Report of group 4: Sophia Schäfer, Katia Lamer, Nikos Papagiannopoulos, María Barrera Verdejo, Umar Saeed

The aim of this group was to calculate the planetary boundary layer height (PBLH) using data from backscatter lidar and radiosonde instruments. The Planetary boundary layer (PBL) is the lowest part of the troposphere which is directly influenced by the changes on the surface of earth on a scale of an hour or so. In clear-sky conditions, PBL exhibits a typical diurnal cycle: During the daytime, the earth surface heats up due to the absorption of solar radiation and a convective boundary layer (CBL) forms with an entrainment zone (EZ) on top of it. The surface heat recedes during the night time and a stable boundary layer (SBL) forms near the surface with an intermittently turbulent layer, called the residual layer (RL), above it. On top of the RL is the capping inversion (CI) which acts as a buffer between the RL and the free troposphere (FT).

PBLH can be traced using the profiles of temperature, vertical wind, aerosols and other trace gases. Depending upon the tracer used and the measuring instruments, different results can be achieved for the same conditions and time of the atmosphere. Here we used the drop in aerosol mixing ratio observed in the lidar backscatter signal at a wavelength of 1064 nm for the determination of PBLH. Since the aerosol extinction is low and molecular scattering is negligible, the backscatter signal intensity is approximately proportional to the particle backscatter coefficient, and therefore, the aerosol mixing ratio. For the retrieval of PBLH the gradient of the backscatter was used. In addition, PBLH was also determined from the sharp change in potential temperature and wind direction in radiosonde profiles taken at Bucharest airport at 12 UTC. Lidar measurement from the INOE multispectral lidar at the wavelengths of 355 nm, 532 nm, and 1064 nm were carried out. However, due to bad weather and calibration issues, only the data from 1064 nm channel was useful.

Group 4 image 1

Figure 1. PBL structure and height (white) on 27 September at time windows 08:33-09:42 (513-582), 10:08-12:12 (608-732), 12:44-14:42 (764-882), and 16:18-16:32 (978-992) UTC etermined from range corrected lidar signal. PBLH derived from the 12:00 UTC radiosonde is shown by the black dot.

Figure 1 shows the PBLH as determined from the range-corrected backscatter lidar signal at 1064 nm wavelength using the gradient method on 27th of September, 2013 in the time windows of 08:33-09:42, 10:08-12:12, 12:44-14:42, and 16:18-16:32 UTC. The time axis is expressed in terms of the number of minutes passed since 00:00 UTC on 27th of September, 2013. During this time the PBLH calculated via the gradient methods varies between 0.8 and 2 km above ground level (agl). Around 12:00 (720) UTC the lidar derived PBLH height could be compared with the radiosonde estimate (black dot) that is also shown in the figure 1. The lidar and the radiosonde show a good agreement around the mean value of the lidar measured PBLH.