Direct assimilation of radar reflectivity data in the Met Office convective scale forecast system

The Met Office is developing a high resolution NWP based forecasting system with the aim of replacing current nowcasting techniques. An hourly-cycling 4D-Var 1.5 km nowcasting system, using the Met Office non-hydrostatic Unified Model, is being trialled on a southern UK domain.

Radar data are an important source of observations for the initialization of high resolution models. The direct assimilation of radar reflectivity data with other observations in 4D-Var allows all available information to be used in a dynamically consistent approach to initialize a forecast, without the need for diagnostic adjustment.

The Met Office Observation Processing System (OPS) has been developed to provide quality controlled radar reflectivity data to the variational assimilation system (VAR), and a system for the monitoring of observation – model background (O-B) differences and standard deviations has been running continuously for one year. The very high resolution radar reflectivity observations are averaged to a coarse grid (super-obbed) prior to assimilation.

The VAR code has been developed to support the direct assimilation of reflectivity data. This involved coding an observation operator, linearization and adjoint, and enhancements to the linear physics in the perturbation (linear) forecast model.

This paper will describe current work and future plans, and present results from assimilation case studies.