American Meteorological Society Demo

FIG. 2. MIMIC-TC advection function for a TC with maximum sustained winds (Vmax) of 100 kt. For Vmax = 50 kt, the function is scaled by 0.5, and so on. FIG. 1a. Example of the MIMIC-TC product for Hurricane Katrina (2005) using three source images shown in Inset 1 from Fig 1. The animation loops from 0000 to 0745 UTC on 28 August. established eyewall at the scale of the microwave imagery are principally nonadvective. That is, radially symmetric eyewalls as viewed in the microwave imagery have no need for rotational operations, and asymmetric eyewalls are often asymmetric resulting from the effects of vertical wind shear. Vertical wind shear is a change in the environmental wind speed or direction with height, which can cause disorganization in the storm structure. During an episode of significant vertical shear, more intense convection normally occurs on the downshear side of the storm core (Corbosiero and Molinari 2002). This can appear as a quasi-static asymmetric eyewall signature in the low-resolution microwave imagery, even though in reality the individual convective cells rotate with the vortex flow. Second, the advection function outside the eyewall peaks at a distance of about 1.5° from the rotation center; this models the transitional zone in which inner spiral bands experience strong horizontal shear and converge toward the eyewall (Rozoff et al. 2006). Also, the advection function at this distance can capture the motion along a secondary eyewall (if present) while retaining the position of any identifiable asymmetries. Third, the tail of the advection function approximates the asymptotic decrease of wind with distance from the Holland profile. Given these parameters, we understand that this advection function is hardly optimized for every tropical cyclone, but 2 yr of experience show that this function is suitable for most storms, and it is especially well suited to cases in which the center of rotation is 1192 | AUGUST 2007 well established, which is normally in strong tropical storm strength TCs and higher (see information online at MIMIC’s Web site; http://cimss.ssec.wisc. edu/mimic.2 One principal drawback, of course, is the absence of a radial component of motion, which is more difficult to generalize and is discussed more in the following sections. The advantage of the rotating fade is that it applies differently at varying scales. At the scale of individual convective cells, it shows those cells to be moving cyclonically with the vortex wind. In the typical fashion for tropical cyclones, these regions of cells emerge from the same convergent areas and generally show a life span of about 2 h (confirmed by land-based radar observations). On the other hand, large regions of convection (>100 km), including the eyewall, outer spiral bands, and both organized and disorganized areas within the closed rotation of the cyclone, are mostly unperturbed by the advection scheme. These areas are subject to change only from the fading scheme, where one microwave image fades into another. There are two main drawbacks to this process. The first is the unavoidable case in which the raw microwave imagery does not update often enough, which results in one image fading unrealistically into the next, for example, a spiral band disappears from one place and reappears in another, or the eyewall changes the orientation of its asymmetry without apparent cause. Although this effect can be quite unnatural, the user can often compensate for the misrepresentation and interpret the underlying phenomenon correctly. Specifically, in the case of the disappearing 2 Readers are encouraged to visit the MIMIC real-time Web site listed above to view an archive of over 100 TCs from 2004 to the present, which include a wide variety of TCs over their entire life spans. This will give readers a more complete impression of the workings of this morphing technique beyond what we are able to present in the few examples in this paper. http://cimss.ssec.wisc.edu/mimic http://cimss.ssec.wisc.edu/mimic

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