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?