********************************************************************************** NOTE: THE NGM ARCHIVE ENDED ON APRIL 15, 1997. USERS OF THE NGM ARCHIVE CAN NOW ACCESS THE ETA DATA ASSIMILATION SYSTEM (EDAS) ARCHIVE AT THE FOLLOWING LOCATION: http://www.arl.noaa.gov/ss/transport/archives.html ftp:/www.arl.noaa.gov/pub/archives/edas ********************************************************************************** NGM ARCHIVE TD-6140 January 1991 - April 1997 Prepared for the National Climatic Data Center (NCDC) by Glenn D. Rolph NOAA Air Resources Laboratory SSMCII, Rm. 3463 1315 East West Highway Silver Spring, MD 20910 (301) 713-0295 x134 http://www.arl.noaa.gov June 1997 NGM ARCHIVE OVERVIEW The National Weather Service's National Center for Environmental Prediction (NCEP) runs a series of computer analyses and forecasts operationally (Petersen and Stackpole, 1989). One of the primary operational systems is the Global Data Assimilation System (GDAS, Kanamitsu, 1989), which uses the spectral Medium Range Forecast model (MRF) for the forecast (Sela, 1980). Another primary system is the Regional Analysis and Forecast System (RAFS), which uses the Nested Grid Model (NGM) for the forecast (Hoke et al., 1989). Output from the RAFS covers North America. In simple terms, for each run, observations are assimilated with "first guess" data fields (forecasts from the previous model run), and dynamic imbalances in the data are reduced, resulting in "analyzed" data fields. Then the forecast is made. The analyzed data should provide a better representation of the real atmosphere than observations alone because of limitations in the observations. Some of these limitations are due to measurement error or other instrument problems, and nonuniform spatial and temporal distributions of the observations. At NOAA's Air Resources Laboratory (ARL), the NCEP model data are used for air quality transport and dispersion modeling. ARL archives both NGM and GDAS data using an ARL packing method and both archives contain basic fields such as the u- and v-wind components, temperature, and humidity. However, the archives differ from each other in the number of surface fields and vertical levels as provided by NCEP. ORIGIN OF DATA The enclosed 2-hourly data come from NCEP's RAFS. The data may be referred to as NGM data because the forecast component of the system is the NGM model. The RAFS was designed to provide improved numerical guidance over the United States out to 48 hours. Improvements over the existing limited-area fine-mesh model (LFM; Gerrity 1977; Newell and Deaven 1981) included improved horizontal and vertical resolution (especially in the lower layers) and better use of observations. Details of the RAFS are described by Hoke, et al. (1989). NCEP post-processing of the RAFS routinely provides NGM forecast output at 6 hour intervals on approximately a 180 km grid (a few fields such as precipitation are output on at 91 km) to the NAS 9000 mainframe computer. ARL archives the NGM forecast data at 2 hour intervals on a 180 km grid (although the data are available to ARL at 1 hour intervals on a 91 km operationally, due to space limitations only 2 hourly, 180 km data are archived). The higher temporal resolution of the data has been found to result in less transport error in an atmospheric dispersion model (Rolph and Draxler, 1990). ARL PROCESSING The ARL archiving program extracts the 2 hourly, 180 km data from the 1 hour, 91 km data onto a 33 by 28 polar stereographic grid covering the United States, Southern Canada, and immediate coastal waters. The ARL program runs on the NAS 9000 mainframe and outputs the data to disk, from which it is later transferred to tape. DATA DESCRIPTION The archive data file contains the data in synoptic time sequence (GMT), without