The DLSM version 4.2 optical property models are currently based upon the Design Atmospheres for use in GTWS Concept Studies provided by the Science Definition Team for the NASA/NOAA Global Tropospheric Wind Sounder.

Having a common scattering target with internally consistent backscatter wavelength dependence enables meaningful "equal resource/equal target" comparisons of GTWS concepts that employ Doppler lidars. While the Science Definition Team (SDT) realizes that aerosol backscatter from the atmosphere will vary over several orders of magnitude, will vary over altitude, latitude and season and will also vary over space/time scales that are not readily modeled, the GLOBE, SABLE/GABLE backscatter surveys, and the AFGL FASCODE aerosol data bases provide a nearly consistent picture of backscatter climatology.

To establish a set of bounding profiles, the SDT has chosen (1) the "background" distribution of backscatter that appears in most stacked histograms of the GLOBE/SABLE/GABLE data sets and (2) the distribution of "enhanced" backscatter opportunities that are most apparent during the summer seasons and more common in the northern hemisphere (Srivastava, et al, 2001). The background mode value should not be interpreted as representing the minimum value of the aerosol cross section to be found. Rather, it represents a low cross section modal peak for aerosols in tropospheric air that does not have loading enhancement due to identifiable aerosol transport. There is a distribution of values and measurements indicating that the lowest aerosol cross sections can be an order of magnitude lower than the mode in some cases. The actual distribution of cross sections in the background mode are not well known. Measurements indicate that the background aerosol mode is present in large regions of the globe, mostly in the upper troposphere but can also be found in the boundary layer. The global distribution of these modes is not known, nor is the correlation of these modes with regions of ageostrophy. 

The DLSM version 4.2 provides background, enhanced and FASCODE optical property data bases for 0.355 µm, 0.532 µm, 1.06 µm and 2.0518 µm DWL wavelengths. The user can choose the median profiles or to use the median profiles with aerosol backscatter randomly distributed with log normal variability as shown in the figure below. The DLSM uses the same random data seed for an entire DWL "shot's " line of sight path. The aerosol attenuation is not adjusted for the varying backscatter. Studies are ongoing to relate the optical properties to the nature run atmospheric variables similar to the DLSM version 3.0's optical property models.

Caveats

1. These atmospheres are meant only for the purpose of enabling "equal target" comparisons of different DWL concepts and their potential LOS data products. Emphasis is on measurement accuracy and not representativeness or coverage. Furthermore, there is no claim to the frequency of occurrence of the two backscatter modes.

2. The wavelength dependency of the backscatter coefficient across the 1-2 orders of magnitude width of the background mode is thought to vary from λ -3 on the left side (lower on the left side (lower β ) to λ -1.5 on the right side (higher on the right side (higher β )). A λ -2.5 was used in these tables going from 1.06 µm data to 2.0518,  0.532 and .355 µm at and above 3 km. Since a different λ coefficient was used below 3 km, some smoothing of the resulting profiles has been done to make the transition more realistic. Even so, there are some "jumps" between the mid-troposphere and boundary layer values due in part to the use of different phase functions for the aerosol attenuation in those two regions.

Last Updated: 02/07/2007