Numerical experiments on extratropical tropopause inversion layer using a mesoscale model

Tropopause inversion layer (TIL) is a persistent layer with high static stability (Birner, 2002). Formation mechanisms of the TIL are not well understood, though some processes which may contribute to the formation have been proposed, such as a dynamical mechanism due to local vertical convergence associated with a synoptic vortex (Wirth, 2004), and a radiative forcing mechanism due to heating by ozone and cooling by water vapor (Randel et al., 2007). Most of the studies so far used some idealized simple models, and numerical experiments with realistic conditions have not been conducted yet.

We perform numerical experiments on a life cycle of an observed extratropical cyclone with Non-Hydrostatic Model (NHM) of Japan Meteorological Agency (JMA), which is originally used for operational numerical weather predictions. The model we modified has 200 layers in the vertical from the surface to 25 km in altitude, and the horizontal domain is 4140 km x 4000 km around Japan with a horizontal resolution of 20 km. The time integration period is 72 hours from 19th to 22nd in February, 2009, during which a typical event of explosive cyclogenesis was observed. For the initial and boundary conditions, we use NCEP/FNL data.

The TIL obtained in the control run has similar characteristics as observations, including dependence on local relative vorticity: stronger TIL in negative vorticity areas while weaker TIL in positive vorticity areas.

In an experimental run, water vapor is removed above 300 hPa level in the initial condition in order to investigate the temperature response to the radiative forcing by water vapor perturbations around the tropopause. The explosive development of the extratropical cyclone is not different from the control run very much, but the TIL becomes stronger in the experimental run. The vertical profile of static stability becomes sharper due to weak cooling above the tropopause with decreased water vapor. Quantitative analyses on the formation of the TIL are done in detail to see the relative importance of dynamical and radiative forcing mechanisms.