Problem with surface and 2 meter temperature, and Tmax

The skin temperature is determined diagnostically.  It is temperature
required to balance the fluxes at the surface.


AIR:
             latent heat flux
             sensible heat flux *
             short wave (up and down)
             long wave (up and down) *

Ground:
	     heat flux into soil *

The terms with with an asterisk have a dependence on the skin temperature.

The problem occurs when the winds are weak (less than 0.75 m/s), and the
stability conditions are just right. Under these conditions, the 
parameterization for the thermal exchange coefficient can break down.  
This caused the model to blow-up (divide by zero).  (After running this
code for years, the bug surfaced.)  A bug fix was introduced (instead
of dividing by zero, dividing by a small number).  However, the
fundamental problem remained.  Under light winds, the thermal exchange 
coefficient can be close to zero.  For argument's sake, let's assume that
the exchange coefficient is zero.  In this case, the sensible heating
is zero, and the solar heating has to be balanced by latent heat flux, the 
net longwave flux and a ground heat flux.  Since the latent heat flux is not 
dependent on the skin temperature, the reduction of the sensible heat
flux must be balanced by an increased upward long wave flux and a
downward ground heat flux.  In order to make this balance work, the
skin temperature will become unreasonably hot.  On the next time step, the 
winds may pickup and skin temperature will return to normal (skin temperature 
has no interia).  The affect on the lowest sigma level is minor as the net 
fluxes remain reasonable.  This phenomenon will not happen over water as
the skin temperature (sst) is fixed.

Affected variables:

Tmax (Tmin?)  Tmax is the worst possible case - it will capture spikes that
              only last 1 time step

surface temperature

2 m Temperature  The 2 m temperature is "interpolated" from the skin
              and bottom sigma-layer temperature

sensible heat flux

upward long wave radiation flux


All variable have not been checked out.  There may be more variable affected.



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                   Test:


The only test I have is to reject <instantaneous> data if

        (1) over land
        (2) 10 meter wind speed < 0.75 m/s

This test rejects ~ 2% of the total # grid points.  It will not work
for averaged quantities and Tmax, Tmin.  In addition, this test
rejects an awful amount of good data.  In the monthly means I will be
not be using this screening test because IMHO rejecting so much good
data would be worse than including a few bad points (which get smoothed
out in the monthly averages).


A test to reject data if > N standard deviations may work too.

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                                       August 18, 1995
                                       Wesley Ebisuzaki