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Tropical Cyclogenesis in Wind Shear: Climatological Relationships and Physical Processes

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Tropical Cyclogenesis in Wind Shear: Climatological Relationships and Physical Processes Tropical Cyclogenesis in Wind Shear: Climatological Relationships and Physical Processes David S. Nolan and Michael G. McGauley Intro What is the purpose of this study? Intro What is the purpose of this study? To study the effects of vertical wind shear on tropical cyclogenesis To discover if there is a preferred magnitude or direction of shear for genesis Methodology Identified genesis events using the International Best Track Archive for Climate Stewardship (IBTrACS) from 1969 to 2008 Focused primarily on genesis events within 20 degrees of the equator to eliminate baroclinic cases Wind shear values computed via NCAR/NCEP reanalysis Used simulations from WRF 3.1.1 Previous Work McBride and Zehr (1981) Analyzed rawinsonde observations and composited their associated wind fields according to developing or non- devoloping disturbances Found the developing composite has an axis of near-zero wind shear over the disturbances (anticyclone overhead) Lee (1989) Developing systems à Light easterly shear Non-developing à Strong westerly shear Tuleya and Kurihara (1981) Idealized modeling study of TC genesis in wind shear Vortex embedded in low-level easterly flow of 5 m/s Found easterly wind shear to be more favorable (peak favorable value was 30 knots!) Hadn’t been systematically verified until this paper Previous Work Cont. Bracken and Bosart (2000) Most frequent values of shear for genesis between 8-9 m/s and no events below 2 m/s Genesis Parameters All indicate a steadily increasing likelihood for genesis with decreasing shear Designed for seasonal forecasts of TC activity • Smallest monthly mean values are still at least 6 m/s • However, the most frequent shear values are lower than the mean • What is the distribution of genesis events by shear? Low shear values are rare! Easterly vs. Westerly Shear • Easterly shear is more favorable for genesis in the deep tropics • Even the normalized results show this • Was TK81 correct? • Although in higher latitudes low shear or light westerly shear is more favorable than easterly shear Time for Simulations Used WRF 3.1.1 Domain is rectangular area using 320x200 grid points with 18 km grid spacing Employed two nested grids with 6 km and 2 km resolution that followed the vortex 40 vertical levels Environment was based off “moist tropical” sounding of Dunion (2011) SST is set to 29° C throughout the domain Vortex embedded in 5 m/s easterly flow Weak mid-level vortex with max tangential winds of 9 m/s at a RMW of 126 km “Hurricane in a box” Simulations with Westerly Shear • Low level flow is 5 m/s and wind shear is westerly at 5 m/s • Unsurprisingly, the more moist the middle atmosphere is, the quicker the intensification is • Top horizontal black line indicates TS intensity • Bottom horizontal black line indicates the 2.5 m/s benchmark for genesis in this modeling study Simulations with Westerly Shear • Low level flow is 5 m/s and wind shear is westerly, but varies in magnitude • Stronger values of westerly shear result in faster genesis, but hinder further development and intensification • 2.5 m/s of westerly shear results in faster development than no shear throughout the time period Why is shear better? • At 24 hours, deep convection is widely dispersed to the north of the circulation center • Over time the mid level circulation center rotates around to the south while convection remains mostly confined to a cyclonically curved band • Finally by 72 hours, a MCV develops near a “coherent surface vortex” Why is shear better? • With westerly shear present, the MCV and surface circulation are both established by 48 hours • The convective structure resembles that of a typical sheared cyclone • More effective in producing net diabatic heating due to concentrated convection? Easterly vs. Westerly Shear • Westerly shear is more favorable for genesis • Note that the 5 m/s easterly shear case did technically achieve genesis faster than the 2.5 m/s westerly shear case, but the smoothed surface pressure does not fall below 1012 hPa and the systems eventually weakens • This disagrees with TK81 What’s up with TK81? • Reproducing the initialization of TK81’s model but with a more advanced mesoscale model found results opposite to those published in TK81 • Yet again, westerly shear is more favorable than easterly shear • TK81 argued easterly shear was more beneficial in maintaining the vertical alignment of the warm core • TK81 resulted in westward propagation faster than the low- level easterly flow with westerly shear à ??? Why is westerly shear more favorable? • Thermal wind balance argument • Westerly shear means colder temperatures in the middle and upper troposphere to the north • Beta drift will cause the storm to move into these areas of colder temps overlying the same SST and temps in the lower atmosphere • Turns out to be not that big of a factor Why is westerly shear more favorable? • Grayscale contours depict surface moisture flux for westerly shear (top) and easterly shear (bottom) at t=36 h • Wind shear in the same direction as the low-level flow was found to be much less favorable for genesis than when shear is in the opposite direction • Reason is asymmetry in the surface fluxes Why are the simulations different than climatology? • Locations where easterly shear is prevalent often are regions of high values of MPI • “Easterly shear is correlated with lower latitudes and with some locations known to be highly favorable for genesis” (e.g. East Atlantic and East Pacific) Conclusions Climatology suggests easterly shear is more favorable for tropical cyclogenesis than westerly shear Normalized wind shear values of 1.25 to 5.0 m/s are found to be the most favorable Unlike TK81, numerical simulations found westerly shear to be more favorable for genesis, especially around 5 m/s The reasons for the disagreement between the simulations and climatology remain undetermined Questions? .
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