DYNAMICAL AND MICROPHYSICAL RETRIEVAL FROM SIMULATED DOPPLER RADAR OBSERVATIONS USING THE 4DVAR ASSIMILATION TECHNIQUE

Based on a cloud model and the four-dimensional variational data assimilation method developed by Sun,et al.,simulated experiments of dynamical and microphysical retrieval from Doppler radar data were performed.The 4DVAR data assimilation technique was applied to a cloud scale model with a warm rain parameterization scheme.The 3D wind,thermodynamical,and microphysical fields were determined by minimizing a cost function,defined by the difference between both radar observed radial velocities reflectivities and their model predictions.The adjoint of the numerical model was used to provide the gradient of the cost function with respect to the control variables.Experiments demonstrated that the 4DVAR assimilation method was able to retrieve the detailed structure of wind,thermodynamics,and microphysics using either dua-Doppler or single-Doppler information.The quality of retrieval depended strongly on the magnitude of const raint with respect to the variables.Retrieving the temperature field,cloud water and water vapor was more difficult than the recovery of the wind field and rainwater.Accurate thermodynamic retrieval requires a longer assimilation period.The inclusion of a backg round term,even mean fields from a single sounding,could be helpful to reduce the retrieval errors.Less accurate velocity fields were obtained when single-Doppler data were used.It was found that the retrieved velocity was sensitive to the location of the retrieval domain relative to the radars while the other fields had very little change.Two radar volumetric scans are generally adequate to providing the evolution,although the use of additional volumes improves the retrieval.As the amount of the observations decreased,the performance of the retrieval is degraded.How ever,the missing observations can be compensated by adding a background term to the cost function.The technique is robust to random errors in radial velocity and calibration errors in reflectivity.The boundary conditions from the dual-Doppler synthesized winds were sufficient for the retrieval.When the retrieval was mainly controlled by the observations in the regions away from the boundaries,the simple boundary conditions from VAD analysis were also available.The microphysical retrieval was sensitive to model errors.