On the relationship between the scattering phase function of cirrus and the atmospheric state

Abstract

Abstract. This is the first paper to investigate the relationship between the shape of the scattering phase function of cirrus and the relative humidity with respect to ice (RH i ), using space-based solar radiometric angle-dependent measurements. The relationship between RH i and the complexity of ice crystals has been previously studied using data from aircraft field campaigns and laboratory cloud chambers. However, to the best of our knowledge, there have been no studies to date that explore this relationship through the use of remotely sensed space-based angle-dependent solar radiometric measurements. In this paper, one case study of semitransparent cirrus, which occurred on 25 January 2010 off the north-east coast of Scotland, is used to explore the possibility of such a relationship. Moreover, for the first time, RH i fields predicted by a high-resolution numerical weather prediction (NWP) model are combined with satellite retrievals of ice crystal complexity. The NWP model was initialised at midnight, on 25 January 2010, and the mid-latitude RH i field was extracted from the NWP model at 13:00 UTC. At about the same time, there was a PARASOL (Polarization and Anisotropy of Reflectance for Atmospheric science coupled with Observations from a Lidar) overpass, and the PARASOL swath covered the NWP-model-predicted RH i field. The cirrus case was located over Scotland and the North Sea. From the satellite channel based at 0.865 µm, the directionally averaged and directional spherical albedos were retrieved between the scattering angles of about 80 and 130• . An ensemble model of cirrus ice crystals is used to predict phase functions that vary between phase functions that exhibit optical features (referred to as pristine) and featureless phase functions. For each of the PARASOL pixels, the phase function that best minimised differences between the spherical albedos was selected. This paper reports, for this one case study, an association between the most featureless phase function model and the highest values of NWP-predicted RH i (i.e. when RH i > 1.0). For pixels associated with NWPmodel-predicted RH i < 1, it was impossible to generally discriminate between phase function models at the 5 % significance level. It is also shown that the NWP model prediction of the vertical profile of RH i is in good agreement with dropsonde, in situ measurements and independent aircraft-based physical retrievals of RH i . Furthermore, the NWP model prediction of the cirrus cloud-top height and its vertical extent is also found to be in good agreement with aircraft-based lidar measurements.

Ice crystal characterization in cirrus clouds: a sun-tracking camera system and automated detection algorithm for halo displays

“…Thus, it is possible that the majority of cirrus clouds during the observation period in Munich contain a large fraction of rough ice crystals. This would support the hypothesis of Knap et al (2005), Baran and Labonnote (2006), and Baran et al (2015), who found that on average rough ice crystals better reproduce remote sensing radiance measurements than assuming crystals with smooth surface. However, if multiple scattering by cirrus clouds or aerosol is accounted for, the minimum fraction of smooth crystals could be much larger in the case of haloproducing cirrus clouds.…”

Quantitative three‐dimensional ice roughness from scanning electron microscopy

“…We think it more likely, however, that when faced with strongly non‐Weibull PDFs (such as the bimodal distribution appearing in Figure c), in situ retrieval algorithms compensate by assigning excessively low values of η W . In terms of scale parameters, our results span a similar range to that reported from the above in situ studies ( σ W up to ~0.25) but are small compared to the results of remote sensing studies ( σ W up to ~0.5) [ Cole et al ., ; van Diedenhoven et al ., ; Baran , ; Hioki et al ., ]. But our results are obtained under a rather narrow range of conditions, −39°C to −29°C and near‐equilibrium humidity, whereas remote sensing studies span a much larger range.…”

“…Cirrus clouds play an important role in the Earth's climate by absorbing and reflecting infrared and solar radiation [Stephens et al, 1990;Lynch, 2002;Baran, 2009Baran, , 2012van Diedenhoven et al, 2014b;Baran, 2015]. Although crystals making up these clouds possess complex geometries, a useful simplification is to consider them as aggregates of simpler components, especially hexagonal-shaped prisms.…”

Halo ratio from ground-based all-sky imaging

“…The asymmetry parameter on the other hand is expected to be positively correlated with the HR (Ulanowski et al, 2006(Ulanowski et al, , 2014Gayet et al, 2011). Many studies (Korolev et al, 2000;Garrett et al, 2001;Baran and Labonnote, 2007;Shcherbakov et al, 2006;Gayet et al, 2011;Baum et al, 2011;Cole et al, 2013;Ulanowski et al, 2014;Baran et al, 2015) suggest that cirrus clouds are mainly formed by rough or complex particles giving rise to typically featureless SPFs, confirming indications from previous observations and explaining the relative rarity of ground-observed halo occurrences (Sassen et al, 1994). Nevertheless, recent findings (Forster et al, 2017) suggest that the fraction of halo-producing cirrus might be larger than previously thought.…”