Upcoming NAEFS implementation

TIGGE-GIFS (NCEP/GEFS metadata)

Effective 12Z 23 February 2010 - NAEFS/GEFS upgrade 
1) Change horizontal resolution from T126 to T190 out to 384hrs
2) Using 8th order horizontal diffusion for all forecast resolutions
3) Introduce ESMF (Earth System Modeling Framework) – Version 3.1.0rp2 for GEFS
4) Add stochastic perturbation scheme to account for random model errors
5) Add new variables (28 more) to pgrba files for NAEFS data exchange

Effective 12Z 4 December 2007 - NAEFS upgrade
1) GFS bias correction
2) Combination of GFS & GEFS forecasts
3) Probabilistic NAEFS forecasts
4) Downscaled NAEFS forecasts 

Effective 12Z April 6 1999:
Increase in initial perturbation amplitude size

Effective 07 December 1998 at 12Z:
Change in regional rescaling procedure for setting initial perturbation amplitudes

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Effective March 1997:

Ensemble forecast data are available on OSO server

How to access ensemble GRIB files via anonymous ftp, and how to pull ensemble member identification information from the PDS section of the ensemble GRIB files, click here .

A bug has been fixed in the regional rescaling procedure of the initial perturbation generation algorithm. The bug was introduced in June 2000 when the resolution of the ensemble forecasts was increased. At one place in the code, where the winds are extracted from the vorticity for use in the kinetic energy scaling, the list of latitudes to use for cosine weighting was not updated to reflect the change from T62 to T126.  The effect of this error was to introduce a smoothly varying error in scaling the kinetic energy in the Northern Hemisphere and a randomly varying error in the Southern Hemisphere.  The actual rescaling process is limited using a geographic mask, so the perturbations were not allowed to have large or "spike" errors, but we were introducing smooth latitudinal variations in the amplitude of the perturbations in the NH and noisy variations in the SH. This error was corrected starting with the 12Z cycle on 20 Dec 2000.

With this implementation, the size of the initial ensemble perturbations were globally increased by a factor of 1.75. This does not affect directly the structure or geographical distribution of the bred initial perturbations.
For the NCEP global ensemble, the size of the initial perturbations averaged over the northern hemisphere extratropics was formerly set to be between 8-9 m in terms of 500 hPa height rms spread around the control analysis. This value was based on estimates from Savijarvi (1995, MWR, 212-221), who provided analysis uncertainty estimates around 8 m (see his Table 1). There are inidications, however, that this level of perturbation amplitude may not be sufficient:
1)    There is a deficit in terms of ensemble spread when compared to errors in the ensemble mean (or control) forecast at short ranges. For example, the 2-day ensemble mean error is 80% larger than the corresponding ensemble spread. "Talagrand" histograms show that at the same lead time 25-30 % of the cases the ensemble misses (does not encompass) the verifying analysis (in excess of what is expected due to limited ensemble size). This also suggests a lack of adequate spread at short lead times.
2)    A comparison of ECMWF and NCEP reanalysis fields for January-February for the years 1988-1994 revealed that the average rms difference between the two reanalyses over the NH extratropics is around 21.6 m. Assuming that these two analyses are random samples of the true state, the expected rms error in the analyses is around (21.6/sqrt(2)=) 15.3 m.
3)    Differences between observations and short-range forecasts or analysis fields also suggest that the error in these fields may be larger than 8-9 m for the 500 hPa height. For example, in February 1999, the 6-hour forecasts (FG) were 15 m away from the radiosonde measurements (OBS) over the northern hemisphere extratropics in an rms sense:
FGERR=FG-OBS=15
Let us assume that the instrumental error affecting the observations (OBSERR) is around 10 m.
One can write the square of the error in the first guess (FGERR) as:
(FG-OBS)**2=(TRUTH+FGERR-(TRUTH+OBSERR))**2=15**2
Assuming that OBSERR and FGERR are uncorrelated:
FGERR=11.2
Using independent data we estimate that the rms error in the analized fields is on the order of 6% smaller than in the first guess forecasts:
ANERR=10.6
Given the assumptions, this is an estimate for the error (with respect to the true state of the atmosphere) in the analyses in the vicinity of existing radiosonde sites. We further assume that the analysis uncertainty is substantially (50-100%larger over areas void of radiosondes, and that these areas constitute roughly half of the NH extratropics. The range of rms uncertainty for the whole NH extratropics then is: ANERR(NH) ~ 10.6 x  1.25    -    10.6 x 1.5, i. e., it is between 13 and 16 m.

