McIDAS User's Guide
Version 2014.1

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GRDDISP

Generates contours, streamlines or plots from grid data.

Format

GRDDISP dataset frame [keywords]

Parameters

Search Keywords

Multiple Contour/Plot Keywords

Derived Grid Keywords

Mathematical Operations Keywords

Navigation Keywords

Contour and Plot Keywords

Contour-Specific Keywords

Plot-Specific Keyword

Output Keywords

Remarks

Individual grids are stored in grid files. Each dataset position points to a single grid file. If you know which grid file contains the grid you want to display, specify its position in the dataset parameter to reduce the searching time.

The GRDDISP command draws contours or streamlines, or plots data from the first grid or set of grids that matches the conditions specified with the search keywords. If you don't specify any search keywords, GRDDISP displays the first grid in the dataset. If, for example, you specify PARAM=T LEV=850 TIME=12 SRC=ETA, GRIDDISP displays the first grid with T in its parameter field, 850 in its level field, 12 in its time field, and ETA in its source field. To choose the exact grid you want, you may need to specify several search keywords.

Use the keyword FHOUR or FRANGE with GRA or NUM to generate a contour or plot using more than one grid with a single GRDDISP command. The keyword GRA displays the grids in the order entered with FHOUR or sequentially from the range bhr to ehr in FRANGE. For example, if you specify SRC=GFS TIME=12 PARAM=Z LEV=500 FHOUR=0 36 24 60 GRA=11 14, the 12 UTC GFS 500 mb height grids with forecast hours 0, 36, 24 and 60 are displayed in frames 11, 12, 13 and 14.

The default contour interval for a grid is taken from the text file CONTOUR.DEF. If no file entry matches the grid's parameter, units and level, the default is calculated to produce a reasonable number of contours, usually between 10 and 20. To change, add or delete entries in CONTOUR.DEF, first back up the current version, then modify it using the format described near the top of the file.

If you specify PARAM=MOVIE, STREAML, WINDB or WINDV, GRDDISP does not search for a grid with that parameter. Instead, it locates the first u-component grid that matches the other search keywords, then it locates the matching v-component grid and uses those grids to contour streamlines, plot wind barbs, or plot wind vectors. In all other cases, the value entered in the keyword PARAM is the value searched for in the grid's parameter field.

When plotting data (with OUT=PLOT or PAR=WINDB or PAR=WINDV), you can assign output plot color based on the parameter's values by using the COLOR keyword format color[min-maxBinc], where color is the beginning graphics color level, min and max indicate the range of parameter values to color-code, and inc is the increment (the dash means "to" and the B means "by"). For example, when plotting integer values, COLOR=2[30-59B10] plots values 30-39 in level 2, values 40-49 in level 3, and values 50-59 in level 4. Values less than 30 are in level 1, and values greater than 59 are in level 5. When plotting floating point values (e.g., FORMAT=F5.2), COLOR=2[30-59.99B10] plots values 30.00-39.99 in level 2, values 40.00-49.99 in level 3, and values 50.00-59.99 in level 4. Values less than 30.00 are in level 1, and values greater than 59.99 are in level 5. Also, when plotting color-coded wind barbs (PAR=WINDB), the units of the 'min' and 'max' values are knots, even if you specify UNIT=MPS.

Use the SUBSECT keyword to analyze a portion of the original grid. The analysis can be performed slightly faster because there is less data to transfer and analyze.

Wind barb symbols use triangular flags, and long and short lines, or barbs, to represent wind speed. For example, the wind barb shown here represents a wind speed of 75 knots or 37.5 meters/second.

• a flag represents 50 knots or 25 meters/second
• a long barb represents 10 knots or 5 meters/second
• a short barb represents 5 knots or 2.5 meters/second.

When using the UNIT keyword, you can specify the following units:

Use the DERIVE keyword to display grids of a variety of common meteorological parameters. The valid options are defined in the table below. In these equations, the following variables appear often:

DERIVE=	Description	Equation
ABV	absolute vorticity	f =coriolis parameter: 2ΩsinΦ (see COR)
BETA	beta parameter	f =coriolis parameter: 2ΩsinΦ (see COR) Ω=angular speed of the rotation of the planet (7.292 × 10-5 radians/second for Earth) Φ=latitude in degrees a=radius of the planet
COR	coriolis parameter	2ΩsinΦ Ω=angular speed of the rotation of the planet (7.292 × 10-5 radians/second for Earth) Φ=latitude in degrees
DIR	wind direction	atan2(-u,-v)
DSH	shearing deformation
DST	stretching deformation
DVG	divergence
SPD	wind speed
TD	dew point temperature	T =temperature in Kelvin RH =relative humidity Rv=moist gas constant: 461.5 Joules per kilogram per degree Kelvin
VOR	relative vorticity

Use the MERIDIONAL keyword to correct for the convergence of longitude lines at the poles. When deriving divergence and stretching deformation grids (DVG and DST), specify MERIDIONAL=YES to subtract the following correction term from the calculation.

