Intercomparison of the NCEP/NCAR and the NASA/DAO
Reanalyses (1985-1993)
The NCEP/NCAR reanalysis gridded fields have been classified into four classes, depending on the relative influence of the observational data and the model on the gridded variable. An A indicates that the analysis variable is strongly influenced by observed data, and hence it is in the most reliable class (e.g., upper air temperature and wind). The designation B indicates that, although there are observational data that directly affect the value of the variable, the model also has a very strong influence on the analysis value (e.g., humidity, and surface temperature). The letter C indicates that there are no observations directly affecting the variable, so that it is derived solely from the model fields forced by the data assimilation to remain close to the atmosphere (e.g., clouds, precipitation, and surface fluxes). Finally, the letter D represents a field that is obtained from climatological values, and does not depend on the model (e.g., plant resistance, land-sea mask). Section 6.2 contains the complete classification of variables. Although this classification is necessarily somewhat subjective, the user should exercise caution in interpreting the results of the reanalysis, especially for variables classified in categories B and C. In addition to this simple guidance, the user should keep in mind that quadratic variables (e.g., kinetic energy, transport of water vapor) are in general less reliable than the components from which they were computed. The mandatory pressure levels, sigma levels and isentropic levels of the output are:
Standard Pressure levels (hPa):
1000 925 850 700 600 500 400 300 250 200 150 100 70 50 30 20 10
Sigma levels:
0.9950 0. 9821 0.9644 0.9425 0.9159 0.8838 0.8458 0.8014 0.7508 0.6943 0.6329 0.5681 0.5017 0.4357 0.3720 0.3125 0.2582 0.2101 0.1682 0.1326 0.1028 0.0782 0.0580 0.0418 0.0288 0.0183 0.0101 0.0027
Isentropic surfaces (K):
650 550 450 400 350 330 315 300 290 280 270
The reanalysis archive has been designed to satisfy two major requirements: 1) the output should be comprehensive, allowing, for example, the performance of detailed budget studies, and 2) it should be easily accessible to the users interested in long time series of data. It is not possible to satisfy both requirements with a single archival format, so the output module includes several different archives. Reanalysis information and selected output is also available online by internet (http://nic.fb4.noaa.gov). In the following section we describe the archives:
6.2 Details of Assimilation Output
(a) BUFR observational archive
Reanalysis observational data undergo multiple processing stages, any of which may influence the quality of subsequent analysis and forecast products. For purposes of monitoring and review, and for research based on the reanalysis, it is useful to be able to trace the progression of QC and related processing to which any particular observation, or group of observations, have been subjected prior to their use (or non-use) in the actual data assimilation. A BUFR observation event archive format (described in Section 5f of Kalnay et al. 1996) has been designed to provide researchers with this capability. This archive contains reanalysis BUFR files, along with each original observation. These files include an indication of the observations source, all QC decisions, a history of modifications made to the observations prior to the analysis and various background quantities relevant to the analysis process. As a result, the observational database produced by the reanalysis system contains a fairly complete processing history of each observation.
Although the details of the BUFR format, and the BUFR structures devised to support the observation events archive, are rather complicated, a FORTRAN programmer interface package has been developed to simplify a user's interaction with these files, and enable fairly straightforward access to all of the archive information, without the need for a great deal of technical expertise in BUFR. These "user-friendly" FORTRAN interface routines, along with appropriate documentation and instructions for their use, will be available to reanalysis investigators.
