Low-level wind error in the tropical East Pacific

 

Glenn.White@noaa.gov

 

          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

 

1 day error in operational

1 day error in parallel

Difference in 1 day error

 

Winds at 800 hPa

 

1 day error in operational

1 day error in parallel

Difference in 1 day error

 

          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

1 day forecasts

2 day forecasts

3 day forecasts

 

1 day error

2 day error

3 day error

 

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)

 

Analyses and 1 day forecasts

Analyses FNOC and NCEP

1 day forecasts FNOC and NCEP

6hr and 1dy forecast errors

1dy forecast errors in FNOC and NCEP

Analyses and 1 day forecast differences in FNOC and NCEP

 

Latitude-pressure cross-sections of the time-mean meridional wind at 95W 30S-30N

 

Analyses and 1 day forecasts

6hr and 1 day forecasts errors

6 and 12 hr  forecast errors valid at 1200UT

 

Time-mean meridional wind at 850 hPa

 

Analyses and 6 hr forecasts

Analyses and 1 day forecasts

6hr and 1day errors

6 and 12 hr errors valid 1200UT

 

Time series of meridional wind at 2.5S

 

1 day fcst errors at 850 hPa

1 day fcst errors at 750 hPa

 

RMS error in meridional wind 20S-20N

 

1 day forecasts

3 day forecasts

RMS error with and without bias

RMS error with and without bias

Effect of bias and bias

Effect of bias and bias

Time series of RMS error

Time series of RMS error

 

Time-mean wind in the East Pacific

 

850 hPa

 

750 hPa

Analyses and 1 day forecasts

Analyses and 1 day forecasts

Analyses and 12 hr forecasts valid at 1200UT

2 and 3 day forecasts

6 hr and 1 day errors

1 day error

6 and 12 hr errors valid at 1200UT

2 day error

 

3 day error

 

Time-mean 750 hPa stream function in East Pacific

 

Analyses and 1 day forecasts

1 and 2 day errors

2 and 3 day forecasts

3 day errors

 

Time-mean errors in absolute vorticity

 

1 day forecasts

2 day forecasts

3 day forecasts

 

Time-mean horizontal divergence

 

5N

 

Analyses and 12 hr forecasts for 1200 UT

6 and 12 hr forecast errors valid at 1200 UT

 

2.5N

 

Analyses and 1 day forecasts

6 hr and 1 day forecast errors

Equator

 

Analyses and 12 hr forecasts for 1200 UT

6 and 12 hr forecast errors valid at 1200 UT

 

95W

 

Analyses and 12 hr forecasts for 1200 UT

6 and 12 hr forecast errors valid at 1200 UT

 

Analyses and 1 day forecasts

6 hr and 1 day forecast errors

 

750 hPa

 

Analyses and 1 day forecasts

6 hr and 1 day forecast errors

 

Time-mean vertical velocity

 

7.5N

 

Analyses and 6 hr forecasts

Analyses and 1 day forecasts

Errors in 6 hr and 1 day forecasts

 

2.5N

 

Analyses and 6 hr forecasts

Analyses and 1 day forecasts

Errors in 6 hr and 1 day forecasts

 

95W

 

Analyses and 12 hr forecasts for 1200 UT

6 and 12 hr forecast errors valid at 1200 UT

 

Analyses and 1 day forecasts

6 hr and 1 day forecast errors

 

500 hPa

 

Analyses and 1 day forecasts

6 hr and 1 day forecast errors

 

Clouds

 

Total

 

0-6 hr and nephanalyses

0-6hr minus nephanalyses

0-6 hr and 0-24 hr

0-24 minus 0-6 hr

 

Boundary layer

 

0-6 hr and nephanalyses

0-6hr minus nephanalyses

0-6 hr and 0-24 hr

0-24 minus 0-6 hr

 

Low

 

0-6 hr and nephanalyses

0-6hr minus nephanalyses

0-6 hr and 0-24 hr

0-24 minus 0-6 hr

 

Middle

 

0-6 hr and nephanalyses

0-6hr minus nephanalyses

 

High

 

0-6 hr and nephanalyses

0-6hr minus nephanalyses

 

Surface short wave radiation

 

Downward

Net

0-6 and 0-24 hour forecasts

0-6 and 0-24 hour forecasts

0-24 minus 0-6 hr

0-24 minus 0-6 hr

 

Precipitation

 

0-6 hr and 0-24 hr

0-24 minus 0-6 hr