During the Department of Energy Atmospheric Radiation Measurement Program (DOE ARM) sponsored Tropical Warm Pool International Cloud Experiment (TWP‐ICE), ice crystals with maximum dimensions (D) < 50 μm were measured in aged cirrus and fresh anvils by a Cloud and Aerosol Spectrometer (CAS) and a Cloud Droplet Probe (CDP). The CAS/CDP ratio of the number concentrations of droplets with 3 < D < 50 μm, N3−50, averaged 0.98 ± 0.69 in liquid clouds. However, N3−50, measured by the CAS averaged 91 ± 127 times larger than N3−50 from the CDP in ice clouds. The CAS/CDP N3−50 ratio had a correlation coefficient of 0.387 with the concentration of particles with D > 100 μm measured by the Cloud Imaging Probe, suggesting that ice crystals may have been shattering or bouncing on the CAS inlet or protruding airflow shroud enhancing N>3−50,CAS. During the Costa Rica Aura Validation Experiment N3−50,CAS measured by a CAS without an airflow shroud were an order of magnitude less than those observed during TWP‐ICE. This, and estimates of the maximum shattering based on the inlet and shroud sizes, suggest that the airflow shroud used during TWP‐ICE was responsible for much of the shattering or bouncing.

The Impact of Arctic Aerosols on Clouds During one flight leg over the water on 4 April, large chunks of ice were seen floating in the Arctic Ocean after breaking up from the ice sheet along the coastline near Barrow, Alaska. Photo by Alexei Korolev.

[1] Cloud and aerosol data acquired by the National Research Council of Canada (NRC) Convair-580 aircraft in, above, and below single-layer arctic stratocumulus cloud during the Indirect and Semi-Direct Aerosol Campaign (ISDAC) in April 2008 were used to test three aerosol indirect effects hypothesized to act in mixed-phase clouds: the riming indirect effect, the glaciation indirect effect, and the thermodynamic indirect effect. The data showed a correlation of R = 0.78 between liquid drop number concentration, N liq inside cloud and ambient aerosol number concentration N PCASP below cloud. This, combined with increasing liquid water content LWC with height above cloud base and the nearly constant vertical profile of N liq , suggested that liquid drops nucleated from aerosol at cloud base. No evidence of a riming indirect effect was observed, but a strong correlation of R = 0.69 between ice crystal number concentration N i and N PCASP above cloud was noted. Increases in ice nuclei (IN) concentration with N PCASP above cloud for 2 flight dates combined with the subadiabatic LWC profiles suggest possible mixing of IN from cloud top consistent with the glaciation indirect effect. The lower N ice and lower effective radius r el for the more polluted ISDAC cases compared to data collected in cleaner single-layer stratocumulus conditions during the Mixed-Phase Arctic Cloud Experiment is consistent with the operation of the thermodynamic indirect effect. However, more data in a wider variety of meteorological and surface conditions, with greater variations in aerosol forcing, are required to identify the dominant aerosol forcing mechanisms in mixed-phase arctic clouds.

Gamma distributions represent particle size distributions (SDs) in mesoscale and cloud-resolving models that predict one, two, or three moments of hydrometeor species. They are characterized by intercept (N0), slope (λ), and shape (μ) parameters prognosed by such schemes or diagnosed based on fits to SDs measured in situ in clouds. Here, ice crystal SDs acquired in arctic cirrus during the Indirect and Semi-Direct Aerosol Campaign (ISDAC) and in hurricanes during the National Aeronautic and Space Administration (NASA) African Monsoon Multidisciplinary Analyses (NAMMA) are fit to gamma distributions using multiple algorithms. It is shown that N0, λ, and μ are not independent parameters but rather exhibit mutual dependence. Although N0, λ, and μ are not highly dependent on choice of fitting routine, they are sensitive to the tolerance permitted by fitting algorithms, meaning a three-dimensional volume in N0–λ–μ phase space is required to represent a single SD. Depending on the uncertainty in the measured SD and on how well a gamma distribution matches the SD, parameters within this volume of equally realizable solutions can vary substantially, with N0, in particular, spanning several orders of magnitude. A method to characterize a family of SDs as an ellipsoid in N0–λ–μ phase space is described, with the associated scatter in N0–λ–μ for such families comparable to scatter in N0, λ, and μ observed in prior field campaigns conducted in different conditions. Ramifications for the development of cloud parameterization schemes and associated calculations of microphysical process rates are discussed.

Mesoscale models that predict the evolution of tropical cyclones (TCs) are sensitive to the representation of cloud microphysical processes. Bulk cloud parametrizations used in such models make assumptions about the particle size distributions (PSDs) of different ice species, and their representativeness for TCs is not well known. In situ cloud probe data acquired in tropical storms, depressions and waves during the NASA African Monsoon Multidisciplinary Analyses (NAMMA) project are used to define PSDs of snow and graupel, and of all ice hydrometeors combined. These PSDs are fitted to gamma functions to determine how the intercept (N0), shape (μ), and slope (λ) vary with cloud and environmental conditions. Families of PSDs are determined for each condition (e.g. PSDs found in updraughts, downdraughts and stratiform regions, for different ranges of ice water content (IWC) and temperature (T), and for differing stages of TC development). A volume of equally plausible solutions in (N0‐μ‐λ) phase space is defined for each environmental condition sampled based on the goodness of the fits and the uncertainty in the measured PSDs due to statistical sampling. Per cent overlap between two families in each environmental and cloud condition was calculated, and results show that areas with sustained vertical velocity with a magnitude of at least 1 m·s−1 lie in a different phase space than stratiform regions, and PSDs corresponding to IWC < 0.01 g·m−3 lie in a different phase space than PSDs corresponding to IWC > 0.1 g·m−3. All other environmental and cloud conditions did not have significant impacts on either the location or uncertainty in the family of ellipsoids.