Low-level wind error in the tropical East
Pacific
During the
Northern Hemisphere summer of 2001, the NCEP global operational model produced better
forecasts of hurricanes than any other model; however, in standardized skill
scores in the tropics it continued to lag other models, particularly at 850
hPa. The RMS vector error is especially
large in the MRF at one day. This study
examines a large time-mean error in the one-day forecasts of meridional wind
(verified against its own analyses) in the tropical East Pacific south of the
equator below 500 hPa. The error occurs
during most, but not all forecasts, and has been present for at least a year. The error is large enough to contribute
substantially to the tropical root-mean-square error in one-day forecasts of
the meridional wind; eliminating the mean error statistically reduces the
tropical mean RMS error by 10%.
The
error occurs near two profiler stations, one
in the Galapagos Islands at 1S, 90W and the other on the coast of Peru at
5S, 81W. The one in the Galapagos
appears to measure only the lower atmosphere to 4 or 5 km; the other appears to
measure the atmosphere to 15 km. The
winds in the latter are blacklisted above 200 hPa. The profiler winds were previously blacklisted. The error in the
one-day forecasts is associated with changes in the divergent circulation in
the eastern portions of the Inter-Tropical Convergence Zone. The error persists in the two-day and
three-day forecasts, but does not grow.
It becomes more rotational with forecast length. One hypothesis is that the error is caused by
either inaccurate profiler data or an incompatibility between the model and the
profiler data. The error appears to be
present in the parallel x test of a higher resolution model; removing the
profiler data from the parallel x would test whether the error is associated
with the profilers.
R. Kistler and R.
Treadon did remove the profiler data from the parallel x for a few weeks; the
results show a dramatic decrease in error in the immediate vicinity of the
Galapagos profiler. However, the larger
east Pacific error remains.
Pressure-longitude
cross-section of v at 2.5S
Winds at 800 hPa
The
model's clouds in the region appear to have substantial differences from
independent estimates, reflecting difficulties in the modeling of low-level
stratus. Whether these problems in
cloudiness force the errors described above or are partly forced by them is not
obvious.
Longitude-pressure
cross-sections of the time-mean meridional wind at the equator
Analyses
and 12 hr forecasts valid at 1200UT (East Pacific)
6hr
and 12 hr forecast errors valid at 1200UT (East Pacific)
Longitude-pressure
cross-sections of the time-mean meridional wind at 2.5S (East Pacific)
Latitude-pressure
cross-sections of the time-mean meridional wind at 95W 30S-30N
Time-mean meridional
wind at 850 hPa
Time series of
meridional wind at 2.5S
RMS error in
meridional wind 20S-20N
|
1 day
forecasts |
3 day
forecasts |
Time-mean wind in
the East Pacific
|
850 hPa |
750 hPa |
|
|
Time-mean 750 hPa
stream function in East Pacific
Time-mean errors in
absolute vorticity
Time-mean horizontal
divergence
5N
2.5N
Equator
95W
750 hPa
Time-mean vertical
velocity
7.5N
2.5N
95W
500 hPa
Clouds
Total
Boundary layer
Low
Middle
High
Surface short wave
radiation
|
Downward |
Net |
Precipitation
