!===============================================================================

!

! README for OMAERUVd (OMI Daily L3 for OMAERUV)

!

! Peter J.T. Leonard (ADNET) - 08/20/08

!

! OMAERUVd High Level Overview:

! This is the main program for the OMI (Ozone Monitoring Instrument)
OMAERUVd

! Product Generation Executive (PGE). The OMAERUVd PGE creates the
OMAERUVd

! data product, which is the primary Level 3 (L3) aerosol product of the
U.S.

! OMI Science Team. The \"d\" at the end of \"OMAERUVd\" represents
\"daily\".

!

! The OMAERUVd PGE creates a (Total Ozone Mapping Spectrometer)
TOMS-like daily

! L3 gridded data product file from (as many as) three consecutive
OMAERUVG

! daily Level 2G (L2G) gridded data product files, where each OMAERUVG
file

! contains 24 consecutive UTC hours of OMAERUV orbital Level 2 (L2)
swath data

! subsetted onto a 0.25-degree by 0.25-degree grid in longitude and
latitude.

!

! A TOMS L3 day is defined as the ensemble of all L2 ground pixels with
pixel

! centers that have the same local calendar date on the ground. There
are two

! reasons behind such a definition. First, a TOMS L3 day provides
complete

! coverage of Earth, since every point on Earth (outside of polar night)

! experiences daylight on each calendar date (in comparison, 24
consecutive

! UTC hours of OMI observations do not completely cover Earth). Second,
the

! TOMS L3 day puts the discontinuity (i.e., where the L2 observations
within a

! given day differ by almost 24 hours) at +/-180 degrees longitude, and,
thus,

! the discontinuity can be placed undistractingly along the extreme left
and

! right edges of several commonly used map projections.

!

! The calendar date of the TOMS L3 day is the calendar date at Greenwich

! midway through the TOMS L3 day, and is specified via the L3 day of
year

! parameter in the PCF (Process Control File) of the OMAERUVd PGE. Note

! that some of the L2 observations at the beginning of a TOMS L3 day
will

! correspond to the previous calendar date at Greenwich, and some of the
L2

! observations at the end of a TOMS L3 day will correspond to the next

! calendar date at Greenwich. Consequently, data from three consecutive
OMI

! L2G files are required to fully populate the L3 grid at all longitudes
for

! any given TOMS L3 day.

!

! The OMAERUVd PGE was developed for Omar Torres (Hampton University),
and is

! based upon the TOMS Level 3 Gridded Software. The latter was developed
over

! a period of many years by several people: W. Byerly, D. Cao, E.
Celarier,

! Q. Choung, S. Huang, B. Irby, D. Lee, L. Liu, R. McPeters, L. Moy, M.
Peng,

! L. Phung, B. Raines, C. Seftor, and, especially, C. Wellemeyer.

!

! Adopted OMAERUVd Grid:

! The adopted L3 grid is a 1.0-degree by 1.0-degree grid in longitude
and

! latitude. The dimensions of the grid are 360 by 180. The center of the

! first grid cell is located at longitude -179.5 and latitude -89.5. The

! center of the final grid cell is located at longitude 179.5 and
latitude

! 89.5. The center of the grid itself is located at longitude 0.0 and

! latitude 0.0, and corresponds to the corners of four grid cells.

!

! The adopted L3 grid is consistent with the document entitled
\"Definition

! of OMI Grids for Level 3 and Level 4 Data Products\" by J.P. Veefkind
et al.

!

! The format of the L3 product files is consistent with the document
entitled

! \"A File Format for Satellite Atmospheric Chemistry Data\" by C. Craig
et al.

!

! OMAERUVd Gridding Algorithm:

! The values in the OMAERUVd data product file for each L3 grid cell are

! unweighted averages of the values for all of the OMI L2 observations

! accepted into the L3 grid cell. No scheme of weights (e.g., based upon

! the fractional area of overlap of each L2 observation with the L3 grid

! cell) is applied.

!

! Each of the OMI L2 observations with ground pixel centers that lie
within

! each L3 grid cell is considered, and compared with several exclusion

! criteria. These criteria are summarized here in sequence.

!

! Let l3_tnoon be the time at noon UTC for the TOMS L3 day, and let
l2g_time

! be the L2 observation time.

!

! A1) As a rough first cut, L2 observations made outside of the 48-hour
time

! interval centered at l3_tnoon are excluded. Thus, L2 observations with

!

! l2g_time \< l3_tnoon - (24 hours - 15 minutes)

!

! or

!

! l2g_time \>= l3_tnoon + (24 hours - 15 minutes)

!

! are excluded.

!

! At any given moment, all points on Earth between the longitude of
midnight

! and the dateline that are on the same side of the dateline have the
same

! calendar date. The calendar dates on opposite sides of the dateline
differ

! by one day, except at the instant when the longitude of midnight and
the

! dateline coincide, in which case the date is the same everywhere on
Earth.

!

! Let l2_lom be the longitude of midnight at l2g_time, and let l2g_lon
be the

! longitude at the center of the L2 observation. The dateline is assumed
to

! lie strictly at a longitude of +/-180 degrees for the sake of
simplicity,

! which ignores the zigs and zags of the actual dateline.

!

! A2) L2 observations with local calendar dates on the ground that
correspond

! to the day before the TOMS L3 day are excluded. This has been

! implemented as L2 observations with

!

