FAQ's

For FAQ's about annual cdroms, see cdrom FAQ's.

• Does reanalysis assimilate the recent broad swath scatterometer data, such as from the Quikscat satellite?[11/29/2001]
  No. The test results of introducing SSM/I wind data into reanalysis assimilation has caused a large shift in the "climatological" surface winds. After much debate, the decision was made not to use the SSM/I data.

• How is sea surface temperature field used in Reanalysis processing?[6/23/2000]
  GISST v2.2 from the UK Met Office was used prior to 1982 and Reynolds SST for later years. GISST was a monthly data set which was interpolated to daily values. The Reynolds SST was a weekly SST analyses interpolated to daily values. The CDAS currently runs using a daily SST analyses using a 6 or 7 day sliding window for it's input data.

• What does NCEP use for the gravity constant in Reanalysis processing?[5/5/2000]
  In nearly all processing, NCEP uses 9.8 exactly as the gravity constant. The only exceptions are that a value of 9.81 is used to compute mass weight variables on isentropic surfaces and to compute pressure levels of the isentropic surfaces.

• Is the field called geopotential height of the earth's surface always the same throughout the entire Reanalysis period?[5/5/2000]
  The geopotential height of the earth's surface that is stored in the grbsanl file on the Gaussian grid is derived from the spectral sigma file and the code that does this derivation has changed three times. There was a change on 21 September 1994 and a change on 27 January 1998 which have no effect on the values. A change made on 22 January 1998 did cause some differences and fields before this date will be different from fields after this date. Otherwise this field is the same throughout the period. This change only affects this particular field and only in the grbsanl file. The same field on the 2.5x2.5 lat-lon grid is not affected and is the same throughout the period.

• Why are the results prior to 1957JUN every 3 hours instead of 6 hours?[3/9/2000]
  >From ftp://ftp.cpc.ncep.noaa.gov/wd51we/reanal/random_notes/pre1957.tim
  Analysis Times ---- Currently the traditional times for launching sondes are 00Z, 06Z, 12Z and 18Z with the 00Z and 12Z having the most launches. However, prior to June 1957, the launches occured at 03Z, 09Z, 15Z, 21Z which most being at the 03Z and 15Z hours. These "off-time" launches had the potential of reducing the accuracy of the reanalyses. So to avoid this situation, the analysis times were changed from 00/06/12/18Z to 03/09/15/21Z for the period before June 1957. In order to make life easy for all the users of the reanalysis, 00/06/12/18Z analyses were made from the 3 hours forecasts. The 0-6 hour forecast averages (fluxes) were calculated from the 3-6 hour and the following 0-3 hour forecasts to produce averages from the traditional 6 hour periods.

  BTW the program that converted the flux data from 03/09/15/21Z to 00/06/12/18Z treated the runoff as a rate rater than an accumulation and incorrectly divided the results by two. SKY noticed some jumps in the radiative quantities and I have to find time to investigate. (Ebisuzaki, NCEP)

• Is there a summary of the NCEP model used for NCEP/NCAR Reanalysis project?[8/31/1998]
  It can be found at model_summary.

• When using gribmap of GrADS to make *.idx file, it failed. How to make it work?[8/13/1998]
  This does happen, for example, the grb2d92070100 file. This is because there is a trailing bad GRIB record which is not part of the grb2d data. To fix the data file, you can use wgrib to filter out the trailing bad record. To do this, enter `wgrib -d all -grib -o grb2d92070100.fix grb2d92070100' assuming wgrib is installed. The 1992JUL-NOV grb2d vsns are fixed as of Oct 17, 2000. There is no change of the data, but the tailing extra incomplete GRIB records are removed.

