Abstract
Abstract. Two-dimensional horizontal fields of cloud optical thickness τ derived from airborne measurements of solar spectral, cloud-reflected radiance are compared with semi-idealized large eddy simulations (LESs) of Arctic stratus performed with the Consortium for Small-scale Modeling (COSMO) atmospheric model. The measurements were collected during the Vertical Distribution of Ice in Arctic Clouds (VERDI) campaign carried out in Inuvik, Canada, in April/May 2012. The input for the LESs is obtained from collocated airborne dropsonde observations of a persistent Arctic stratus above the sea-ice-free Beaufort Sea. Simulations are performed for spatial resolutions of 50 m (1.6 km × 1.6 km domain) and 100 m (6.4 km × 6.4 km domain). Macrophysical cloud properties, such as cloud top altitude and vertical extent, are well captured by the COSMO simulations. However, COSMO produces rather homogeneous clouds compared to the measurements, in particular for the simulations with coarser spatial resolution. For both spatial resolutions, the directional structure of the cloud inhomogeneity is well represented by the model. Differences between the individual cases are mainly associated with the wind shear near cloud top and the vertical structure of the atmospheric boundary layer. A sensitivity study changing the wind velocity in COSMO by a vertically constant scaling factor shows that the directional, small-scale cloud inhomogeneity structures can range from 250 to 800 m, depending on the mean wind speed, if the simulated domain is large enough to capture also large-scale structures, which then influence the small-scale structures. For those cases, a threshold wind velocity is identified, which determines when the cloud inhomogeneity stops increasing with increasing wind velocity.
Highlights
Arctic clouds are expected to be a major contributor to the so-called Arctic amplification (Serreze and Barry, 2011; Wendisch et al, 2017) and need to be represented adequately in model projections of the future Arctic climate (Vavrus, 2004)
The dropsondes were released to sample profiles of meteorological parameters – air pressure (p), air temperature (T ), relative humidity (RH), wind speed (v), and wind direction (WD) – below the aircraft, which typically operated at about 3 km altitude to sample the entire cloud and ABL structure
Remote sensing of cloud optical thickness and atmospheric dropsonde measurements from the airborne Vertical Distribution of Ice in Arctic Clouds (VERDI) campaign conducted in April/May 2012 are exploited
Summary
Arctic clouds are expected to be a major contributor to the so-called Arctic amplification (Serreze and Barry, 2011; Wendisch et al, 2017) and need to be represented adequately in model projections of the future Arctic climate (Vavrus, 2004). Horizontal small-scale cloud inhomogeneities in the size range of less than 1 km in simulations and measurements can be investigated with LES to better understand the radiative properties of Arctic mixed-phase clouds. Airborne imaging spectrometer measurements, obtained during the Vertical Distribution of Ice in Arctic Clouds (VERDI) campaign, are used to analyze small-scale cloud inhomogeneities (less than 1 km), which are compared to COSMO simulations using the same model setup as proposed by Loewe et al (2017) with 64 by 64 grid points and 100 m spatial resolution as well as a setup with higher resolution of 32 by 32 grid points and 50 m spatial resolution.
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