Latest development of 3DVAR system for ARPS and its application to a tornadic supercell storm

As reported in the previous NWP conferences, an incremental 3DVAR data assimilation scheme for a storm scale nonhydrostatic NWP model, the Advanced Regional Prediction System model (ARPS), was developed at Center for Analysis and Prediction of Storms (CAPS). The emphasis of this 3DVAR system is given to the use of radar data, though other types of data, including Oklahoma Mesonet observations, rawinsondes and wind profiler data, are also used. In our recent development, the cost function is extended by including two new weak constraints to couple pressure and temperature fields with wind field, which are derived from ARPS model equations. By doing so, the usage of radar data can be maximized by also providing thermodynamic information. On the other hand, an anisotropic recursive filter is implemented in the system to model the effect of flow-dependent background error covariance. With the anisotropic filter, the pattern of the background error can be stretched in directions oblique to the alignment of the grid coordinates and is constructed by applying at each point six one-dimensional recursive filters along six directions corresponding to a special configuration of oblique lines of the grid. The application of new 3DVAR system to a real data case (3 May, 1999 tornado outbreak) has been performed and preliminary results exhibit improvement in the forecast. Several new numerical experiments will be conducted and their performance will be reported at the conference.