On the importance of small ice crystals in tropical anvil cirrus

Abstract

Abstract. 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, and therefore is expected to be less susceptible to shattering artifacts than instruments such as CAS. 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 to 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. It appears that the numerous small crystals detrained from convective updrafts do not persist in the anvil cirrus sampled during TC-4. We hypothesize that isolated occurrences of higher ice concentrations in aged anvil cirrus are caused by ice nucleation driven by gravity waves.

Relativistic electron avalanches as a thunderstorm discharge competing with lightning

“…The attachment length, l cl , depends on the density of cloud particles, the total water content, the charge on the cloud particles and the electric field 19,32,33 . The CDP is not designed to measure non-spherical ice, but if we assume a particle density of roughly 10 particles per cm 3 and a particle diameter of 40 mm, which is consistent with both the CDP measurement during the glow and the literature for similar meteorological situations [34][35][36] , we find l cl to be 27 m. The value of 10 particles per cm 3 was chosen to be conservative by a factor of 2 compared with the numbers shown in Fig. 4 when calculating the discharge current.…”

Cirrus Clouds and Climate Engineering: New Findings on Ice Nucleation and Theoretical Basis

“…As noted, historical PSD measurements in cirrus clouds have suffered from ice particles shattering on the probe inlet tube which artificially enhanced the concentration of small (D < 100 µm) ice crystals. The problem of ice particle shattering is greatly reduced in the 2D-S probe due to both probe design and the removal of shattered ice particles based on ice particle interarrival times (Lawson et al, 2006a;Jensen et al, 2009;Baker et al, 2009a,b;Lawson et al, 2010). Thus the PSDs measured during these field programs appear to be much more realistic than previous PSD measurements and render more realistic estimates of ice particle concentrations and V m .…”

“…A common problem to this and many other studies addressing this nucleation issue from a measurement angle is that the ice PSD has only recently been measured with reasonable accuracy. Earlier measurements were plagued with the problem of ice particle shattering, whereby ice particles sampled by a probe impact and shatter on the inlet tube, producing anomalously high concentrations of small ice crystals (McFarquhar et al 2007, Jensen et al 2009, Zhao et al 2011. The subsequent sampling of these artifact ice fragments along with natural ice particles made it virtually impossible to say anything about ice nucleation processes in the atmosphere based on aircraft probe measurements.…”

“…The subsequent sampling of these artifact ice fragments along with natural ice particles made it virtually impossible to say anything about ice nucleation processes in the atmosphere based on aircraft probe measurements. Recently PSD measurement probes have been improved to greatly reduce the shattering problem (Lawson et al 2006a, Jensen et al 2009, Korolev et al 2011, Lawson 2011. These relatively recent PSD measurements are discussed below in relation to several field campaigns sampling cirrus clouds.…”

Characterizations of Cloud Droplet Shatter Artifacts in Two Airborne Aerosol Inlets

“…With most aerosol inlets, the extent of the shatter problem is different in warm and cold clouds, with the shatter problem, generally, less significant with ice particles than liquid droplets (Weber et al 1998;Craig et al in press). Droplet shatter has also been a problem for cloud and precipitation measurements (Gardiner and Hallett 1985;Field et al 2003;Korolev and Isaac 2005;Jensen et al 2009), but advances in cloud probe tip designs and introduction of new highspeed data analyses approaches have helped alleviate some of these problems related to size distribution measurements of activated droplets (Korolev and Isaac 2005;Lawson 2011). There have, however, been very limited efforts to address the problems related to aircraft-based aerosol measurements in clouds.…”