Knowledge of the vertical profile, composition, concentration, and size of aerosols is re-quired for assessing the direct impact of aerosols on radiation, as well as determining the effects of aerosols on clouds and precipitation. When used in conjunction with the exten-sive suite of aerosol-related measurements on the A-train constellation of satellites, the multiwavelength aerosol backscatter and extinction profiles retrieved from data acquired by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) sensor on board the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite will provide key information for assessing aerosol direct and indirect effects.

Retrievals of both aerosol backscatter and extinction from a backscatter lidar such as the CALIOP sensor rely on accurately specifying the relationship between aerosol extinction and backscattering, which is typically expressed as a ratio (i.e. lidar ratio or Sa). Uncer-tainties in the lidar ratio are a significant source of uncertainty in the CALIOP extinction retrievals.

One method of determining the lidar ratio and potentially reducing the uncertainty in the CALIOP aerosol extinction retrievals relies on constraining the solution with aerosol opt-ical thickness from coincident satellite measurements. A method that uses aerosol optical thickness (AOT) measurements from the Moderate Resolution Imaging Spectroradiome-ter (MODIS), the Multiangle Imaging Spectroradiometer (MISR), and/or PARASOL (Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar) is presented. This method is examined using lidar backscatter data provided by the NASA Langley Research Center (LaRC) airborne High Spectral Resolution Lidar (HSRL) and by the CALIOP sensor. The LaRC airborne HSRL uses the spectral distribution of the lidar return signal to measure aerosol extinction and backscatter profiles independently at 532 nm; consequently, these HSRL measurements provide an ideal dataset to evaluate the constrained retrieval of extinction from backscatter. The HSRL also uses standard backscatter lidar techniques to derive aerosol backscatter and extinction profiles at 1064 nm. Aerosol depolarization profiles are measured at both wavelengths. The HSRL instrument has flown aboard the NASA Langley King Air B200 on more than 150 flights since 2006 and collected data coincident with CALIOP during portions of 68 of these research flights thereby providing an extensive dataset for evaluating CALIOP aerosol retrievals.

Retrievals of aerosol extinction from HSRL and CALIOP backscatter measurements con-strained using MODIS, MISR, and PARASOL AOT measurements are presented. Pre-liminary comparisons with aerosol extinction profiles derived using the HSRL technique show that the retrieved profiles depend strongly on the uncertainty in the AOT measure-ment. The AOT uncertainty depends on the magnitude of AOT and the underlying sur-face (land vs. water). With these caveats, preliminary results show the combined retrieval from HSRL backscatter data and MODIS Aqua AOT over land, limited to AOT greater than 0.15, produces extinction profiles within 30% of the value determined using the HSRL technique for 65% of retrievals. Results will be discussed for other cases as well. Retrievals using CALIOP data require careful application of the vertical feature mask to set the starting point of the retrieval as well as additional smoothing. Examples of these retrievals will be presented as well.