Ice crystal number concentration estimates from lidar–radar satellite remote sensing – Part 1: Method and evaluation

Abstract

Abstract. The number concentration of cloud particles is a key quantity for understanding aerosol–cloud interactions and describing clouds in climate and numerical weather prediction models. In contrast with recent advances for liquid clouds, few observational constraints exist regarding the ice crystal number concentration (Ni). This study investigates how combined lidar–radar measurements can be used to provide satellite estimates of Ni, using a methodology that constrains moments of a parameterized particle size distribution (PSD). The operational liDAR–raDAR (DARDAR) product serves as an existing base for this method, which focuses on ice clouds with temperatures Tc<-30 ∘C. Theoretical considerations demonstrate the capability for accurate retrievals of Ni, apart from a possible bias in the concentration in small crystals when Tc≳−50 ∘C, due to the assumption of a monomodal PSD shape in the current method. This is verified via a comparison of satellite estimates to coincident in situ measurements, which additionally demonstrates the sufficient sensitivity of lidar–radar observations to Ni. Following these results, satellite estimates of Ni are evaluated in the context of a case study and a preliminary climatological analysis based on 10 years of global data. Despite a lack of other large-scale references, this evaluation shows a reasonable physical consistency in Ni spatial distribution patterns. Notably, increases in Ni are found towards cold temperatures and, more significantly, in the presence of strong updrafts, such as those related to convective or orographic uplifts. Further evaluation and improvement of this method are necessary, although these results already constitute a first encouraging step towards large-scale observational constraints for Ni. Part 2 of this series uses this new dataset to examine the controls on Ni.

CALIPSO (IIR–CALIOP) retrievals of cirrus cloud ice-particle concentrations

“…While hom does not always result in relatively high N (e.g., Spichtinger and Krämer, 2013), hom can produce much higher N than het within sustained appreciable updrafts and is generally associated with higher N (e.g., Barahona and Nenes, 2008). Other observational studies indicating that hom is often important in determining the microphysics of cirrus clouds are , Zhao et al (2018), Sourdeval et al (2018a) and Gryspeerdt et al (2018).…”

“…The geographical distributions are qualitatively similar, including higher N over mountainous regions and over the southern oceans, with minimal N over the tropics. Sourdeval et al (2018a) have developed a new satellite retrieval for N regarding ice clouds, based on the CloudSat radar and the CALIPSO lidar (built upon the so-called DAR-DAR retrieval and referred to as the DARDAR-LIM scheme; LIM standing for Leipzig Institute for Meteorology). This retrieval uses the operational DARDAR retrieval products of IWC and N * 0 (a PSD normalization factor that is a function of the IWC and D m , where D m is the ice-particle mean volume diameter, defined as the PSD moment ratio M 4 /M 3 ), and also the "universal" normalized PSD described in Delanoë et al (2005), to estimate N. Although their retrieval scheme is based on different physics than our scheme, these schemes yield similar results.…”

“…These results are discussed in Sect. 6, which includes comparisons with previous work using the combined radar-lidar approach (Sourdeval et al, 2018a;Grysperdt et al, 2018). The discussion also addresses the radiative significance of these cirrus cloud retrievals and a potential link between Arctic cirrus and mid-latitude weather.…”

“…The retrieval of N as a function of latitude and topography is of particular importance as it provides insight into specific physical processes controlling N. The satellite remote sensing study by Zhao et al (2018) has advanced our understanding of the complex relationship between aerosol particles and cirrus clouds, showing the importance of homogeneous ice nucleation (henceforth hom) under relatively clean (i.e., relatively low aerosol optical depth) conditions. A satellite retrieval for N has been proposed that builds upon the lidar-radar (hence DARDAR) retrieval described in Hogan (2008, 2010), as described in Gryspeerdt et al (2018) and Sourdeval et al (2018a). Relating satellite retrievals of N and D e to mineral dust observations (Gryspeerdt et al, 2018) and to cirrus cloud-aerosol modeling outcomes (Zhao et al, 2018) have yielded insights into the relative importance of hom and heterogeneous ice nucleation (henceforth het) in cirrus clouds as a function of aerosol concentrations.…”

Characteristics of Ice Clouds Over Mountain Regions Detected by CALIPSO and CloudSat Satellite Observations

“…This phenomenon suggests that orography triggers convective clouds over mountain regions. Sourdeval et al, 2018) and the CALIPSO satellite product (Mitchell et al, 2018).…”