[AAS03-P10] Large-scale deformation of tropical cloud band in the ITCZ detected in NICAM simulations
Keywords:tropics, ITCZ, cirrus
We found a similar separation event in an output of the simulation for one year from June 2004 and horizontal resolution 3.5 km. We examined the life cycle of the cirrus bands after separation. One interesting point is on the height of the cirrus clouds in the separated bands. The simulated cirrus clouds in the bands lie in the 13-14 km height level, while the height of the maximum meridional wind that contributes to the meridional movement of the cirrus bands is about 12 km. This difference of the heights means that the cirrus clouds were not just advected by meridional wind but were possibly developed in the course of the separation.
The trigger of zonally simultaneous separation of a long band is another our interest. Inertial instability may be a candidate since vertical exchange of zonal momentum by deep convection would occur in an original band and result in the large meridional shear of the zonal wind in the upper troposphere. We tried to confirm necessary condition for inertial instability by calculating geostrophic vorticity, following the general procedure. However, the position of the bands is too close to the equator (3-7N) to establish geostrophic relationship, and the spatial pattern of the geostrophic vorticity is too noisy to analyze. Therefore, we instead calculate simple absolute vorticity. We found that the absolute vorticity around the separated bands was almost negative. It suggests the possibility of the inertial (or similar kind of) instability.
We are now analyzing another numerical experiment including four kinds of runs using a cloud scheme with different cloud parameter settings. In the real atmosphere, a separation event occurred on 16-17 June 2008 at the just east of the date line. The event can be simulated in all of four runs at 36 hours from the initial time. It suggests that the basic mechanism of the separation is not sensitive to the detail of cloud systems. By analyzing the dynamical fields of these outputs, we are trying to find the key mechanisms to trigger the meridional separation of the cloud bands.