WHAT'S NEW ABOUT THE RUC-2?
1. Frequency of Assimilating Observations
In the old version of the Rapid Update Cycle (RUC-1),
observations are assimilated once every three hours. In the
new version (RUC-2) observations are assimilated hourly.
2. Time of Availability
In the 60km RUC-1, the data cutoff time is +1h, 20 min. For
the 40km RUC-2, the data cutoff time is about +0h 20 min,
meaning that RUC-2 analyses and forecasts are available
almost an hour earlier than those from the RUC-1. A catch-up cycle will
be used to assimilate late-arriving rawinsonde data
at 0000 and 1200 UTC, and forecasts will be run from the catch-up
cycle rather than the first cycle at these times.
3. Computational Grid
RUC-2 covers a geographical domain about 50% larger
in area than the 60km RUC-1, extending further in all
directions, but especially in the southeast, north, and
west, and covering considerably more oceanic area. Moving the
boundaries slightly farther from the coasts has significantly
improved forecasts in these areas. The old
RUC-1 grid has 60-km resolution (81 x 62 grid) and 25 levels.
The new grid has 40-km resolution (151 x 113) and 40 levels.
The RUC-2 continues to use an isentropic-sigma hybrid vertical
coordinate.
4. Data Assimilated
There are several new types of observations assimilated into RUC-2 not present in the 60km RUC. These include:
5. Moist Physics
Clouds must be inferred from the relative humidity field in
RUC-1. In RUC-2, a comprehensive cloud physics
package has been imported from the NCAR/Penn State MM5
mesoscale model. Mixing ratios of five types of
hydrometeors are explicitly predicted: cloud water, rain
water, snow, ice crystals, and graupel. Even the number
density of ice crystals is predicted as part of the
microphysical processes.
Cloud fields initialize each RUC-2 model run, using the
most recent hydrometeor forecast to avoid cloud spin-up
problems.
6. Surface Processes
The RUC-1 calculates surface fluxes using a simple surface slab with constant soil moisture availability. The RUC-2 features a multi-level soil and vegetation model with evolving soil moisture and temperature fields that are far more accurate than climatology. Snow accumulation and melting are accounted for.
The 60-km RUC employs a coarse land use data and derives sea
surface temperatures from climatology and does not consider
snow cover. The RUC-2 employs improved land use data,
including vegetation class, monthly vegetation fraction from
NESDIS, and soil type; it utilizes daily detailed fields of
sea-surface and lake-surface (for the Great Lakes)
temperature. It also cycles snow cover and
canopy water along with the soil fields to further improve
short-range forecasts.
7. Turbulence
The RUC-1 computes turbulent fluxes from a formulation due
to Mellor and Yamada (Level 2.0). The RUC-2 calculates the
kinetic energy associated with turbulence (TKE) explicitly
from equations derived by Burk and Thompson (Level 3.0).
Boundary fluxes are improved using the explicit turbulence
parameterization, and explicit TKE maxima are commonly found
in upper-level frontal zones.
8. Radiation
There was no explicit calculation of atmospheric radiative
fluxes in the RUC-1, a simplification partly responsible for its warm bias.
Full atmospheric radiation imported from the MM5 model is
included in RUC-2. The radiative heating/cooling is
influenced by hydrometeors (clouds) at each model level.
9. Lateral boundary conditions
The old RUC-1 used lateral boundary conditions specified from the NGM model at 6-h intervals. RUC-2 uses early Eta model boundary values specified at 3-h intervals.
Most dramatic improvements in RUC-2 fields over those from RUC-1. There are improvements in almost every field from RUC-2, and also many useful new fields not provided in RUC-1. These are some of the highlights