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Hurricane Intensity:
Hurricane Opal of 1995 illustrates the limited progress in intensity forecasting. Opal intensified rapidly overnight as it accelerated toward the United States Gulf Coast ( Figure h3 ). It would have been a repeat of Hurricane Camille's devastating landfall if it had not weakened equally abruptly. Neither the intensification nor the weakening were forecast with enough lead time to permit appropriate response. A simple air-sea interaction model developed at Massachusetts Institute of Technology (Emanuel, K. A., 1999: Nature, 401, 665-669) appears to demonstrate a dominant role for oceanic forcing relative to atmospheric surroundings or the cyclones' internal dynamics. As impressive as this result is, it is accurate only to about one Saffir-Simpson category. A possible improvement is replacement of the climatological representation of upper ocean structure with that observed from satellite altimetry. Detailed observations of ocean response from aircraft, combined with buoy and dropwindsonde observations of the hurricane's atmospheric boundary layer, also show promise.

Shear of the environmental wind is known to have a significant role in day-to-day intensity changes of individual storms, and it appears to be the mechanism by which atmospheric teleconnections modulate Atlantic hurricane activity. Airborne Doppler and reflectivity radar show that an environmental shear >10 m s-1 imposes a wavenumber one structure on the eyewall convection (Figure h4 ). Individual cells form 45 degto the right of the down-shear direction, reach maturity with reflectivities >45 dBZ on the left side of the shear vector, and have largely rained out by the time they detach from the eyewall as they advect back to the right side of the shear. Observations of chemical tracers offer an opportunity to validate meteorological theories of hurricane development. Already, these insights have led to a reevaluation of hurricane eye thermodynamics in which air has a long residence time inside the eye, in contrast with the rapid recycling through the eye postulated earlier.

The boundary layer is the place where hurricanes impact people and property. Hurricane surface winds are the main emphasis of the Hurricanes at Landfall (HaL) focus of the United States Weather Research Project (USWRP). Since the mid-1980s, HRD has provided forecasters with quasi-operational analyses that are based on data from reconnaissance aircraft and surface anemometers ( Figure h5 ). Recent addition of satellite cloud-drift winds and surface winds deduced from both spaceborne and airborne remote sensing extends the spatial domain and accuracy of this product. It is used routinely as guidance for watches and warnings and is proving useful for early evaluation of insurance losses and impacts on infrastructure during landfall.

The GPS-based dropwindsonde developed for synoptic surveillance are superb boundary layer probes ( Figure h6 ) because they report independent wind and thermodynamic observations every 5 m as they fall to the surface. In the convective regions of hurricanes, GPS sondes have revealed previously unsuspected low-level jets at 100 to 300 m altitude with winds 20-40% stronger than those at either 3 km or the surface. Another operationally important property of hurricanes is the onset of gale-force winds (17 ms-1) after which preparations for landfall generally must cease. The surface wind analyses provide forecasts and validation of this key parameter, particularly so since their augmentation with remote sensing and dropwindsondes (Figure h7). Dedicated aircraft missions with detailed radar observations in hurricanes as they pass onshore, data from land-based intercept teams from universities such as Texas Tech, Clemson, and the University of Oklahoma, and careful post-storm damage assessments will produce increasingly refined quantitative models of hurricane wind impacts. Evacuation in response to effective and timely warnings have reduced deaths from storm surge to the point that inland flooding is the primary cause of U.S. mortality, as the somber experience in Hurricane Floyd of 1999 dramatized.