Based on the above considerations a decision was made to increase the initial perturbation size over the NH extratropics from around 8.5 m, by a factor of 1.75, to around 15.5 m.  This new perturbation level is consistent with analysis/short-range forecast error estimates discussed above.

The increase in perturbation amplitude size is applied globally and should be beneficial to users of forecasts over the NH extratropics and the tropics. The factor of 1.75 increase may have neutral or somewhat negative effect over the SH extratropics where the perturbation amplitudes appeared to be large enough before the implementation. The current perturbation rescaling procedure, that uses monthly varying geographically dependent estimates of analysis uncertainty (based on a comparison between two independent analysis cycles), however, did not allow for controlling the perturbation amplitudes separately over the two hemispheres. Our plans call for the implementation of an adaptive rescaling procedure that would be based on the distribution of analysis and first guess increments to the actual observed data.
Zoltan Toth, Richard Wobus and Istvan Szunyogh (credits to Eugenia Kalnay, Tim Marchok and Suranjana Saha)

A bug has been fixed in the initial ensemble perturbation generation code. Earlier, the pertubation amplitudes were too small at low, and too large at high latitudes, due to an incorrect computation of the norm (kinetic energy) used in the regional rescaling procedure (Fig. 1): Smoothed average rotational kinetic energy of initial pertubations around the 500 hPa level for the period February 1-28, 1998, with the bug). With the correction, the overall perturbation amplitudes over high latitudes decreased, while over low latitudes increased; the midlatitude amplitudes remained unchanged (Fig.2): Same as Fig. 1 except with corrected code). The geographical distribution of the resulting new initial perturbations is consistent with that of estimated analysis uncertainty (Fig.3): Geographical "mask" used in the regional rescaling procedure - Smoothed average rms difference in rotational kinetic energy around the 500 hPa level between reanalized and operational NCEP analyses for the period February 1-28, 1995). After the code correction, the global pertubation amplitude was set at such a level that the average size midlatitude pertubation amplitudes, which were consistent with different estimates of analysis uncertainty, remained unchanged.
Zoltan Toth, Istvan Szunyogh and Richard Wobus

On May 6, 1998 at 12Z a change was implemented in the NCEP global ensemble forecasts. The time independent geographical mask (that sets the amplitude to which the 24-hr bred perturbations are rescaled) was replaced by a new mask. This mask is based on the average difference between a series of operational T126 analyses and T62 reanalyses. Monthly average rms difference fields are used (based on one year of data) and are linearly interpolated for each day. This change should provide more realistic initial perturbation amplitudes. In particular, initial perturbation amplitudes will be somewhat reduced at high latitudes whereas somewhat increased at low latitudes.

Beginning March, 1997 the NCEP global ensemble data are placed onto the OSO server on a daily basis (in addition to having the data on the nic server). Please see:

 nic anonymous ftp server ( ftp://140.90.50.22/pub/ens/ )

 The planned retaining period for the data on the OSO server is 24 hours (though initially this period may be shorter). Currently, the retaining period for the data on the nic server is about 4 days. For additional information on the OSO server, go to:

OSO server information

Yuejian Zhu and Zoltan

Beginning Feb. 11, 1997 on our ftp server we provide grib files containing the probability that 24-hr forecast precipitation amounts will exceed certain threshold values. There will be one file per day, based on the latest 17 ensemble members available at 00Z with a filename like:

The grib files have a special extension in their headers describing the data. Please consult the appropriate NCEP GRIB documentation for details. For those who cannot (or does not want to) decode the special grib header information, each file contains 133 records. The records contain the following information:

GRIB Record Number	Description
1	0-24 hrs probability that the precip amount exceeds .254 mm
2	Same as Record 1, except exceeding 1.0 mm
3	Same as Record 1, except exceeding 2.54 mm
4	Same as Record 1, except exceeding 6.35 mm
5	Same as Record 1, except exceeding 12.7 mm
6	Same as Record 1, except exceeding 25.4 mm
7	Same as Record 1, except exceeding 50.8 mm
8-14	Same as Records 1-7, except for 12-36 hrs forecast interval
15-21	Same as Records 1-7, except for 24-48 hrs forecast interval
22-28	Same as Records 1-7, except for 36-60 hrs forecast interval
.	.
.	.
.	.
127-133	Same as Records 1-7, except for 216-240 hrs forecast interval

The forecast probability is estimated directly from the 17-member global ensemble. At each gridpoint the number of ensemble members having a 24-hour precipitation amount greater than the limit considered is counted (M) and the probability is expressed as 100*(M/17)

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