A version of the University of Oklahoma's Advanced Regional
Prediction System (ARPS) has been enhanced with an ability for
grid points to adapt in the vertical direction. Thus vertical resolution can be increased in the layer with strong vertical thermodynamic or dynamic gradient like a cloud-capped boundary layer.
The advantage of CDGA (Continuous Dynamic Grid Adaptation)
compared to vertical stretched coordinate is that the high vertical
resolution region follows the movement of feature, such as strong
vertical gradient region, rather than fixes at certain area. On the basis of traditional CDGA, a newly designed grid distribution scheme is developed. In a numerical domain three layers (i.e. surface, middle, and top) are defined. The depths of these layers are adjustable. The grid points in the surface and top layers are relatively stable while the CDGA works in the middle layer. So relatively high resolution near surface can be preserved and unexpected numerical noise near the top is restrained. This scheme is very useful for atmospheric boundary layer study, especially
when surface process and phenomena play a key role in whole system.
The scheme and capabilities of CDGA are tested and demonstrated with several idealized simulations. Propagation of gravity waves along the interface between two neutral layers which produces an oscillating feature is executed to exam the response of CDGA. The scheme is also used to simulate the development of a convective boundary layer covered by a deep stable layer. With continuous supply of heat and moist from surface and turbulence mixing, the top of boundary layer arises. ARPS with CDGA performances satisfactorily in these experiments.
The purpose of configuring CDGA in ARPS is to forecast low-clouds
and marine fog off the west coast of North America. During late spring and summer, a high pressure area over eastern Pacific produces a large scale subsidence and a northerly wind which provide a favorable condition for the formation of low clouds and marine fog. An 100 km X 100 km area over Pacific ocean with same horizontal resolution and CDGA in vertical direction is nested in non-CDGA ARPS to simulate an event on July 25, 1997 in which fog and low cloud are observed. The advantages and disadvantages of the forecast model with CDGA in this simulation are documented.