Parameterizing anvil cirrus size distributions and ice sedimentation rates
Authors
David L. Mitchell — Desert Research Institute
Paul Lawson — SPEC, Inc.
Category
Cloud Properties
Description
Ice particle size distributions (PSDs) for fresh anvil cirrus sampled during the TC4 field campaign. PSD were normalized by their ice water content (IWC) to compare number concentrations directly. The threshold for homogeneous freezing nucleation is ~ -40°C, precisely where the PSDs transition from monomodal to bimodal. This appears consistent with the higher nucleation rates predicted from homogeneous freezing, increasing the concentration of the smaller crystals.
The life cycle of cirrus clouds depends strongly on their ice sedimentation rates, which have been difficult to characterize due to uncertainties in the concentration of small ice crystals. The 2D-S probe has the capability to remove ice artifacts (e.g., small ice particles produced by larger particles that shatter on the probe tips). The technique uses software processing and has measured the size-resolved number, area, and mass concentration of anvil and in situ cirrus particles during two recent field campaigns. These measurements have been used to develop the relationships needed for parameterizing Vf, the median mass-flux fall speed of the ice particle size distribution (PSD): (1) the ice particle projected area- and mass-dimension relationships and (2) a parameterization of the PSD. Regarding (1), these relationships appear fairly insensitive to temperature and cloud type (i.e., anvil cirrus, aged anvils, in situ cirrus, and turrets). Since ice water contents (IWCs) measured by the 2D-S agreed well with CVI-measured IWCs, IWCs predicted by these relationships should also be consistent with CVI measurements. Regarding (2), the PSD parameterization allows for bimodality, following the method published in Mitchell et al. (2010 JAS).
The observed (aircraft leg-averaged) and parameterized PSD fall into two categories: (1) monomodal for temperature T < -40°C, and bimodal for T > -40°C. The secondary maximum for the bimodal PSD occurs ~ D = 220 µm. For smaller particles, the number concentration in the monomodal PSDs is ~ 10 times larger relative to bimodal PSDs of similar IWC, resulting in lower Vf relative to the bimodal PSDs. This suggests that recently formed anvil cirrus colder than -40°C will have longer lifetimes than warmer cirrus.
The clean separation of PSD type at -40°C suggests a physical mechanism may be responsible. Size-sorting by fall velocity from higher to lower levels could possibly explain the PSD evolution. If the ice crystals formed at temperatures similar to sampling conditions (questionable for anvil cirrus), then a transition in dominant nucleation mechanism, from heterogeneous to homogeneous freezing, may explain the observations. The PSD for in situ cirrus clouds were colder than -40°C and were monomodal, consistent with this reasoning. Moreover, PSD number concentration normalized by IWC for T < -40°C was a factor of 7 higher on average relative to the warmer concentrations.
