"Trevor Harley" wrote in message
news:2006011709535616807-taharley@dundeeacuk...

Is there any simple way to convert thickness (see, I think I've
learned something here) to likely surface maxima and minima?


.... The thickness value (500-1000 hPa/TTHK) is one of the *worst*
parameters to use to try and forecast surface temperature data,
particularly for winter, continental situations (and airmasses coming
out of such areas).

The problem is very well illustrated by the situation over east and
northern Poland at the moment:
Using the 12Z radiosonde data yesterday (Tuesday) from Legionowo
(12374), the thickness value is given as 5319m (or 532 dam).

Day-time maxima across a large swathe of north, central and eastern
Poland yesterday (Tuesday) were generally -6 to -8degC. Yet 532 dam is
hardly a very low thickness value. If it were to 'obey' a standard (ISA)
profile, you might be expecting a screen maximum around 4degC.

The key of course lies in the snow-cover over eastern Europe. If you
look at the ascent here

( http://makeashorterlink.com/?G29A25C7C )

you'll see that the lowest few hundred metres of the troposphere has
become totally divorced (or decoupled) from the 'main' ascent, and heat
exchanges (radiation/conduction) are occurring without reference to what
is happening higher up. Note also that the 850hPa level is also just
above the boundary-layer region, so even using 850 hPa temperatures
could lead you astray (this applies to such as 850-1000 hPa thicknesses
of course).

Will was talking about this in his summary of the situation: by all
means use the TTHK values as a 'first guess' to chase the broadscale
airmass changes around the synoptic map, but in these situations where
the character of the surface is highly important, then it is best to use
a 'tracer' at much lower levels (if available) such as 900 hPa.

Martin.


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