HR: 0800h
AN: A51A-0020    [Abstracts]
TI: Interaction of dynamic and thermodynamic controls on deep convection in an idealized model of the ITCZ
AU: * Sobel, A H
EM: ahs129@columbia.edu
AF: Columbia University, 500 W. 120th St., New York, NY 10025
AU: Neelin, J D
EM: neelin@atmos.ucla.edu
AF: University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90024-1565
AB: Theories for the position and intensity of precipitation over tropical oceans on climatic time scales fall broadly into two classes. Some focus on the momentum budget of the planetary boundary layer (PBL), while others focus on local thermodynamic factors. Particularly in the case of narrow intertropical convergence zones (ITCZs), such as occur in the eastern Pacific there is some evidence for both classes of theories. This is perplexing, since the two focus on quite different physical mechanisms. We develop a simple model in which both sorts of mechanisms can operate, and the interaction between them can be analyzed. The model includes a mixed layer representation of the PBL, coupled to a free troposphere whose vertical structure is that of the quasi-equilibrium tropical circulation model developed by Neelin and Zeng, except that the vertical structure functions now begin at PBL top instead of the surface. The model is axisymmetric and run over a fixed sea surface temperature (SST) lower boundary condition, with an off-equatorial SST maximum superimposed on an otherwise uniform SST field. An ITCZ develops in the model whose width and intensity are, for best-guess parameters, controlled by horizontal diffusion of moisture, which can be regarded as a proxy for mixing by transient disturbances, or perhaps zonal advection (which would be present to some degree in a 3D case). As diffusion is made small, the simulated ITCZ becomes unrealistically narrow and intense, though analysis suggests that the nondiffusive limit is not singular. By contrast, the original Neelin-Zeng model, which lacks a boundary layer, produces a reasonable ITCZ (by comparison to observations) in the absence of diffusion for the same configuration. In the present model, the negative gross moist stability associated with shallow ascent driven by PBL momentum dynamics seems to play a large role in causing this difference, and the direct SST-induced A��,��"Lindzen-NigamA��,���� component is important in the PBL momentum budget (as found in EPIC). The suggestion from the model is that transients such as easterly waves may be crucial to the structure of the observed time-mean ITCZ, though by a different mechanism than postulated in the past.
DE: 3309 Climatology (1616, 1620, 3305, 4215, 8408)
DE: 3319 General circulation (1223)
DE: 3354 Precipitation (1854)
DE: 3367 Theoretical modeling
DE: 3374 Tropical meteorology
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005