• Is there any information on the vertical structure of liquid water content (clouds and rain drops) available from Reanalysis?[6/9/98]
  Liquid water is not carried as a distinct variable in Reanalysis. Specific humidity, relative humidity, and total column precipitable water are the primary moisture variables in the atmosphere. (Kanamitsu)

• What is the reason to rerun the 1997MAR-DEC reanalysis?[3/23/98]
  According to NCEP, the reason follows:
  In response to a question by Dr. Rupa Kumar (visiting scientist at ENM/URF/UMT Meteo France) concerning the large 1997 Indian monsoon rainfall, Bob Kistler found a problem with our handling of the Indian station data since March 1997. These analyses are not "wrong" but use more Indian station data than intended. Preliminary plans are to redo the analyses starting from March 1997 with an expected completion date in February 1998. The updated results of 1997 data (including 1997MAR-DEC rerun) are released on March 23, 1998.

  CDAS/Reanalysis did not use all the Indian station data (i.e., stations were "blacklisted") which was NCEP practice when Reanalysis was developed. Since then, operational practice has changed so that Indian stations are not blacklisted and PAOBS are not used. In March 1997, CDAS's GTS decoder had to be changed to the new operational decoder because NCEP's mainframes were removed from service. Through an oversight, the Indian data were not blacklisted for processing by CDAS. This oversight will be corrected. At present, there are no plans to add PAOBS decoding to the new decoder.

  The effect of using the "blacklisted" Indian stations was to drastically increase the Indian monsoon precipitation. According to Dr. Kumar, the assimilation's too low precipitation became too large. Removing the "blacklisted" stations is necessary for the sake of temporal continuity over the Indian subcontinent. On the bright side, Dr. Kumar found the interannual variability was "reasonably well captured" (about 0.8 correlation between the total Indian JJAS precipitation anomalies and Reanalysis for the years 1974-1996.) The same info is also at NCEP.
  PS: 1979 Reanalysis did not use the blacklisted stations

• What is the nature of snow cover problem in the Reanalysis results between 1974 and 1994?[1/15/98]
  Details are discussed as Some Problems with the Snow Cover.

• Why are there sometimes negative humidity values in the Reanalysis fields?[1/13/98]
  There is a discussion of this issue by Wesley Ebisuzaki (NCEP) available at negative.h2o.

• How are the soil layer structured?[12/2/97]
  The soil model consists of 3 layer:

  Layer	Depth	GRIB Description	Abbreviation	Comment
  Top	0 - 10 cm	0-10cm	10.dlr
  Middle	10 - 200 cm	10-200cm	200.dlr
  Bottom	200 - 400 cm	300cm	300.dpl	Defined as a level in GRIB, it actually represents a layer

• How are temperatures of soil layers handled?[12/2/97]
  The annual cycle in the soil temperature decreases in amplitude with depth. The model assumes that the bottom layer has no time variations of temperature (not a bad assumption). The temperature of the bottom layer is set to the annual-mean surface temperature climatology so that the net heat flux from the bottom layer should approach zero. (Wesley Ebisuzaki, NCEP)

• How useful are upper level specific humidity fields on sigma surfaces?[11/15/97]
  The higher levels in this stack do not contain sufficient precision for many applications. For levels above the 0.1682 sigma level, the fields may contain only constant zero values.

• How does reanalysis handle snow cover?[11/14/97]
  The snow coverage was estimated from satellite imagery (once per week) and snow coverage was converted into a snow depth by assuming it is a simple function of the surface temperature. (Yes, it is unrealistic but we have no snow depth observations.) For the purposes of data assimilation, the snow depth is not critical. Radiative properties depend on the coverage rather than depth. In addition, if the snow coverage happened to melt away (because the snow wasn't deep enough), the snow coverage gets reinitialized with "observations" every six hours. It is suggested that snow coverage be used rather than the snow depth; i.e., convert the snow depth (WEASD) into 1's or 0's. The skin temperature over a snow surface is determined by a balance of the fluxes. Net solar + net long wave + sensible + latent + conduction through the snow layer + snow melt = 0. The snow melt term is set to the minimum amount required to keep the the skin temperature at or below freezing. Within the snow layer, the temperature is assumed to vary linearly between the skin temperature and soil temperature (top layer). If the soil temperature is above freezing, there will be snow melt which will reduce the soil temperature. (Of course, the snow depth is suspect.) (Wesley Ebisuzaki, NCEP)

• Are there corrected prate fields to fix the spurious moisture source in the Reanalysis data?[11/7/97]
  The "spurious moisture source" is equivalent to "spectral snow". The corrected daily and monthly values are available in our anonymous ftp area. The 6-hourly precipitation rate problem has not been visited yet.

