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GrADS ousmane/Students/grads/gadoc151.pdf · PDF fileGrADS implements a 4-Dimensional data model, where the dimensions are usually latitude, longitude, level, and time. Each data

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  • The Grid Analysis and Display System

    GrADS

    V1.5.1.12

    Brian Doty

    [email protected]

    10 September, 1995

    Manual reformatted and updated by:

    Tom Holt,Climatic Research Unit,University of East Anglia, Norwich, [email protected]

    and

    Mike FiorinoProgram for Climate Model Diagnosis and IntercomparisonLawrence Livermore National Laboratory L-264Livermore, CA [email protected]

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    Table of Contents

    TABLE OF CONTENTS 2

    ABSTRACT 9

    SUGGESTIONS 10

    HOW TO USE THIS MANUAL 11

    INTRODUCTORY GUIDE 12

    1.0 STARTING AND QUITTING GrADS 13

    Help 13

    Diagnostics at startup 13

    Startup options 13

    Leaving GrADS 14

    2.0 BASIC CONCEPT OF OPERATION 15

    3.0 TUTORIAL 16

    4.0 USING GrADS DATA FILES 21

    Default file extension 21

    Introduction to GrADS Data Sets 21Gridded Data Sets 22

    The options record in the Data Descriptor File 27Station Data Sets 28Station Data Descriptor File 29STNMAP Utility 30

    Creating Data Files 30Examples of Creating a Gridded Data Set 30Examples of Creating Station Data Sets 31

    5.0 DIMENSION ENVIRONMENT 34

    6.0 VARIABLE NAMES 35

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    7.0 EXPRESSIONS 36

    8.0 DEFINED VARIABLES 37

    Defining new variables 37

    Undefining new variables 39

    9.0 DISPLAYING DATA PLOTS 40

    Displaying your data 40

    Clearing the Display 40

    10.0 GRAPHICS OUTPUT TYPES 41

    11.0 ANIMATION 43

    12.0 PAGE CONTROL 44

    Real and virtual pages 44

    Controlling the plot area 44

    13.0 GRAPHICS PRIMITIVES 45

    Drawing commands 45

    Controlling drawing commands 46

    Plot clipping 47

    14.0 HARDCOPY OUTPUT 48

    Producing a GrADS print file 48

    Printing a GrADS print file 48

    15.0 EXEC COMMAND 49

    16.0 USING STATION DATA 50

    Operating on station data 50

    Station Models 51

    17.0 INTRODUCTION TO GrADS SCRIPTS 52

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    What scripts can do 52

    Running scripts 52

    Automatic script execution 52

    Storing GrADS scripts 53

    18.0 ADDITIONAL FACILITIES 54

    Shell commands 54

    Command line options on GrADS utilities 54

    Reinitialisation of GrADS 54

    Displaying GrADS Metafiles 55

    REFERENCE SECTION 56

    19.0 GRAPHICS OPTIONS 57

    1-D Graphics 57Line Graphs (gxout = line): 57Bar Graphs (gxout = bar) 57Error Bars (gxout = errbar) 58Line Graph Shading (gxout = linefill) 58

    2-D Gridded Graphics 58Line Contour Plots (gxout = contour) 58Shaded or Grid Fill Contour Plots (gxout = shaded or grfill) 60Grid Value Plot (gxout = grid) 61Vector Plot (gxout = vector) 61Wind Barb Plot (gxout = barb) 62Scatter Plot (gxout = scatter) 62Specific Value Grid Fill Plot (gxout = fgrid) 63Streamline Plot (gxout = stream) 63

    1-D Station Graphics 64Plot time series of wind barbs at a point (gxout = tserbarb) 64Plot time series of weather symbols at a point (gxout = tserwx) 64

    2-D Station Graphics 64Plot station values (gxout = value) 64Plot wind barb at station (gxout = barb) 64Plot weather symbol at station (gxout = wxsym) 65Plot station model (gxout = model) 65

    Other Display Options 66Find closest station to x,y point (gxout = findstn) 66Write data to file (gxout = fwrite) 66Display information about data (gxout = stat) 66

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    Set Commands to Control Graphics Display 67Set range for plotting 1-D or scatter plots 67To control log scaling when the Z dimension is plotted on any plot: 67To control axis orientation: 68To control axis labelling 68To control displayed map projections 69To control map drawing: 69To control annotation 70To control console display 70To control the frame 70To control logo display 70

    20.0 GrADS FUNCTIONS 71

    Averaging Functions 71aave 71amean 72ave 72mean 74vint 74

    Filtering Functions 75smth9 75

    Finite Difference Functions 75cdiff 75

    Grid Functions 76const 76maskout 77skip 78

    Math Functions 78abs 78acos 78asin 79atan2 79cos 79exp 79gint 79gint(expr) 79log 79log10 79pow 80sin 80sqrt 80tan 80

    Meteorological Functions 80tvrh2q 80tvrh2t 81

    Special Purpose Functions 81tloop 81

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    Station Data Functions 83gr2stn 83oacres 83stnave 85stnmin 85stnmax 85

