Validation of ceilometer aerosol profile retrievals using sun–sky photometer and balloon-borne in situ measurements

Abstract. This study evaluates two approaches for retrieving aerosol properties from ceilometer observations, using aerosol optical depth (AOD) from AERONET and synergistic aerosol profiles obtained by combining the AERONET sun–sky photometer and ceilometer measurements through the GRASPpac algorithm. The two retrieval techniques considered for the ceilometer retrievals are the traditional Klett–Fernald backward inversion and a forward iterative method including an independent calibration procedure. Observations collected at three European stations (Granada, Spain; Payerne, Switzerland; and Lindenberg, Germany) during 2019–2020 are analyzed to assess the performance of both approaches under a wide range of aerosol conditions. The results show that the forward iterative approach generally provides better agreement with the reference datasets than the Klett–Fernald backward approach. Under high aerosol load conditions, particularly during coarse-mode-dominated events, the forward retrieval reduces AOD uncertainties by ∼50 % and achieves root-mean-square errors comparable to those reported in previous validation studies. Vertical comparisons against GRASPpac profiles indicate that the forward method maintains consistent accuracy throughout the troposphere, whereas the backward approach exhibits altitude-dependent biases, especially within dust layers. Additional evaluations using COBALD balloon-borne backscatter measurements confirm that the forward retrieval reproduces observed aerosol structures within 10 %–30 % deviation. These results demonstrate the significant performance gains achieved by operational ceilometer networks when applying forward retrievals with independent calibration made under nighttime cloud-free conditions with negligible aerosol loading at the reference altitude.