Comparison of Airborne In-situ with Ground- and Satellite- based LIDAR Aerosol Light Extinction Coefficient during JATAC/CAVA-AW 2021/2022 Campaigns

The JATAC campaign in September 2021 and September 2022 on and above Cape Verde Islands was carried out with the main objective to calibrate and validate the ESA satellite Aeolus ALADIN lidar. The campaign also featured secondary scientific objectives related to climate change. Constraining remote sensing measurements with those provided by in-situ instrumentation is crucial for proper characterization and accurate description of the 3-D structure of the atmosphere.

We present the results performed with an instrumented light aircraft (Advantic WT-10) set-up for in-situ aerosol measurements. Twenty-seven flights were conducted over the Atlantic Ocean at altitudes up to 3000 m above sea level during intense dust transport events. Simultaneous measurements with Polly^XT, and eVe ground-based lidars took place, determining the vertical profiles of aerosol optical properties.

The aerosol light extinction coefficient was determined at three different wavelengths as a combination of the absorption coefficients determined at 467, 529 and 653 nm using Continuous Light Absorption Photometers (CLAP) and the scattering coefficients 450, 525 and 635 nm measured with an Ecotech Aurora 4000 nephelometer. The particle size distributions above 0.3 µm diameter were measured with two Grimm 11-D Optical Particle Size Spectrometers (OPSS). Moreover, CO2 concentration, temperature, aircraft GPS position and altitude, air and ground speed were also measured.

We compare the in-situ aircraft measurements of the aerosol extinction coefficients with the ALADIN lidar derived extinction coefficients, as well as with the ground-based eVe and Polly^XT lidar extinction coefficients when measurements overlapped in space and time. The direct comparison is performed at 532 nm, where the Polly^XT lidar wavelength closely matches that of the in-situ measurements. The comparisons at 355 nm are performed by extrapolating the in-situ aerosol extinction coefficients to match the measurement wavelengths of the Polly^XT, eVe, and ALADIN lidar systems.

In general we find an underestimation of the extinction coefficient obtained by lidars compared to the in-situ extinction coefficient. The slopes of Deming regression lines of ground-based lidars, Polly^XT and eVe, against the in-situ measurements are characterised by values ranging from 0.74 to 0.94 and R² between 0.74 and 0.89 at 355 nm. At the 532 nm, the slope of the regression characterizing the relationship between the Polly^XT and in-situ measurements is 0.69, with an R² value of 0.92 Comparison further suggests better agreement between ALADIN lidar and the in-situ measurements. Relationship described by fitting the Aeolus to in-situ data is characterised by the slope value 0.98 and R² of 0.53 at 355 nm.

The presented results show the importance of the comparison of the remote with in-situ measurements for the support of the research on evolution, dynamics, and predictability of tropical weather systems and provide input into and verification of the climate models.