Formation of a hurricane in a sheared environment John Molinari, David Vollaro, and Kristen L. Corbosiero Note: these figures should be examined along with a copy of the discussion of a paper in preparation of the same title. The text is in Word format in file jm2.doc. In this powerpoint file, not all figures from the paper are shown.

Figure 1. Post-processed (right) and real-time (left) track of Hurricane Danny (1997). The numbers in the left panel are advisory numbers. Note the “jump” in the center in one hour between advisories 4 and 5, followed by a cyclonic loop.

Figure 4. Radius-time plot of lightning ground flash density per unit time in Hurricane Danny (1997). Contours are logarithmic: 40, 80, 160, 320 flashes per 100 X 100 km box per day.

Figure 6. Azimuthal distribution of lightning with respect to vertical wind shear within 100 km of the center of Hurricane Danny. Contours increase by a factor of two beginning with 10 flashes. Small outbreaks occurred downshear on the 16th, followed by two large outbreaks on the 17th, after which lightning became much less frequent.

Figure 7. Aircraft reconnaissance winds and conventional surface winds (both yellow), wind speed contours (black; m s -1 ), infrared cloud image (light blue shading begins at -58°C), and lightning locations (blue dots) at 2200 UTC 16 July. The storm is at depression stage. This is a classic sheared storm, with convection and higher winds downshear (shear is from the northwest).

Figure 8a. As in Figure 7, but for 1200 UTC 17 July, during the major convective outbreak in the storm. Three vortices are present: the original tropical depression, visible in low cloud swirls to the west; a tropical storm that reformed downshear, shown in lightning, winds from oil rigs (black), and clouds; and an intense vortex shown by the cluster of hundreds of lightning flashes downshear from the tropical storm. It will be argued the latter cluster became the locus for hurricane formation.

Figure 8b. As in Fig 8a, but one hour later. The “X” shows the center location found by reconnaissance aircraft less than 3 hours after this image. It is argued that the small intense vortex represented by the lightning cluster (which we argue generates 1 X s -1 of vorticity in three hours) will become the dominant vortex over the following several hours.

t = 0 h t = 6 h t = 12 h Figure 9. Vorticity evolution in a barotropic simulation of two interacting vortices. From Enagonio and Montgomery, Time zero resembles the situation in the previous figure.

Figure 10a. As in Figure 8a, except at 1900 UTC 17 July 1997, 6 hours after Figure 8b. Similar to the idealized vortex interaction in Figure 9, vorticity to the west of the center appears to be drawn to the south, as indicated by lightning and high cloud to the south but no longer to the west of the evolving center.

Figure 10b. As before, but for 0100 UTC 18 July As in hour 12 of the idealized simulation, no evidence of vorticity is left to the west or southwest, and only a single narrow filament to the east, around a tight core.

Figure 11a. Radial scatterplot of  e during the depression phase of Hurricane Danny from Air Force reconnaissance. Values are taken every 30 s (roughly 2.5 km) and are all below 500 m of elevation.

Figure 11b. As in Fig 11a, but for early tropical storm stage.

Figure 11c. As in Fig 11a, but for late tropical storm stage.

Figure 11d. As in Fig 11a, but for early hurricane stage.

For interpretation of these results, see the accompanying discussion section.