Seong Soo Yum

Maritime-Continental Drizzle Contrasts in Small Cumuli

Abstract Continuous aircraft measurements of cloud condensation nuclei (CCN) were made during 16 summertime flights in eastern Florida. The air masses were divisible into maritime and continental regimes that respectively corresponded to wind direction—easterly (onshore) and westerly (offshore). Throughout these small cumulus clouds there were consistently higher concentrations of smaller droplets in the continental air. There was much more drizzle (diameter > 50 μm) in the maritime clouds where drizzle was associated with larger mean cloud droplet (2–50-μm diameter) sizes, higher concentrations of large cloud droplets, and greater amounts of cloud droplet liquid water. An apparent cloud droplet mean size threshold for the onset of drizzle was almost never exceeded in the continental clouds but was often exceeded in the maritime clouds, especially at higher altitudes. All together these results demonstrate that higher CCN concentrations suppressed drizzle.

Droplet spectral broadening in marine stratus

Abstract Broadening of the cloud droplet (diameter 50 μm, i.e., drizzle). Broad droplet spectra were most closely associated with drizzle drops. Both the concentration, C, and slope, k, of the cloud condensation nuclei (CCN) spectra were theoretically found to affect droplet spectral width. For individual cloud parcels a higher C and lower k each contributed to broader droplet spectra. When mixing among cloud parcels with different updrafts was considered, the predictions deviated especially at larger mean droplet diameters. Variations in updraft velocity result in differences in droplet concentrations ...

Vertical distributions of cloud condensation nuclei spectra over the springtime Arctic Ocean

More than 23 hours of continuous condensation nuclei (CN) and cloud condensation nuclei (CCN) spectral data from the Arctic Clouds Experiment are presented and analyzed. These measurements were made at altitudes ranging from 6 km to 30 m in eight flights during May 1998 over the mostly frozen Arctic Ocean at least 500 km north of the Alaskan coast. Concentrations generally increased with altitude with a pronounced deficit in the boundary layer when low stratus clouds were present. The low-level vertical gradient could be demonstrated to be a result of cloud scavenging. Boundary layer concentrations at 0.8% supersaturation averaged 76 cm -3 with low cloud (below the low cloud) and 250 cm -3 when no low cloud was present. The ratio of CCN to CN (total particles) was relatively high, usually exceeding 0.6. The relatively high concentrations at higher altitudes, the high CCN/CN ratio, and the lower CCN spectral slopes are characteristic of an aged aerosol probably due to long-range transport.

Comparisons of cloud microphysics with cloud condensation nuclei spectra over the summertime Southern Ocean

In spite of the many factors that determine cloud droplet concentrations, relationships were found between cloud condensation nuclei (CCN) and cloud droplet concentrations for the First Aerosol Characterization Experiment (ACE 1). Measurements were made in summertime stratocumulus clouds over the Southern Ocean far from anthropogenic sources. The closest relationship of CCN and droplet concentrations was found for CCN measured just below near-adiabatic cloud parcels. Moreover, an adiabatic droplet growth model successfully predicted these droplet concentrations from the CCN spectra and updraft velocities. Correlations of the averages of CCN and cloud droplet concentrations over the entirety of each of the flights were also good. The model also predicted, with reasonable accuracy, these flight-wide average droplet concentrations from flight-wide average CCN spectra and flight-wide average cloud updraft velocities. Drizzle was a major reason for the larger variations in droplet concentrations than in CCN concentrations. Vertical differences in CCN concentrations may have also affected cloud droplet concentrations. Despite variabilities in droplet concentrations, there was a clear relationship between CCN and droplet concentrations. Many of the inferred supersaturations (0.5 to 1.4%) in the “adiabatic” clouds ranged higher than some previous estimates (e.g., 0.2%) for stratus clouds. This may have been due to the cleaner air of the Southern Ocean but is more likely attributable to the more intensive analysis used in this study. Nonetheless, even where the inferred cloud supersaturations were lower (i.e., 0.2%), due to reductions of the droplet concentrations, there was still a relationship between CCN and droplet concentrations.

