Taking Stratospheric Effects Into Account Improves
Extended-Range Weather Forecasts
Minoru Kadota and Kevin Hamilton
Error at stratosphere (150 mb)
Error near surface (850 mb)
Root mean square errors in the NAM index
as a function of forecast lead time. Results
only for cases with a strong initial
stratospheric vortex. Results for raw
ensemble forecast (black), with a mean drift
statistical correction (blue) and with a
strong-vortex bias correction (red). Errors
expressed in standard deviations of the
observed time series.
Structure of NAM in geopotential computed from
NCEP reanalyses at 6 pressure levels.
Results from the NOAA Reforecast Project
were used to identify systematic effects of the
stratospheric winter vortex circulation on
tropospheric weather forecasts. The Northern
Annular Mode (NAM) structure and NAM
index time series were computed for the
November through April period from daily
geopotential observational analyses and
compared to the NAM index in the ensemblemean reforecasts produced by NOAA for
1979-2007. As a first step, the mean drift of
all reforecasts away from observations was
removed from individual forecasts. These
corrected forecasts were then examined for
any remaining systematic biases associated
with strong stratospheric vortex anomalies
that existed in the initial conditions. The
forecasts conducted when the winter vortex
was anomalously strong are significantly
biased on average compared to observations.
When each forecast in this group was
adjusted with a statistical bias correction, the
average skill of the forecasts improved
considerably in the stratosphere and also
near the surface.