(b) Synoptic archive
This is the most comprehensive archive of the reanalysis, and contains a large number of analysis and first guess "pressure" fields at 00, 06, 12, and 18 UTC on a 2.5° latitude-longitude grid; "flux", "diagnostic" and "sigma" files on the model gaussian grid, 192*94 points, in order to maintain maximum accuracy, and restart files at full resolution in order to ensure reproducibility. The complete list of output fields with their classification is given below:
Standard GRIB output
Pressure: Pressure coordinate output
... Regular latitude-longitude grid (2.5° x 2.5°)
... All fields are instantaneous values at a given time
A Geopotential height (gpm) at 17 levels
A u-wind (m s-1) 17 levels
A v-wind (m s-1) 17 levels
A Temperature (K) 17 levels
B Pressure vertical velocity (Pa s-1) 12 levels
B Relative humidity (%) 8 levels
A Absolute vorticity (s-1) 17 levels
A u-wind of the lowest 30 hPa layer (m s-1)
A v-wind of the lowest 30 hPa layer (m s-1)
B Temperature of the lowest 30 hPa layer (K)
B Relative humidity of the lowest 30 hPa (%)
B Pressure at the surface (Pa)
B Precipitable water (kg m-2)
B Relative humidity of the total atmospheric column (%)
A Temperature at the tropopause (K)
A Pressure at the tropopause (Pa)
A u-wind at the tropopause (m s-1)
A v-wind at the tropopause (m s-1)
A Vertical speed shear at the tropopause (s-1)
B Surface lifted index (K)
B "Best" (4-layer) lifted index (K)
A Temperature at the maximum wind level (K)
A Pressure at the maximum wind level (Pa)
A u-wind at the maximum wind level (m s-1)
A v-wind at the maximum wind level (m s-1)
D Geopotential height at the surface (gpm)
A Pressure reduced to MSL (Pa)
B Relative humidity in 3 sigma layers: 0.44-0.72, 0.72-0.94, 0.44-1.0 (%)
B Potential temperature at the lowest sigma level (K)
B Temperature at the lowest sigma level (K)
B Pressure vertical velocity at the lowest sigma level (Pa s-1)
B Relative humidity at the lowest sigma level (%)
B u-wind at the lowest sigma level (m s-1)
B v-wind at the lowest sigma level (m s-1)
Grb2d...2-dimensional diagnostic file
C Cloud forcing net longwave flux at the top of atmosphere (W m-2)
C Cloud forcing net longwave flux at the surface (W m-2)
C Cloud forcing net longwave flux for total atmospheric column (W m-2)
C Cloud forcing net solar flux at the top of the atmosphere (W m-2)
C Cloud forcing net solar flux at the surface (W m-2)
C Cloud forcing net solar flux for total atmospheric column (W m-2)
C Convective precipitation rate (kg m-2 s-1)
C Clear sky downward longwave flux at the surface (W m-2)
C Clear sky downward solar flux at the surface (W m-2)
C Clear sky upward longwave flux at the top of the atmosphere (W m-2)
C Clear sky upward solar flux at the top of atmosphere (W m-2)
C Clear sky upward solar flux at the surface (W m-2)
C Cloud work function (J Kg-1)
C Downward longwave radiation flux at the surface (W m-2)
C Downward solar radiation flux at the top of the atmosphere (W m-2)
C Downward solar radiation flux at the surface (W m-2)
C Ground heat flux (W m-2)
D Ice concentration (ice=1;no ice=0) (1/0)
D Land-sea mask (1=land;0=sea) (integer)
C Latent heat flux (W m-2)
C Near IR beam downward solar flux at the surface (W m-2)
C Near IR diffuse downward solar flux at the surface (W m-2)
C Potential evaporation rate (W m-2)
C Precipitation rate (kg m-2 s-1)
C Pressure at high cloud top (Pa)
C Pressure at high cloud base (Pa)
C Pressure at middle cloud top (Pa)
C Pressure at middle cloud base (Pa)
C Pressure at low cloud top (Pa)
C Pressure at low cloud base (Pa)
C Pressure at the surface (Pa)
C Run off (kg m-2 per 6-hour interval)
D Surface roughness (m)
C Nearby model level of high cloud top (integer)
C Nearby model level of high cloud base (integer)
C Nearby model level of middle cloud top (integer)
C Nearby model level of middle cloud base (integer)
C Nearby model level of low cloud top (integer)
