Differential absorption lidar (DIAL) systems have been used for the measurements of water vapor from ground-based and aircraft platform for two decades. The technology and applications of the DIAL technique have progressed significantly in that time period. The German Aerospace Research Center (DLR) has long-term experience in design, development and operation of airborne water vapor DIAL systems. In the DLR Institute for Atmospheric Physics, such systems are routinely operated and steadily improved using state-of-the-art technology since 1989.
The most recent development is the implementation of an all-solid-state laser transmitter that enables precise water vapor measurements in the upper troposphere and lower stratosphere (UT/LS). Based on a high peak and average power optical parametric oscillator (OPO), its transmitter fulfils the stringent spectral requirements for precise water vapor DIAL measurements in the troposphere as well as in the stratosphere. The residual systematic error caused by the spectral properties of the new radiation source amounts to only 2%. The transmitter is designed to operate at either 920 nm suitable for tropospheric measurements or at 940 nm for regions of low water vapor content at upper tropospheric and lower stratospheric heights.
This new airborne H2O DIAL system was successfully employed in the investigation of stratospheric intrusions. The figure displays a vertical water vapor concentration profile along a 650 km long path across Europe measured with the airborne H2O DIAL in comparison to ECMWF analysis data. One notes the large variability of the water vapor concentration within the range of a tropopause fold. The measurement exhibits a larger variability than the model analysis.
Based on the expertise gained with airborne DIAL water vapor measurements, the DLR lidar group proposed WALES (Water Vapor Lidar Experiment in Space) for an ESA Earth Explorer opportunity mission. Extensive studies have been carried out by the group, clearly demonstrating both the technical feasibility of a space borne DIAL and its positive impact on weather prediction and climate change issues.