5. A note about sea-surface temperatures and skin temperatures
The way sea-surface temperatures are archived seems to vary widely from
dataset to dataset. In REGRID parlance, we make a
distinction between sea-surface temperature and skin
temperature. A sea-surface temperature field represents the
temperature of the water (whether that's the temperature at the
water surface or temperature of some layer at the top of the
water). Over land, this field doesn't really have any meaning,
and in many datasets the values are smoothly interpolated from
water points. Example:
.
A skin temperature field represents the temperature
of the surface of the earth, whether that surface is land or
water. Thus, in summer for example, you may wind up with large
temperature gradients between warm land points and cold water
points. Example:
.
The skin temperature can be tricky to use within the MM5 modeling
system, because of the horizontal interpolation to the MM5 grid.
Horizontal interpolation to a point near the coast can wind up
being an interpolation from a land point and a water point. If,
for example, the summertime ground temperature is considerably
warmer than the water temperature, you may wind up with
unreasonably cold ground temperatures and unreasonably warm water
temperatures at the coast. This will be particularly noticeable
when interpolating to an MM5 grid that has a much higher
horizontal resolution that the source dataset.
At the REGRID stage, fields SST and SKINTEMP are treated
identically. They are simply interpolated to the MM5 horizontal
grid, with no concern for what may happen at the coast.
Later in the MM5 modeling system, in program INTERPF, fields
called SST and SKINTEMP are interpreted differently. If an SST
field is found, INTERPF will interpret that field as one that can
be used as an instantaneous field appropriate for water
temperatures, which may vary in time. INTERPF will create a
lower-boundary-conditions file (LOWBDY) with an SST field for each
analysis time. This field will be called TSEASFC. If there is no
SST field, but a SKINTEMP field is found, INTERPF will average
SKINTEMP for all times, and write that average as a single,
time-invariant TSEASFC field to the LOWBDY file. This averaging
is done to remove the diurnal variation of the skin temperature
over land in the averaged, time-invariant TSEASFC field.
In short, fields called SST will be interpreted as instantaneous
water-temperature fields and will allow time-varying sea-surface
temperatures
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Last modified: Thu Aug 31 15:08:27 MDT 2000