Large-eddy simulation of cloud streets over the East China Sea during cold-air outbreak events

Convective boundary layer develops as very cold air originated from Siberia and China breaks out over the Japan Sea, the Yellow Sea, and the East China Sea during winter seasons. The CBL quickly deepens away from the coastline of China with increasing fetch length and increasing sea surface temperature. As the depth of the CBL changes, the embedded roll vortices (cloud streets) grow in size. The convection eventually becomes three-dimensional in the farther downstream region.

We have used the National Taiwan University / Purdue mesoscale numerical model to simulate the early development stage of the phenomena. The model solves a fully compressible, nonhydrostatic system of equations explicitly with a two-stage forward-backward time integration scheme. Since the numerical procedure is neutral with respect to both sound waves and internal gravity waves, there is no need to impose any time-smoother in the model. Thus, the model results can be very accurate and numerically stable. In addition, the explicit algorithm is particularly suited for parallel computation. Our computer program is efficiently parallelized and it is suited for this very demanding problem in terms of computer resources.

With the use of an open lateral boundary condition along the mean wind direction on both ends of the calculating domain, we are able to simulate the roll vortices under heterogeneous environment. Our preliminary results showed that the size of convection rolls indeed changes along the mean wind direction and the model reproduces all major characteristics of the CBL.