Lidar stands for light detection and ranging, the optical analog of radar. By sending a very short pulse of light (<10 ns, < 1.5m) up into the atmosphere and measuring the time of flight for the returned signal which reflects off air molecules and particles, one can profile the atmospheric structure of aerosols above the instrument. The reflection of the light depends on the optical characteristics of the aerosol. For particulates, Mie Scattering occurs and has a dependence on particle number, particle size distribution, and particle indices of refraction. For gases, scattering is from Rayleigh scattering and has a gaseous backscatter coefficient of 1 x 10-7 m-1 sr-1 at 1.064 Ám wavelength. For other wavelengths the scattering scales as (1.064 Ám/λ)4, therefore the Rayleigh return can be a way of calibrating a lidar return signal if a region of particle free air is found in a lidar profile.
The MPL lidar utilizes a relatively low power (1ÁJ) 527 nm pulse at high repetition rate (2500 Hz). The combination gives a useable average power output from the laser, which if averaged over several seconds, can be quantitatively analyzed for the lidar return. The lidar equation is:
where P(r) is the power returned from range, r(m), C is a system constant, O(r) is the optical overlap between the transmitted and received fields of view, βpit (m-1 sr-1) is the total backscatter (Rayleigh plus aerosol), α (m-1) is the extinction coefficient of the atmosphere, the integral is the two way transmission of the atmosphere out to range r and Pb is the background light signal from the sky. This "elastic" lidar signal is complicated by the two unknowns in the equation, β and α. Since both are related to the optical properties of the aerosol, often a simple scaling relationship is used between them:
(α/β) = Sa (sr) = "lidar ratio"
and the lidar equation can be solved for either β or α. Sa values are typically 30-80 sr. The "attenuated backscatter coefficient" is given the terminology NRB (normalized relative backscatter) in the MPL community and is:
The design of the micropulse lidar is such that the transmitted beam and the received beam utilize the same telescope. For this reason, the outgoing beam and the received beam are well matched in divergence in the far field but in the near field there is incomplete overlap between the two beams and the MPL signal is weak near the ground. The telescope on the UMBC MPL has some thermal characteristics which make the O(r) unstable for relative minor temperature changes. In its configuration at UMBC, the MPL is in a temperature controlled housing which stabilizes this thermal structure, but in DISCOVER-AQ the MPL is running in a larger trailer with some thermal drift. Characterizing the O(r) is challenging in this environment. O(r) approaches unity (complete overlap) at about 4 kilometers altitude. To improve the sensitivity and stability of the MPL to very low altitudes, UMBC has added a "minireceiver", a second channel with a small 1" wide angle lens and detector. This minireceiver will reach full overlap by 200-300 m above the MPL and will allow detection of structures in the atmosphere near the surface.
Figure 1: UMBC MPL as installed in the UMBC Research Trailer. The MPL telescope has a top flange with the minireceiver installed (red cable runs to minireceiver).