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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| 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. |
| These products can be used to double-check the "goodness" of the model, because at times the model is "slow" or "quick" with respect to the speed of particular weather system. At other times NOGAPS may miss the intensity of a system. Skilled users can infer these discrepancies from the overlayed images and adjust their forecasts accordingly. For example, a frontal cloud band on the satellite picture may lag behind (to the west) of an advancing trough based on the overlayed NOGAPS surface contours. A forecaster with responsibility for a region in the expected track of the front might rightly believe that all of the forecast frontal positions from NOGAPS are too quick. She/he might decide therefore to delay the expected onset of precipitation in the region. |
| The overlayed NOGAPS contours and barbs are based on the analyses and short-term forecasts from a very reliable global model. However, they should not be thought as "ground truth" or observations. Especially in unobserved regions like the tropics, the overlays may not depict important features accurately. Furthermore, the NOGAPS contours will not show mesoscale features near the surface of the earth. For example, satellite pictures sometimes show marine cloud plumes downwind of coastal canyons. These plumes are associated with strong low-level jets, or "gap winds" moving out over the ocean. The global NOGAPS model will not capture these highly-localized wind systems. |
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| On the left a 500-mb overlayed image shows a deep cyclone off the Japanese Coast. The associated frontal cloud band shows the marks the boundary between the shortwave trough and a downsteram shortwave ridge. On the right a 300 mb overlay image indicates a jet maximum (winds in excess of 100 kt) at the rear of the cold frontal band. | |
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| The IR image on the left shows a surface wind shift along the back edge of the cold frontal cloud band. Ahead of the cloud band winds are from the southwest through southeast. Behind the front they shift to westerly or northwesterly. Surface isobars overlayed on the image on the right suggests strong flow over Japan around an intense cyclonic low. | |
Author: Tom Lee Last Updated: Wed Dec 18 07:47:33 2002 Produced by: The Composer (Ver: 1.1.2 ) |
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