Diurnal Cycle of Boundary Layer Aerosol over Munich

This is an example of lidar measurements of the development of the boundary layer of a "full" day (April 1., 1999). Measurement site was the platform at the roof of our institute in Munich. The time series started at 8:00 GMT (10:00 MESZ) and ended just before sunset (18:00 GMT). During the first hours, 500 shot were averaged, since 12 GMT the temporal resolution was 100 shots (10 Hz prf). All measurements were made under a zenith angle of 20°. During the "white areas" in Fig. 1 scans (20°, 50° and 62° zenith angle) and quasi-horizontal measurements (87° zenith angle) were performed.

In the figure the range corrected signals (@ 532 nm) were color-coded and plotted against height (above ground) between 0.25 km and 2.0 km. The color scale gives a first approximation of the aerosol load and the extent of the layer in the lower troposhere. The top of the boundary layer can clearly be seen between 1 and 1.5 km, where the green color changes to blue.

Measurements below 250 m could not be evaluated because of the unknown overlapp between lidar emitter and receiver. However, information on the aerosol distribution below 250 m can be obtained if quasi-horizontal measurements are evaluated by means of the slope method. In cases of piecewise horizontal homogeneity it results in extinction coefficients. Results are shown in the figure (below) for a mean height of 50 m above the lidar site. The measurement times correspond to the (white) gaps in the previous figure. The red, green and blue marks denote the extinction coefficients at 1064 nm, 532 nm and 355 nm wavelengths, respectively. Error bars are included. It can be clearly seen that in the course of the day the extinction close to the surface (z = 50 m) has decreased by roughly a factor of 2.

Fig. 2

Application of the two angle approach, combined with the Klett method, also results in extinction coefficients. For this purpose measurements under different zenith angles (20, 50, 62°) were combined. Another advantage of the consideration of slant lines of sight is the reduction of the gap between the surface and the range of complete overlap (250 m, see above): Extinction profiles can be extended down to approximately 150 m above ground. Fig. 3 shows vertical profiles for four times and the 532 nm wavelength. It can be seen that the profiles change from a general decrease with height (08:33 UTC; note that the extinction near the surface is about 0.22 km^-1 according to Fig. 2) to a sharp top and an aerosol layer elevated from the ground (approx. 0.1 km^-1 at z=0.05 km). At 17:19 UTC the pronounced top of the planetary boundary layers is lower (z=1 km) but significant aerosol extinction is observed till 3 km.

Fig. 3

It can be concluded that under favorable conditions, i.e., aerosols that are "sufficiently homogeneous" in horizontal planes so that the two angle approach can be applied, it is possible to derive extinction coefficients of the full vertical extent of the boundary layer from backscatter lidar measurements. The temporal evolution of the lower troposhere can be determined qualitatively (by means of range-corrected signals) anyway.

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