Geoscience Reference
In-Depth Information
The ultimate aim is to use 4D-VAR, if affordable and beneficial, in a real-time, routinely
running NWP-based nowcast system. This will exploit high resolution (in time and space)
Doppler radar measured radial winds and reflectivity or derived surface rain rates directly
within the variational analysis scheme. Direct use in 4D-VAR should allow optimum
extraction of information through interaction with other data sources, and the potential to
modify the dynamical and physical forcing of precipitation and convective storms.
Research is also proceeding to investigate the background errors, balances and control
variables required for use in convective scale data assimilation. The aim is to have a real-
time system running continuously from summer 2012 for southern England.
5.4.2 A case study comparison of conventional and NWP-based nowcasts
Figure 4
compares T+1 hour, T+2 hour and T+3 hour STEPS control member nowcasts of
surface precipitation rate, all valid at 2100 UTC on 3 June 2007 with a radar-based analysis
of surface precipitation rate for the same time. At T+3 h, the STEPS nowcast is a combination
of an extrapolation nowcast and UK4 forecast precipitation. The UK4 forecast tends to
produce individual convective precipitation elements that are too large. It also fails to
predict the full extent of the bands of convective precipitation to the east of the precipitation
area lying through south-west England and west Wales. At T+2 hours, the STEPS scheme
has re-produced the line of convection in the east but this is too narrow, possibly due to
convergence in the diagnosed advection velocity field. By T+1 h, a reasonable nowcast has
been produced.
Fig. 4. STEPS T+0 h, T+1 h, T+2 h and T+3 h nowcasts of surface rain rate all valid at 2100
UTC on 3 June 2007. The key shown on the right-hand side represents precipitation rate in
units of mm/h. Dry areas are shown in white. Note that dark blue areas in the STEPS
nowcasts are not included in the colour key. These represent light drizzle.
