On the importance of small ice crystals in tropical anvil cirrus 1NASA Ames Research Center, Moffett Field, CA, USA
2SPEC Inc., Boulder, CO, USA
3National Center for Atmospheric Research, Boulder, CO, USA
4NASA Goddard Space Flight Center, Greenbelt, MD, USA
5Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
Received: 29 Jan 2009 – Published in Atmos. Chem. Phys. Discuss.: 02 Mar 2009Abstract. In situ measurements of ice crystal concentrations and sizes made with aircraft
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
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.
Revised: 21 Jul 2009 – Accepted: 22 Jul 2009 – Published: 06 Aug 2009
Citation: Jensen, E. J., Lawson, P., Baker, B., Pilson, B., Mo, Q., Heymsfield, A. J., Bansemer, A., Bui, T. P., McGill, M., Hlavka, D., Heymsfield, G., Platnick, S., Arnold, G. T., and Tanelli, S.: On the importance of small ice crystals in tropical anvil cirrus, Atmos. Chem. Phys., 9, 5519-5537, doi:10.5194/acp-9-5519-2009, 2009.