Na layer observation

From December 2005 to April 2007, 53 night observations were obtained. Most of the observations cover the period of 20:00LT-04:30LT. The raw data consists of a series of photon counts corresponding to consecutive range bins. Each profile has an integration time of 4min with a vertical resolution of 75 m.

3.1.1. Nocturnal variation

A typical photocount profile is plotted in Fig. 1(a). The profile was obtained at Hefei during the night of 23 December 2005. The resonant scattering from the Na layer between 80-105 km is clearly evident. The wavelike structure in the layer is caused by the wind perturbation associated with low-frequency internal gravity waves. The non-zero count level above 110 km is caused primarily by background noise from scattered moonlight and starlight.1 Fig. 1(b) shows the sodium density variations with the local time and altitudes from 20:15LT 22 December 2005 to 04:00LT 23 December 2005. We can find during the midnight of 22 December, the sodium density increased to a very high value (^7000cm~3), which is called sporadic sodium layer and considered to be linked to the formation of sporadic E,2 or the atmospheric dynamic procession (tide, gravity waves).

3.1.2. Seasonal variation

To investigate the seasonal variation, 53 nights' observational data were selected (exclude the period of sporadic sodium layer). It has been proved that the unperturbed sodium layer is a Gaussian profile:

V2nCTNa

24a where ANa is column abundance, zNa is centroid height and aNa is RMS width.

Seasonal variations of sodium column abundance (a), centroid height (b) and RMS width (c) are given in Fig. 2. Triangles stand for daily mean, error bars are daily standard deviations and dash lines are monthly mean values. Sodium column abundance reaches a maximum value of 6.014 x 109 cm~2 in December and a minimum value of 1.126 x 109 cm~2 in June. It reveals a significant summertime depletion followed by a maximum concentration in wintertime, especially in December. The monthly mean column abundance profile shows a minimum value in April due to that there is only one night data which lasted for 1 h in April, and we use it to fix the data gap during April. The same seasonal variations in abundance have been reported in other lidar sites,3'4 and our results are consistent with those previous studies. Daily centroid height varies from 91.5 km to 92.5 km and has no obvious seasonal variation generally. It is higher in summertime and lower in wintertime. RMS width has a semiannual variation. It increases in summer and wintertime, decreases in spring and

0 20 40 60 80 100 120 140

Height (km)

0 20 40 60 80 100 120 140

Height (km)

20:30 21:30 22:30 23:30 00:30 01:30 02:30 03:30 Local Time

1000 2000 3000 4000 5000 6000 Na (cm-3)

Fig. 1. (a) A typical lidar photocount profile at 589.0 nm, integrated time 4min, with a vertical resolution of 75 m, at the night of 23 December 2005; (b) Sodium density variation during the night of 23 December 2005.

10 11 12

10 11 12

10 11 12

Fig. 2. Seasonal variations of sodium column abundance (a), centroid height (b) and RMS width (c). Triangle stands for daily mean, error bar is standard deviation, dash line is monthly mean.

autumn, i.e. it's 6.61 and 5.37km in June and January, and 4.56 and 4.58 km in March and September, respectively.

3.2. Atmosphere density and temperature observation

In Rayleigh mode, the photocounts are integrated for 500 s with a vertical resolution of 150 m. The density and temperature profiles are inferred from the relative atmosphere density profile by using the hydrostatic equation and the ideal gas law. Figure 3 shows an example of the observation during 19:26-22:02LT, 6 December 2005. The solid lines represent the observational results and the dash lines show the monthly mean results of density and temperature obtained from CIRA-86. Below 70 km, the temperature difference between the observation and CIRA-86 is less than 10 K, while above 70 km is much bigger. This may due to the mesospheric temperature inversion.

Density (cm 3) Temperature (K)

Fig. 3. Observed density and temperature profile. Dash lines are observed results and solid lines are CIRA-86 results.

Fig. 4. Aerosol extinction profile obtained on the night of 1 June 2006.
0 0

Post a comment