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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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| Notice the jet stream on the image on the left which roughly parallels the frontal cloud band over the west coast of the United States. Over Oregon the wind exceeds 100 kt. This product is excellent for support of long- distance flight missions. On the right 500 mb contours shows troughs to the west of two major frontal bands. One trough is associated with the cloud band moving on to the west coast. The other trough is associated with a system well off to sea. | |
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| The surface wind barbs on the left image show a definite wind shift around the frontal cloud band on the west coast. Within the cloud band winds are from the south. But behind it to the west the winds shift to more westerly. The surface pressure pattern on the right image shows a series of lows in the Pacific each associated with its own frontal cloud system. | |
Author: Tom Lee Last Updated: Tue Dec 10 16:41:20 2002 Produced by: The Composer (Ver: 1.1.2 ) |
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