An extended lidar-based cirrus cloud retrieval scheme: first application over an Arctic site

Abstract

Accurate and precise characterization of cirrus cloud geometrical and optical properties is essential for better constraining their radiative footprint. A lidar-based retrieval scheme is proposed here, with its performance assessed on fine spatio-temporal observations over the Arctic site of Ny-Ålesund, Svalbard. Two contributions related to cirrus geometrical (dynamic Wavelet Covariance Transform (WCT)) and optical properties (constrained Klett) are reported. The dynamic WCT rendered cirrus detection more robust, especially for thin cirrus layers that frequently remained undetected by the classical WCT method. Regarding optical characterization, we developed an iterative scheme for determining the cirrus lidar ratio (LR ci ) that is a crucial parameter for aerosol - cloud discrimination. Building upon the Klett-Fernald method, the LR ci was constrained by an additional reference value. In established methods, such as the double-ended Klett, an aerosol-free reference value is applied. In the proposed constrained Klett, however, the reference value was approximated from cloud-free or low cloud optical depth (COD up to 0.2) profiles and proved to agree with independent Raman estimates. For optically thin cirrus, the constrained Klett inherent uncertainties reached 50% (60-74%) in terms of COD (LR ci ). However, for opaque cirrus COD (LR ci ) uncertainties were lower than 10% (15%). The detection method discrepancies (dynamic versus static WCT) had a higher impact on the optical properties of low COD layers (up to 90%) compared to optically thicker ones (less than 10%). The constrained Klett presented high agreement with two established retrievals. For an exemplary cirrus cloud, the constrained Klett estimated the COD355 ( L R c i 355 ) at 0.28 ± 0.17 (29 ± 4 sr), the double-ended Klett at 0.27 ± 0.15 (32 ± 4 sr) and the Raman retrievals at 0.22 ± 0.12 (26 ± 11 sr). Our approach to determine the necessary reference value can also be applied in established methods and increase their accuracy. In contrast, the classical aerosol-free assumption led to 44 sr LR ci overestimation in optically thin layers and 2-8 sr in thicker ones. The multiple scattering effect was corrected using Eloranta (1998) and accounted for 50-60% extinction underestimation near the cloud base and 20-30% within the cirrus layers.

Properties of Cirrus Clouds over the European Arctic (Ny-Ålesund, Svalbard)

“…The retrieval of cirrus geometrical and optical properties was based on a newly developed scheme [20]. The detection scheme extended the Wavelet Covariance Transform (WCT) method [25] by dynamic thresholds (dynamic WCT) so as to provide the cirrus C base and C top .…”

“…The optical characterization scheme (constrained Klett) extended the Klett-Fernald retrieval [28,29] by an iterative lidar ratio (LR) selection. The iterative process was constrained by a reference value, which indicated the aerosol concentration beneath the cirrus layers (Section 2.3 and Figure 4 of Nakoudi et al [20]). Contrary to existing approaches (e.g., double-ended Klett [30]), the aerosol-free assumption was not adopted, but the aerosol conditions were approximated by an initial guess.…”

“…Thanks to the more realistic reference value, the accuracy of the optical properties was increased. The scheme is described in detail in Nakoudi et al [20] and it is publicly available [31].…”

“…Placing emphasis on optically-and geometrically-thinner cirrus, Nakoudi et al [20] developed an extended lidar-based retrieval scheme (more details in Section 2.2). In this work, this scheme is applied on a 10 year cirrus lidar data set over Ny-Ålesund, Svalbard.…”

Empirical model of multiple scattering effect on single-wavelength lidar data of aerosols and clouds

“…Usually, that assumption is implicitly implied or mentioned with the relation to the following factors: a narrow RFOV, a small footprint, and a quite low value of the extinction coefficient. The only exception is cirrus clouds observed with a ground-based lidar, that is, the majority of works take into account multiple scattering employing one of possible multiple-scattering functions (MSF) (see the discussion in Appendix A) or models (see, e.g., Nakoudi et al, 2021 and references therein).…”