A51P-0327
Instabilities of Tropical Cyclones and their Nonlinear Saturation in Moist-Convective Rotating Shallow Water Model

Friday, 18 December 2015
Poster Hall (Moscone South)
Noe Lahaye, University of California San Diego, La Jolla, CA, United States and Vladimir Zeitlin, Laboratoire de Météorologie Dynamique ENS, Paris, France
Abstract:
Studies of stability of tropical cyclones (TC) are mostly performed either in over-simplified (2D Euler, e.g. [1]), or in over-complexified "all-inclusive", e.g. [2], models. TC have very high Rossby numbers, so Lighthill radiation is operational and instabilities are radiative. Yet, the quantitative results for radiative instabilities of vortices are available only for simplified vortex profiles, e.g. [3]. TC evolve in the essentially moist and precipitating atmosphere, yet studies of precise dynamical role of moisture in developing instability are scarce [4].

We use the moist-convective Rotating Shallow Water model of [5], the simplest possible one which includes inertia-gravity gravity waves (IGW) and the effects of moisture and precipitation. Unstable modes are investigated by means of a linear stability analysis, then the nonlinear saturation is simulated in cases with precipitation off (dry), precipitation on but evaporation off (moist-precipitating), and precipitation and evaporation on (moist-precipitating-evaporating).

Our main results are:

Linear stability:

  • Main instability: ageostrophic barotropic instability
  • Unstable modes: mixed Rossby - inertia gravity waves.

Dry saturation:

  • Axisymmetrization of the TC
  • Intensification of winds inside the radius of maximum wind
  • Bursts in the IGW emission

Moist-precipitating saturation:

  • Amplification of the IGW emission with respect to the dry case
  • Amplification of the wind intensification mechanism

Moist-precipitating-evaporating saturation:

  • Appearance of convectively-coupled IGWs
  • Net intensification of wind (even at the radius of maximum wind)

References:

  1. J.P. Kossin and W.H. Schubert, J. Atmos. Sci., 58, 2196, 2001.
  2. Y.C. Kwon and W.M. Frank, J. Atmos. Sci., 65, 106, 2008.
  3. S. Le Dizes and P. Billant, Phys. Fluids, 21, 1, 2009.
  4. D.A. Schecter and M.T. Montgomery, J. Atmos. Sci., 64, 314, 2007.
  5. F. Bouchut, J. Lambaerts, G. Lapeyre, and V. Zeitlin, Phys. Fluids, 21, 126601, 2009.


Figure: Nondimensional vorticity (colors) and precipitation (green isolignes) in course of the nonlinear evolution of an unstable tropical cyclone.

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