C Nearby model level of low cloud base (integer)
C Sensible heat flux (W m-2)
C Volumetric soil moisture content (fraction) (2 layers)
B Specific humidity at 2 m (kg/kg)
C Total cloud cover of high cloud layer (%)
C Total cloud cover of middle cloud layer (%)
C Total cloud cover of low cloud layer (%)
B Maximum temperature at 2 m (K)
B Minimum temperature at 2 m (K)
AB Temperature at the surface (skin temperature) (K)
C Temperature of the soil layer (3 layers) (K)
B Temperature at 2 m (K)
C Temperature of high cloud top (K)
C Temperature of low cloud top (K)
C Temperature of middle cloud top (K)
C Zonal gravity wave stress (N m-2)
B Zonal component of momentum flux (N m-2)
B u-wind at 10 m (m s-1)
C Upward longwave radiation flux at the top of the atmosphere (W m-2)
C Upward longwave radiation flux at the surface (W m-2)
C Upward solar radiation flux at the top of the atmosphere (W m-2)
C Upward solar radiation flux at the surface (W m-2)
C Meridional gravity wave stress (N m-2)
C Visible beam downward solar flux at the surface (W m-2)
C Visible diffuse downward solar flux at the surface (W m-2)
C Meridional component of momentum flux (N m-2)
B v-wind at 10 m (m s-1)
C Water equivalent of accum. snow depth (kg m-2)
Grb3d...3-dimensional diagnostic file
... Gaussian grid (192 x 94) on 28 model levels
... All fields are average of 6-hour integration starting from a given time
C Deep convective heating rate (K s-1)
C Deep convective moistening rate (kg kg-1 s-1)
C Large scale condensation heating rate (K s-1)
C Longwave radiative heating rate (K s-1)
C Shallow convective heating rate (K s-1)
C Shallow convective moistening rate (kg kg-1 s-1)
C Solar radiative heating rate (K s-1)
C Vertical diffusion heating rate (K s-1)
C Vertical diffusion moistening rate (kg kg-1 s-1)
C Vertical diffusion zonal accel. (m s-2)
C Vertical diffusion meridional accel. (m s-2)
Sigma
... Gaussian grid (192 x 94) on 28 model levels or surface
... All fields are instantaneous values at a specified time
A Relative vorticity (28 levels) (s-1)
B Divergence (28 levels) (s-1)
A Temperature (28 levels) (K)
B Specific humidity (28 levels) (kg kg-1)
A x-gradient of log pressure (surface) (m-1)
A y-gradient of log pressure (surface) (m-1)
A u-wind (28 levels) (m s-1)
A v-wind (28 levels) (m s-1)
A Pressure (surface) (Pa)
A Geopotential height (surface) (gpm)
A x-gradient of height (surface) (m m-1)
A y-gradient of height (surface) (m m-1)
Isen...Isentropic coordinate output
... Gaussian grid (192 x 94) most on 10 isentropic levels
... All fields are instantaneous values at a specified time
A Potential temperature (surface) (K)
A Temperature (K)
A u-wind (m s-1)
A v-wind (m s-1)
B Pressure vertical velocity (Pa s-1)
B Relative humidity (%)
A Montgomery stream function (m2 s-2)
B Brunt-Vaisala frequency squared (s-2)
B Potential vorticity (m2 s-1 kg-1)
Other non-GRIB output files
Zonal diagnostic file (binary)
... Average over 90o S-60o S, 60o S-30o S, 30o S-30o N, 30o N-60o N, 60o N-90o N and global
... Unmarked fields are instantaneous values at a given time
... (Av) indicates average during the 6-hour integration
A u component of wind (m s-1) at 28 model levels
A v component of wind (m s-1) at 28 model levels
A virtual temperature (K) at 28 model levels
B specific humidity (g g-1) at 28 model levels
B squared vorticity (s-2) at 28 model levels
C squared divergence (s-2) at 28 model levels
B pressure vertical velocity (Pa s-1) at 28 model levels
A temperature (K) at 28 model levels
B relative humidity (%) at 28 model levels
B kinetic energy (m2 s-2) at 28 model levels
C convective heating (K s-1) at 28 model levels (Av)
C large scale heating (K s-1) at 28 model levels (Av)
C shallow convection heating (K s-1) at 28 model levels (Av)
C vertical diffusion heating (K s-1) at 28 model levels (Av)
C convective moistening (g g-1 s-1) at 28 model levels (Av)
C shallow convection moistening (g g-1 s-1 at 28 model levels (Av)
