The excitation of equatorial waves by deep convection in the NCAR Community Climate Model (CCM3)

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 |ω| < 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.