I’ll be talking at AMS Hurricanes 28 on the 1st of May in Orlando!

by Joe Archive on January 14, 2008

http://ams.confex.com/ams/28Hurricanes/techprogram/paper_138350.htmbr /br /Abstract:br /b/bblockquotebA toy model of the instability in the equatorially trapped convectively coupled waves/bp bJoseph Allan Andersen/b, Harvard University, Cambridge, MA; and Z. Kuang/pp The equatorial atmospheric variability shows a spectrum of significant peaks in the wave number-frequency domain. These peaks have been identified (Wheeler and Kiladis, 1999) with the equatorially trapped wave modes of rotating shallow water wave theory (Matsuno, 1966). We are attempting to address the observation that the various wave types (e.g. Kelvin, Rossby, etc.) and wave numbers show differing amplitudes. It is hypothesized that this is due to the (linear) stability of the atmosphere to the various wave types depending both upon the specific wave type and the wavenumber in question. /ppWe use the simplified model of the convectively coupled waves in first two baroclinic vertical modes developed in Kuang 2007, extended to the equatorial beta plane. The convective parameterization for this model is based upon the quasi-equilibrium concept. The inclusion of tropospheric moisture as an input to the convection calculation to represent the effect of lateral entrainment of the (generally) drier tropospheric air into a rising convective cloud, controls the depth of convection. /ppThe linear instability spectrum of the resulting coupled system is found by eigenvalue analysis. We use realistic model parameters estimated from cloud system resolving model (CSRM) studies of the convectively coupled waves. The instability analysis produces unstable waves with phase speeds, growth rates and structures (vertical and horizontal) that compare well with the results from CSRM simulations and observations. /ppThe linear system shows peak unstable “Kelvin” waves around planetary wavenumber seven with peak growth rates of 0.09/day (e-folding time of eleven days). The system also shows unstable “Mixed Rossby Gravity” (MRG) and “Inertia-Gravity” waves with significant growth in the wavenumber range from negative ten to positive ten. The peak MRG growth rate is around one third that of the Kelvin wave and occurs at planetary wavenumber three. /ppThis demonstrates that one aspect of the convectively coupled waves which cannot be captured without a beta plane (or some other representation of meridional variation) is the instability’s dependence upon wave type. We believe this is related to the differing roles of divergent and rotational flows in the convective parameterization. We further conducted analytical investigation of simplified cases to explore the details of the mechanisms responsible for wave number and wave type selection in the instability spectrum.!–Abstract 138350 modified by on 11-15-2007– /pa href=”http://ams.confex.com/ams/28Hurricanes/techprogram/session_21821.htm”Session 15D, Convectively Coupled Waves II/abr /bThursday, 1 May 2008, 1:15 PM-3:00 PM/bbr //blockquotebr /It’s not directly hurricane related… but the Hurricane conference seems to be a catch-all for tropical and convective stuff. The convective waves seem to have an influence on cyclogenesis – the hurricane formation rate does seem to be correlated with the various wave activities/phases.br /br /We’re trying to determine the physical mechanisms that cause the specific waves that are observed to exist, and not other waves of similar wavelength but with different frequencies (or vice-versa) , or different wave profiles – for example, the spectrum has a lot of power in the Kelvin wave part of the spectrum with wavelengths of around 7000km, but much less in the Rossby waves or for Kelvin waves with shorter or longer wavelengths.br /br /One of the mechanisms involved is Moist Convective Damping (eg Emanuel 94) – If the convection occurs slightly in the cold part of the wave, then the convection will damp the wave rather than amplifying it. As the time delay for convection to respond tot he wave is pretty much contant, the higher the frequency of the wave, the further out of phase the heating is with the temperature perturbation. This leads to a preferential damping of higher frequency waves.br /br /As I mention in the abstract, we have some ideas about other mechanisms that further constrain the wave activity.br /br /This will be my first conference talk since back in my old life as an atomic physicist at UQ – I gave a talk at Non-Linear Guided Waves in 01 (also in Florida). I’m looking forward to it.

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