any missing records (missing data will be represented by negative 1 and the forecast hour by negative 1). Therefore it is possible to position randomly to any point within a data file. At each time period the surface (or single level) data come first, followed by the data at each sigma level from the ground up. A sigma level (Phillips, 1957) is defined as P - Pt sigma = --------; Ps - Pt where P is the reference pressure, Pt is the top model pressure (0 mb for the NGM), and Ps is the model surface pressure along the model terrain surface. NGM data are archived at 10 sigma levels ranging from 0.9823 to 0.43414, or approximately 980 to 434 mb. Tape Specifications Data for six months are archived on a tape. NOTE: The NGM archive ended on April 15, 1997. TYPE 3480 1/2" Cartridge, ASCII* LABEL Non-Labeled RECORD FORMAT Fixed Block RECORD LENGTH 974 BLOCK SIZE 32142 * Note that each data record is composed of a 50-character header in ASCII, followed by the binary packed data. Therefore, an ASCII-EBCDIC conversion on the entire data record or tape file is not possible. Beginning with July 1994 there are two files per month with the first file containing data for days 1 through 15 and the second file containing the rest of the month. Data Grid The data are on a 33 by 28 polar stereographic grid (Fig. 1). In Table 1 the data grid is identified by the model that produced the data, a grid identification number, the number of X and Y grid points, the grid spacing ( ) which is true at the indicated latitude, the longitude to which the Y axis is aligned, and the pole position in grid units. The given pole position results in the lowest left grid point to have a value of (1,1). The NGM archive grid has an ID of #6. Table 1. Data Grid Specifications Model ID X Y True Align X Y Type # Max Max Km Lat. Lon. Pole Pole NGM 06 33 28 182.9 60N 105W 13.25 42.75 Meteorological Fields and Vertical Structure The archived data file only contains some of the fields normally produced by the model at NCEP. These were selected according to what is most relevant for transport and dispersion studies and disk space limitations. In Table 2, the fields are identified by a description, the units, and a unique four character identification label that is written to the header label (see Data Grid Unpacking procedure in a later section) at the beginning of each record. Data order in the file is given by a two digit code. The first digit indicates if it is a surface (or single) level variable (S) or an upper level variable (U). The second digit indicates the order in which that variable appears in the file. The upper level NGM data are output on the following sigma surfaces: 0.98230, 0.94317, 0.89671, 0.84367, 0.78483, 0.72101, 0.65307, 0.58196, 0.50864, 0.43414. Table 3 gives the level number, corresponding to each data level, which is also written to each header label. NOTE: on December 29, 1992, the surface level was changed from level 1 to level 0. This also implies that all sigma levels were reduced by one as well. Table 2. Meteorological Fields for NGM Archive Data. Field Units Label Data Order Ice Covered Water Areas (0=no, 1=yes) ---- ICWT S1 Snow Covered Areas (0=no, 1=yes) ---- SNOW S2 Terrain Height m SHGT S3 Mean Sea Level Pressure hpa MSLP S4 Convective Precipitation m CPPT S5 Total Precipitation m TPPT S6 Exchange Coefficient at surface kg/m2/s EXCO S7 Upward Turbulent Flux of Sensible Heat W/m2 HFLX S8 Upward Turbulent Flux of Water kg/m2/s WFLX S9 Surface Pressure hpa PRSS S10 Number of Mixed Layers next to surface ----- MXLR S11 U Component of Wind wrt Grid m/s UWND U1 V Component of Wind wrt Grid m/s VWND U2 Vertical Velocity (dp/dt) hpa/s WWND U3 Specific Humidity kg/kg SPHU U4 Temperature K TEMP U5 Table 3. Description of Vertical Levels Level Sigma (å) 10 .43414 9 .50864 8 .58196 7 .65307 6 .72101 5 .78483 4 .84367 3 .89671 2 .94317 1 .98230 0 surface Missing Data When the NGM data are not available or there is a problem with the ARL archiving process, missing data are written as an array of -1. Usually the field label is as given in Table 2, but sometimes it is defined as "NULL." No attempt at present is made to fill in missing data with data from another source. Data Grid Unpacking Procedure NCEP typically saves their data in NCEP Office Note 84 (ON-84) format. However, here the data are stored differently because the ARL format is a bit more compact and it can be directly used on a variety of computing platforms with direct access I/O. The data array is packed and stored into one byte characters. To preserve as much data precision as possible the difference between the values at grid points is saved and packed rather than the actual values. The grid is then reconstructed by adding the differences between grid values starting with the first value, which is stored in unpacked ASCII form in the header record. To illustrate the process, assume that a grid of real data, R, of dimensions i,j is given by the below example. 1,j 2,j .... i-1,j i,j 1,j-1 2,j-1 .... i-1,j-1 i,j-1 .... .... .... .... .... 1,2 2,2 .... i-1,2 i,2 1,1 2,1 .... i-1,1 i,1 The packed value, P, is then given by Pi,j = (Ri,j - Ri-1,j) 2(7-N), where the scaling exponent N = ln Rmax / ln 2 . The value of Rmax is the maximum difference between any two adjacent grid points for the entire array. It is computed from the differences along each i index holding j constant. The difference at index (1,j) is computed from index (1,j-1), and at 1,1 the difference is always zero. The packed values are one byte unsigned integers, where values from 0 to 126 represent -127 to - 1, 127 represents zero, and values of 128 to 254 represent 1 to 127. Each record length is then equal in bytes to the number of array elements plus 50 bytes for the label information. The 50 byte label field precedes each packed data field and contains the following ASCII data. Field Format Description ÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ Year I2 Greenwich date for which data valid Month I2 " Day I2 " Hour I2 " Forecast* I2 Hours forecast, zero for analysis Level I2 Level from the surface up (see Table 3) Grid I2 Grid identification (see Table 1) Variable A4 Variable label (see Table 2) Exponent I4 Scaling exponent needed for unpacking Precision E14.7 Precision of unpacked data Value 1,1 E14.7 Unpacked data value at grid point 1,1 *Forecast hour is -1 for missing data. Notes The convective and total precipitation (convective plus grid scale) fields are accumulated from the 2- to 12-hour forecast time. Therefore when looking at any particular forecast hour, for example forecast hour 04, the precipitation is really a 4 hour accumulated total, and at forecast hour 12, the precipitation is a 12 hour total. Sample Program In the following FORTRAN program, the unpacking subroutine is used to read the first record. The value at grid point 1,1 from the header and the unpacked value are printed, along with the other header values. In the program the packed data array is in the variable QPACK and the unpacked real data array is returned in variable QVAR. NX and NY are the number of grid points in the horizontal and vertical directions, respectively. NXY is just the product of NX and NY. The variable NEXP and VAR1, are the packing exponent and value of the field at the 1,1 grid point. These values are obtained from the header label information. Also attached is a sample program to determine the latitude and longitude of any grid point on the model domain. This program can be helpful for a user who must transform the data into another coordinate