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GLA08 Records: Release 33

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Elevated Aerosol Layers

The Level-2 planetary boundary layer (PBL) and elevated aerosol layer heights product (GLA08) contains top and bottom heights of elevated aerosols below 20.5 km (4 sec sampling rate) up to five layers, and up to three layers between 20.5 km to 41 km (20 sec sampling rate).

Elevated aerosol layers are determined from 532 nm data. If a layer top is found above 20.5 km, but the bottom is below 20.5 km, the layer is treated as if it were above 20.5 km. You should examine the layer height flag values ("i_LayHgt_Flag," bytes 301-332) to view the number, quality, and type of layers found. The type is either "normal" aerosol or polar stratospheric cloud (PSC). The PSC flag is set only when the layer meets certain requirements for its average temperature, latitude, and height.

The quality flags ("i20_aer_qf" and "i4_aer_qf") assign a layer quality based on the ratio of the average signal within the layer, divided by the average above the layer. Values of "1" or "2" indicate a very tenuous layer with a higher chance of being a false positive. The maximum value for this flag is 13, which indicates a layer that contains very strong backscatter and is not likely to be a false positive.

Planetary Boundary Layer Height

Planetary Boundary Layer (PBL) height is determined at a 4 sec sampling rate from the 532 nm channel only. The PBL height is the lowest layer that can be resolved with the ground as its bottom. The PBL thickness (top minus ground height) cannot exceed 6 km. If the layer top is greater than 6 km above the local ground height, it is not identified as the PBL top. The PBL top is first searched by using a 4 sec average profile. If the top is found at that resolution, then the PBL top is located from each of the 20 (5 Hz) shots that make up that 4 sec period. Each PBL top retrieval is given a confidence rating similar to the elevated aerosol layer flag. The values run from "1" (lowest) to "13" (highest) confidence of a good PBL height retrieval. This number is computed by taking the ratio of the average signal within the PBL to the signal level above the PBL. See the "i_LayHgt_Flag" description for more information.

Values of 1-13 in "i_LayHgt_Flag" describe confidence levels of the quality of the height retrieval; "1" indicating very low confidence and "13" very high. A value of "14" indicates bad PBL height retrieval because of either bad input data or a mistake in the retrieval algorithm.

The PBL quality flag is computed from the ratio of the average signal (attenuated, calibrated backscatter) within the PBL to the average signal 500 m above the PBL. Normally, the backscatter increases significantly at the top of the PBL and remains higher within the PBL unless it is attenuated by a cloud, extremely dense dust, or smoke; thus, the quality flag is proportional to the magnitude of the gradient of scattering at the PBL top. The larger this gradient, the easier it is to find the PBL top and hence the higher confidence in its detection.

The boundary layer at night over land collapses down to form the nocturnal boundary layer, usually 100 m or less in thickness. GLAS cannot pick this up, so over land at night if the boundary layer retrieval algorithm sees anything, it is certain to be the residual boundary layer from the day before.


Follow the links in the table below to the GLAS Atmosphere Data Dictionary for details of each record, including units and scaling factors. The GLAS science team created this dictionary. Units and scaling factors with a "d" indicate double-precision constants; for example, a value of "1.0d5" is equivalent to 100,000. The variable "pe/bin" represents photo electrons per bin.

The following codes denote data types throughout the remainder of this document.

i1b: 1-byte integer
i2b: 2-byte (short) integer
i4b: 4-byte (long) integer
r4b: 4-byte real
r8b: 8-byte real

Values in parentheses indicate the record size, for example:

i2b (39): 39 records of 2-byte integers
i1b (48,40): 48-record x 40-record array of 1-byte integers

Nearly all integers are signed. Exceptions are noted in the following record table.


