Low-level moisture analysis from refractivity data derived from a network of S-band and X-band Radars using ARPS 3DVAR

The near-surface atmospheric refractivity can be calculated using phase change measurements between any two ground clutter targets aligned along the radar beams. These refractivity observations are most sensitive to atmosphere moisture content, and therefore can provide high-resolution information on the often highly spatially varying low-level moisture, with the spacing of good ground targets determining the data resolution. The assimilation of such refractivity data into a Numerical Weather Prediction (NWP) model is expected to improve convective initiation and quantitative precipitation forecasting.

The Advanced Regional Prediction System (ARPS) Three-dimensional Variational Analysis (3DVAR) system is enhanced to include the analysis of radar-derived refractivity measurements. This work expands upon an earlier effort that analyzes simulated phase change data between ground targets assuming that the temperature and pressure are known. The current work simultaneously analyzes water vapor, temperature, and pressure that all affect refractivity. The analysis is performed together with other available data, including those from the Oklahoma Mesonet, and uses an NWP model grid as the background. The Mesonet data serves as an important anchor in the analysis because it provides direct observations of the state variables. This procedure will be tested using the refractivity data collected by the Oklahoma City (KTLX) and Fredrick (KFDR), Oklahoma S-band WSR-88D radars. In the future, data from X-band radars of the IP1 located between the Oklahoma City and Fredrick radars will be included in the analysis, providing better overall spatial coverage of refractivity. The impact of assimilating such data on convective initiation and precipitation forecast will be evaluated for selected cases.