! l2g_time \< l3_tnoon - 15 minutes

!

! and

!

! -180 degrees \<= l2g_lon \< l2_lom

!

! are excluded.

!

! A3) L2 observations with local calendar dates on the ground that
correspond

! to the day after the TOMS L3 day are excluded. This has been

! implemented as L2 observations with

!

! l2g_time \>= l3_tnoon + 15 minutes

!

! and

!

! l2_lom \<= l2g_lon \< 180 degrees

!

! are excluded.

!

! Let bit5 be bit 5 (the 6th bit) of the \"ground pixel quality flag\"
of the L2

! observation. This is the solar eclipse possibility flag.

!

! A4) L2 observations with the solar eclipse possibility flag set are
excluded.

! Thus, L2 observations with

!

! bit5 /= 0

!

! are excluded.

!

! Let l2g_scene be the scene number (from 1 to 60) of the L2
observation.

!

! Let l3_year be the year of the L2 observation, and let l3_month be the
day

! of the L2 observation.

!

! A5) L2 observations with scene number equal to 54 or 55 (1-based) on
or

! after June 1, 2007 are excluded. Thus, L2 observations with

!

! l2g_scene == 54 or l2g_scene == 55

!

! and

!

! l3_year == 2007 and l3_month \>= 6 or l3_year \>= 2008

!

! are excluded.

!

! A6) L2 observations with scene number in the range from 38 to 43
(1-based)

! on or after May 1, 2008 are excluded. Thus, L2 observations with

!

! (l2g_scene \>= 38 and l2g_scene \<= 43)

!

! and

!

! ((l3_year == 2008 and l3_month \>= 5) or (l3_year \>= 2009))

!

! are excluded.

!

! After this point there are significant differences in how L2
observations

! are excluded from 1) the L3 grids for the aerosol optical depths, and
2) the

! L3 grids for the aerosol indices.

!

! OMAERUVd Gridding Algorithm for Aerosol Optical Depths:

! There are four criteria in addition to A1 through A5 (above) for
excluding

! L2 observations from the L3 grid for the aerosol optical depths.

!

! Let l2g_aflg be the algorithm flag for the L2 observation.

!

! B6) L2 aerosol absorption optical depth observations with an algorithm
flag

! not equal to 0 and 1 are excluded. Thus, L2 aerosol absorption optical

! depth observations with

!

! l2g_aflg /= 0

!

! and

!

! l2g_aflg /= 1

!

! are excluded.

!

! B7) L2 aerosol extinction optical depth observations with an algorithm
flag

! not equal to 0 are excluded. Thus, L2 aerosol extinction optical depth

! observations with

!

! l2g_aflg /= 0

!

! are excluded.

!

! Let l2g_faaod be the aerosol absorption optical depth for the L2
observation.

!

! B8) L2 observations with an aerosol absorption optical depth less than
zero

! are excluded. Thus, L2 aerosol absorption optical depth observations

! with

!

! l2g_faaod \< 0.0

!

! are excluded.

!

! Let l2g_faeod be the aerosol extinction optical depth for the L2
observation.

!

! B9) L2 observations with an aerosol extinction optical depth less than
zero

! are excluded. Thus, L2 aerosol extinction optical depth observations

! with

!

! l2g_faeod \< 0.0

!

! are excluded.

!

! OMAERUVd Gridding Algorithm for Aerosol Indices:

! There are four criteria in addition to A1 through A5 (above) for
excluding

! L2 observations from the L3 grid for the aerosol indices.

!

! C6) L2 observations with a solar zenith angle greater than or equal to
70.0

! degrees are excluded. Thus, L2 observations with

!

! l2g_sza \>= 70.0

!

! are excluded.

!

! Let first4bits be bits 0 through 3 (the first four bits) of the
\"ground

! pixel quality flag\" of the L2 observation, which has the following
values:

! 0 - shallow ocean

! 1 - land

! 2 - shallow inland water

! 3 - ocean coastline/lake shoreline

! 4 - ephemeral (intermittent) water

! 5 - deep inland water

! 6 - continental shelf ocean

! 7 - deep ocean

! 8-14 - not used

! 15 - error flag for land/water

!

! Let glint_angle be the \"glint angle\" of an L2 observation. This is
equal to

! the inverse cosine of

! ( cos(l2g_sza) \* cos(l2g_vza) + sin(l2g_sza) \* sin(l2g_vza) \*
cos(l2g_raa) )

! where l2g_raa is the relative azimuth angle of the observation.

!

! C7) L2 observations with water at the ground pixel center and a glint
angle

! less than or equal to than 20.0 degrees are excluded. Thus, L2

! observations with both

!

! first4bits /= 1

!

! and

!

! glint_angle \<= 20.0

!

! are excluded.

!

! Let l2g_uvai be the UV aerosol index for the L2 observation.

!

! C8) Values of UV aerosol index less than 0.0 are excluded. Thus, L2

! observations with

!

! l2g_uvai \< 0.0

!

! are excluded.

!

! Let l2g_visai be the visual aerosol index for the L2 observation.

!

! C9) Values of visual aerosol index less than 0.0 are excluded. Thus,
L2

! observations with

!

! l2g_visai \< 0.0

!

! are excluded.

!

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