The RL operates by transmitting pulses of laser radiation and recording radiation backscattered from the atmosphere as a function of time to provide range information similar to a radar system. The return signal contains a strong elastically scattered component due to scattering from clouds, aerosols, and molecular Raman Scattering. The return also contains weaker inelastically scattered components that provide chemical-specific information.
As the beam propagates through the cloud, it is strongly attenuated and this often results in a shadowing effect above the cloud. We note that the elastic channels operate at 355 nm, which is in a so-called solar blind region of the spectrum due to absorption of solar radiation by ozone in the stratosphere. As a result, the elastic channels are essentially unaffected to solar radiation.
The nitrogen and water channels are not sensitive to aerosol backscatter, however it is affected by aerosol extinction. The signal will not exhibit a strong return for the cloud base, but the signal is strongly attenuated in the cloud and will exhibit a shadowing effect similar to the elastic return. These channels are also sensitive to solar radiation.
The RL generally runs smoothly with little to no problems. As such, the metrics should appear green for all variables for routine data. When individual variables flag as red (bad data) or grey (missing data), a problem may be ongoing.
List of past problems for this instrument that DO need to be mentioned in your DQA's:
Instrument out of alignment
The most frequent problem from which the RL suffers are due to a misalignment of the mirrors inside the instrument. This has been known to occur after particularly heavy thunderstorms and after strong earthquakes (because yes, that is a problem in Oklahoma!) The instrument sometimes even gets out of alignment during the middle of the day for no apparent reason. When an alignment issue is suspected, the metrics may indicate so by flagging the data in yellow. The best way to confirm an alignment issue is by looking at the data.
The RL experienced what looked to be a case of the mirrors out of alignment, but it turned out to be a layer of bugs and dew covering the window.