Abstract
Abstract. The combination of downward-looking airborne lidar, radar, microwave, and imaging spectrometer measurements was exploited to characterize the vertical and small-scale (down to 10 m) horizontal distribution of the thermodynamic phase of low-level Arctic mixed-layer clouds. Two cloud cases observed in a cold air outbreak and a warm air advection event observed during the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) campaign were investigated. Both cloud cases exhibited the typical vertical mixed-phase structure with mostly liquid water droplets at cloud top and ice crystals in lower layers. The horizontal, small-scale distribution of the thermodynamic phase as observed during the cold air outbreak is dominated by the liquid water close to the cloud top and shows no indication of ice in lower cloud layers. Contrastingly, the cloud top variability in the case observed during a warm air advection showed some ice in areas of low reflectivity or cloud holes. Radiative transfer simulations considering homogeneous mixtures of liquid water droplets and ice crystals were able to reproduce the horizontal variability in this warm air advection. Large eddy simulations (LESs) were performed to reconstruct the observed cloud properties, which were used subsequently as input for radiative transfer simulations. The LESs of the cloud case observed during the cold air outbreak, with mostly liquid water at cloud top, realistically reproduced the observations. For the warm air advection case, the simulated ice water content (IWC) was systematically lower than the measured IWC. Nevertheless, the LESs revealed the presence of ice particles close to the cloud top and confirmed the observed horizontal variability in the cloud field. It is concluded that the cloud top small-scale horizontal variability is directly linked to changes in the vertical distribution of the cloud thermodynamic phase. Passive satellite-borne imaging spectrometer observations with pixel sizes larger than 100 m miss the small-scale cloud top structures.
Highlights
In the Arctic, low-level stratus and stratocumulus clouds are present around 40 % of the time on annual average (Shupe et al, 2006; Shupe, 2011), and they may persist up to several weeks (Shupe, 2011; Morrison et al, 2012)
Based on airborne active and passive remote sensing conducted by a passive imaging spectrometer and vertically resolving instruments, such as lidar and radar, the horizontal and vertical structure of the thermodynamic phase in Arctic mixed-phase cloud cases was characterized for two example clouds observed during a cold air outbreak and a warm air intrusion event
While the spectral imaging was used to identify the structure of the horizontal distribution of the cloud ice at scales down to 10 m, the combined radar and lidar observations revealed the general vertical thermodynamic phase distribution of the clouds
Summary
In the Arctic, low-level stratus and stratocumulus clouds are present around 40 % of the time on annual average (Shupe et al, 2006; Shupe, 2011), and they may persist up to several weeks (Shupe, 2011; Morrison et al, 2012). On smaller scales of a few hundred meters, the vertical motion is driven by evaporative cooling, associated with entrainment of moist air supplied from upper layers (Mellado, 2017) This entrainment process ensures the formation of liquid water droplets and balances the loss of cloud water by precipitating ice crystals (Korolev, 2007; Shupe et al, 2008; Morrison et al, 2012). Understanding the radiative properties and temporal evolution of Arctic mixed-phase clouds requires a three-dimensional (3D) characterization of the thermodynamic phase partitioning, which relates the vertical distribution of liquid droplets and ice crystals to the small-scale structures observed close to the cloud top. The active remote sensing instruments provide the general vertical stratification of ice particles and liquid water droplets, which is needed to interpret the 2D maps of cloud phase observed by the spectral imager.
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call