Lidar Simulation Model: GADS Optical Property Models

The AGM provides two options for computing aerosol backscatter and attenuation (molecular and aerosol) coefficients.

Optical Property Model: AFGL

The first option in the AGM uses the AFGL models MODTRAN ( Kneizys et al., 1996) and FASCODE (Gallery et al., 1983) to compute optical properties. These models, coupled with a SWA created global data base of AFGL model inputs (Wood et al., 1993) and ECMWF meteorological profiles produce aerosol backscatter and attenuation profiles as a function of laser wavelengths. The optical property's natural variability due to altitude, location, seasons, and meteorological conditions are taken into consideration in the model.

The AGM optical property algorithm reads in an AFGL input index from a FORTRAN 90 direct access data file, OPTTOP, based on global location. Each index is stored as a function of latitude and longitude at a 1 deg x 1 deg resolution. The index is used as a pointer for 5 pre-defined AFGL inputs. The inputs are location profile model, haze model, coastal influence parameter, stratosphere model and upper atmosphere model. Given the ECMWF meteorological profile and location, the AGM can estimate the optical properties as shown in the table below.

The OPTTOP data base is considered to be a baseline nominal data base. The model location index is a function of latitude. The haze model is a function of continent vs. desert vs. ocean location. The current version of the AGM does not use the urban, ocean or fog models, but the data bases are stored in the AGM for future upgrades. The coastal influence is a function of distance from a land mass (only three of the AFGL options are used).

The stratospheric and upper atmospheric variables are set to the clean background mode. Any variations to the computed baseline optical properties are made in the LSM. For example, if one wanted to include a volcanic stratospheric dust level or an advection of maritime aerosols over Europe, one would use the LSM to modify the baseline inputs as a function of latitude, longitude and altitude.

Given location and the OPTTOP pre-defined inputs, along with the vertical profile of meteorological data from the ECMWF data set, the AGM computes the extinction (scattering and absorption) coefficient, backscatter phase function, Rayleigh scattering coefficient and the aerosol scaling parameter as a function of altitude. Aerosol backscatter is computed with consideration of aerosol natural variability due to local wind speed, temperature, relative humidity and standard visual range ( Hanel, 1972). The AGM computes the attenuation coefficient from the molecular absorption, aerosol absorption, molecular scattering and aerosol scattering.

ß_p = (s_s × P_p × SCL)/1000

a = a_m + s_m + (a_a + s_a) × SCL + RAY

where

ß_p (m^-1sr^-1) - aerosol backscatter coefficient

P_p (sr^-1) - backscatter phase function

SCL - aerosol scaling function,

a (km^-1) - attenuation coefficient

a_m (km^-1) - molecular absorption coefficient

a_a (km^-1) - aerosol absorption coefficient

s_m (km^-1) - molecular scattering coefficient

s_s (km^-1) - aerosol scattering coefficient

RAY (km^-1) - Rayleigh scattering coefficient.

Optical Property Model: GLOBE

The second option in the AGM is a hybrid between the AFGL models and GLOBE data. Below the 850 mb level, the AGM uses the AFGL models to generate the aerosol backscatter. Above the 850 mb level to the tropopause, one of three median 9.11 um GLOBE profiles is used, depending upon latitude and season. The three profiles obtained during GLOBE II and were provided by Robert Menzies (Jet Propulsion Laboratory). Variability around the median values is slewed to variations in the relative humidity profiles provided by the ECMWF nature run. All the attenuation coefficients are computed from the AFGL models.