"kieler.sprotte" wrote in message
...
Hi,

Does someone know criteria (empirical, theoretical,...) which hleps
me forecasting drizzle or rain in the warm sector.

Perhaps there is a link between the cloudlayerthickness and what
kind of precipitation we have to forecast.
I know that if the layer is to thin only small droplets will be
build (warm rain)an not by the Bergeron-Findeisen process.

To produce drizzle requires coalescence of very tiny cloud droplets to
such a size (albeit still very small) that they can just leave the
cloud against any weak upcurrents that are involved in the
cloud-formation process. As you write, the clouds are 'warm' cloud
type and the clouds must have a high liquid water content to
precipitate (which is one factor that modern high-resolution NWP
models can handle reasonably well), or evaporation will offset the
precipitation-formation process.

The relative humidity below the cloud base must also be high (well
above 90%) or again, evaporation will come into play - and in general
terms, the higher the cloud base, the thicker must be the cloud for
any PPN to reach the surface. Although vertical (upward) motion must
be small (or the elements would not fall from the cloud), there must
be a reasonable amount of turbulence through the cloud-layer to lead
to efficient collision & coalescence.

The cloud must also be reasonably thick - various studies suggest
somewhere between 400 & 600 metres (or roughly more than 1300 ft up to
~2000 ft), but this is highly dependent upon such things as the
concentration of condensation nuclei present, humidity content,
updraught strength etc. If the cloud layer is thicker than these
values and/or the updraught strength is greater, then this tends to
lead to rain (by standard definition) rather than drizzle - in
practice, as I'm sure you're aware, it is devilishly difficult to
forecast when one will form rather than the other! I've been
underneath cloud that *ought* to have produced something, but it stays
dry all day.

And, as I see you have cross-posted to world-wide groups, it is worth
emphasizing that the studies that gave rise to the figures I have
quoted were performed within the British, essentially maritime,
environment: others may want to comment relating to different
environments.



Martin.


--
Martin Rowley