• Where can users find information on the usefulness of the Cloud layer variables?[8/15/97]
  Wesley Ebisuzaki at NCEP has put together some notes on this subject. These can be found at NCEP, or NCAR.

• In the Reanalysis archive, what does NCEP use for saturation vapor pressure?[7/23/97]
  Regardless of temperature, the saturation vapor pressure relative to water is used. (Kanamitsu)

• This applies to ANNUAL_CDROM only
  How can the sea level pressure be obtained from the annual CDROMs?[6/26/97]
  The sea level pressure fields in the NCEP/NCAR Reanalysis data archive are derived directly from the 1000mb and 500mb geopotential height fields. Sea level pressure fields are not included on the CDROMs, but these height fields are. Sea level pressure in Pascals can be derived using the following formula. slp=100000.*exp(z1000/(1.5422885*(z500-z1000)))

• This applies to ANNUAL_CDROM only
  Date-time of the Surface Pressure Fields on the Annual CDROM Series[6/11/97]
  The pressure fields in the pres.sfc file in the at00z12z directory are actually 6 hour forecast fields. The valid time for these fields is 06Z and 18Z rather than 00Z and 12Z. The GRIB identifiers are correct and indicate the appropriate reference date-time and the 6 hour forecast. This pressure field was extracted from the grb2d Reanalysis subset and all fields in this subset are either 6 hour accumuations from the reference time to 6 hours later or 6 hour forecasts valid at the reference time plus 6 hours. The GrADS index file on the CDROM is defined so that the fields appear to be valid at 00Z and 12Z. Users should take this time difference into account when using GrADS on this file, or the GrADS GRIB index file should be regenerated with proper times by rerunning the GRIBMAP utility.

• This applies to ANNUAL_CDROM only
  What is the lable of 1993 annual cdrom?[4/28/97]
  The labels of annual cdroms follow 'REANALyy' convention where 'yy' is the last two digits of the year except the 1993 cdrom. The 1993 annual cdrom is labelled as 'REANL93'. We don't expect this inconsistency to cause users any problems.

• This applies to ANNUAL_CDROM only
  Why are daily tmax.2m, tmin.2m, tmp.sfc fields on 1994-1995 annual cdroms only?[4/28/97]
  The problem with the boundary layer parameterization (the sensible heat flux went to zero when the wind speed went to zero) forced NCEP to eliminate tmin.2m, tmax.2m, tmp.sfc fields for 1986-1993. In addition some of the other near surface fields are blanked-out when the winds are very weak.

• How are grids defined in Reanalysis?[1/21/97]
  The orientation, grid increment, etc are defined in the Grid Definition Section (GDS) of a GRIB record. To know the exact grid definition, users need to understand the details of GDS. In general, reanalysis grids are global and start at the North Pole, go eastwards along a latitudinal circle and then move southwards. For example, the 2.5x2.5 global grids start at 0E,90N. The second grid is at 2.5E,90N. The 145th grid is at 0E,87.5N. The last grid is at 357.5E,90S.

• Are isentropic data on Gaussian grid as shown in March 1996 BAMS paper?[12/13/96]
  No, the isentropic data (file_type: theta) are on 2.5x2.5 latitude, longitude grids, not on Gaussian grids.

• Is the unit of potential vorticity [m**2(s kg**-1)**-1] as shown on page 464 in March 1996 BAMS paper?[12/13/96]
  No, the unit of potential vorticiy should be [m**2/s/kg].

