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GLA08 Records

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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.

Page last updated: 10/01/08