Performance modeling of an airborne Raman water-vapor lidar.

We have developed a sophisticated Raman lidar numerical model to simulate the performance of two ground-based Raman water-vapor lidar systems. After verifying the model using these ground-based measurements, we then used the model to simulate the water-vapor measurement capability of an airborne Raman lidar under both daytime and nighttime conditions for a wide range of water-vapor conditions. The results indicate that, under many circumstances, the daytime measurements possess comparable quality to an existing airborne differential absorption water-vapor lidar whereas the nighttime measurements have improved spatial and temporal resolution. In addition, an airborne Raman lidar can offer measurements that are difficult or impossible with the differential absorption lidar technique.

Remote detection of Raman scattering by use of a holographic optical element as a dispersive telescope.

We describe the retrieval of nighttime lidar profiles by use of a large holographic optical element to simultaneously collect and spectrally disperse Raman-shifted return signals. Results obtained with a 20-Hz, 6-mJ/pulse , frequency-tripled Nd:YAG source demonstrate profiles for atmospheric nitrogen with a range greater than 1 km for a time average of 26 s.

Measurement of an isosbestic point in the Raman spectrum of liquid water by use of a backscattering geometry.

Measurements presented here confirm that a temperature-insensitive point occurs in the backscattered Raman spectrum from liquid water. This result, coupled with existing laboratory measurements of Raman scattering from liquid droplets, indicates that a Raman lidar measurement of cloud liquid water is feasible.

Application of statistical methods to the determination of slope in lidar data.

Assumptions made in the analysis of both Raman lidar measurements of aerosol extinction and differential absorption lidar (DIAL) measurements of an absorbing species are tested. Statistical analysis techniques are used to enhance the estimation of aerosol extinction and aerosol extinction error that is usually handled using a linear model. It is determined that the most probable extinction value can differ from that of the linear assumption by up to 10% and that differences larger than 50% can occur in the calculation of extinction error. Ignoring error in the number density alters the calculated extinction by up to 3% and that of extinction error by up to 10%. The preceding results were obtained using the least-squares technique. The least-squares technique assumes that the data being regressed are normally distributed. However, the quantity that is usually regressed in aerosol extinction and DIAL calculations is not normally distributed. A technique is presented that allows the required numerical derivative to be determined by regressing only normally distributed data. The results from this technique are compared with the usual procedure. The same concerns raised here regarding appropriate choice of a model in the context of aerosol extinction calculations should apply to DIAL calculations of absorbing species such as water vapor or ozone as well because the numerical derivative that is required is identical.

Upper tropospheric temperature measurements with the use of a Raman lidar.

Upper tropospheric temperature profiles were measured with the NASA Goddard Space Flight Center scanning Raman lidar five months after the eruption of Mt. Pinatubo. To derive temperatures in regions of high aerosol content, the aerosol transmission is calculated for the Raman N(2) return signals under cloud-free conditions. The lidar-derived aerosol backscattering ratio and an estimate of the aerosol extinction-to-backscatter ratio were used to compute the aerosol transmission. With a model reference temperature at 25 km, temperature profiles with a root-mean-square difference between the lidar and radiosonde temperatures of <2 K were obtained over an altitude range of 5-10 km for a 10-min integrated measurement with 300-m resolution.

Raman lidar system for the measurement of water vapor and aerosols in the Earth's atmosphere.

A nighttime operating Raman lidar system that is designed for the measurement of high vertical and temporal resolution profiles of the water vapor mixing ratio and the aerosol backscattering ratio is described. The theory of the measurements is presented. Particular attention is given to operational problems that have been solved during the development of the system. Data are presented from Sept. 1987 and described in their meteorological context.