system. **************************************************************************** * SAMPLE PROGRAM TO UNPACK DATA ARRAY IN FIRST RECORD **************************************************************************** * QVAR = UNPACKED REAL DATA ARRAY * QPACK = PACKED DATA ARRAY * NX = MAXIMUM GRID DIMENSION IN X-DIRECTION * NY = MAXIMUM GRID DIMENSION IN Y-DIRECTION * NXY = NX * NY * LABEL = 50 CHARACTER ASCII HEADER, CONSISTING OF THE FOLLOWING: * IY = YEAR FOR WHICH DATA ARE VALID * IM = MONTH FOR WHICH DATA ARE VALID * ID = DAY FOR WHICH DATA ARE VALID * IHR = HOUR FOR WHICH DATA ARE VALID (GREENWICH) * IFHR = HOURS AFTER INITIALIZATION * LVL = LEVEL FROM THE SURFACE UP * IG = GRID ID * VARB = 4 CHARACTER VARIABLE LABEL * NEXP = PACKING EXPONENT * PREC = PRECISION OF UNPACKED DATA * VAR1 = UNPACKED DATA VALUE AT GRID POINT (1,1) **************************************************************************** PARAMETER ( NX=33, NY=28, NXY=NX*NY) REAL*4 QVAR(NX,NY) CHARACTER*1 QPACK(NXY) CHARACTER*4 VARB CHARACTER*50 LABEL LREC=NXY+50 WRITE(*,*) 'ENTER NGM FILENAME' OPEN(10,FILE=' ',ACCESS='DIRECT',RECL=LREC,FORM='UNFORMATTED') READ(10,REC=1) LABEL,QPACK READ(LABEL,100) IY,IM,ID,IHR,IFHR,LVL,IG,VARB,NEXP,PREC,VAR1 100 FORMAT(7I2,A4,I4,2E14.7) CALL UNPACK(QVAR,QPACK,NX,NY,NXY,NEXP,VAR1) WRITE(*,200) IY,IM,ID,IHR,IFHR,LVL,IG,VARB 200 FORMAT(1X,'YEAR=',I2,' MONTH=',I2,' DAY=',I2,' HOUR=',I2, : ' FORECAST HOUR=',I2,' LEVEL=',I2,' GRID=',I2, : ' VARIABLE=',A4) WRITE(*,300) VAR1 300 FORMAT(1X,'FROM HEADER, QVAR(1,1)= ',F7.1) WRITE(*,400) QVAR(1,1) 400 FORMAT(1X,'FROM UNPACKED DATA, QVAR(1,1)= ',F7.1) STOP END ************************************************************************ SUBROUTINE UNPACK(QVAR,QPACK,NX,NY,NXY,NEXP,VAR1) ************************************************************************ CHARACTER*1 QPACK(NXY) REAL*4 QVAR(NX,NY) SCALE=2.0**(7-NEXP) INDX=0 QOLD=VAR1 DO J=1,NY DO I=1,NX INDX=INDX+1 QVAR(I,J)=(ICHAR(QPACK(INDX))-127.)/SCALE+QOLD QOLD=QVAR(I,J) END DO QOLD=QVAR(1,J) END DO RETURN END C**************************************************************** C PROGRAM TO GIVE LAT/LON FOR ENTERED GRID POINTS C**************************************************************** C NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION C AIR RESOURCES LABORATORY C GLENN D. ROLPH C**************************************************************** CALL SETNMG(90.,105.,13.25,42.75,60.,105.,182.9) 10 WRITE(6,*)'ENTER x y position to convert to lat lon' READ(5,*)X,Y CALL XYTOLL(X,Y,XLAT,YLON) WRITE(6,*)'LAT,LON',XLAT,YLON GO TO 10 STOP END SUBROUTINE LLTOXY (XLAT,YLON,XI,YJ) C**************************************************************** C POLAR STEREOGRAPHIC CONVERSION OF LATITUDE AND LONGITUDE * C Dr. A. D. Taylor NOAA/OAR/ARL * C REVISION: 01/14/92 to allow oblique stereographic * C projection * C**************************************************************** C CONSTANTS BASED ON ASSUMPTION THAT RADIUS OF SPHERICAL C EARTH IS 6371.2 KILOMETERS, AS USED BY NCEP IN ITS GRID C MODELS. real * 4 radpdg,drthnm,drthkm,DEARTH,POLLON,XP,YP,sclfct real*4 cpolat,spolat,clat,slat,clon,slon,phi real*4 x1,y1,z1,rho0,y2,r3,cosmu,sinmu DATA RADPDG/1.745 3293E-2/ ,DRTHNM/ 6 880.35/ DATA DRTHKM/12 742.4/ DATA DEARTH,POLLON,XP,YP,SCLFCT/31.2043,260.,33., :33.,4.53527E-3/ data cpolat, spolat / 0.0, 1.0 / clat = cos(radpdg * xlat) slat = sin(radpdg * xlat) phi = (ylon-pollon)*radpdg clon = cos(phi) slon = sin(phi) x1 = clat * slon y1 = cpolat * slat + spolat * clat * clon z1 = spolat * slat - cpolat * clat * clon if (1.