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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| Integrated Water Vapor Image | Track map of Kate for case example |
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| The integrated water vapor (IWV) algorithm shows variations of low-level water vapor over ocean areas. It gives important mesoscale information for regions that are otherwise unobserved. It can give important information about the low-level moisture environment that surrounds tropical cyclones.The IWV image above (left) shows an example of Kate duringApril 1999 in the Western Pacific. Kate will be discussed inthe examples below (track map to the right above). Integrated Water Vapor (IWV) over the open ocean is retrieved from data collected near the center of a weak water-vapor absorption line at 22 GHz (Alishouse et al., 1990). The IWV mainly corresponds to low level water vapor (i.e., 700 mb and below). The error is ~10% when compared to radiosonde measurements. This product is one of the most accurate and useful of the SSM/I products. It is generally considered to be as accurate as rawinsonde values of integrated vapor. Accordingly, it is assimilated into numerical models routinely, including the US Navy Operational Global Atmospheric Prediction System (NOGAPS). Alishouse, J., S. Snyder, J. Vongsathorn, and R. Ferraro, Determination of oceanic total precipitable water from the SSM/I, IEEE Trans. Geosci. Remote Sens., 28, 811-816, 1990. | |
| Link to Paul McCrone's Tutorial on SSM/I, AFWA/Metsat Applications:Tutorial |
| Low-level water vapor, a necessary but not singularly sufficientingredient for tropical cyclone development, is captured well by the IWVproduct. Low values indicate a poor regime for storms, and a storm movinginto such a region is likely to weaken. High values suggest thepossibility of sustained development or even intensification. Furthermore,the IWV product can be used together with Geostationary water vapor imagesfor a three-dimensional view of storm moisture: the IWV gives moisturevalues near the surface, and the Geostationary water vapor indicates thedistribution of moisture aloft. |
| There are several disadvantages associated with the IWV product. It is useful over open-ocean only, and is of limited value closer than ~50 nm (~80 km) to coastlines or islands. It cannot give information about water vapor at multiple levels in the atmosphere, andonly an integrated value is given. Since the marine boundary layer (approximately the lowest 1 km of the atmosphere) usually holds the bulk of the water vapor, the IWV product heavily represents vapor in that layer, and will often not show upper-level moisture gradients.For middle to upper-level moisture (at ~700 mb or above), Geostationary-satellite water vapor channels at 6.7 micron (infrared) give more usefulinformation. |
| High IWV favorable for intensification | Color Composite shows developing eye |
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| The image pair above shows Kate in an intensification phaseeast of the Phillipines. A clearly-defined storm eye in the colorcomposite image (right) is nearly commpletely enclosed. The IWVimage (left) shows a water vapor rich storm environment. Reds indicatevalues of 60 kg/squaremeter or greater. Such abundant moisture iscapable of feeding a developing storm, and it is therefore notsuprising that a subsequent image pair of Kate (just below) showscontinued intensification. | |
| Typhoon Kate moving into drier marine air mass | Convection in red; dry air to north in dark gray |
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| This image pair shows Kate near her peak intensity. The color compositeimage (right) shows a well-defined typhoon circulation with a enclosedeye. Light blue cyclonic bands indicate low clouds. The dark gray bandin the north of the image indicates a drier subtropical air mass. The IWVimage (left) shows a moisture distribution that is being shaped by thestorm circulation. On the east side of the storm, the southerly flow ispumping moisture (reds) to the north. To the west and north of the storm,northerly flow is advecting drier air (yellows, blues, violets, andblacks) into the storm environment. | |
| Dry air entering Kate on west side | Blue-green traces low-level cyclonic circulation |
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| The IWV image on the left shows low values on the west sideas dry air is pulled in from the north. The dry air is evenbeing pulled in on the southern flank of the storm (blue tongue). Highvalues of IWV (reds) continue to be advected into the storm by thesoutherly flow to the east of the system. The color compositeimage (right) shows the low-level circulation in blue-green. Onlya small area of convection (red) remains near the center of thestorm. The remainder of the convection has been sheared off bythe flow aloft and appears to the east. | |
| Storm center cutoff from moisture | Storm becomes extratropical |
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| This image pair shows Kate continuing to weaken and becomingextratropical. The IWV image on the left shows that the low-levelmoisture content of the air mass has been reduced considerably. Unlikethe previous IWV images in this series, red coloration (very high values)is nearly absent. The storm center (circular orange pool in west side ofimage) is now surrounded by relatively dry air (light blue) and cutofffrom the moist flow to the east (yellows). The color composite imageshows a low-level cloud circulation (blue-green). Deep convection (red)appears to the east but over the remains of the low-level center. | |
Author: 7541tl\@nrlmry.navy.mil Last Updated: Mon Nov 25 16:47:07 2002 Produced by: The Composer (Ver: 1.09 ) |
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