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Message: fixes: freezing level, sfc temp, precip type

Posted by Stan Benjamin on 28 Jan 98, 9:38 MT

A set of changes to the RUC-2 post-processing went in at 12Z this morning in response to comments from users:

Freezing level:

1) bottom-up freezing level now done as follows:

if Tsfc < 0 deg C, return sfc but overwrite this with bottom of warm layer aloft (above freezing) if there is one. (suggested by WFO BTV and agreed to by Aviation Weather Center)

if Tsfc > 0 deg C, return 1st level at which temp goes below 0 deg C

2) top-down freezing level:

searches from top down for 1st level at which temp goes above freezing. Returns sfc if entire column is below freezing. Returns top of warm layer aloft if there is one.

The top-down freezing level was not correct until the fix this morning.

Both levels are interpolated between RUC-2 native levels to return the best estimate of the actual freezing level. These fields are pretty interesting -- they really highlight the warm layers aloft.

Precipitation type:

Was previously keying only on instantaneous precip at sfc, but now also looks at accumulated total precip, snow, and rain. A "snow ratio" is calculated based on snow/precip. If this ratio exceeds 25%, snow is diagnosed. If the ratio is
less than 75%, rain is diagnosed. Thus, for a snow ratio in the 25-75% range, the algorithm would return both rain and snow.

The precip type is also influenced by the next change.

Diagnosis of sfc temp

The sfc temp and dewpoint from RUC-2 are diagnosed using a "minimum topography" field. This is discussed in http://maps.fsl.noaa.gov/MAPS.40km.html.

In this diagnosis, a lapse rate is calculated between levels 1 and 5 in the native RUC-2 output with which to extrapolate down to the minimum topography level. Previously, this lapse rate had been bounded by the dry adiabatic (10 K/km) on the high end and the standard lapse rate (6.5 K/km) on the low end. The lower bound of the standard lapse rate was too restrictive, we decided, so it is now changed to an isothermal lapse rate (0 K/km). This gives colder sfc temperatures in rough terrain regions at night or other very stable situations. In the cases we've looked at, it seems to help fit the sfc obs better and take away a warm bias. We think there is still a slight warm bias at night -- the RUC-2 doesn't cool off enough. We've given this a lot of thought and believe it is related to either radiation or night-time mixing in the sfc layer -- we hope for a fix in the next few months but it will take some work. But this extrapolation change will help some.

Hope you find these changes helpful.


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