+z1 .gt. .5e-6) then rho0 = 1. / (1. + z1) xi = xp + dearth * rho0 * x1 yj = yp + dearth * rho0 * y1 scale = 2. * sclfct * rho0 else write(*,*)'LLTOXY Error:',xlat,',',ylon,' represents', C ' point at infinity.' stop end if RETURN ENTRY XYTOLL (XI,YJ,XLAT,YLON) x1 = (xi - xp) / dearth y1 = (yj - yp) / dearth rho0 = .5 * (1. + x1 * x1 + y1 * y1 ) z1 = 1. - rho0 SCALE = 2. * SCLFCT * rho0 slat = cpolat * y1 + spolat * z1 y2 = cpolat * z1 - spolat * y1 clat = sqrt( x1 * x1 + y2 * y2) xlat = atan2( slat , clat ) /radpdg if (clat.ge..5e-6) then clon = -y2/clat slon = x1/clat YLON = AMOD(POLLON+180.+ATAN2(slon,clon)/RADPDG,360.)-180. else clon = 1. slon = 0. ylon = 0. end if RETURN ENTRY UVG2N(UG,VG,UE,VN) C**************************************************************** C Converts grid wind components (ug,vg) into North-South * C components (vn,ue) for the location of the last point * C transformed by lltoxy or xytoll. * C**************************************************************** if (abs(xlat).le.89.) then r3 = sqrt(clat*clat+slat*slat) cosmu = - clon * (r3 + slat * spolat) + clat * cpolat sinmu = - slon * (r3 * spolat + slat) r3 = sqrt (cosmu*cosmu + sinmu*sinmu) vn = (vg*cosmu + ug*sinmu)/r3 ue = (ug*cosmu - vg*sinmu)/r3 else C**************************************************************** C In the regions around the North or South Pole, component * C winds have to be redefined. The standard adopted here for* C sites within one degree of the poles is that winds are * C assigned directions according to the meridian from which * C they are blowing; 0 or 360 (North) for winds from the * C Greenwich meridian, 90 (East) for winds from the 90W * C meridian, etc. Thus, VN is the component in the direction* C of Greenwich and UE is the component in the direction of * C 90W. For the South polar region, replace 90W in the above* C with 90E. * C**************************************************************** cosmu = cos(radpdg * pollon) sinmu = sin(radpdg * pollon) if (slat.lt.0.) then cosmu = - cosmu sinmu = - sinmu end if ue = ug * cosmu + vg * sinmu vn = vg * cosmu - ug * sinmu end if RETURN ENTRY UVN2G(UE,VN,UG,VG) C**************************************************************** C Converts North-South wind components (vn,ue) into grid * C components (ug,vg) for the location of the last point * C transformed by lltoxy or xytoll. * C**************************************************************** if (abs(xlat).le.89.) then r3 = sqrt(clat*clat+slat*slat) cosmu = - clon * (r3 + slat * spolat) + clat * cpolat sinmu = - slon * (r3 * spolat + slat) r3 = sqrt (cosmu*cosmu + sinmu*sinmu) vg = (vn*cosmu - ue*sinmu)/r3 ug = (ue*cosmu + vn*sinmu)/r3 else C**************************************************************** C In the regions around the North or South Pole, component * C winds have to be redefined. The standard adopted here for* C sites within one degree of the poles is that winds are * C assigned directions according to the meridian from which * C they are blowing; 0 or 360 (North) for winds from the * C Greenwich meridian, 90 (East) for winds from the 90W * C meridian, etc. Thus, VN is the component in the direction* C of Greenwich and UE is the component in the direction of * C 90W. For the South polar region, replace 90W in the above* C with 90E. * C**************************************************************** cosmu = cos(radpdg * pollon) sinmu = sin(radpdg * pollon) if (slat.lt.0.) then cosmu = - cosmu sinmu = - sinmu end if ug = ue * cosmu - vn * sinmu vg = vn * cosmu + ue * sinmu end if RETURN ENTRY SCALER(SIZENM) C**************************************************************** C returns the size of a grid cell in nautical miles at the point* C last referenced * C**************************************************************** SIZENM=SCALE RETURN ENTRY SETNMC C**************************************************************** C restores grid settings to standard NMC grid: North Polar * C Stereographic, oriented with 80W longitude vertical in the * C sense of North up, tangent point (North Pole) sited at 33,33, * C and