Rayleigh and Sodium Lidars

The JPL Rayleigh/Raman lidar at Mauna Loa Observatory (MLO, 19.5◦N, 155.6◦W) has been making regular measurements of ozone, temperature and aerosol profiles for the Network for Detection of Stratospheric Change (NDSC) program since July 1993 [Mcdermid et al., 1995]. The current system of JPL Rayleigh/Raman lidar system at MLO employs a Nd:YAG laser emitting at 355 nm, a 1-m Cassegrain telescope collecting the laser light backscattered by the atmospheric molecules, and a set of beam-splitters dividing the collected signals into three temperature-dedicated channels. Two of these channels receive elastic Rayleigh/Mie-backscattered light at 355 nm for the retrieval of temperature between ∼30

and 90 km, and one receives Raman-shifted light backscattered by atmospheric nitrogen at 387 nm, which allows temperature retrieval from 40 km down to about 15 km even in the presence of thin volcanic aerosols and clouds. A detailed description can be found in [Leblanc et al., 1999a,b]. The raw signals were initially collected in 300-m vertical bins and saved every 4-10 minutes. Depending on the signal-to-noise ratio of the data and the nature of application, the analysis programs further average the raw photon profiles over a longer time interval and over a larger vertical range. In the raw data analysis, the raw photon profiles were smoothed with a hamming window function (2 km is the full width half maximum).

Laser radiation transmitted into the atmosphere is backscattered by the molecules in the atmosphere. The number of the received photons is proportional to the number of photons emitted in the laser pulse and also the air density. The relative air density can be deduced based on the Rayleigh lidar equation and temperature is then derived from the relative air density, according to the hydrostatic balance and ideal gas law assumption [Leblanc et al., 1999a]. The selected reference point is dependent on the signal-to-noise ratio and usually between 80 and 90 km. The reference temperature comes from MSIS-90 model. The 15 K difference between the reference atmosphere and the real temperature is common at the top of the profile. The uncertainty due to this temperature difference decreases when integrating the profile downward, reaching about only 3 K at 80 km and less than 1K near 75 km [Leblanc et al., 1998]. Therefore, the temperature uncertainty we derived below 80 km is mainly due to the photon noise. The relative uncertainty of temperature is inversely proportional to the square root of received photon counts. As the altitude increases, received photons decrease exponentially with the altitude, and temperature errors increase rapidly. The JPL system at MLO was upgraded in early 2001, leading to more output power at 355 nm (∼10 W), and higher signal-to-noise ratio of return signals in the stratosphere and mesosphere [Li et al., 2008].

The University of Illinois at Urbana-Champaign (UIUC) Na wind/temperature lidar system [Gardner and Papen, 1995] measured winds and temperature from ∼80 to 105 km.

The Na lidar emits a laser pulse to the atmosphere and collects the scattered photons by co-located telescopes. When the laser frequency is tuned to a resonant line of Na, a Na atom will absorb a photon and re-emits a photon to the atmosphere with the same energy as it absorbs. This is the so-called resonance fluorescence scattering process. The shape of the absorption cross-section is sensitive to the temperature as Figure 2.1a shows [Chu and Papen, 2005]. So the temperature can be inferred from the broadening of the absorption spectrum. The wind information is derived from the Doppler frequency shift of the central frequency as Figure 2.1b shows [Chu and Papen, 2005]. A three-frequency technique was used to determine the shape of the absorption spectrum for the Maui Na lidar [She and Yu, 1994; States and Gardner , 2000b].

Figure 2.1: (a) Na absorption cross section for three temperatures, the radial velocity equals to zero. (b) Na absorption cross section for three different radial velocities: 0, 50 and 100 m s−1 at T = 200 K, [Chu and Papen, 2005].

The UIUC Na wind/temperature lidar system was located on Mt. Haleakala in Maui, HI (20.7◦N, 156.3◦W), which is ∼ 150 km away from MLO. The lidar system was coupled with a steerable 3.67 m diameter astronomical telescope at the Air Force Maui Optical Station. It has made high-resolution measurements of Na density, temperature, and winds in 35 nights during the period from Jan 2002 to Mar 2005. The temporal resolution of the temperature

measurement is ∼2 min and vertical resolution is 480 m. The lidar was directed to the zenith (Z), 30◦ off zenith toward north (N), east (E), south (S) and west (W) in the sequence of ZNEZSW. The relations between the horizontal winds and the off-zenith line-of-sight (LOS) winds are [Liu et al., 2002; Li et al., 2005a]:

VE = uEsin θ + w cos θ (2.1)

VW = −uWsin θ + w cos θ (2.2)

VN = vNsin θ + w cos θ (2.3)

VS = −vSsin θ + w cos θ (2.4)

where V is the LOS wind and θ is the zenith angle. The subscripts denote the lidar beam positions. The vertical wind is much smaller than the horizontal winds and can be ignored. Then, the zonal and meridional winds are derived as:

u0_E = VE/ sin θ (2.5)

u0_W = −VW/ sin θ (2.6)

v_N0 = VN/ sin θ (2.7)

v_S0 = −VS/ sin θ (2.8)

On the night of October 28, 2003, the Rayleigh lidar observation started from 5:01 to 15:13 UT and Na lidar started from 5:19 to 15:55 UT. The overlapping observation period between 5:19 and 15:13 UT was selected for the GW study in Chapter 3. The Na lidar data were smoothed temporally by using a 12-min Hanning window to obtain the same temporal resolution as the Rayleigh lidar data. At this resolution, the minimum nightly averaged uncertainty of temperature from Na lidar is ∼0.5 K at 92.3 km and increases to ∼1.5 K

below 84 km and ∼3.5 K above 103 km. So only the observations between 84 and 103 km were used.