The effective density of small ice particles obtained from in situ aircraft observations of mid‐latitude cirrus

Abstract

The effective ice-particle density, parametrized through a mass-dimension relation, is widely used in ice microphysical schemes for weather and climate models. In this study, we use aircraft-based observations in mid-latitude cirrus taken during the Constrain field programme in 2010. The low temperatures and a humidity often close to ice saturation meant that the typical ice particles observed were small (maximum dimension 20-800 µm) and ice water contents were low (0.001-0.05 g m −3 ). Two new instruments are included in this study: the Small Ice Detector Mark-2 (SID-2) and the deep-cone Nevzorov Total Water Content probe. SID-2 is a new singleparticle light-scattering instrument and was used to identify and size small ice particles (10-150 µm). The deep-cone Nevzorov probe is shown to be able to collect small ice masses with sufficient sensitivity. The focus of this article is on the effective density of small ice particles (both pristine ice crystals and small aggregates up to 600 µm maximum dimension). Due to instrument limitations in previous studies, the effective density of small ice particles is questionable.Aircraft flights in six cirrus cases provided ice-particle measurements throughout the depth of the cirrus. The particle size distribution (PSD) was mostly bimodal. The smaller ice-crystal mode dominated the PSD near cloud top and the larger ice-aggregate mode dominated near cloud base. A mass-dimension relation valid for both ice crystals and aggregates was found that provided a best fit to the observations. For small ice particles (less than 70 µm diameter) the density is constant (700 kg m −3 ), while for larger ice crystals or aggregates the mass-dimension relation is m(D) = 0.0257D 2.0 . These measurements allow testing of the diagnostic split between ice crystals and aggregates used in the Met Office Unified Model.

Growth of ice particle mass and projected area during riming

“…Thus, the BL06 m − A expression is not expected to be universally valid for all ice crystal habits. On the other hand, when applied to A measurements in cirrus clouds, it yields ice particle mass estimates that are very consistent with two other studies that estimated m − D expressions for cirrus clouds Cotton et al, 2013), as described in Sect. 3.…”

“…As explained in EM16, the important problem with airborne measurements is that they are unable to measure single ice particle mass. Nonetheless, we compare our results with m − D relationships from previous studies Cotton et al, 2013;EM16) that are based on airborne measurements. EM16 provided m − D curve fits based on cloud particle imager (CPI) measurements from the Department of Energy (DOE)-Atmospheric Radiation Measurement (ARM)-funded Small Particles In Cirrus (SPartICus) field campaign for D < 100 µm and a subset of SCPP data for D > 100 µm.…”

“…They are determined via direct measurements of ice particle mass and dimension (Locatelli and Hobbs, 1974;M90), or are constrained through aircraft measurements of the ice particle size distribution (PSD) and IWC Cotton et al, 2013). The prefactor α was considered to contain information on particle density and thickness, whereas β was believed to have information on particle shape.…”

“…4.1. Also displayed are the m − D power law expressions from Cotton et al (2013) and which were acquired from synoptic ice clouds for −60 • C < T < −20 • C and from both synoptic and anvil ice clouds for −60 • C < T < 0 • C, respectively. The grey line, corresponding to spherical particles having density equal to 0.917 g m −3 (solid ice density), serves as an upper limit to ice particle mass.…”

Retrieval of an ice water path over the ocean from ISMAR and MARSS millimeter and submillimeter brightness temperatures

“…Secondly, the scattering properties of snow have to be investigated especially in the submillimeter range, because data for the scattering properties of this range of the electromagnetic spectrum are rare and partially inconsistent with measurements. The mass of the taken Hong aggregates is proportional to the third power of the maximum dimension of these aggregates (see also 3.2), whereas the measurements show that the mass is approximately proportional to the second power of the maximum dimension (Cotton et al, 2013). This is especially important in view of retrievals for the upcoming ICI sensor, because the retrieval results will strongly depend on the goodness of the scattering properties.…”