When deriving vorticity, absolute vorticity, or shear deformation grids (VOR, ABV, or DSH), specify MERIDIONAL=YES to add the following correction term to the calculation.

For both the correction terms above:

The MERIDIONAL keyword has no effect with other DERIVE keyword options.

Use the Gn keywords to specify the grids to be used with the MATH keyword. Each Gn keyword specifies a single grid. Gn is followed by a list of select clauses in single quotes. By default, subsequent Gn keywords have the same select clauses, unless specified differently. For example, to request 850 and 1000 mb height grids from the 0:00 UTC GFS run, specify

G1='LEV 850;PARAM Z;TIME 0;SRC GFS' G2='LEV 1000'

The MATH keyword defines the operation to perform on the specified grids, for example, MATH='G1**(SQRT(G2))'. The table below shows the operations available with the MATH keyword.

MATH= option	Function
+	addition
-	subtraction
*	multiplication
/	division
**	power
SQRT	square root
EXP	exponential (the number e raised to a power)
LOG	natural logarithm
LOG10	base 10 logarithm
SIN	sine
COS	cosine
TAN	tangent
ASIN	arcsine
ACOS	arccosine
ATAN	arctangent
ABS	absolute value
MIN	minimum of two grids
MAX	maximum of two grids
DDX	derivative with respect to x
DDY	derivative with respect to y
DELSQ	laplacian
COR	coriolis parameter (2ΩsinΦ)
BETA	beta parameter
LAT	creates a grid of latitude
LON	creates a grid of longitude

When specifying powers of ten, such as 10² or 10^-3 in the MATH keyword, use one of the two methods shown in the examples below.

MATH='2*7.292*(1e-5)*(SIN(G1))'

MATH='2*7.292*(10**(-5))*(SIN(G1))'

In these entries both 1e-5 and 10**(-5) represent 10^-5. You must include the parentheses with 10**(-5); 10**-5 will not be interpreted correctly.

When using the MATH keyword and the range of output data values is larger than five orders of magnitude, the output grid is scaled based on the maximum end of the range. This means that small data values may be replaced by zeros in the output grid.

Use the FORMAT keyword options below to format output based on FORTRAN designators for data output. To add text to the numeric output, follow the designator with a text in single quotes.

To specify a parameter's output format, use a designator followed by a positive number. For example, I4 specifies an integer of up to four digits; F10.1 specifies a floating point decimal of up to 10 digits (including the decimal point) with one digit to the right of the decimal; E10.3 specifies an exponential number of up to 10 characters (including the decimal point and the four-character exponential flag) with three digits to the right of the decimal. Specify an X to use the parameter's default format. If the specified integer or floating point format is too small for the parameter's values, a series of asterisks (***) is displayed.

Examples

GRDDISP G/LOCAL

This entry draws a contour using the first grid in the first grid file in dataset G/LOCAL. The map and contours cover the grid's entire domain. The contour interval is taken from the text file CONTOUR.DEF if it has an entry that matches the grid's parameter, units and level. If CONTOUR.DEF does not have a matching entry, GRDDISP calculates the contour interval.

GRDDISP G/LOCAL.9 GRID=5 OUT=PLOT PINT=1 3 NAV=C

This entry plots grid point values from grid 5 in the grid file in position 9 of dataset G/LOCAL. Every grid point in the row direction and every third grid point in the column direction is plotted using the navigation of the current frame.

GRDDISP NMC/ETA LEV=1000 PARAM=WINDV DAY=#Y TIME=0 FDAY=#Y FTIME=18 MAP=MID PRO=MERC TITLE='ETA 1000 mb wind vectors, valid at 18 UTC today'

This entry plots the 1000 mb wind vector forecast valid at 18 UTC today over the Midwest. The vectors are created using the u- and v-component grids from today's 00 UTC model run in dataset NMC/ETA. The default title below the plot is replaced with the text ETA 1000 mb wind vectors, valid at 18 UTC today.

GRDDISP NCEP/GFS LEV=500 PARAM=Z CINT=60 FHOUR=24 LAT=-40 -40 LON=-90 90 SLAT=-40 PRO=CONF

This entry draws a contour of the 24-hour forecast 500 mb heights with an interval of 60 over the South Pole. The data is taken from the first 24-hour forecast 500 mb height grid in dataset NCEP/GFS.

GRDDISP NCEP/GFS LEV=500 PARAM=Z CINT=60 DAY=#Y TIME=12 FRANGE=0 120 24 GRA=1 6

This entry draws a contour of the 0-, 24-, 48-, 72-, 96- and 120-hour forecast 500 mb heights with an interval of 60. The contours are displayed over a map of the grids' domain in frames 1, 2, 3, 4, 5 and 6. The data is taken from today's 12 UTC model run grids in dataset NCEP/GFS.

