FORMAT OF NORTHERN HEMISPHERE 5-DEGREE LAT-LON GRIDS

General description:

NCAR has northern hemisphere daily and monthly 5-degree lat-lon grids for various levels and parameters. Currently, most grids were derived from the NMC octagonal grid. For these grids there is no original data south of 20N, all these points are set to missing. Grids derived from other sources may have data to the equator. Some early grids in some of the series have some interior points missing. Packed values must be checked for missing.

Format of tape records (each record is 346 sixty-bit words long):

Words 1 and 2 are packed integer identification fields:
bits 1-6	Format number (= 1,2,3)
bits 7-13	Year, e.g. 55
bits 14-17	Month, e.g. 11
bits 18-22	Day
bits 23-27	Hour, GMT e.g. 15
bits 28-37	Pressure level True value = 1023 - Packed value The following pressure values have special meanings: 1013 = Sea-level 1001 = Surface 980 = Tropopause
bits 38-46	Parameter code: 1 = Height 6 = Sea-level Pressure 10 = Temperature 28 = Sea-level Pressure 30 = u-component of the wind 31 = v-component of the wind 57 = Sea-surface Temperature
bits 47-55	Forecast (= 0 for analyses)
bits 56-65	Second pressure level, not applicable
bits 66-71	Source of analysis: 1 - NMC 2 - USAF 3 - Navy 4 - ESSPO 5 - NMC B-3's 6 - NCAR digitized 9 - extended fcst center, 700mb 9 - Univ. of Wisconsin, 300mb
bits 72-76	Grid status (= 0)
bits 77-91	Bias used in packing the data
bits 92-106	Scale factor, True value = Packed value - bias
bits 107-114	Number of days in mean if yrmo grid, otherwise = 0
bits 115-120	Unused
Word 3 is the base value in sign-magnitude CDC 60-bit floating point format.
Words 4-345 contain 1368 (72x19) packed integer grid values in 15-bit bytes.
Word 346 is a 60-bit checksum of words 1 through 345.

Data and grid characteristics:

True grid values are obtained from the packed values as follows:
true value = base + (packed value - bias) x 2^scale   (1)

Note: A packed value of zero indicates the true value is missing. Therefore, the packed value should be checked before applying (1).

The 1368 packed data points (DP) have the following (I,J) values:
DP#1 is (1,1), DP#2 is (2,1), DP#73 is (1,2), DP#1368 is (72,19)

As I varies 1 to 72, longitude varies 0E to 355E; as J varies 1 to 19, latitude varies 0N to 90N.