C vertical diffusion moistening (g g-1 s-1) at 28 model levels (Av)
C zonal accel by vertical diffusion (m s-2) at 28 model levels (Av)
C meridional accel by vertical diffusion (m s-2) at 28 model levels
C short wave radiation heating (K s-1) at 28 model levels (Av)
C long wave radiation heating (K s-1) at 28 model levels (Av)
C total precipitation (Kg m-2) (Av)
C convective precipitation (Kg m-2) (Av)
C sensible heat flux (w m-2) (Av)
C latent heat flux (w m-2) (Av)
B zonal stress (dyn m-2) (Av)
B meridional stress (dyn m-2) (Av)
C rain area coverage (%)
C convective rain area coverage (%)
B surface pressure (hPa)
C surface skin temperature (K)
C soil wetness (cm)
C snow depth (m)
C 10 cm deep soil temperature (K)
C 50 cm deep soil temperature (K)
D 500 cm deep soil temperature (K)
C surface net short wave flux (W m-2) (Av)
C surface net long wave flux (W m-2) (Av)
B relative humidity at the lowest model level (%)
B virtual temp at the lowest model level (K)
B temperature at the lowest model level (K)
B specific humidity at the lowest model level (K)
D surface roughness (m)
D land sea sea-ice mask (int)
C zonal accel by gravity wave drag (m s-2) (Av)
C meridional accel by gravity wave (m s-2) (Av)
B surface torque (g m-2 s-2) (Av)
C gravity wave drag torque (g m-2 s-2) (Av)
B mountain torque (g m-2 s-2) (Av)
B total angular momentum (m2 s-1)
B planetary angular momentum (m2 s-1)
RESTART FILES (binary)
... Spectral (28 model levels) or Gaussian grid (192 x 94)
... All fields are instantaneous values at a specified time
Sigma spectral coefficient file
D Surface geopotential
B Natural log of surface pressure
A Virtual temperature
B Divergence
A Vorticity
B Specific humidity
Surface file (on Gaussian grid)
C earth surface temperature (K)
C soil moisture level 1 (% volume)
C soil moisture level 2 (% volume)
C snow depth (m)
C soil temperature level 1 (K)
C soil temperature level 2 (K)
C soil temperature level 3 (K)
D surface roughness length (m)
C convective cloud cover (%)
C convective cloud bottom height (sigma)
C convective cloud top height (sigma)
C albedo (fraction)
C snow/ice/land mask
D minimum stomatal resistance (s m-1)
C canopy water content (m)
C ratio of 10 m and lowest sigma level winds (fraction)
(c) Reduced timeseries archive
This archive contains basic upper air parameters on standard pressure levels, selected surface flux fields, and diabatic heating and radiation terms for each analysis cycle for the entire reanalysis period. Most of the data will be saved in GRIB format. The pressure level data are saved on a 2.5 degree lat/lon grid, while the surface flux fields and radiation/diabatic heating data are saved on a T62 Gaussian grid (192 x 94). In addition, monthly means of vorticity, divergence, virtual temperature, specific humidity, and surface pressure are saved in spherical harmonic form on sigma levels. Monthly means of the flux terms are stored on the gaussian grid.
The radiation/diabatic heating data are composed of two radiative terms (short and long wave) and four diabatic heating terms (large-scale condensation, deep convection, shallow convection, and vertical diffusion). Monthly means of these data are stored at each sigma level of
the 00, 06, 12, and 18 UTC cycles separately, so that the monthly mean diurnal cycle in these fields is preserved.
The data storage order will be markedly different from the manner in which model data have traditionally been stored at NCEP. Since the climate research community generally uses individual parameters at a single atmospheric level but for many time periods (rather than all parameters for a single time), much of the data are stored in chronological, not synoptic order. That is, individual fields for a single atmospheric level are available for "all time" from a single data structure. The basic pressure level data and surface flux data will be stored in this order, which is referred to as "time series".
