NOGAPS 300mb Wind Overlay - Mediterranean Tutorial

Introduction

Satellite pictures provide a realistic depiction of cloud cover, texture, and movement. They can be used to estimate where precipitation might fall or where skies are clear. However, satellite pictures can not provide quantitative information such as temperature, pressure, winds, or geopotential height at a specific level. The combination of both satellite and model output produces a powerful tool for the synoptic meteorologist, the aviator, or the marine scientist. NOGAPS output and geostationary satellite data are two very different kind of inputs: NOGAPS is generally available every twelve hours, while geostationary images are available as often as every 15 min. Thus, traditional model overlay images have only been available every 12 hours at 00 and 12 UTC, matching the availability of the model output. However, for the products displayed here, the NOGAPS has been time-interpolated between these output times to match the valid time of each satellite picture. Thus, if the current picture is at 06 GMT, the model overlay field is derived by averaging the 00 UTC analysis and the 12 UTC forecast to arrive at a 06 UTC overlay field. We choose four fields to overlay here: 500 mb geopotential heights, 300 mb winds, surface pressure and surface winds. Surface pressure (mb) is the standard parameter for following weather-producing systems near the surface of the earth. 500 mb geopotential height (m above sea level) represents the mid- troposphere. 300 mb (kt) winds show the approximate levels of upper-level jet streams. This level shows winds often encountered during overseas flights. Surface winds (kt) give the winds expected for mariners at sea. Wind barbs are used to indicate wind speed and direction. An attached perpendicular line segment (flag) adds 10 kt; half a flag adds 5 kt; a pennant adds 50 kt. Summing the contributions yields the wind speed. The flagged end of each barb points in the direction from which the wind blows; the point end of each barb points in the direction toward which the wind blows.

Advantages

Limits

Examples

300 MB WINDS SHOW JET STREAMS	500 MB TROUGHS COINCIDE WITH FRONTAL CLOUD SYSTEMS
The image on the left shows two major jet streams. The polar jet streamis associated with a frontal cloud system off Spain and thencrosses the west coasts of Spain and France and into Scandanavia. Thesubtropical jet stream appears at about 30 N through Africa. It isassociated with little cloudiness. The image on the right is the 500 mbimage overlayed on the meteosat IR image. It shows three importantshort wave troughs each associated with cloud systems: one off Spain,the next over the eastern Meditteranean, and a third over the Persian Gulf.

SURFACE WINDS SWIRL AROUND FRONTAL SYSTEM OFF SPAIN	ISOBARS MARK INTERACTION OF ATMOSPHERIC FLOW WITH CLOUD SYSTEMS
The image on the left shows NOGAPS surface winds overlayed on theMeteosat IR picutre. The winds are generally stronger over oceans thanover the land. Winds spiral ina counterclockwise fashion into cyclonic cloud systems such as in thecloud system off Spain. The image on the right shows surface isobariccontours on the IR image. Notice that clouds are more common wherethe contours are tigher. This is because tight gradients in pressureare often associated with frontal systems which produce clouds.