AIDJEX Bulletin #40 - Polar Science Center - University of Washington

error in this prediction. The error in the pressure field has several components.
There is the inherent error in the accuracy of the instruments that
provide the initial and verifying pressure fields. The accuracy required
for NWS is no greater than 20.5 mb for individual instruments, while that
for AIDJEX pressures is k0.3 mb. When the individual pressures are processed
by hemispheric objective surface analysis (as is done by NWS), the analysis
errors will be largest where the density of data points is low and the distribution
thin. The sparseness of weather stations in the Arctic results in a
lack of detail in the pressure field analysis. Since the ice responds to
large-scale averages, this may affect the ice model prediction.
For the persod of investigation, the average error in the NWS analysis
and prognosis can be estimated from the AIDJEX surface analysis since the
latter involves a more rigorous local pressure analysis and includes many
stations: the buoy array, three camps, shore weather stations, and a weakly
weighted border of points taken from the NWS analysis over the northern
portion of the ocean. A sixth-order polynomial least squares fit to the
AIDJEX data produces the surface pressure analysis and a readily obtained
pressure gradient field. This contrasts to the NWS analysis, which generally
contains only the shore stations in a computerized hemispheric analysis and
therefore produces a much smoother field. The prognoses then use this analysis
as initial values.
From this comparison, the NWS maps contain errors averaging from t0.5 mb
at the shore to k3.0 mb at the AIDJEX camp approximately 300 km offshore.
During the period in question, this error translates into an error in geostrophic
wind (derived from the pressure gradient) of up to 50% in both the
NWS surface analysis and the 24-hour prognosis. This is shown in Table 1,
whic5, in a comparison betreen NWS analyses and proguoses and AIiIJZX analyses,
estiulates errors for a representative point within the XIDJEX buoy array
during the period covered. The 24- aiii 48-hour prognoses on the bottom line
(the estimated error in the geostrophic flow derived from the different
pressure fields) are taken from Walsh (1977).
Surface pressure maps for 30 January 1976 are shown in Figure16 (made
from NWS analyses) and Figure 17 (from AIDJEX analyses).
16 8