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Rapid Refresh (RAP) and High-Resolution Rapid Refresh (HRRR) Forum

RUC vs ETA - Convergence forecasts

Posted By: Jack Glendening
Date: 24-June-04 2113Z

Last year (25 June) I reported on this forum that RUC predictions of
convergence lines (actually the maximum predicted vertical velocity
within the boundary layer) created by topography in the northwest
region of California's Central Valley have been found to be
qualitatively correct based on verifications by sailplane pilots who
use them for long distance flights. The RUC predictions are accurate
enough that pilots have come to rely on them for their flight
planning, in evaluating whether they should expect to be able to find
and utilize such a line.

This year I am producing similar convergence predictions based on the
ETA model. ETA of course has a smaller grid resolution (12 km) so
would on that basis be expected to provide better convergence
predictions. But these observed (and RUC-forecast) lines do _not_
appear in the ETA predictions. One can speculate on why that is so -
perhaps it is an effect of the coarser vertical resolution near the
surface used by the non-terrain-following ETA or perhaps artificial
smoothing is greater over terrain in the ETA.

However, on occasion relatively strong ETA convergence lines _have_
been noted near the coastline, over flatter terrain, whereas RUC has
not previously predicted significant convergence there. Verification
of these lines is spotty because they occur in locations where pilots
do not normally find lift and typically do not fly, so most are
reluctant to go there and chance "landing out". But the few pilots
who have made that attempt (when a line was marked by clouds, which
gave visual assurance that lift actually existed there) did find
sufficient vertical motion to keep themselves aloft as they followed
that line, and those cases did occur when ETA was predicting a
convergence there.

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