ISLSCP II GlobalView: Atmospheric Methane Concentrations ############################################################################# File Naming Convention ---------------------- The file names for the GLOBALVIEW-CH4 data set use the following format: 1 2 3 4 5 6 [site/prog][data group]_[lab#][sampling strategy][plat]_[qualifier]_ch4.dat 1. [Sampling site/program] * 3-character alphanumeric field specifying site or program code. See Section 6.2.1 below or section 9 of the "2_gv_ch4_2001_doc.pdf" document for a complete list of the site abbreviations. 2. [Grouping of data within the file] * If not specified then the sampling site is at a single fixed position. [brw_, prs_] * If an aircraft then identifier is a 3-character numeric field with units of 102 meters (hm) above sea level. [car040_, aia005_] * If a tower then identifier is a 3-character numeric field with units of meters (m) above sea level. [lef051_, hun048_] * If a ship and binned by longitude then identifier is a 3-character numeric with units of degrees (000-360). [npo140_, nao350_] * If a ship and binned by latitude, identifier is a 3-character alphanumeric field with units of degrees. (00-90). Bins in the northern and southern hemispheres are denoted as n## and s## respectively. The equatorial bin is denoted as 000. [pocs25_, poc000_, scsn03_] * Note: A binned file requires further explanation regarding the bin width, e.g., car050 is a 1000m bin centered on 5km. 3. [lab# (contributing laboratory)] Two-character numeric field identifies the measurement laboratory (00-99). See section 2 of the "2_gv_ch4_2001_doc.pdf" document. 4. [Sampling strategy] Single alphanumeric character (0-9,a-z,A-Z) indicates the sampling strategy. _??D Discrete _??C Continuous/Quasi-continuous _??E Event _??I Integrated 5. [Sampling platform] Single alphanumeric character (0-9,a-z,A-Z) indicates the sampling platform. _???0 Single Fixed Position _???1 Ship _???2 Aircraft _???3 Tower _???4 Kite _???5 Balloon _???6 Firn/Ice Core 6. [Qualifier] Multiple alphanumeric character field (0-9,a-z,A-Z) identifies the file's contents. _????_ext Extended Record _????_wts Extension Weights _????_var Average Atmospheric Variability _????_seas Average Seasonal Cycle _????_diu Average Diurnal Cycle _????_tod Sampling Time-Of-Day Summary _????_mtx MBL Reference Matrix File Name Examples cgo_02D0_ext_ch4.dat Extended CH4 record derived from CSIRO discrete measurements at Cape Grim. mlo_00D0_ext_ch4.dat Extended CH4 record derived from CMDL discrete measurements at Mauna Loa. There are 5 types of files that are included in GLOBALVIEW-CH4. Each type is distinguished by its file name qualifier (see 6 above). Files with an "ext" qualifier contain extended records, i.e., records that contain synchronized smoothed values, and interpolated and extrapolated values derived using the latitude reference data extension method. Files with a "wts" qualifier contain weights that were applied by CMDL when fitting smooth curves to weekly distributions of CH4 mole fraction as a function of latitude. Files with a "var" qualifier contain a statistical summary of atmospheric variability by month. Files with a "seas" qualifier contain a statistical summary of the average seasonal pattern by month. Files with a "diu" qualifier contain a statistical summary of average diurnal cycle patterns by month accumulated for all complete measurement years. Files with the "ext", "wts", "var", and "seas" qualifier exist for all sites described in GLOBALVIEW-CH4. Files with the "diu" qualifier accompany a subset of extended records derived from high-resolution measurement records where the diurnal cycle is a dominant feature of the observations. Files with the "tod" qualifier accompany a subset of extended records derived from discrete measurements where sample collection times have been made available. ############################################################################ File Contents: ------------- All file types (except for reference MBL matrix) have 16 lines of descriptive information that include + Extended record name + Measurement organization or institution + Type of measurement program + Type of sampling site + Name of organization collecting air + Position of sampling site + Conversion from Universal Coordinated Time (UTC) to Local Standard Time (LST) + Creation date of the file + Number of rows in the file following the column description + Column descriptions There are no blank fields in any column. Missing values are denoted with a standard default value, -999.999. All units are in _mol mol-1 CH4 unless otherwise specified. Extended Record Files (ext) --------------------------- Following the descriptive information detailed above, the four (4) columns in the extended record files are: UTC: "Weekly" synchronized time steps in Universal Coordinated Time (UTC) as decimal dates, i.e., year plus fraction of the year. Each year has 48 "weekly" steps. "Synchronized" means that the synchronization period and the time steps are the same for all extended record files. S(t): Smoothed values extracted from a curve fitted to measurement data that have been selected for conditions where the sampled air is thought to be representative of large well-mixed air parcels. Internal and external gaps in the measurement record are denoted as default values. REF(t): The latitude reference time-series, based on marine boundary layer sites, constructed at the sine (latitude) of the measurement site. The latitude reference is defined at all time steps. diff: The difference climatology describes how the site differs from marine boundary layer (MBL) sites that are nearby in latitude. The difference climatology is defined at all time steps. Extension Weights Files (wts) ----------------------------- Any method used to fill spatial and temporal gaps in observational records is forced to make assumptions creating uncertainty in the resulting data product. Each extended record included in GLOBALVIEW-CH4 has a corresponding weight file that suggests a relative significance for each value in the extended file. All smooth values (derived directly from the actual measurements) receive a relative weight (ranging from 2 to 10) that depends on sampling density and measurement variability. All filled values (interpolated and extrapolated) receive a fixed weight of 1. We strongly recommend that users