Fields on standard pressure levels
A Zonal wind
A Meridional wind
A Geopotential Height
A Virtual Temperature
A Absolute Vorticity
B Vertical Velocity (1000 to 100 hPa only)
B Specific Humidity (1000 to 300 hPa only)
17 levels: 1000, 925, 850, 700, 600, 500, 400, 300, 250, 200, 150,100, 70, 50, 30, 20, 10
hPa
Surface flux data
B Surface Temperature B Precipitable Water
C Skin Temperature C Snow Depth
B 2 m Temperature B Snow Cover
B Surface Pressure C Precipitation (total & convective)
D Albedo B Mean Relative Humidity
C Surf. Sens. & Lat. Fluxes C Soil Wetness, Temperature
C Top of atmos. Fluxes C Surface Runoff
B Zonal wind at 10 meters C Cloud fraction (hi, mid, low)
B Meridional " " " C Cloud forcing, clear sky fluxes
C Surface Wind Stress C Gravity Wave Drag
A Mean Sea Level Press. B Max and Min temperature
(a) Quick look CDROM output
We are creating a "Quick-look" data base that can fit into a relatively small (1 per year) number of CDROMs. Currently two types of CDROMs are planned. The first would contain reanalysis products for a single year (1 CDROM per year). The second type would be produced after about 10 years and would contain time series of relatively few variables. We believe this is an efficient way to satisfy the requirements of most members of the meteorological community, many of whom were consulted in the preparation of the output list. Additional reanalysis information and selected output is also available online by internet (http://wesley.wwb.noaa.gov/reanalysis.html). The following is the plan for the first type of CDROMs.
00Z and 12Z analyses:
U, V, temp. at 850, 500, 200 hPa
geopotential height 1000, 850, 700, 500, 300, 200 hPa
(sea level pressure can derived from above fields)
surface pressure
omega at 500 hPa
precipitable water
temperature at 2 m
specific humidity at 2 m
U, V at 10 m
RH at 500, 200 hPa
Daily averaged analyses:
zonal, meridional wind stress
net short/long wave flux at surface
precipitation
latent/sensible heat flux
Model OLR
downward short wave flux at surface
outgoing short wave flux at top
Tmin, Tmax (24 hour period)
skin temperature (includes SST)
snow (liquid water equivalent)
OOZ (isentropic) and 12Z (stratosphere) analyses:
height, temperature at 100, 50 and 20 hPa
U, V at 100, 50 and 20 hPa
potential vorticity on 3 surfaces (315, 330, 450 K)
U, V on 3 theta surfaces (315, 330, 450 K)
temperature on 3 theta surfaces (315, 330, 450 K)
All cross-sections (monthly averaged)
Monthly means, variances and covariances
Observed OLR
GrADS control and index files
Documentation:
BAMS paper (Office Note 401 with updates)
Office Note 388, (GRIB table of local definitions, documentation)
Miscellaneous
Total documentation volume
Software to read grib (PC-GrADS, wgrib)
(b) BAMS CDROM
The BAMS CDROM, the first ever included with the Bulletin of the AMS, includes four types of files: climatologies (13-year average monthly fields), monthly fields (for each of the 13 years), selected daily fields for 1993, and selected observed fields. All the fields have been interpolated to a uniform latitude-longitude 2.5o resolution grid (144 by 73 grid points). The 17 pressure levels are 1000, 925, 850, 700, 600, 500, 400, 300, 250, 200,150, 100, 70, 50, 30, 20, and 10 hPa for the climatology and monthly mean fields, and a subset of 5 levels (850, 700, 500, 200, and 30 hPa) for daily values. There are other single level fields (e.g., precipitation), and isentropic potential vorticity (IPV) on 11 isentropic levels (650, 550, 450, 400, 350, 330, 315, 300, 290, 280, and 270K) for the monthly fields, and 3 selected levels (450, 330, and 315 K) for the daily fields.
