (vertical line running down the length of the image) between 75 and 80 degrees North. Winds in the middle levels of the atmosphere (orange arrows) were swirling in a clockwise direction around the center. The highest speeds appear to be occurring in the southwestern quadrant: many of the arrows have three or four bars on the tail, which indicates wind speed of 30 or 40 knots. In the Northern Hemisphere, clockwise winds signal the presence of an area of high atmospheric pressure at the core.
By tracking the movement of clouds and water vapor in consecutive, overlapping satellite passes over the Earth’s poles, scientists at the Cooperative Institute for Meteorological Satellite Studies create estimates of polar winds that are improving the medium-range forecasts (5-10 days) generated by NWP centers around the world. To read more about how MODIS-based wind estimates are improving weather forecasts, see “Polar Wind Data Blow New Life into Forecasts.”