Resolution Dependence of Equatorial Precipitation Activities Represented in a General Circulation Model with No Cumulus Parameterization

Abstract:

An aquaplanet experiment is performed to explore how representation of equatorial precipitation activities in a GCM depends on its horizontal resolution. The numerical model utilized is AFES, an AGCM optimized for the Earth Simulator (Ohfuchi et al 2004). For the sake of simplicity, we adopt no cumulus parametarizetion scheme, although large scale condensation scheme is retained. We compare the results with increasing horizontal resolution from T39 to T319. Other setups follows those of the CONTROL case of the Aqua Planet Experiment (APE) Project (Neale and Hoskins, 2000; Blackburn and Hoskins, 2013). We can identify two kinds of resolution dependences; one is zonal and the other is meridional.

Zonally, at lower horizontal resolution, precipitation occurs as disturbances with the scale of a few gridpints, and propagate both eastward and westward. As the resolution increases, zonal scale of precipitation disturbances decreases, and there appears a hierarchical structure: grid scale disturbances move westward, while they are organized into eastward moving, Kelvin wave like envelopes. The westward motion of grid scale precipitation structure can be related to the advection of inhomogeniety of moisture by the zonal mean wind.

Another kind of large scale organization identified is the north-south "zigzag" structure of the distribution of grid scale precipitation disturbances, which has zonal wavenumber two and propagates westward. This structure appear only when the model can resolve the moist equatorial radius of deformation, and can be interpreted as moist mixed-Rossby gravity wave. The Mixed-Rossby gravity wave and the Kelvin wave like structure occur superposed to each other.

Features similar to the above can also be noted in some of the AGCM runs with cumuls parameterizations in the APE (Blackburn et al, 2013; Williamson et al 2013; Nakajima et al 2013).

Acknowledgement:
Numerical experiments were performed at the Earth Simulator, JAMSTEC.