Because of the horizontal and vertical interpolation, it is recommended that these fields not be used for budget studies, which generally require access to the original data. Z, U, V, T, MSLP are of type A (analysis variable is strongly influenced by observed data); W, RH, Q, PWAT, U10, V10, T2M, IPV can be considered of type B (although there are observational data that directly affects the value of the variable, the model also has a very strong influence on the analysis value); most other variables are of type C (indicating that there are no observations directly affecting the variable, so that it is derived solely from the model fields forced by the data assimilation to remain close to the atmosphere. Users of the diabatic heating fields ("HEATCL" file) should note that these data were converted from sigma level to pressure level after temporal averaging. The conversion process is not exact and the process may result in questionable values, particularly near the surface.
All of the data on this CDROM are stored in GRIdded Binary ("GRIB") which is a packed binary format and thus are not directly readable. However, two simple and powerful software packages are included that allow users to view and manipulate the data on this CDROM and to unpack the data so that users can import the data into their own application software. These software packages are "GrADS" and "wgrib"; details on how to use these applications, which require no programming, are found on the CDROM. For users who wish to have a quick-look at maps of the NCEP/NCAR Reanalysis CDROM, a menu-driven "demo" program that can display most of the fields on this CDROM has been included.
The following fields are included in the monthly and climatological file directories. The * indicates they are also included in the daily (1993) directory file. With the exception of the first 7, these are single level fields.
Z Geopotential height (gpm)*
U u wind (m s-1)*
V v wind (m s-1)*
T Temperature (K)*
W Pressure vertical velocity (Pa s-1)* (500 hPa only)
Q Spec humidity (kg kg-1) *
PWAT Precipitable water (kg m-2)*
MSLP Pressure reduced to MSL (Pa)*
CPRATE Convective precipitation rate (kg m-2 s-1)*
CSDLFSFC Clear sky downward long wave flux (W m-2)
CSDSFSFC Clear sky downward short wave flux (W m-2)
CSULFSFC Clear sky upward long wave flux (W m-2)
CSUSFTOA Clear sky upward short wave flux at top of atmosphere (W m-2)
CSUSFSFC Clear sky upward short wave flux at the surface (W m-2)
DSWRFSFC Downward long wave radiation flux at the surface (W m-2)
DSWRFTOA Downward short wave radiation flux at top of atmosphere (W m-2)* DSWRFSFC Downward short wave radiation flux at the surface (W m-2)*
ICEC Ice concentration (ice=1; no ice=0) (1/0)
LHTFL Latent heat flux (W m-2)*
PRATE Total Precipitation rate (kg m-2 s-1)
RUNOFF Runoff (kg m-2)*
SFCR Surface roughness (m)
SHTFL Sensible heat flux (W m-2)*
SOILW10 Volumetric soil moisture content 0-10 cm layer (fraction)*
SOILW200 Volumetric soil moisture content at 10-200 cm layer (fraction)*
Q2M Specific humidity at 2 m above ground (kg kg-1)*
HCLDCOV Hi-cloud cover (percent)
MCLDCOV Middle-cloud cover (percent)
LCLDCOV Low-cloud cover (percent)
TSFC Skin Temperature (K)*
T2M Temperature at 2 m above ground (K)*
UGWD Zonal Gravity wave stress (N m-2)*
UFLX Zonal component of momentum flux (N m-2)*
U10M u wind at 10 m above ground (m s-1)*
ULWRFTOA Upward long wave radiation flux at top of the atmosphere OLR (W m-2)*
ULWRFSFC Upward long wave radiation flux at the surface (W m-2)*
USWRFTOA Upward short wave radiation flux at top of the atmosphere (W m-2)*
USWRFSFC Upward short wave radiation flux at the surface (W m-2)
VGWD Meridional gravity wave stress (N m-2)*
VFLX Meridional component of momentum flux (N m-2)*
V10M v wind at 10 m above ground (m s-1)*
The following components of the heat and moisture budget are only available for the 13 year climatology (17 pressure levels):
LRGHR Large scale condensation heating rate (K s-1)
CNVHR Deep convective heating rate (K s-1)
SHAHR Shallow convective heating rate (K s-1)
VDFHR Vertical diffusion heating rate (K s-1)
SWHR Shortwave radiative heating rate (K s-1)
LWHR Longwave radiative heating rate (K s-1)
The isentropic potential vorticity is available in the monthly mean (11levels) and 1993 daily (3 levels):
IPV Isentropic Potential Vorticity (m2 s-1 kg-1)
The fixed fields (type D) included are:
OROG Orography (m) (surface elevations in meters on a 2.5 x 2.5 degree grid)
MASK Land-sea mask, 1 for land, 0 for sea (useful to mask out land or ocean values).
