Operating Principles: The aerosol backscatter is a sharp peak superposed at the central wavelengths of the doppler broadened molecular signal. This peak can be removed by an iodine vapor absorption cell, which leaves only the wings of the broad molecular signal intact. The full molecular signal can be restored if the atmospheric pressure and temperature profiles are known. The HSRL is operated at room temperature; it has a very fast transmission rate (4000 Hz), allowing a 15-30m vertical resolution while remaining nominally eye safe. Successive pulses are horizontally and vertically polarized allowing fast depolarization measurements. A state-of-the-art photon counting system gives the HSRL a huge dynamic range. Aerosol backscatter signal strength is used to measure cloud optical depth. Depolarization measurements are used to determine the nature of the hydrometeors. The molecular signal provides a good check of whether the molecular-aerosol signal separation is being performed correctly. Figure 2.1 shows an example of an inversion. Figure 2.2 shows a scan image of a cirrus cloud. Depolarization measurements are able to distinguish between ice and water. Results indicate that ice is only found below 0° C, and water only above -35 ° C (to 5° C accuracy). Depolarization measurements also sometimes show the existence of high (4-6km) irregular aerosol particles (soil or pollen perhaps) which may have been lofted up by deep convection.
The strong point of the HSRL is the retrieval of optical depth. The practical optical depth range is 0.01 to 3 (for a several minute integration). This implies a huge dynamic range of e-6 (round-trip photon travel). The range can be extended by using more power but a broader beam (to keep the lidar eye safe) and a telescope to collect the beam. Photon counting performance is the dominant limitation.
In a study when the lidar was turned on only to study cirrus clouds, it was found that 40% of the time some liquid water clouds were also present. This highlights the need to take water clouds into account when making cirrus observations.
The possibility of measuring effective radius via diffraction peak width and variable field-of-view measurements is an exciting new development. It is found that there is a significant relationship between effective radius and diffraction peak width. The width of the diffraction peak is obtained from measurements at different field-of-view widths.