Study on how the direction of wind shear impacts tropical cyclone intensity published in Monthly Weather Review

This is the first study to examine how the direction of the wind shear changes how much heat and moisture is available to the TC and how it influences TC intensity change.  These results provide TC forecasters another tool to predict intensity change.

■ Summary: 

The amount of wind shear, the change of winds with height, is one of the most commonly used predictors of TC intensity change, with large amounts of wind shear unfavorable for intensification. Regardless of the direction of the wind shear,  TCs in the north Atlantic basin generally have warm, moist air from the environment near the sea surface on the east side (solid red arrows in Fig. 1) and cool, dry air from the environment on the west side (solid blue arrows in Fig. 1).  Nearer the TC center, for all TCs, there is an area of warm and moist air moving inward to the TC center right-of-shear (dashed red lines in Fig. 1) and an area of cool and dry air from downward moving air left-of-shear (blue dashed lines in Fig. 1). This is the first study to examine how the direction of the wind shear (see Fig. 2) changes how much heat and moisture is available to the TC and how it influences TC intensity change.

Figure 1: A schematic of the different environmental (solid arrows) and storm (dashed arrows) processes for TCs in the north Atlantic basin exposed to (a) southerly wind shear and (b) northerly wind shear. Blue indicates cool, dry air, and red indicates warm, moist air.

■ Important Conclusions:

  • Multiple previous studies show that when a TC has  temperature and humidity the same all around the eyewall (what we call symmetry), conditions are  favorable for intensification.  When wind shear is southerly (Fig. 1a), the downward moving cool, dry air left-of-shear is in the same location (northwest quadrant) as the cool, dry air in the environment. Additionally, the area of environmental warm, moist air coincides with the area of inward moving warm, moist air (southeast quadrant). This overlapping cool, dry air in the northwest quadrant and overlapping warm, moist air in the southeast quadrant leads to an asymmetric boundary layer temperature and moisture distribution which is unfavorable for intensification because the warm, moist air TCs require to sustain strong thunderstorms and intensify is limited. 
  • In contrast, when the wind shear is northerly (Fig. 1b), the downward moving cool, dry air left-of-shear is in the northeast quadrant, in a region of warm, moist environmental air. Additionally, in this setup, the inward moving warm, moist air coincides with the region of cool, dry environmental air. Since these partially cancel each other out, in northerly wind-shear environments, the temperature and moisture near the ocean surface is more symmetrically distributed.
  • So, temperature and moisture is more symmetrically distributed near the ocean surface when TCs are exposed to northerly wind shear and more asymmetrically distributed with TCs are exposed to southerly wind shear
  • The more symmetrically distributed temperature and moisture for TCs in northerly wind shear environments makes those storms more likely to intensify than storms in southerly wind shear environments.

Figure 2.  Wind shear and wind shear direction explained.

The full study can be found at https://doi.org/10.1175/MWR-D-21-0022.1.

For more information, contact aoml.communications@noaa.gov.

This research was carried out[in part under the auspices of the Cooperative Institute for Marine and Atmospheric Studies (CIMAS), a Cooperative Institute of the University of Miami and the National Oceanic and Atmospheric Administration, cooperative agreement #NA20OAR4320472. Evan Kalina is supported by funding from NOAA Award Number 688NA17OAR4320101. Jun Zhang is supported by funding from ONR Award Number N00014-20-6891-2071.

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