GRDDISP MODEL.2 LEV=850 PARAM=T UNIT=C CINT=5 SRC=GFS DASH=NEG COLOR=5

This entry draws a contour of the first 850 mb GFS temperature grid in the grid file in position 2 of the dataset with the alias name MODEL. The map and contours cover the grid's entire domain. The contours are drawn in graphics color level 5 with an interval of 5 degrees Celsius. Negative contours are dashed.

TE SNOW "0 10 20 32 40 60
GRDDISP NMC/ETA LEV=850 PARAM=T UNIT=F CINT=SNOW MAP=USA PRO=CONF

The TE command entry creates a string called SNOW containing contour values for the GRDDISP entry. The GRDDISP entry draws a contour of the first 850 mb temperature grid in dataset NMC/ETA. The map and contours cover the USA, and only the 0, 10, 20, 32, 40 and 60 degree Fahrenheit contours are drawn. Do not specify a pound sign (#) before the string name in the CINT keyword.

GRDDISP GRIDS/GFS LEVEL=SFC PARAM=T DAY=97300 FHOUR=24 MAP=USA SUBSECT=25 50 60 100

This entry draws 24-hour forecast surface temperature contours from the dataset GRIDS/GFS. A map of the United States is drawn, but the contours are drawn only for the geographic region specified in the SUBSECT keyword (25-50°N and 60-100°W). If the SUBSECT keyword was not specified, GRDDISP would subsect a region slightly larger than the map and draws contours in the entire frame.

GRDDISP GRD.8000 4 LAT=20 50 LON=75 145 PRO=CONF DERIVE=SPD LEV=500 FHOUR=48

This entry draws a 500 mb wind speed contour with a 48-hour forecast time. The component grids used to derive wind speed are the u- and v-components of the wind from position 8000 in the dataset with the alias GRD. The contour is displayed on frame 4 over a polar stereographic map between 20° and 50° latitude and 75° and 145° longitude.

GRDDISP RTGRIDS/GFS MAP=USA PRO=MERC DERIVE=VOR LEV=500 DAY=96284 TIME=12 FHOUR=12 24 36 48 GRA=1 4 DASH=NEG

This entry draws contours for a sequence of 500 mb relative vorticity grids from day 96284 at 12:00 UTC. The component grids are from dataset RTGRIDS/GFS. The 12-, 24-, 36-, and 48-hour forecast contours are displayed on frames 1, 2, 3, and 4, and displayed over the USA in a Mercator projection. All negative contours are dashed.

GRDDISP RTGRIDS/GFS PARAM=MOVIE LEV=500 DAY=#Y TIME=0 FHOUR=18 MAP=USA MOVIE=2 8
TE MOVIECOLORS "GU MAKE !1 0 !2 0
REPEAT MOVIECOLORS 2 TO 8 BY 1 0 BY 40
CM 2 8 GO=YES

The GRDDISP entry draws the 18-hour GFS forecast 500 mb streamlines with colored line segments on a map of the U.S. The TE and REPEAT entries set graphics color levels 2 to 8 to increase linearly from black (level 2) to bright green (level 8). The CM entry then circulates the graphics color levels to animate the wind flow.

GRDDISP RTGRIDS/NAM-USLC DAY=#Y TIME=12 FHOUR=12 PAR=T LEV=SFC UNIT=F MAP=USA COLOR=3[10-69B10] OUT=PLOT FORMAT=I4 PINT=2 3

This entry plots the 12-hour forecast surface temperatures over the United States from today's 12 UTC NAM model run. The temperatures are color coded: less than 10°F in graphics color level 2, 10-19°F in level 3, 20-29°F in level 4, 30-39°F in level 5, 40-49°F in level 6, 50-59°F in level 7, 60-69°F in level 8, and 70°F and greater in level 9.

GRDDISP RTGRIDS/GFS-GLME DAY=#Y TIME=0 FHOUR=24 PAR=WINDB LEV=700 UNIT=KTS MAP=NA COLOR=2[1-20B5] PINT=3 3

This entry plots the 24-hour forecast 700 mb wind barbs over North America from today's 00 UTC GFS model run. The flags are color coded by speed: less than 1 knot in graphics color level 1, 1-5 knots in level 2, 6-10 knots in level 3, 11-15 knots in level 4, 16-20 knots in level 5, and greater than 20 knots in level 6. Note that each flag symbol may appear in two different colors because the speeds are rounded to the closest five knot increment (i.e., 2.5-7.4 knot speeds are displayed as 5 knot flags, 7.5-12.4 knot speeds are displayed as 10 knot flags, 12.5-17.4 knot speeds are displayed as 15 knot flags, etc.). Thus, 5 knot flags can be in graphics color level 2 or 3, 10 knot flags can be in level 3 or 4, 15 knot flags can be in level 4 or 5, etc.

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