    Vector Functions 86hcurl 86hdivg 86mag 86

    21.0 USER DEFINED FUNCTIONS (UDFS): 88

    Overview of User Defined Functions 88

    The user defined function table 88

    Format of the function data transfer file 89

    Format of the function result file 91

    Example: Linear Regression Function 91

    22.0 FURTHER FEATURES OF GRADS DATA SETS 94

    File and time group headers 94

    Variable format/structure control 94

    Multiple file time series 99

    Enhanced data formats and structures 101

    23.0 PROGRAMMING GRADS: USING THE SCRIPTING LANGUAGE 102

    Overview of the Scripting Language 102

    Elements of the Language 102Variables 103

    String variables 103Predefined variables 103Global scripting variables 103Compound scripting variables 103

    Operators 104Expressions 105Flow control 106

    IF Blocks 106WHILE Blocks 106

    Functions 107Assignment 108Standard input/output 108

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    Sending Commands to GrADS 108Intrinsic Functions 109

    String functions 109Input/output functions 109

    Commands that complement the scripting language 110

    Widgets 112On screen buttons 113Rubber banding 113

    Examples 114

    24.0 USING MAP PROJECTIONS IN GrADS 115

    Using Preprojected Grids 115Polar Stereo Preprojected Data (coarse accuracy for NMC Models) 116Lambert Conformal Preprojected Data 117NMC Eta model (unstaggered grids) 120NMC high accuracy polar stereo for SSM/I data 122CSU RAMS Oblique Polar Stereo Grids 124Pitfalls when using preprojected data 128

    GrADS Display Projections 128

    Summary and Plans 129

    APPENDICES 130

    APPENDIX A: SUPPLEMENTARY SCRIPTS 131

    1) Correlation between two horizontal grids (corr.gs) 131

    2) GrADS Color Table Script (cmap.gs) 131

    3) Font Display (font.gs) 134

    4) Plot a color bar (cbar.gs) 134

    5) Stack commands and display on flush (stack.gs) 134

    6) Draw all WX Symbols (wxsym.gs) 134

    7) (draw.gs) 134

    8) (string.gs) 134

    9) (loop.gs) 134

    10) (bsamp.gs) 135

    11) Expanded Color Bar Script (cbarn.gs) 135

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    12) Computing Standard Deviation (sd.gs) 135

    13) Draw an x,y Plot (xyplot.gs) 135

    APPENDIX B: USING GRIB DATA IN GRADS 136

    Gribscan 136File options: 136Processing Options: 136Special note to NMC users 137Display options: 137Some examples: 137

    Gribmap 138

    APPENDIX C: COMMAND LINE EDITING AND HISTORY UNDER UNIX 142

    APPENDIX D: 32-BIT IEEE FLOATS ON A CRAY 144

    APPENDIX E: USING GRADS ON THE IBM PC 145

    Hardware considerations 145

    Some limitations of the PC version: 145

    Data sets from other platforms 145

    Printing on non-postscript printers 146

    Incorporating GrADS pictures into PC software 146

    APPENDIX F: GRADS-RELATED NETWORK FACILITIES 147

    ftp Sites 147

    Listserver 147

    WWW Sites 147

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    Abstract

    The Grid Analysis and Display System (GrADS) is an interactive desktop tool that is currently in useworldwide for the analysis and display of earth science data. GrADS is implemented on allcommonly available UNIX workstations and DOS based PCs, and is freely distributed over theInternet. GrADS provides an integrated environment for access, manipulation, and display of earthscience data.

    GrADS implements a 4-Dimensional data model, where the dimensions are usually latitude,longitude, level, and time. Each data set is located within this 4-Dimensional space by use of a datadescription file. Both gridded and station data may be described. Gridded data may be non-linearlyspaced; Gaussian grids and variable resolution ocean model grids are directly supported. Theinternal data representation in a file may be either binary or GRIB.

    Since each data set is located within the 4-D data space, intercomparison of disparate data sets isgreatly facilitated. Operations may be performed between data on different grids, or between griddedand observational data. Data from different data sets may be graphically overlaid, with correct spatialand time registration.

    The user accesses data from the perspective of the 4-D data model. A dimension environment isdescribed by the user as a desired subset of the 4-D space. Data is accessed, manipulated, anddisplayed within this subset.

    Operations may be performed on the data directly, and interactively, by entering FORTRAN-likeexpressions at the command line. A rich set of built-in functions are provided. In addition, users mayadd their own functions as external routines written in any programming language. The expressionsyntax allows complex operations that range over very large amounts of data to be performed withsimple expressions.

    Once the data have been accessed and manipulated, they may be displayed using a variety ofgraphical output techniques, including line, bar, and scatter plots, as well as contour, shaded contour,streamline, wind vector, grid box, shaded grid box, and station model plots. Graphics may also beoutput in PostScript format for printing on monochrome or color PostScript printers. The user haswide control over all aspects of graphics output, or may choose to use the geophysically intuitivedefaults.

    A programmable interface is provided in the form of an interpreted scripting language. A script maydisplay widgets as well as graphics, and take actions based on user point-and-clicks. Quitesophisticated data graphical interfaces can, and have, been built. The scripting language can also

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