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Thermodynamic Characteristics of Downdrafts in Tropical Cyclones as Seen in Idealized Simulations of Different Intensities

Joshua B. WadleraNOAA/Atlantic Oceanographic and Meteorological Laboratory/Hurricane Research Division, Miami, Florida
bCooperative Institute for Marine and Atmospheric Studies, University of Miami, Miami, Florida

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David S. NolancRosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida

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Jun A. ZhangaNOAA/Atlantic Oceanographic and Meteorological Laboratory/Hurricane Research Division, Miami, Florida
bCooperative Institute for Marine and Atmospheric Studies, University of Miami, Miami, Florida

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Lynn K. ShaycRosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida

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Abstract

The thermodynamic effect of downdrafts on the boundary layer and nearby updrafts are explored in idealized simulations of category-3 and category-5 tropical cyclones (TCs) (Ideal3 and Ideal5). In Ideal5, downdrafts underneath the eyewall pose no negative thermodynamic influence because of eye–eyewall mixing below 2-km altitude. Additionally, a layer of higher θe between 1- and 2-km altitude associated with low-level outflow that extends 40 km outward from the eyewall region creates a “thermodynamic shield” that prevents negative effects from downdrafts. In Ideal3, parcel trajectories from downdrafts directly underneath the eyewall reveal that low-θe air initially moves radially inward allowing for some recovery in the eye, but still enters eyewall updrafts with a mean θe deficit of 5.2 K. Parcels originating in low-level downdrafts often stay below 400 m for over an hour and increase their θe by 10–14 K, showing that air–sea enthalpy fluxes cause sufficient energetic recovery. The most thermodynamically unfavorable downdrafts occur ~5 km radially outward from an updraft and transport low-θe midtropospheric air toward the inflow layer. Here, the low-θe air entrains into the updraft in less than 5 min with a mean θe deficit of 8.2 K. In general, θe recovery is a function of minimum parcel altitude such that downdrafts with the most negative influence are those entrained into the top of the inflow layer. With both simulated TCs exposed to environmental vertical wind shear, this study underscores that storm structure and individual downdraft characteristics must be considered when discussing paradigms for TC intensity evolution.

Significance Statement

It is known that downdrafts transport cool and dry air into the hurricane boundary layer, where it can enter the eyewall and weaken the storm. Simulated hurricanes are used to understand how the effects of individual downdrafts are related to their location within the storm and the hurricane’s structure. Downdrafts near the upper part of the boundary layer have the greatest weakening effect, while downdrafts that reach the surface are mitigated by energy transferred from the ocean. Additionally, when warm and moist air is transported away from the eyewall aloft, it shields the boundary layer from unfavorable downdraft air, mitigating its effect. The results highlight the importance of storm structure and air–sea interactions for understanding how downdrafts influence hurricane intensity.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Joshua B. Wadler, joshua.wadler@noaa.gov

Abstract

The thermodynamic effect of downdrafts on the boundary layer and nearby updrafts are explored in idealized simulations of category-3 and category-5 tropical cyclones (TCs) (Ideal3 and Ideal5). In Ideal5, downdrafts underneath the eyewall pose no negative thermodynamic influence because of eye–eyewall mixing below 2-km altitude. Additionally, a layer of higher θe between 1- and 2-km altitude associated with low-level outflow that extends 40 km outward from the eyewall region creates a “thermodynamic shield” that prevents negative effects from downdrafts. In Ideal3, parcel trajectories from downdrafts directly underneath the eyewall reveal that low-θe air initially moves radially inward allowing for some recovery in the eye, but still enters eyewall updrafts with a mean θe deficit of 5.2 K. Parcels originating in low-level downdrafts often stay below 400 m for over an hour and increase their θe by 10–14 K, showing that air–sea enthalpy fluxes cause sufficient energetic recovery. The most thermodynamically unfavorable downdrafts occur ~5 km radially outward from an updraft and transport low-θe midtropospheric air toward the inflow layer. Here, the low-θe air entrains into the updraft in less than 5 min with a mean θe deficit of 8.2 K. In general, θe recovery is a function of minimum parcel altitude such that downdrafts with the most negative influence are those entrained into the top of the inflow layer. With both simulated TCs exposed to environmental vertical wind shear, this study underscores that storm structure and individual downdraft characteristics must be considered when discussing paradigms for TC intensity evolution.

Significance Statement

It is known that downdrafts transport cool and dry air into the hurricane boundary layer, where it can enter the eyewall and weaken the storm. Simulated hurricanes are used to understand how the effects of individual downdrafts are related to their location within the storm and the hurricane’s structure. Downdrafts near the upper part of the boundary layer have the greatest weakening effect, while downdrafts that reach the surface are mitigated by energy transferred from the ocean. Additionally, when warm and moist air is transported away from the eyewall aloft, it shields the boundary layer from unfavorable downdraft air, mitigating its effect. The results highlight the importance of storm structure and air–sea interactions for understanding how downdrafts influence hurricane intensity.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Joshua B. Wadler, joshua.wadler@noaa.gov
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