The observed fields (in the "OBSERVED" directory) included are:
OBS93OLR Daily Outgoing Long-wave Radiation for 1993 (W m-2)
OBSMNOLR Monthly means Outgoing Long-wave Radiation (W m-2)
GPCPRAIN GPCP estimated rainfall rates
NCEPRAIN Xie-Arkin estimated rainfall rates
MRGDRAIN Schemm estimated rainfall rates
JAGRAIN Jaeger (1976) precipitation climatology
LWRAIN Legates-Willmott (1990) precipitation climatology
For the precipitation analyses and climatologies that are included, all values are expressed in units of "kg m-2 s-1" to match the model forecasts. To convert to units of "mm day-1", multiply the values by "86400".
Measurements of outgoing longwave radiation (OLR) are obtained from the Advanced Very High Resolution Radiometer (AVHRR) aboard the NOAA polar orbiting spacecraft (Gruber and Krueger, 1984). The data units are watts per square meter and each value represents the areal average OLR flux for a 2.5 x 2.5 degree lat/lon "box". The observations of OLR during the 1979-1994 time period that are included on this CDROM are exclusively from the "afternoon" satellite, i.e. one which observes at the equator near 0230/1430 LST. It should be noted that considerable observing time drift occurs during the lifetime of the "afternoon" polar orbiting satellites, and observing times can be up to 5 hours later toward the end of a satellites lifetime compared to the initial launch observing time.
The Xie-Arkin precipitation analysis (Xie and Arkin, 1996) is derived in a two-stage process from monthly raingauge observations and several estimates based on satellite data. First, the satellite estimates are combined using a weighted average where the weights are proportional to the estimated errors of the various estimates. This weighted average is then merged with an analysis of gauge observations over land and with observations from atoll gauges over the ocean. In general, gauge values are used wherever available.
The merged precipitation dataset for 1979 - 1992, prepared by Jae Kyung E. Schemm, was generated by combining observed monthly total precipitation data from the world surface station climatology (Spangler and Jenne 1990) from NCAR and estimated oceanic precipitation from the MSU measurements (Spencer, 1993). The station data were interpolated to a resolution of 2.5 degree longitude/latitude by averaging station values within a 200 km radius with weights proportional to the inverse of square distance (Schemm et al. 1992). An attempt was made to control the quality of the dataset by removing station data reporting total precipitation amounts over 1000 mm. The MSU estimates were screened for sea ice contamination by removing data with monthly totals greater than 900 mm in regions poleward of 50 degrees latitude.
The Global Precipitation Climatology Project (GPCP) which is administered by the Global Energy and Water Cycle Experiment (GEWEX) has produced a monthly mean 2.5 degree gridded precipitation data set for the period July 1987 through December 1994 (December, 1987 is missing). This data set has been produced by blending gauge and infrared and microwave satellite estimates of precipitation. While the instantaneous microwave-based precipitation estimates are more accurate than IR-based estimates, the microwave estimates suffer from reduced temporal sampling (twice-daily) relative to the IR (8-times daily) due to the polar orbit of the spacecraft that house the SSM/I instruments (most of the IR data are from geostationary satellites). Thus, an adjustment procedure has been developed that attempts to meld the strengths of these two estimates, i.e. increased accuracy of the microwave combined with better temporal sampling of the IR. The adjustment procedure is an adaptation of earlier work by Huffman et al. (1995) and consists of steps that first remove the biases in infrared estimates by adjusting to coincident microwave estimates of precipitation. The microwave estimates are obtained from the SSM/I instrument aboard the Defense Meteorological Satellite Program (DMSP) series of satellites and utilize a scattering model for estimates over land and an emission model for over ocean estimates. The final analysis step adjusts the merged satellite data to the gauge observations and combines them using weights that depend on the estimated local error of each field. The gauge data are analyses from the Global Precipitation Climatology Centre and reflect approximately 6,700 gauges which have been carefully quality controlled. This is a new data set and we request that users provide comments to Arnold Gruber, Manager of the GPCP at agruber@orbit.nesdis.noaa.gov. For more information about the Global Precipitation Climatology Project see the GPCP Home Page on the World-Wide Web (WWW): http://orbit-net.nesdis.noaa.gov/gpcp/
The two relatively long-term climatologies that are on this CDROM, namely Jaeger (1976) and Legates and Willmott (1990), are based on gauge data over land and estimates over the oceans. In the Jaeger climatology, data were assembled from contemporary precipitation atlases over land. Over the oceans, Jaeger inserted digitized values from subjectively analyzed data from the U. S. Marine Climatic Atlas, and adjusted the oceanic rainfall estimates to yield an arbitrary global annual mean of 1000 mm. Legates and Willmott applied bulk corrections to the gauge values over land (to correct for evaporation and wind catchment problems). Over the oceans, they incorporated the estimates of Dorman and Bourke (1978, 1981) who estimated monthly rainfall from synoptic observation reports for "present weather" from ships.
