Title: Adapting the UKMO linear model for NWP-based nowcasting

Authors: C. L. Charlton-Perez (UK Met Office)
S. P. Ballard (UK Met Office)
Z. Li (UK Met Office)
D. Simonin (UK Met Office)
H. Buttery (UK Met Office)
N. Gaussiat (UK Met Office)
L. Hawkness-Smith (UK Met Office)

The UK Met Office is developing an NWP-based nowcasting system which will operate at high resolution in space (1.5 km) and time (0-6 hour forecast). The main aim is to forecast convective storms and then use these forecasts to predict floods. We will present results from testing the ability of the linear model used for 4D-Var to represent the dynamics and microphysics of the nonlinear model at convection-permitting scales.

Our team's work is focussed on the assimilation of high resolution, novel observation types to enhance this nowcasting system. These observation types include Doppler radar winds, radar reflectivity, radar-derived rain rates and in the future may include radar refractivity. Initial trials have used a data assimilation system run with hourly cycling either in 3D-Var or 4D-Var mode along with latent heat nudging of radar-derived precipitation rates and humidity nudging based on 3D cloud cover analyses. For the trials the model domain covers southern England and Wales with boundary conditions provided by a 4km resolution forecast with a domain covering the whole of the UK.

Unlike the Unified Model at coarser resolutions, both the linear and nonlinear models in the nowcasting system operate without a convective parameterization. Therefore, we subject the linear model to a series of linearization tests to confirm that the linear model is simulating the nonlinear model well at the nowcasting scales. Several areas for improvement have been identified using measures of linearization error. In the linear model, parameters for both the precipitation efficiency and the boundary layer scheme for vertical diffusion of moisture and temperature are being re-tuned. The treatment of moisture in the linear model microphysical scheme is being re-assessed in conjunction with the assimilation of radar reflectivity which demands a different model operator for reflectivity of ice vs. liquid water.

GMAO Head: Michele Rienecker Global Modeling and Assimilation Office NASA Goddard Space Flight Center

Curator: Nikki Privé Last Updated: May 27 2011