Banded Convection Activity Associated with Gravity Waves over Southern China

Wavelike banded convective activity that occurred near the south coast of China on 30 Jan 2018 (Fig. 1) was investigated using convective-permitting simulations of a nonhydrostatic mesoscale model. The cloud-resolved model can well capture the wavelike features (Fig. 2). The banded convection was coupled with gravity waves with a northeastward wave speed of around 11.5 m s^-1. The power spectrum indicates that the main (secondary) period of the waves was 45 (30) min and the main wavelength was around 35 (20) km. The wavelike cloud water mixing ratio, potential temperature, and vertical motion are nearly in phases and coupled. The gravity waves could be amplified under the development of convection via the wave-CISK. The sensitivity simulations that turn off the latent heating failed to capture such wavelike features, which verified that convective heating plays a key role on the waves.

A low-level thick stable layer below 1.5 km was capped by a reflecting layer with low stability within 1.5-4 km. A single critical level exists in the less stable layer with Ri < 0.25. Such environment provided a good ducting of gravity waves that supports to guide and maintain waves near the surface. It was found that the vertical and horizontal wavelength and intrinsic ducted wave speed of the gravity wave are all consistent with the theory of wave ducting. In order to further investigate the impact of the wave ducting, the terrain height was raised to reduce the stable layer depth. The vertical and horizontal wavelength deceased accordingly in the sensitivity test. Therefore, wave ducting and wave-CISK co-exist (ducted wave-CSIK mode) in the banded convection activity.