Name Short Description Byte Offset Data Type Total Bytes
i_rec_ndx GLAS record index 0 i4b 4
i_UTCTime Transmit time of first shot in frame in J2000 (referenced from noon on 01 January 2000) 4 i4b (2) 8
i_beam_coelev Co-elevation 12 i4b (4) 16
i_beam_azimuth Azimuth 28 i4b (4) 16
i_pad_angle PAD angle 44 i4b (4) 16
i_spare0 Spares 60 i1b (40) 40
i_AttFlg1 Attitude flag 100 i2b (4) 8
i_lat Profile location, latitude 108 i4b (4) 16
i_lon Profile location, longitude 124 i4b (4) 16
i_OrbFlg Orbit flag 140 i1b (2,4) 8
i_LidarQF Lidar frame quality flag 152 i2b (4) 8
i_atm_dem DEM value at current location from 1 km x 1 km grid 160 i4b (4) 16
i4_aer_bot Below 20 km aerosol layer bottom 176 i2b (5) 10
i4_aer_top Below 20 km aerosol layer top 186 i2b (5) 10
i20_aer_bot 20-40 km aerosol layer bottom 196 i2b (3) 6
i20_aer_top 20-40 km aerosol layer top 202 i2b (3) 6
i_LRpbl_ht Low-resolution PBL height 208 i2b 2
i_LRpbl_grd Ground detection for low-resolution PBL 210 i2b 2
i_HRpbl_ht High-resolution PBL height 212 i2b (20) 40
i_HRpbl_grd Ground detection for high-resolution PBL 252 i2b (20) 40
i4_aer_pct Percentage of saturated bins in below 20 km aerosol layers 292 i1b (5) 5
i20_aer_pct Percentage of saturated bins in 20-40 km aerosol layers 297 i1b (3) 3
i_LRpbl_pct Percentage of saturated bins in low-resolution PBL layer 300 i1b 1
i_LayHgt_Flag Layer height flag (view byte structure) 301 i1b (32) 32
i_AttFlg3 Attitude flag 3 333 i1b 1
i_timecorflg Time correction flag 334 i2b 2
i_SolarAngle Solar angle 336 i4b (4) 16
i_Aer_top_b20_temp Temperature of top of aerosol layers in bottom 20 km of atmosphere at 532 nm 352 i2b (5) 10
i_Aer_top_b20_pres Pressure of top of aerosol layers in bottom 20 km of atmosphere at 532 nm 362 i2b (5) 10
i_Aer_top_b20_relh Relative humidity of top of aerosol layers in bottom 20 km of atmosphere at 532 nm 372 i2b (5) 10
i_Aer_bot_b20_temp Temperature of bottom of aerosol layers in bottom 20 km of atmosphere at 532 nm 382 i2b (5) 10
i_Aer_bot_b20_pres Pressure of bottom of aerosol layers in bottom 20 km of atmosphere at 532 nm 392 i2b (5) 10
i_Aer_bot_b20_relh Relative humidity of bottom of aerosol layers in bottom 20 km of atmosphere at 532 nm 402 i2b (5) 10
i_Aer_top_a20_temp Temperature of top of aerosol layers above 20 km of atmosphere at 532 nm 412 i2b (3) 6
i_Aer_top_a20_pres Pressure of top of aerosol layers above 20 km of atmosphere at 532 nm 418 i2b (3) 6
i_Aer_top_a20_relh Relative humidity of top of aerosol layers above 20 km of atmosphere at 532 nm 424 i2b (3) 6
i_Aer_bot_a20_temp Temperature of bottom of aerosol layers above 20 km of atmosphere at 532 nm 430 i2b (3) 6
i_Aer_bot_a20_pres Pressure of bottom of aerosol layers above 20 km of atmosphere at 532 nm 436 i2b (3) 6
i_Aer_bot_a20_relh Relative humidity of bottom of aerosol layers above 20 km of atmosphere at 532 nm 442 i2b (3) 6
i_Aer_PBL_LR_temp Temperature of low resolution planetary boundary layer top at 532 nm 448 i2b 2
i_Aer_PBL_LR_pres Pressure of low resolution planetary boundary layer top at 532 nm 450 i2b 2
i_Aer_PBL_LR_relh Relative humidity of low resolution planetary boundary layer top at 532 nm 452 i2b 2
i_Aer_ir_top Elevation of top of aerosol layers detected in 1064 nm 454 i2b (2) 4
i_Aer_ir_bot Elevation of bottom of aerosol layers detected in 1064 nm 458 i2b (2) 4
i_Aer_ir_layflg Layer flag for 1064 Aerosol 462 i1b (2) 2
i_Aer_ir_top_temp Temperature of top of aerosol layers detected in 1064 nm 464 i2b (2) 4
i_Aer_ir_top_pres Pressure of top of aerosol layers detected in 1064 nm 468 i2b (2) 4
i_Aer_ir_top_relh Relative humidity of top of aerosol layers detected in 1064 nm 472 i2b (2) 4
i_Aer_ir_bot_temp Temperature of bottom of aerosol layers detected in 1064 nm 476 i2b (2) 4
i_Aer_ir_bot_pres Pressure of bottom of aerosol layers detected in 1064 nm 480 i2b (2) 4
i_Aer_ir_bot_relh Relative humidity of bottom of aerosol layers detected in 1064 nm 484 i2b (2) 4
i_Surface_temp Surface temperature 488 i2b (4) 8
i_Surface_pres Surface pressure 496 i2b (4) 8
i_Surface_relh Surface relative humidity 504 i2b (4) 8
i_Surface_wind Surface wind speed 512 i2b (4) 8
i_Surface_wdir Surface wind direction azimuth from north 520 i2b (4) 8
i_PBL_Layer_ht PBL Layer Height from Met Data 528 i2b (4) 8
i_Spec_Humid Specific Humidity 536 i2b (4) 8
i_Temp2mAbvGrnd Temperature 2m Above Ground Level 544 i2b (4) 8
i_Total_CloudCov Total Cloud Cover 552 i2b (4) 8
i_spare2 Spares 560 i1b (232) 232

Page last updated: 01/11/13