phil882 Posted August 23, 2012 Share Posted August 23, 2012 Hi guys. Give the upcoming importance of the interaction of Isaac with Hispaniola, I thought it would be appropriate to give a quick breakdown of the interaction that TCs often encounter with higher terrain. Let's first look at some literature. Starting with the oldest, Bender et al. (1987) discusses numerical modeling results of tropical cyclones interacting with high topography over specific islands (Hispaniola, Luzon, and Taiwan). In one case where a TC interacted with the landmass of Taiwan, the original surface reflection of the TC dissipated, while the mid-level circulation continued across the island, eventually coupling with the lee cyclone that formed in advanced of the surface cyclone. More recently, another study by Lan-Lin and Witcraft (2008) indicates that forward motion of a tropical cyclone is also critically important with these type of interactions. A slower TC allows for more time for surface air parcels that cross a mountain chain to then descend on the backside of the mountain, producing adiabatic warming and significant vortex stretching, which helps to develop a lee cyclone. The slow forward motion makes it difficult for the original surface TC to cross the mountain pass, so it re-develops on the other side in direct association with the lee cyclone. In this paper, they compaire two TCs that crossed Taiwan, Billis (2000) and Toraji (2001), showing that Billis was able to maintain a relatively consistent track and circulation due to its fast forward motion. Toraji on the other hand, redeveloped on the lee side of the mountain chain since it was too slow to cross Taiwan. Fortunately for us, we have two tropical cyclones that are about to have substancial island mountain interaction, TCs Tembin and Isaac are both about to undergo major mountain interaction over Island chains. We have already extensively talked about Isaac's land interaction, so lets look at Tembin first. Typhoon Tembin Figure 1. Typhoon Tembin makes landfall in Taiwan. Plotted are dynamic tropopause (color shaded) and low-level relative vorticity (black contours) From what the literature describes above, this is definitely a slow moving type island mountain interaction. There is plenty of time to allow the lee cyclone on the other side of Taiwan to develop sufficient depth that by the time Tembin makes landfall and quickly starts to fill, it quickly takes over the circulation of Tembin and it "jumps" across the island. This has already happened on several different occasions through TCs this year. You do see some sort of binary interaction occur with the two systems, but due to the small size of Tembin, it simply doesn't have time to have a major interaction before it is absorbed into the lee cyclone. Now lets look at Isaac. Tropical Storm Isaac Figure 2. Tropical Storm Isaac in the Caribbean Sea. Figure variables are the same as figure 1. With Isaac we have a few different characteristics. First, the storm is moving at a much faster clip than Tembin thanks to the amount of mid-level ridging in place. Secondly, the storm is also less organized, and the overall cyclonic vorticity is larger. Both of these factors will probably enable the low level circulation associated with Isaac to survive the passage of Hispaniola without a new lee cyclone replacing the old center. This is what the ECMWF was suggesting a few runs ago, but has backed off that solution and now looks rather similar to the GFS. Based off previous research, that seems like the safer bet at this time, without much deviation in the WNW track the system makes as it strikes Hispaniola. Link to comment Share on other sites More sharing options...
HM Posted August 23, 2012 Share Posted August 23, 2012 Phil great post. These concepts can also be applied during the cold season too when we starting talking mountain torques etc. The katabatic, vortex stretching allowing for lee-cyclogenesis also has a significant effect on angular momentum exchange as you perfectly implied in picture form with those DT/rel. vort. maps. Link to comment Share on other sites More sharing options...
phil882 Posted August 23, 2012 Author Share Posted August 23, 2012 Phil great post. These concepts can also be applied during the cold season too when we starting talking mountain torques etc. The katabatic, vortex stretching allowing for lee-cyclogenesis also has a significant effect on angular momentum exchange as you perfectly implied in picture form with those DT/rel. vort. maps. Exactly... this has much more application than just merely tropical cyclones. However, TCs do represent a more extreme lee cyclogenesis event thanks in part to the strong low level flow that is quickly forced over and then back descending on the lee side of a tall mountain chain. It is really fascinating to watch the cyclonic vorticty curl in like it does on the animations above. It does this because the mountains don't only produce cyclonic vorticity, but also anticyclonic vorticity on the windward side, which is then curled around the circulation of the TC. Thus, you end up with this banana like structure that aids in stretching the initial low level vortex that leads to dissipation in slow moving TC/terrain interaction cases. Link to comment Share on other sites More sharing options...
aslkahuna Posted August 23, 2012 Share Posted August 23, 2012 The development of the leeside low off Luzon was critical to our wind experiences with storm passing to the north of us as we would usually see our strongest winds occurring after the mid level circulation had coupled up with the surface low and the latter began to intensify. Also, our surface winds would often reflect the presence of the leeside system even as the original system was making landfall. Our winds would be light south to southwest with the system NE of us with the SW winds picking up strongly when the leeside began to crank. Steve Link to comment Share on other sites More sharing options...
phil882 Posted August 23, 2012 Author Share Posted August 23, 2012 The development of the leeside low off Luzon was critical to our wind experiences with storm passing to the north of us as we would usually see our strongest winds occurring after the mid level circulation had coupled up with the surface low and the latter began to intensify. Also, our surface winds would often reflect the presence of the leeside system even as the original system was making landfall. Our winds would be light south to southwest with the system NE of us with the SW winds picking up strongly when the leeside began to crank. Steve Hi Steve, Thanks for your input! The mesoscale flow that occurs with these types of events is really fascinating. I am very interested in how Tembin will look on radar imagery as it gets ready to interact with the mountainous terrain. Perhaps we will see some of these mesoscale effects as the system crosses the island. Link to comment Share on other sites More sharing options...
aslkahuna Posted August 23, 2012 Share Posted August 23, 2012 One of the typhoons chased to Guam by Jim Leonard in the early 1990's was a smallish storm that developed a low in the lee of GUAM-a 12X26 mile island. At the time he was wondering as to the peculiar circumstances of the storm and JTWC in its ATCR documented the development of the leeside low. Steve Link to comment Share on other sites More sharing options...
dan88 Posted August 24, 2012 Share Posted August 24, 2012 Seeing some interaction on the 850mb vort map on the lee side of Hispaniola already with Isaac Link to comment Share on other sites More sharing options...
Amped Posted August 25, 2012 Share Posted August 25, 2012 Isaacs center has jumped to the exact spot you'd expect a lee cyclone to form. Now it has slowed as if the spot were magnetic and has finally begun deepening after 3 days. This trend will end shortly however. Pretty good indication that topography played a role in the path. As usual great explanation and graphics by Phil Link to comment Share on other sites More sharing options...
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