A numerical experiment of conditional symmetric baroclinic instability (CSBI) is carried out including Kessler's parameterization, in order to show a possible mechanism for formation of meso-β scale rainbands associated with extratropical cyclones and Baiu front.
The unstable condition of CSBI is controlled by Richardson number defined by equivalent potential temperature. The flow is unstable for CSBI when the equivalent Richardson number is less than unity.
In the numerical model, horizontal and vertical space resolutions are 2 and 0.5km, respectively, in a 250km×10km domain. We assumed an initial zonal flow with vertical shear, which is geostrophically balanced with the meridional gradient of virtual potential temperature. All the derivatives with respect to zonal direction vanish due to zonal symmetric assumption. An unstable condition for CSBI is initially set everywhere in the domain, adjusting the water vapor content included. To set on CSBI, a few percents of supersaturation is initially incorporated at the center portion of the domain together with initial random vertical velocity disturbances.
A typical slantwise circulation of CSBI is seen with horizonal scale of about 150km independent of the initial supersaturation area. The total meridional heat (equivalent potential temperature) transport is northward. The total meridional transport of zonal momentum is not clear from the present experiment. The total transport of heat is upward, while the total zonal momentum transport is downward. A slantwise stratiform cloud appears and is sustained along the updraft of CSBI. A mid-tropospheric convective unstable layer is induced above the stratiform cloud due to the differential advection of equivalent potential temperature by the CSBI circulation. Convective cells appear in this layer.Thus, CSBI prepares the favorable environment for the seeder-feeder precipitation mchanism to work efficiently. At the top of the boundary layer, upward velocity exceeds 0.02ms^-1 due to the convergence induced by the CSBI, horizontally extending over 30km and lasting for more than 15hrs. A low level jet is formed due mainly to the deceleration of zonal wind at the mid-troposphere, together with a slight acceleration below.