AVHRR Files

The Advanced Very High Resolution Radiometer, flown on a NOAA satellite, measures aerosol data which are analyzed for aerosol optical depth and posted daily at an FTP site.

Format of the data

AEROSOL WEEKLY 100KM ANALYZED FIELD FILE

CEMSCS DSN = PRD.AEROSOL.FIELD.KM100

1) DATASET ABSTRACT

An aerosol optical thickness 100km analyzed field file consists of a specific set of information pertaining to global latitude and longitude intersections. The 1° resolution file, includes the area from -180° to +179° longitude and from -70° to +70° latitude.

2) FILE STRUCTURE DESCRIPTION

The file consists of one documentation record (record number one), followed by one record for each latitude or row of the field. Record 2 or the first latitude row is the southernmost row or the 70° South row. Each row consists of seven words (28 bytes) of information for each longitude column forming a grid intersection plus one seven-word unit containing the row number identification and the date and time of the last analysis made for the field. The first grid intersection of each row is the 180° west meridian or the date line. Gridpoints proceed to the east across the record from left to right ending with the 179° East meridian . The documentation record is created from a namelist dataset and is displayed in namelist format, although it is stored as a binary record. The record size is 10,108 bytes.

3) DOCUMENTATION RECORD FORMAT

A detailed listing of the documentation record format is provided here. This is the first record of the file, and contains the valid time, minimum and maximum latitudes and longitudes of the area covered, together with the grid resolution. If also contains tne number of rows (latitudes), the number of columns (number of longitudes plus one), and information about the currency and reliability of the data.

4) DATA RECORD FORMAT

Each data record (latitudinal row) consists of a series of grid intersection points. These points are 28 bytes in length. Each longitude (column) reflects one grid intersection. At the end of the data record (i.e., immediately following the last column) is a 28 byte row identifier.

GRID INTERSECTION

WORDNO. -- LENGTH in BYTES

DESCRIPTION -- UNITS -- COMMENTS

1 -- 2 Optical Thickness -- (x1000) -- 0 to 2440

1 -- 2 Average Gradient -- units/100KM (x1000) -- 0 to 300

2 -- 2 Gradient X+ -- units/100KM (x1000) -- 0 to 300

2 -- 2 Gradient X- -- units/100KM (x1000) -- 0 to 300

3 -- 2 Gradient Y+ -- units/100KM (x1000) -- 0 to 300

3 -- 2 Gradient Y- -- units/100KM (x1000) -- 0 to 300

4 -- 1 Physiographic Descriptor -- 0 = sea 1 = land -- 0 to 15

4 -- 1 Spare -- Undefined -- 0 to 255

4 -- 1 Number Observations -- Integer -- 0 to 255

4 -- 1 Age Recent Observation -- Hours -- 0 to 255

5 -- 2 (Weight) Wxy -- Integer -- 0 to 32767

5 -- 2 Class 1 Coverage -- Bits -- 0 to 1

6 -- 1 Spatial Covariance X+ -- Grid Units -- 0 to 10

6 -- 1 Spatial Covariance X- -- Grid Units -- 0 to 10

6 -- 1 Spatial Covariance Y+ -- Grid Units -- 0 to 10

6 -- 1 Spatial Covariance Y- -- Grid Units -- 0 to 10

7 -- 2 Climatological Temp. -- oC (x10) -- -850 to +610

7 -- 2 Spare -- Undefined -- 0 to 32767

All parameters are stored as integer values.

DEFINITION OF TERMS IN GRID INTERSECTION

Optical Thickness - The latest Aerosol Optical Thickness calculated based on the previous analysis optical thickness, weighted according to its reliability, combined with a weighted average of current observations within a surrounding area which is determined according to the grid point's gradient.

Average Gradient - The average of the gradients in all four directions (N, S, E, W) from the grid intersection.

Gradient in X+ Direction - Change in optical thickness between the grid point and neighbor points within the field in the positive direction along the X axis.

Gradient in X- Direction - Change in optical thickness in the negative direction along the X axis.

Gradient in Y+ Direction - Change in optical thickness in the positive direction along the Y axis.

Gradient in Y- Direction - Change in optical thickness in the negative direction along the Y axis.

Physiographic Descriptor - The land/sea tag indicating whether a grid intersection is a land or sea point.

Spare - Unused parameter.

Number of Observations - The total number of current observations used in the analysis of the new optical thickness for the grid intersection.

Age of Most Recent Observation - The age, in hours from the time of last analysis, of the most recent observation used to determine the new optical thickness for a grid intersection.

Reliability -

New reliability associated with the new optical thickness, based on the previous reliability combined with the weighted reliability of all observations used in the last analysis.

Class 1 Temporal Coverage - Set of bits (0-15) of which bit 1 is set to 1 for each analysis which included observations with a reliability greater than or equal to a specific minimum reliability considered as class 1. Bit 0 always remains a 0, and all bits are shifted right during each analysis leaving bit 1 a 0 when no class 1 reliability observations are used for a grid intersection.

Spatial Covariance X+ - The distance in grid units from the grid intersection to the nearest land mass in the positive direction along the X axis.

Spatial Covariance X- - The distance in grid units from the grid intersection to the nearest land mass in the negative direction along the X axis.

Spatial Covariance Y+ - The distance in grid units from the grid intersection to the nearest land mass in the positive direction along the Y axis.

Spatial Covariance Y- - The distance in grid units from the grid intersection to the nearest land mass in the negative direction along the Y axis.

Independent Grid Temperature - The average sea surface temperature of a grid intersection for a particular month over a number of years, taken from the global climatology file.

ROW IDENTIFICATION INFORMATION

Full Word -- Length Bytes

Description -- Units -- Comments

1 -- 4

Row -- Integer -- 1-141

2 -- 4

Spare -- Undefined --

3 -- 4

Spare -- -- Undefined

4 -- 1

Marker -- Integer -- Always 255

4 -- 3

Spare -- Undefined --

5 -- 4

Hour of Day, Minutes of Hour -- (100 x Hours) +Minutes -- 0-2359

6 -- 4

Day of Year -- Days -- 1-366

7 -- 4

Year -- Years --

NOTE: All parameters are stored as integer values. Words 5 to 7 are the date and time at which analysis was performed.



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