(c) CDAS
The Reanalysis Project originated with the idea of performing a "post analysis" with a Climate Data Assimilation System (CDAS), which would remain frozen into the future. In 1990, Profs. Mark Cane and Julia Nogués-Paegle of the Advisory Committee suggested that a very long reanalysis would be more useful than the CDAS alone. The development of the reanalysis system was then started and became the largest component of the project. It is clear that the combination of reanalysis for the past, and the CDAS into the future, both using the same frozen system, will be much more helpful to researchers than either component alone.
The CDAS is performed within 3 days of the end of the month, with the same software as the reanalysis. This allows time to capture the bulk of any delayed data, and serves as the basis for the generation of the monthly Climate Diagnostics Bulletin of CPC. As noted before our plans include a second phase of the CDAS/Reanalysis to start sometime in 1997, after the first phase is completed. In the second phase, the Reanalysis-2 will be performed with a 1998 state-of-the-art system, coupled with a corresponding CDAS-2 into the future. Such reanalyses would then be repeated every five years or so using the most advanced systems and the additional recovered data from the past. The CDAS-1, however, will be continued into the foreseeable future in order to maintain the longest homogeneous data assimilation product possible. Given that the CDAS-1 will become less expensive with time, it may be feasible to consider running a fixed observation system for comparison with the current reanalysis, which has considerable variations in the observing systems.
(d) Reanalysis Archives at NCAR, CDC, NCDC
Full copies of the NCEP/NCAR reanalysis archive are located at the National Center For Atmospheric Research (NCAR) and the National Climatic Data Center (NCDC) while selected products in NetCDF format are available from CDC. Detailed descriptions of the products available at each of these sites are found at the following internet addresses:
NCAR: http://www.scd.ucar.edu/dss/pub/reanalysis/index.html
NCDC: http://www.ncdc.noaa.gov
CDC: http://www.cdc.noaa.gov:80/cdc/reanalysis
A brief description of the value-added reanalysis products provided by NCAR and CDC to users is given below.
NCAR provides more than 20 different types of data products from the reanalysis, model runs and model forecasts. These products are defined in terms of the NCAR archive names, physical variables, resolutions and media storage size. On-line documentation includes a "Data Product Description" (e.g. data formats, data file types, CDROM information, detailed lists of all variables, etc.), a "Data Archive Summary" (e.g. tables of data availability by file type, projected timelines, etc.), and a "Data Users Home Page" (e.g. information on software, a problem list, a guide to accessing reanalysis data, data ordering information, plans for making reanalysis data subsets and reanalysis FAQ's).
CDC maintains a netCDF-based internet accessible subset of the NCEP/NCAR reanalysis data products. On-line documentation (at the address above) includes the current project status, the data inventory/specifications (e.g. description of the data on-line at CDC, specifications for netCDF files including size estimates), a library of compatible software (e.g. netopen FORTRAN library, CRDtools, FERRET, Freud, MATLAB) and an electronic atlas of derived reanalysis products.