of this data product consider the weight files, which provide an estimate of the relative significance of each value in the extended record. Following the descriptive information detailed above, the four (4) columns in the weight files are: UTC: Synchronization year where the number of years is determined by the synchronization period. rsd: Residual standard deviation (RSD) of the measurements about the smooth curve, S(t), with annual resolution. Years with fewer than six (6) measurements are assigned default values. #: The number of residuals per year used in the RSD determination. weight: Scaled weights determined using the relative weighting scheme described by Masarie and Tans, [1995]. Years where weights cannot be determined are assigned a default minimum weight of one (1). The first row past the descriptive information specifies the residual standard deviation, number of residuals, and derived weight for all years, all observations. Average Atmospheric Monthly Variability Files (var) --------------------------------------------------- A statistical summary of average atmospheric variability is provided for each measurement record. A residual distribution is determined by fitting a smooth curve, S(t), to the observations, C(t), and computing residuals C(t)-S(t). The residuals for all Januarys, Februarys, etc are aggregated and statistics are determined with monthly resolution. The aggregated monthly statistics include within month and year-to-year variability. Information pertaining to the diurnal cycle is not considered here. Following the descriptive information detailed above, the six (6) columns in the "var" files are: mo: Month (1-12) specification. stdev: Standard deviation of the residual distribution computed monthly for all years. 50%ile: The 50th percentile or median of the residual distribution. 16%ile: The 16th percentile of the residual distribution. 84%ile: The 84th percentile of the residual distribution. #: The number of aggregated monthly residual values used to compute the monthly statistics. Average Seasonal Cycle Files (seas) ----------------------------------- A statistical summary of the average seasonal cycle is provided for each measurement record. Monthly means are computed from a detrended smooth fit, S(t)_T(t), to the observations. The monthly means for all Januarys, Februarys, etc. are aggregated and statistics are determined with monthly resolution. The standard deviation of each aggregated monthly mean value is a measure of the year-to-year variability in the monthly mean values. The standard error of the aggregated monthly mean value is an estimate of the uncertainty in the aggregated monthly mean value. Following the descriptive information detailed above, the five (5) columns in the "seas" files are: mo: Month (1-12) specification. mean: Mean of the aggregated detrended monthly means for all years. stdev: Standard deviation of the aggregated monthly mean distribution. std err: Standard error of the aggregated monthly mean distribution. #: The number of monthly mean values used to compute the aggregated monthly statistics. Average Diurnal Cycle Files (diu) --------------------------------- A statistical summary of average diurnal cycles by month compiled using data from complete years is provided for each measurement record with hour resolution and where the diurnal cycle is a dominant feature in the observations. The residual distribution is determined by subtracting the 24-hour average mixing ratio for each day from every observation for that day. Note that for tall tower measurements, the 24-hour average is determined from measurements at the highest level. Following the descriptive information detailed above, the six (6) columns in the "diu" files are mo: Month (1-12) specification. hr: Hour (0-23) specification in UTC. 50%ile: The 50th percentile or median of the residual distribution computed monthly for all complete years. 16%ile: The 16th percentile of the residual distribution. 84%ile: The 84th percentile of the residual distribution. #: The number of residual values from complete years used to compute the monthly statistics. Marine Boundary Layer (MBL) Reference Matrix Files (mtx) -------------------------------------------------------- The reference marine boundary layer matrix contains CH4 mixing ratios as a function of time and sine of latitude and is a by-product of the data extension procedure (see Masarie and Tans, [1995] and Appendix A of the gv_ch4_2001.pdf document for details). Be aware that significant information contained in the actual data may be lost in this matrix. In addition, the reference MBL matrix itself may give an unrealistic impression of the comprehensiveness of global atmospheric CH4 measurements since it contains CH4 values at locations and times when no measurements exist. There is a single header line in the matrix file that specifies the format of the reference matrix. + Matrix format: FORMAT="(F12.6, 41(1X,F12.4))" Following the single header line above, the 42 columns are UTC: "Weekly" synchronized time steps in Universal Coordinated Time (UTC) as decimal dates, i.e., year plus fraction of the year. Each year has 48 "weekly" steps. "Synchronized" means that the synchronization period and time steps in the matrix are identical to those in the extended record files. sine of latitude: [columns 2-42] There are 41 even intervals of 0.05 sine of latitude from 90¡S to 90¡N, i.e., column 2 represents a reference MBL value at -1.00 (90¡S), column 3 at -0.95 (71.8¡S), column 4 at -0.90 (64.2¡S), and so on. ################################################################################ ASCII File Format ----------------- All of the files in the ISLSCP Initiative II data collection are in the ASCII, or text format. The data files in this data set contain multiple header lines which contain the site name, the location, and the date of the data collection (see above). The actual data follows in a series of columns with fixed width. Following the header information detailed above, the format (width) of each type of file is as follows: Extended "ext" F12.6, 3(F12.4) Weight "wts" F12.6, 3(F12.4) Atmospheric "var" I5, 4(F12.4), I6 Variability Seasonal Cycle "seas" I5, 3(F12.4), I6 Diurnal Cycle "diu" 2(I5), 3(F9.4), I6 Sample Collection "tod" I10, F10.2, I10 Times Reference Matrix "mtx" F12.6, 41(1X,F12.4)