On the importance of small ice crystals in tropical anvil cirrus

In situ measurements of ice crystal concentrations and sizes made with aircraft instrumentation over the past two decades have often indicated the presence of numerous relatively small (< 50 μm diameter) crystals in cirrus clouds. Further, these measurements frequently indicate that small crystals account for a large fraction of the extinction in cirrus clouds. The fact that the instruments used to make these measurements, such as the Forward Scattering Spectrometer Probe (FSSP) and the Cloud Aerosol Spectrometer (CAS), ingest ice crystals into the sample volume through inlets has led to suspicion that the indications of numerous small-crystals could be artifacts of large-crystal shattering on the instrument inlets. We present new aircraft measurements in anvil cirrus sampled during the Tropical Composition, Cloud, and Climate Coupling (TC4) campaign with the 2-Dimensional Stereo (2D-S) probe, which detects particles as small as 10 μm. The 2D-S has detector "arms" instead of an inlet tube. Since the 2D-S probe surfaces are much further from the sample volume than is the case for the instruments with inlets, it is expected that 2D-S will be less susceptible to shattering artifacts. In addition, particle inter-arrival times are used to identify and remove shattering artifacts that occur even with the 2D-S probe. The number of shattering artifacts identified by the 2D-S interarrival time analysis ranges from a negligible contribution to an order of magnitude or more enhancement in apparent ice concentration over the natural ice concentration, depending on the abundance of large crystals and the natural small-crystal concentration. The 2D-S measurements in tropical anvil cirrus suggest that natural small-crystal concentrations are typically one to two orders of magnitude lower than those inferred from CAS. The strong correlation between the CAS/2D-S ratio of small-crystal concentrations and large-crystal concentration suggests that the discrepancy is likely caused by shattering of large crystals on the CAS inlet. We argue that past measurements with CAS in cirrus with large crystals present may contain errors due to crystal shattering, and past conclusions derived from these measurements may need to be revisited. Further, we present correlations between CAS spurious concentration and 2D-S large-crystal mass from spatially uniform anvil cirrus sampling periods as an approximate guide for estimating quantitative impact of large-crystal shattering on CAS concentrations in previous datasets. We use radiative transfer calculations to demonstrate that in the maritime anvil cirrus sampled during TC4, small crystals indicated by 2D-S contribute relatively little cloud extinction, radiative forcing, or radiative heating in the anvils, regardless of anvil age or vertical location in the clouds. While 2D-S ice concentrations in fresh anvil cirrus may often exceed 1 cm⁻³, and are observed to exceed 10 cm⁻³ in turrets, they are typically ~0.1 cm⁻³ and rarely exceed 1 cm⁻³ (<1.4% of the time) in aged anvil cirrus. We hypothesize that isolated occurrences of higher ice concentrations in aged anvil cirrus may be caused by ice nucleation driven by either small-scale convection or gravity waves. It appears that the numerous small crystals detrained from convective updrafts do not persist in the anvil cirrus sampled during TC-4.