• How was the monthly mean surface stress and fluxes computed?[12/11/96]
  The fluxes and stresses are computed in the model during the 6 hour prediction in the assimilation cycle. The reanalysis products are the averages of fluxes and stresses during this 6 hour prediction. The time step of the model is about 15 min.

• Does reanalysis use a constant drag coefficient to compute the surface wind stress?[12/11/96]
  Drag coefficient is a function of bulk Richardson number and is not constant.

• How does NCEP/NCAR reanalaysis compute the potential vorticity on isentropic surfaces?[12/06/96]
  First the static stability N**2=g/T*(dT/dz+g/cp) is computed on model levels. Then the winds, temperature and static stability are interpolated to isentropic surfaces linearly in log(theta). (Outside the model domain, the fields are held constant for now and will later be compressed out of the final product.) Then the absolute vorticity zeta is spectrally computed on the isentropic surfaces, where the shortest wavelength in the spectral domain is about 4 grid lengths. (The vorticity is computed in the T36 spectral domain for the 2.5x2.5 degree grid.) The density rho=(T/theta)**(cp/R)*p0/(R*T) is also computed at this time directly on the isentropic surfaces. Finally, the NCEP potential vorticity is computed as zeta*N**2/(g*rho). Thus the units of NCEP PV are m**2/s/kg, which is different from the usual units of K*m**2/s/kg; one must multiply the NCEP PV by theta to compare them. The NCEP PV is still a form of the Ertel potential vorticity, since log(theta) is as well conserved as theta. Packing the PV in these units is simpler. The NCEP PV is currently rounded to the nearest 1e-10 m**2/s/kg for packing. The physical constants used are g=9.8, R=287.05, cp=1004.6 and omega=7.2921e-5. (by Dr. Mark Iredell)

• Different GRIB tables used at NCEP operations and NCEP/NCAR reanalysis.[10/24/96]
  Please be aware that NCEP operations and the NCEP/NCAR Reanalysis Project are have been using different grib tables. For example, operations uses number 189 to identify "virtual potential temperature" whereas Reanalysis uses 189 for the "standard deviation of the temperature". Most of the entries are identical as the tables have only started to diverge in the last few years. If we had known that there were 2 NCEP tables, we would have changed the table identification code. Unfortunately we now have the situation where there can be ambiguity in the GRIB files. In addition, it is too late for CDAS/Reanalysis to change grib tables. Please make sure that you are using the correct grib table when reading operational and CDAS/Reanalysis data.

  The operational grib table can be found at opn_nceptab

  The Reanalysis grib table can be found at reanal_nceptab

• What is NOSAT run?[9/17/96]
  The NOSAT run does not use any satellite wind, TOVS, drifting buoys, or Southern Hemisphere surface pressure bogus (PAOBS) data. The results of NOSAT run could be used to evaluate the impack of satellite data on the analysis.

• What is PAOBS?[9/10/96]
  PAOBS (Australian Surface Pressure Bogus data for the Southern Hemisphere) is a product of human analysts who estimate sea-level pressure based on satellite data, conventional data and time continuity. The PAOBS were mislocated in the 1996 version of reanalysis products.

• What is the period of record for PAOBS problem existed?[9/10/96]
  Data in the primary archive had mislocated PAOBS for input during the period 01Jan1980 through 02Apr1993. PAOBS are used only in the Southern Hemisphere and many of these PAOBS were rejected by the data assimilation. Long range Reanalysis plans call for rerunning the analysis every few years, and PAOBS positions will be handled correctly in future runs.

• How does Reanalysis handle the surface water budget?[8/30/96]
  Reanalysis keeps track of soil wetness at various levels in the soil as well as on the canopy, and the fields themselves are available as a part of the reanalysis dataset. The budget equation is based on precipitation, evaporation, runoff and water flow to lower layers of the soil. The details are in the paper by F. Chen et al., JGR(1996, pp. 7251-7268). There are several people looking into the water budget using reanalysis. The person most closely worked on the equation and evaluation of the budget using reanalysis is Dr. John Roads of Scripps. [From Kanamitsu]

• What are the UFLX/VFLX fields? [8/27/96]
  UFLX/VFLX are the negative of the wind stress at the surface. The conversion to UFLX and VFLX was done because GRIB defines a positive vertical direction which can be different from normal scientific usage.

