Weather Data and GRIB2 Local Use Section

About MDL and Local Use Section

Certain weather elements have provided particular challenges in encoding them in GRIB2. Using Section 2 to encode various forms of metadata provides one solution. For the NBM element Weather ("Wx"), MDL GRIB2 encoding software uses the Local Use section for packing metadata unique to each grid. At this time, for parameters other than weather, the Local Use section is omitted.

 

MDL Local Use Template 2.1

Octet No. Content
1-4 Length of the section in octets (N)
5 Number of the section (2)
6 MDL Section 2 template number (1)
7-8 Total number of data groups in the local use section
9-12 Number of values in first group of local use data

13-16

Reference value of first group of data (R) (IEEE 32-bit floating-point value)

17-18

Decimal scale factor (D)

19

Number of bits to pack each value of the first group of data with

20

Type of data in first group ("0"=floating point, "1"=integer)

21-N

The first group of data packed using the simple packing method

 

Data Encoding Details

Grids of weather are stored as two distinct entities. The first entity is a grid of small integer values that will generally store very compactly in Section 5. The second entity is a series of null-terminated strings of ASCII characters called keys (weather keys). The integers on the weather grid serve as indices, each pointing to one of the unique keys in the series. Each key is comprised of one or more subkeys, separated by a caret "^".

The metadata packed in the Local Use Section for the weather element is a lookup table. The ASCII characters are loaded, one per octet, and then compressed using the Simple Packing Scheme for integers defined for Section 5. The Simple Packing Scheme treats the set of keys as an integer array. Each integer in the array is a decimal representation that corresponds to an ASCII character. For instance, the integer 65 translates to the ASCII character "A". The null character is represented by the integer 0, so that whenever a 0 is encountered in the array, it marks the ending of a unique key. The tables that follow show the specific contents of the weather keys/subkeys.
 

Data: Weather Keys and Subkeys

A weather subkey can have up to five parts, each part separated by colons ":". The parts are:

  • one weather type code. See Weather Type Codes, below.

  • one coverage/probability code. See Coverage/probability Codes, below.

  • one intensity code. See Intensity Codes, below.

  • one visibility code. See Visibility Codes, below.

  • zero or more attribute codes, separated by commas ",". See Attribute Codes, below.

Weather Type Codes

Code Value Code Value

<NoWx>

No Weather

T

Thunder

R

Rain

RW

Rain Showers

ZR

Freezing Rain

IP

Ice Pellets (sleet)

S

Snow

SW

Snow Showers
F Fog    
Weather Types Not Currently Used in NBM Weather

A

Hail

BN

Blowing Sand

ZL

Freezing Drizzle

L

 Drizzle

H

Haze

BS

Blowing Snow

K

Smoke

BD

Blowing Dust

 

Coverage/Probability Codes

Code Meaning Code Meaning

SChc

Slight Chance

Iso

Isolated

Chc

Chance of

Sct

Scattered

Lkly

Likely

Num

Numerous

Def

Definite

<NoCov>

No Coverage / probability
Coverage/Probability Codes Not Currently Used in NBM Weather

Wide

Widespread

Ocnl

Occasional

Patchy

Patchy

Areas

Areas of

Pds

Periods of

Frq

Frequent

Inter

Intermittent

Brf

Brief

 

Intensity Codes

Code Meaning Code Meaning

<NoInten>

No Intensity

+

Heavy

-

Light

m

Moderate
Coverage/Probability Codes Not Currently Used in NBM Weather

--

Very Light

 

Visibility Codes

The following visibility codes may appear in NBM GRIB2 bulletins. The units are statute miles: <NoVis>, 0SM, 1/4SM, 1/2SM, 3/4SM, 1SM, 11/2SM, 2SM, 21/2SM, 3SM, 4SM, 5SM, 6SM, P6SM.

Attribute Codes

NBM weather grids will not generally have attribute codes.

Size Limitations for NBM Weather Grids

There is no (practical) limit to the number of weather keys, subkeys, etc. For NBM weather grids, a maximum of 5 weather subkeys in a weather key will be present.

 

Weather Key Examples

Sample Weather Key Meaning

Sct:SW:-:<NoVis>:

Scattered snow showers

Ocnl:R:-:<NoVis>:^S:Ocnl:-:<NoVis>:^
SChc:ZR:-:<NoVis>:

Occasional rain and snow, with a slight chance of freezing rain

Wide:FR:-:<NoVis>:OLA

Widespread frost in the outlying areas

<NoCov>:<NoWx>:<NoInten>:<NoVis>:

No weather

Sct:RW:-:<NoVis>:^T:Iso:m:<NoVis>:

Scattered showers with isolated thunderstorms

Sct:T:+:<NoVis>:DmgW,LgA

Scattered severe thunderstorms with damaging winds and large hail

NBM