scaled 281km at 60N. * C**************************************************************** DEARTH=31.2043 POLLON=260. XP=33. YP=33. SCLFCT=4.53527E-3 cpolat = 0.0 spolat = 1.0 RETURN ENTRY SETNMG(tnplat,tnplon,tnpx,tnpy,reflat,reflon,GRIDSZ) C*************************************************************** C Sets grid settings to specification. Tnp is the "tangent * C point" for the oblique stereographic projection; tnplat and * C tnplon specify the latitude and longitude, resp, of the * C tangent point, while tnpx and tnpy specify the grid * C coordinates x and y of the tangent point. Reflat and reflon * C specify the latitude and longitude of the scale reference * C point, while gridsz is the size in kilometers of a grid cell * C at that point. The meridian through the tangent point is * C assumed vertical in the sense of North up, unless the tangent* C point is the North or South pole, in which case the trulon * C longitude is vertical in the sense of North up. * C setnmc is equivalent to setnmg(90.,80.,33.,33.,60.,0.,281.) * C**************************************************************** xp = tnpx yp = tnpy cpolat = cos(radpdg * tnplat) spolat = sin(radpdg * tnplat) POLLON=AMOD( AMOD(tnplon,360.) + 540. , 360.) z1 = spolat * sin(radpdg * reflat) - 1 cpolat*cos(radpdg*reflat)*cos(radpdg*(reflon-pollon)) dearth = drthkm * .5 * (1. + z1) / gridsz SCLFCT = DEARTH / DRTHNM RETURN end REFERENCES Gerrity, J., 1977: The LFM model - 1976: A documentation. NOAA Tech. Memo. NWS NMC 60, 68 pp. Hoke, J.E., N. A. Phillips, G.J. DiMego, J.J. Tuccillo, and J.G. Sela, 1989: The Regional Analysis and Forecast System of the National Meteorological Center, Weather and Forecasting, 4 (323- 334). Kanamitsu, M., 1989: Description of the NMC Global Data Assimilation and Forecast System, Weather and Forecasting, 4(335-342). Newell, J.E. and D.G. Deaven, 1981: The LFM-II Model - 1980. NOAA Tech. Memo. NWS NMC 66, U.S. Department of Commerce, Washington, D.C., 20 pp. Petersen, R.A. and J.D. Stackpole, 1989: Overview of the NMC Production Suite, Weather and Forecasting, 4 (313-322). Phillips, N.A., 1957: A Coordinate System Having Some Special Advantages for Numerical Forecasting. J. Meteor., 14 (184-185). Rolph, G.D. and R.R. Draxler, 1990: Sensitivity of Three- Dimensional Trajectories to the Spatial and Temporal Densities of the Wind Field, Journal of Applied Meteorology, 29 (1043-1054). Sela, J.G., 1980: Spectral modeling at the National Meteorological Center, Mon. Wea. Rev., 108 (1279-1292). ****************************************************************** DATA AVAILABILITY -- January - April 15, 1997 NOTE: THIS IS THE END OF THE NGM ARL ARCHIVE. Month/Year Date Time Missing data JAN 97 None FEB 97 None MAR 97 09 00Z initialization* 12 12Z initialization* APR 97 04 02-12Z Missing 16-30 Missing _______________________________________________________________ * Initialization means that the 0 hour data were written instead of the 12 hour forecast data. In these cases, a few of the surface (or single) level fields may be filled with 0's. The NGM does not compute these fields at the initialization hour, which is why the 02- to 12-hour forecast fields are normally archived. DATA AVAILABILITY -- July - December, 1996 Month/Year Date Time Missing data JUL 96 None AUG 96 None SEP 96 06 14-22Z All data missing 07 00Z initialization* OCT96 10 12Z initialization* NOV 96 None DEC 96 None _________________________________________________________________ * Initialization means that the 0 hour data were written instead of the 12 hour forecast data. In these cases, a few of the surface (or single) level fields may be filled with 0's. The NGM does not compute these fields at the initialization hour, which is why the 02- to 12-hour forecast fields are normally archived. DATA AVAILABILITY -- January - June, 1996 Month/Year Date Time Missing data JAN 96 None FEB 96 None MAR 96 None APR 96 None MAY 96 None JUN 96 4 00Z initialization* _________________________________________________________________ * Initialization means that the 0 hour data were written instead of the 12 hour forecast data. In these cases, a few of the surface (or single) level fields may be filled with 0's. The NGM does not compute these fields at the initialization hour, which is why the 02- to 12-hour forecast fields are normally archived. DATA AVAILABILITY -- July - December, 1995 Month/Year Date Time Missing data JUL 95 None AUG 95 11 12Z Initialization* SEP 95 None OCT 95 None NOV 95 None DEC 95 None _________________________________________________________________ * Initialization means that the 0 hour data were written instead of the 12 hour forecast data. In these cases, a few of the surface (or single) level fields may be filled with 0's. The NGM does not compute these fields at the initialization hour, which is why the 02- to 12-hour forecast fields are normally archived. DATA AVAILABILITY -- January - June, 1995 Month/Year Date Time Missing data JAN 95 26 14-22Z All data missing 27 00Z initialization* FEB 95 None MAR 95 None APR 95 None MAY 95 27 00Z initialization* JUN 95 24 00Z initialization* _________________________________________________________________ * Initialization means that the 0 hour data were written instead of the 12 hour forecast data. In these cases, a few of the surface (or single) level fields may be filled with 0's. The NGM does not compute these fields at the initialization hour, which is why the 02- to 12-hour forecast fields are normally archived. DATA AVAILABILITY -- July - December, 1994 Month/Year Date Time Missing data JUL 94 28 00Z Initialization* AUG 94 None SEP 94 None OCT 94 None NOV 94 16 00Z Initialization* DEC 94 21 12Z Initialization* _________________________________________________________________ * Initialization means that the 0 hour data were written instead of the 12 hour forecast data. In these cases, a few of the surface (or single) level fields may be filled with 0's. The NGM does not compute these fields at the initialization hour, which is why the 02- to 12-hour forecast fields are normally archived. DATA AVAILABILITY -- January - June, 1994 Month/Year Date Time Missing data JAN 94 17 14-22Z All data missing 18 00Z initialization* FEB 94 None MAR 94 None APR 94 None MAY 94 None JUN 94 None _________________________________________________________________ * Initialization means that the 0 hour data were written instead of the 12 hour forecast data. In these cases, a few of the surface (or single) level fields may be filled with 0's. The NGM does not compute these fields at the initialization hour, which is why the 02- to 12-hour forecast fields are normally archived. DATA AVAILABILITY -- July - December, 1993 Month/Year Date Time Missing data JUL 93 None AUG 93 None SEP 93 None OCT 93 None NOV 93 None DEC 93 None _________________________________________________________________ * Initialization means that the 0 hour data were written instead of the 12 hour forecast data. In these cases, a few of the surface (or single) level fields may be filled with 0's. The NGM does not compute these fields at the initialization hour, which is why the 02- to 12-hour forecast fields are normally archived. DATA AVAILABILITY -- January - June, 1993 Month/Year Date Time Missing data JAN 93 08 00Z Initialization* FEB 93 17 00Z Data missing only this hour MAR 93 08 12Z Initialization* APR 93 None MAY 93 None JUN 93 None _________________________________________________________________ * Initialization means that the 0 hour data were written instead of the 12 hour forecast data. In these cases, a few of the surface (or