Journal Article

On the Spirality of the Asymmetric Rain Field of Tropical Cyclones Under Vertical Wind Shear

Toumi, R., Lau, K. H.

2024

Overview

The downshear‐left enhancement of tropical cyclone rainfall has been demonstrated previously, but the radial dependence of this effect was not analyzed in detail. This study quantifies the progressive upwind shift of the wavenumber‐1 maximum rain position with radius relative to the vertical wind shear direction. This shift is visualized as a distinctive upwind spiral of the maximum. It is shown that this spiral pattern is generally observed across various storm intensities, shear strength, and ocean basins. Detailed examination revealed that the maximum downwind deflection angle of the wavenumber‐1 rain maximum relative to the shear direction is smaller for tropical storms than hurricanes, but insensitive to hurricane intensity. It is proposed that the spirality is produced by a continuous decline in angular advection of air parcels with radius. The stability of the deflection angle in hurricanes may be accounted for by a corresponding increase in vertical ascent under strengthening angular flow. Plain Language Summary Tropical cyclones (TCs) can produce torrential rainfall that generates floods, causing significant socio‐economic losses. Understanding the spatial structure of the TC rain field is crucial for improving disaster preparedness. The TC rain field can be thought of as the combination of a symmetric and an asymmetric part. By using a technique called Fourier decomposition, we can break down the asymmetric part into individual wavenumber components. The first component, wavenumber‐1 (WN‐1), is dominant and tends to be larger in the downshear quadrants under vertical wind shear. Using 21 years of global WN‐1 rain fields, we produced composite images aligned with the shear direction. We discovered that the positions at which the WN‐1 maximum occurs progressively shift upwind with increasing distance from the TC center, forming a spiral. We provided the first quantification of the observed spirals and showed that this is a general pattern that exists across different TC intensities, shear strength, and ocean basins. We also identified detailed changes in the pattern with storm intensity and introduced simple models as a first attempt to comprehend these changes. The findings can improve weather forecasts and risk predictions, making us better prepared for hazards associated with TC rainfall. Key Points There exists a general and progressive upwind shift in the wavenumber‐1 maximum with radius in global shear‐relative rainfall composites Maximum downwind deflection of the wavenumber‐1 maxima rises with storm intensity up to Category 1 on the Saffir–Simpson Hurricane Scale Linearity between the angular velocity and the vertical velocity of the storm may stabilize the downwind deflection beyond Category 1

• Publisher: John Wiley & Sons, Inc
• Subject: Advection / Air parcels / Asymmetry / Atmospheric precipitations / Cyclones / Deflection / downshear‐left rain enhancement / Economic impact / Emergency preparedness / Flow stability / Hazard identification / Hurricanes / Ocean basins / Ocean models / Oceans / Precipitation / Rain / Rainfall / Shear strength / Storms / Tropical cyclone intensities / tropical cyclone rain / tropical cyclone rain asymmetry / tropical cyclone rain composites / Tropical cyclone rainfall / tropical cyclone under vertical wind shear / tropical cyclone wavenumber‐1 rain / Tropical cyclones / Tropical depressions / Tropical storms / Vertical orientation / Vertical wind shear / Wavelengths / Weather forecasting / Wind / Wind effects / Wind shear