The Excitation of Equatorial Waves by Deep Convection in the NCAR Community Climate Model (CCM3)

Abstract

The forcing of equatorial waves by convective heating in the National Center for Atmospheric Research Community Climate Model (CCM3) is investigated and compared with the forcing deduced from observations of convective clouds. The analysis is performed on two different simulations, wherein convection is represented by the Zhang-McFarlane and the Hack parameterization schemes, respectively. Spectra of equatorial waves excited by convective heating (Rossby, Kelvin, and gravity waves) are obtained by projecting the heating field onto Hough modes; the dynamical response to the heating is then calculated in terms of the vertical component of the Eliassen-Palm flux, F z , focusing on waves that are able to propagate into the middle atmosphere. The same analysis is repeated using observations of outgoing longwave radiation as a proxy for tropical convection. Comparison of CCM3 results with those derived from observations indicates that high-frequency heating variability is underestimated in both CCM3 simulations, despite the fact that time-mean values of convective heating are well represented. Moreover, the two convective parameterization schemes differ substantially from each other: Compared to observations, F z is severely underestimated at most frequencies when CCM3 is run with the Zhang-McFarlane scheme. When the Hack scheme is used, F z at frequencies || Ͻ 0.5 cycles per day is comparable to the observations, but it is underestimated at higher frequencies. Misrepresentation of the variability of convective heating is likely to have important consequences for the dynamical simulation of the middle atmosphere and even the troposphere.

Climatology and Forcing of the Quasi-Biennial Oscillation in the MAECHAM5 Model

“…The key problem is the simulation of tropical convection, which must excite the resolved wave spectrum. Differences in the temporal characteristic of modeled convection can be substantial, as shown by Horinouchi et al (2003), which can be attributed partly to the characteristics of the different parameterizations of convection (Ricciardulli and Garcia 2000). Wave excitation from a particular parameterization can be highly sensitive to the choice of the internal parameters and may occur also as a result of resolved convective overturning if the parameterized overturning does not remove the instabilities efficiently enough, as demonstrated by Scinocca and McFarlane (2004).…”

Properties of the average distribution of equatorial Kelvin waves investigated with the GROGRAT ray tracer

“…Another reason is that both OLR and rain rates are directly coupled with the latent heat release acting as source mechanism for Kelvin and other equatorial waves. In a theoretical framework the dynamic response of the atmosphere (e.g., geopotential, temperature or wind spectra) in the near field (just above the top of the convective source) is usually obtained by projecting this spectrum on a set of Hough modes representing the wave modes that are able to propagate vertically in the atmosphere (e.g., Salby and Garcia, 1987;Bergman and Salby, 1994;Ricciardulli and Garcia, 2000). In doing so the spectral shape is changed.…”

“…Therefore spectral signatures of Kelvin waves can be found in tropospheric space-time spectra of, for example, outgoing longwave radiation (OLR) or rain rates (e.g., Wheeler and Kiladis, 1999;Straub and Kiladis, 2003;Cho et al, 2004). These parameters are linked to the latent heat released in convective systems which is the source process of the wave generation (e.g., Salby and Garcia, 1987;Bergman and Salby, 1994;Ricciardulli and Garcia, 2000).…”

“…Studies on the QBO and its driving by different equatorial wave modes were either highly idealized (e.g. Lindzen and Holton, 1968;Holton and Lindzen, 1972;Dunkerton, 1997;Campbell and Shepherd, 2005a,b) or were based on complex general circulation model (GCM) simulations (Horinouchi et al, 2003;Giorgetta et al, 2006;Mayr et al, 2007) providing little insight in the detailed mechanisms and based on only partially realistic convective forcing (Ricciardulli and Garcia, 2000;Horinouchi et al, 2003). Highly idealized simulations which assume for instance only a single wavenumber 1 Kelvin wave and a continuous omnipresent forcing were justified in terms of thought experiments.…”

Comparison of stochastic parametrization approaches in a single-column model

“…For example, many GCMs are known to have insufficient high-frequency, small-scale variability of convective heating rates and precipitation in the tropics, which may damage their ability to represent low-frequency, large-scale climate variability (Ricciardulli & Garcia 2000;Horinouchi et al 2003). A wide variety of plausible stochastic methods continue to be suggested and actively investigated, including perturbing the inputs to a parametrization (e.g.…”