single) level fields may be filled with 0's. The NGM does not compute these fields at the initialization hour, which is why the 02- to 12-hour forecast fields are normally archived. DATA AVAILABILITY -- July - December, 1992 Month/Year Date Time Missing data JUL 92 None AUG 92 None SEP 92 None OCT 92 None NOV 92 01 14-22Z All data missing 02 00Z initialization* DEC 92 None _________________________________________________________________ * Initialization means that the 0 hour data were written instead of the 12 hour forecast data. In these cases, a few of the surface (or single) level fields may be filled with 0's. The NGM does not compute these fields at the initialization hour, which is why the 02- to 12-hour forecast fields are normally archived. NOTE: on December 29, 1992, the surface level was changed from level 1 to level 0. This also implies that all sigma levels were reduced by one as well. DATA AVAILABILITY -- January - June, 1992 Month/Year Date/Time Missing data or comments JAN 92 01/01Z - 17/11Z Odd hours written instead of even hours (1,3,5...) 17/12Z Initialization* FEB 92 None MAR 92 04/02Z - 04/12Z Missing data 26/14Z - 27/22Z Missing data 28/00Z Initialization* 28/14Z - 28/22Z Missing data 29/00Z Initialization* 29/14Z - 29/22Z Missing data 30/00Z Initialization* APR 92 None MAY 92 None JUN 92 None _________________________________________________________________ * Initialization means that the 0 hour data were written instead of the 12 hour forecast data. In these cases, a few of the surface (or single) level fields may be filled with 0's. The NGM does not compute these fields at the initialization hour, which is why the 02- to 12-hour forecast fields are normally archived. DATA AVAILABILITY -- July - December, 1991 Month/Year Date Time Missing data JUL 91 03 12Z initialization* 04 12Z initialization* 06 12Z initialization* 07 12Z initialization* AUG 91 17 00Z initialization* 18 00Z initialization* SEP 91 None OCT 91 25 12Z all data 29 00Z all data NOV 91 26 12Z initialization* DEC 91 02 12Z initialization* 04 12Z initialization* _________________________________________________________________ * Initialization means that the 0 hour data were written instead of the 12 hour forecast data. In these cases, a few of the surface (or single) level fields may be filled with 0's. The NGM does not compute these fields at the initialization hour, which is why the 02- to 12-hour forecast fields are normally archived. Note: During 1991 the odd hour (01,03,...11) convective and total precipitation fields were inadvertently added to the even hour (02,04,...12) precipitation fields. Therefore, to estimate the correct accumulated precipitation one should divide the convective and total precipitation by 2. This error was corrected on January 1, 1992. DATA AVAILABILITY -- January - June, 1991 Month/Year Date Time Missing data JAN 91 None FEB 91 None MAR 91 None APR 91 04 12Z initialization* 05 00Z initialization* 22 12Z all data 23 00Z initialization* 24 00Z initialization* MAY 91 07 12Z all data 08 00Z initialization* 23 12Z all data JUN 91 None _________________________________________________________________ * Initialization means that the 0 hour data were written instead of the 12 hour forecast data. In these cases, a few of the surface (or single) level fields may be filled with 0's. The NGM does not compute these fields at the initialization hour, which is why the 02- to 12-hour forecast fields are normally archived. Note: During 1991 the odd hour (01,03,...11) convective and total precipitation fields were inadvertently added to the even hour (02,04,...12) precipitation fields. Therefore, to estimate the correct accumulated precipitation one should divide the convective and total precipitation by 2. This error was corrected on January 1, 1992.