• What is the meaning of a GRIB field with a header section (PDS), but no data values? [7/10/96]
  According to GRIB convention, when the entire field contains a constant value, the field width (octet 11) is set to zero and no data values are packed. The value for all points in the field is the reference value.

• What should be used as the valid time for fields in the grb2d file? [6/19/96]
  Fields in the grb2d file are either 6 hour averages or 6 hour forecasts. The GRIB reference time is used in the file name, BUT the valid time is actually 6 hours later for the forecast fields. The average fields are averages of the 6 hour period from the reference time to 6 hours later. The GRIB PDS information correctly specifies the time and the GrADS package will assign the proper time.

• PRESmsl vs. PRMSLmsl: [6/19/96]
  These two variables are derived using slightly different algorithms for the same sea-level-pressure. Check this pres.msl for exact definitions.

• Negative PWAT values: [6/19/96]
  This is a common phenomenon in numerical models. The negative values are to be flagged for special processing, or simply reset to zero.

• According to BAMS paper, temperature tendency by physics, non-physics, and radiation (GRIB Table 2 #215-217) are available as reanalysis products. Where are they? [6/19/96]
  Since GRIB ID 215 through 217 can be computed from other heatings in reanalysis grb3D diagnostic file, they are not provided.

• How are vertical diffusion parameters (GRIB Table 2 #246-248) calculated? [6/19/96]
  They are picked up and accumulated in the forecast model as they are computed every time step. The vertical diffusion coefficients are the function of the Richardson number, and the formulation is based on Miyakoda and Sirutis (1986). Unfortunately it is not easy to obtain this paper. If you need more details, please contact Kanamitsu.

• Does reanalysis GRIB data files follow a fixed record/order pattern? [6/19/96]
  No. There is no definite structure in data record/order in reanalysis GRIB data files. User should select a GRIB record according to its Product Definition Section.

• How is the tropopause level calculated in the reanalysis? [6/19/96]
  The model level analyses are used to calculate tropopause data. (A post processor is used). The procedure at NCEP looks for the lowest level above 450 mb where the lapse rate is under 2K per km. A more exact location for the tropopause between two model levels is determined, and the pressure and temperature are calculated for that spot in the vertical. The trop is not allowed to be above 85 mb. In spite of changing observations, the analyses should be quite stable over time. Therefore the derived tropopause data should be stable over time. [From Kanamitsu. NCEP has a listing of the program used.]

• The NCEP/NCAR Reanalyses provides diabatic heating components, with unit K/s. Is this heating the tendency of the POTENTIAL temperature, or just of temperature itself? [Jan, 1999]
  The diabatic heating (K/s) is the temperature tendancy and not the potential temperature tendancy.

• How is the PV (potential vorticity) on the theta surface computed? [Jan 1999]
  The PV is first interpolated to theta surfaces and then the PV is calculated.

• How are 2 meter temperatures derived?[Feb 1999]
  Please contact Hua-Lu Pan at NCEP.

• How is 4LFTXsfc computed?[Jun 1999]
  The Best Lifted Index is computed by finding the parcel with the warmest equivalent potential temperature. Then raising it to 500mb and subtracting its parcel temperature from the environment temperature.

• How are SW radiative fluxes derived?[Jun 1999]
  The SW parameterization used is obtained from GFDL. The clear sky part is from Lacis and Hansen, 1974, JAS. The SW fluxes are broad band fluxes over the solar spectrum from 0.2-4.0 microns.

• Is the concentration of CO2 constant throughout the reanalysis or is it changing according to some measurements? what is the concentration used?
  The model used has a constant 330 ppm CO2 concentration. Please review the following CO2 Note