Because the NEXRAD radar network provides only single-Doppler scanning over most areas in the U. S., research efforts have been undertaken in recent years to develop various methods for meteorological parameter retrievals from single-Doppler observations. The previous efforts, however, were focused mainly on retrievals with zero background information. When these methods are upgraded for Doppler radar data assimilation, it is necessary to introduce a background term in the costfunction to utilize the information provided by model predictions. Ideally, the background term should be introduced in connection with the general formulation derived for the conditional maximum likelihood estimate. How should this be done? Specifically, how should the general formulation be simplified (with certain assumptions) into a form suitable for assimilating Doppler observations? These and related problems will be addressed at the conference. Examples and results of numerical experiments will be also be presented to show how the previous developed simple adjoint method and least-squares method are upgraded for Doppler radar data assimilation.