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Message: the change package - probably next week

Posted by Stan Benjamin on 17 Dec 98, 14:16 MT

Here is the latest on the RUC change package:

It has been sent to NCO and tested successfully today by NCO. They will try to implement it next Monday. If you've followed RUC or Eta or other NCEP changes before, you know that other things (critical weather, etc.) can push back these plans, but this is the current schedule.

Let's review again, just for the record, what is in the change package:

Y2K fixes

Analysis changes

- smaller horizontal error correlation near sfc for T/Td, slightly less dependence on stability => improved sfc T/Td/wind fit in mtns, better near sfc and lower tropospheric structure overall

- fix to use of cloud drift winds => will have much more effect (over water only). Weight had been getting set to very low values, and this is being fixed. West Coast RUC users should start seeing some differences.

Model changes

- fixes to radiation - more cooling at night, more heating in daytime -- A big improvement in the RUC2 diurnal cycle. This fix was removal of the application of the water vapor mixing ratio from the model top level (~60 mb) up to 0 mb. This extra water vapor was acting a bit like a cloud at the top of the model, and damping both longwave and shortwave temperature changes beneath.

- fixes to evaporation physics - further reduction of cool bias in daytime

- fix to convective precipitation to average over deeper layer near surface within the Grell scheme - reduces excessive precipitation over warm water and improves convective precipitation over Florida and along Gulf Coast and along south Atlantic Coast.

Diagnostic fixes

- CAPE/CIN - mix lowest 30-40 mb instead of only 20-25 mb - less jumpiness from analysis to 1h fcst. CAPE users will like this improvement (developed in collaboration with SPC forecasters),

- tropopause level fix - smoother trop fields due to a fix in the algorithm. See http://maps.fsl.noaa.gov/vartxt.cgi for more details on RUC tropopause levels fields and other diagnosed RUC fields (e.g., SLP, precip type, snow accumulation...)

- max wind level fix in case wind is zero at all levels ( not likely, but...)

GRIB table fixes

- improved precision in several variables, including soil moisture. This fix is very important since the RUC soil moisture has been unable to dry out properly since the GRIB precision was not sufficient to resolve 1-h changes in this field. The RUC soil moisture is too high overall now due to this problem, and will finally start to adjust with this change, but it will take a few weeks or longer.

Eta boundary conditions

- The Eta model changed its definition of relative humidity on 11/3/98 from with respect to water to a water/ice combination. Although this change gave more realistic RHs for forecasters thinking about ice clouds, it was unannounced and started causing problems for the RUC and local models using Eta for initial conditions and still assuming that the Eta RH was with respect to water. There is far too much ice cloud in the RUC since 11/3 because of this problem, and especially near boundaries.

NCEP is going to back out the 11/3 Eta RH change, but it looks like a RUC fix to this problem will go in a few days beforehand. The RUC fix will be backed out at the same time the Eta RH change is backed out. This is the current plan -- we'll post other info as we find out more.

Another post will be made when the RUC changes actually go in.

As always, please post any questions you may have.



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