Tuesday, 31 July 2001: 8:59 AM
The 20-km version of the RUC
In spring 2001, a new version of the Rapid Update Cycle (RUC) will be implemented
at NCEP, with extensive changes to the analysis and forecast model as well as
the doubling of horizontal resolution from the previous 40km resolution.
This is the first major overhaul of the RUC since April 1998.
This paper will summarize these differences and changes in forecast
and analysis characteristics resulting from the changes. The assimilation
changes include a 3d variational analysis, assimilation of GOES cloud-top data,
and improved treatment of near-surface data. The 20-km RUC model includes
a new version of the Grell convective parameterization, significant
modifications to the MM5/RUC mixed-phase cloud microphysics, and much
more detailed treatment of the land surface. The paper will summarize
the integrated effects of these changes -- in particular, improved near-surface
and precipitation forecasts.
The RUC is still the only operational assimilation system with a 1h intermittent
forward cycle, but now with 20-km resolution. This means that the forecast
errors that the hourly assimilation is trying to correct have still smaller
spatial scales, while the data are of essentially the same resolution as before. Results
will be presented comparing accuracy of RUC 1h forecasts vs. 3h/6h forecasts
for different variables and levels as a means to investigate how much aliasing
from high-resolution high-frequency assimilation may be affecting results.