Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 9 3 2009 10.5194/acpd-9-10711-2009 http://www.atmos-chem-phys-discuss.net/9/10711/2009/ http://www.atmos-chem-phys-discuss.net/9/10711/2009/acpd-9-10711-2009.html http://www.atmos-chem-phys-discuss.net/9/10711/2009/acpd-9-10711-2009.pdf 10711 10775 2009-05-04 A new paradigm for intensity modification of tropical cyclones: thermodynamic impact of vertical wind shear on the inflow layer M. Riemer mriemer@nps.edu M. T. Montgomery M. E. Nicholls Department of Meteorology, Naval Postgraduate School, Monterey, CA, USA NOAA's Hurricane Research Division, Miami, FL, USA University of Colorado, Cooperative Institute for Research in Environmental Sciences, Boulder, CO, USA 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 classical idealised numerical experiment of tropical cyclones (TCs) in vertical wind shear on an f-plane. 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. A suite of experiments is performed with intense TCs in moderate to strong vertical shear. In all experiments the TC is resilient to shear but significant differences in the intensity evolution occur. 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 downdrafts flux low θe air from the lower and middle troposphere into the boundary layer, 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 strong correlation between 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 inflow layer with low θe air are associated with a quasi-stationary region of convective activity outside the TC's eyewall. 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