National Snow and Ice Data Center

GLA10 contains the optical inversion results from 532 nm backscatter and extinction cross sections, separated into cloud and aerosol components. It contains top and bottom heights of all the layers that were optically processed. These are the same heights detected by the 532 nm channel that were reported in GLA08 and GLA09. Cloud products such as backscatter, extinction, extinction-to-backscatter ratio (S), and cloud layer tops and bottoms, are reported at a 1 sec sampling rate, while aerosol products are reported at a 4 sec rate only. Polar stratospheric clouds (PSC) are part of the aerosol category.

To obtain the complete vertical optical structure, you should merge the aerosol and cloud components. For every atmospheric layer detected by 532 nm, an extinction-to-backscatter ratio (S) value is calculated or estimated, which reflects the current multiple scattering condition. This calculation is essential to complete the optical inversion. "True S" indicates conditions with no multiple scattering. The Aerosol True S Values flag ("i_aer4_sval1") refers to whether the calculated (flag = 2) or estimated (flag = 1) S value was used in the optical inversion process.

The "ground detection" parameter refers to the height where the lidar sensed the ground. If the ground was not sensed, this parameter is set to "invalid." This parameter is useful for determining whether the last layer bottom sensed was a true bottom or if it marks the time when lidar signal was extinguished. All layer locations are referenced from the geoid, and all extinction profiles were corrected for multiple scattering. The multiple scattering correction factor is not stored in GLA10, but rather in GLA11. Each GLA10 record is 4 sec long and contains one group of aerosol products and four groups of cloud products (one for each second). The time stamp refers to the J2000 time of the first shot of the record. J2000 time refers to the number of seconds since 01 January 2000 at 12:00 UTC.

Cloud cross-section data from the 532 nm channel are stored as 1 sec average profiles from 20.4 km (bin 1) to -1 km (bin 280) above the geoid. The 532 nm channel aerosol cross section data are stored as 4 sec average profiles from 41.0 km (bin 1) to -1.0 km (bin 548). Each vertical bin is 76.8 meters thick. The extinction-to-backscatter ratio (S) values are recorded one value per layer. There are up to 10 cloud layers in each cloud profile (obtained from the 1 sec cloud layers detected by GLA09 using the 532 channel) and up to 9 aerosol layers in each aerosol profile (obtained from the 4 sec aerosol layers detected by GLA08 using the 532 channel). Active layers are not necessarily packed toward the front of the array sequence. For aerosols, the top three layer positions are reserved for layers above 20.4 km and the last (ninth) position is reserved for the PBL.

All the backscatter and extinction profiles have an associated composite flag variable that represents conditions in each layer that makes up the profile. The composite flag is made up of two main components: the layer quality flag and layer usage flag. In all cases, the quality flag is a categorized value of the percent error calculation. For backscatter cross section, the usage flag gives saturation status. For extinction cross section, the usage flag designates layer type category.

See 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: 01/11/13