A new paradigm for intensity modification of tropical cyclones: thermodynamic impact of vertical wind shear on the inflow layer 1Department of Meteorology, Naval Postgraduate School, Monterey, CA, USA
2NOAA's Hurricane Research Division, Miami, FL, USA
3University of Colorado, Cooperative Institute for Research in Environmental Sciences, Boulder, CO, USA
Received: 16 Mar 2009 – Published in Atmos. Chem. Phys. Discuss.: 04 May 2009 Abstract. An important roadblock to improved intensity forecasts for tropical cyclones
(TCs) is our incomplete understanding of the interaction of a TC with the
environmental flow. In this paper we re-visit the canonical problem of a TC in vertical wind shear on an f-plane.
A suite of numerical experiments is performed with intense TCs in moderate to strong
vertical shear. We employ a set of simplified model physics – a simple bulk aerodynamic boundary layer
scheme and "warm rain" microphysics – to foster better understanding of
the dynamics and thermodynamics that govern the modification of TC intensity.
In all experiments the TC is resilient to shear but
significant differences in the intensity evolution occur.
Revised: 02 Oct 2009 – Accepted: 02 Mar 2010 – Published: 01 Apr 2010
The ventilation of the TC core with dry environmental air at mid-levels and
the dilution of the upper-level warm core are two prevailing hypotheses for
the adverse effect of vertical shear on storm intensity. Here we propose an
alternative and arguably more effective mechanism how cooler and drier (lower
θe) air – "anti-fuel" for the TC power machine – can enter the
core region of the TC. Strong and persistent, shear-induced downdrafts flux low θe air into the boundary layer from above,
significantly depressing the θe values in the storm's inflow layer.
Air with lower θe values enters the eyewall updrafts, considerably
reducing eyewall θe values in the azimuthal mean. When viewed from
the perspective of an idealised Carnot-cycle heat engine a decrease of storm
intensity can thus be expected. Although the Carnot cycle model is – if at
all – only valid for stationary and axisymmetric TCs, a close association of
the downward transport of low θe into the boundary layer and
the intensity evolution offers further evidence in support of our hypothesis.
The downdrafts that flush the boundary layer with low θe air are
tied to a quasi-stationary, azimuthal wave number 1 convective
asymmetry outside of the
eyewall. This convective asymmetry and the associated downdraft
pattern extends outwards to approximately 150 km. Downdrafts occur on
the vortex scale and form when
precipitation falls out from sloping updrafts and evaporates in the
unsaturated air below. It is argued that, to zero order, the formation of
the convective asymmetry is forced by frictional convergence
associated with the azimuthal wave number 1 vortex Rossby wave
structure of the outer-vortex tilt.
This work points to an important connection between the thermodynamic
impact in the near-core boundary layer and the asymmetric balanced
dynamics governing the TC vortex evolution.
Citation: Riemer, M., Montgomery, M. T., and Nicholls, M. E.: A new paradigm for intensity modification of tropical cyclones: thermodynamic impact of vertical wind shear on the inflow layer, Atmos. Chem. Phys., 10, 3163-3188, doi:10.5194/acp-10-3163-2010, 2010.