Ventilation of Tropical Cyclones Brian Tang ATM 741 3/21/16

Zeng et al. (2008) Tropical cyclone intensity distribution tends to smaller values as vertical wind shear (VWS) increases.

One possible mechanism by which VWS weakens TCs is ventilation. Ventilation is the inward flux of low-entropy (θ e ) air into the inner core of a TC or the outward flux of high-entropy (θ e ) air out of the inner core of a TC. A B AB >360 K <340 K θeθe Houze (2010)

Weakening the radial gradient of entropy causes the TC intensity to decrease. Thermal wind equation for an axisymmetric, balanced vortex (Emanuel 1986, Montgomery et al. 2006, Bryan and Rotunno 2009) V g,max = maximum gradient wind M = angular momentum (using gradient wind) T b -T o = temperature difference b/t top of boundary layer and outflow layer ds/dM = entropy gradient

θ ʹ + Gray (1968) Frank and Ritchie (2001) Wong and Chan (2004) Kwon and Frank (2005) Environmental flow Upper-level warm core mixed outward. TC weakens from the top down.

time (h) Frank and Ritchie (2001) K m s −1 “The level of maximum outward eddy flux descends with time, and the storm weakens throughout this period.”

Simpson and Riehl (1958) Cram et al. (2007) Tang and Emanuel (2010, 2012) low entropy Environmental flow Low-entropy parcels mixed directly into mid-level eyewall.

Storm-relative flow acts as a “constraint on the hurricane heat engine” (Simpson and Riehl 1958). Back trajectories in a simulation of a sheared TC indicate entrainment of environmental parcels, causing a ~1K decrease in eyewall θ e (Cram et al. 2007).

Powell (1990) Riemer et al. (2010, 2013) Zhang et al. (2013) low entropy Environmental flow Downdrafts flush low entropy air down into boundary layer.

z=1.5 km K m s −1 Riemer et al. (2013)

Tang and Emanuel (2012)

ASPECH experiments with parameterized ventilation Entropy and Ventilation J kg -1 K -1 Tang and Emanuel (2012)

H03H06H09H12H15 3 km6 km9 km12 km15 km Low-level and mid-level ventilation most harmful to TC

Ventilation is a hypothesized thermodynamic constraint on TCs. Some open questions… How does ventilation occur, i.e. what aspects of the TC structure interacting the with environmental VWS cause the ventilation (VRWs, quasi-mode, rainbands)? Does ventilation affect TC convection through entrainment or through the production of downdrafts and cold pools? Does the ventilation “flavor” change depending on the shape of the vertical wind shear profile or the TC structure/size/intensity?