Vertical distributions of cloud condensation nuclei spectra over the summertime Southern Ocean

More than 60 hours of airborne cloud condensation nuclei (CCN) measurements made during the First Aerosol Characterization Experiment (ACE 1) south of Australia are presented and analyzed. These ambient measurements covered more than half of the aircraft time on station between 7 km and 30 m altitude during the late spring/early summer season over the Southern Ocean. The complete CCN spectra from 1.2 to 0.02% and condensation nuclei (CN), particles greater than 0.01 μm, concentrations are presented. The CN and CCN concentrations at 1.2 and 0.2% supersaturation compared well with long-term surface measurements at Cape Grim, Tasmania. Lower concentrations, especially for CN, were associated with cloudier regions. Higher concentrations, especially for CN, were found in air masses that were or had recently been closer to land. Nonetheless, these data should be representative of clean maritime conditions.

Cloud Condensation Nuclei and Ship Tracks

Abstract Enhancements of droplet concentrations in clouds affected by four ships were fairly accurately predicted from ship emission factors and plume and background cloud condensation nucleus (CCN) spectra. Ship exhausts thus accounted for the increased droplet concentrations in these “ship tracks.” Derived supersaturations were typical of marine stratus clouds, although there was evidence of some lowering of supersaturations in some ship tracks closer to the ships where CCN and droplet concentrations were very high. Systematic differences were measured in the emission rates of CCN for different engines and fuels. Diesel engines burning low-grade marine fuel oil produced order of magnitude higher CCN emissions than turbine engines burning higher-grade fuel. Consequently, diesel ships burning low-grade fuel were responsible for nearly all of the observed ship track clouds. There is some evidence that fuel type is a better predictor of ship track potential than engine type.

Observations of marine stratocumulus microphysics and implications for processes controlling droplet spectra: Results from the Marine Stratus/Stratocumulus Experiment

During the Marine Stratus/Stratocumulus Experiment, cloud and aerosol microphysics were measured in the eastern Pacific off the coast of northern California on board Department of Energy Gulfstream-1 in July 2005. Three cases with uniform aerosol concentration and minimal drizzle concentration were examined to study cloud microphysical behavior. For these three cases, the average droplet number concentration increased with increasing altitude, while the average interstitial aerosol concentration decreased with altitude. The data show enhanced growth of large droplets and spectral broadening in cloud parcels with low liquid water mixing ratio. Three mixing models, including inhomogeneous mixing, entity type entrainment mixing, and circulation mixing proposed in this study, are examined with regard to their influences on cloud microphysics. The observed cloud microphysical behavior is most consistent with the circulation mixing, which describes the mixing between cloud parcels with different lifting condensation levels during their circulations driven by evaporative and radiative cooling. The enhanced growth and spectrum broadening resulting from the circulation mixing reduce cloud albedo at the same liquid water path and facilitate the formation of precipitation embryos.

Microphysical relationships in warm clouds

Abstract Relationships among cloud microphysical parameters from six aircraft projects carried out over different parts of the globe in different seasons are presented. Comparisons of project averages showed that projects with higher droplet (2–50 μm diameter) concentrations generally had smaller droplets and less drizzle (diameter >50 μm). Hence, when all projects were combined there were negative correlations between droplet concentrations and sizes. However, this was not the case for individual cloud penetrations where only 24% had negative droplet concentration–size correlations. For the stratocumulus clouds, these negative correlations were more likely to be found in clouds with greater droplet spectral broadening and drizzle. The small cumulus clouds in this study were unique in that 95% of the droplet concentration–size correlations were positive regardless of spectral broadening or drizzle. Thus, for the majority of clouds, droplets were smaller in parcels with lower droplet concentrations. Entrainment in these clouds was then consistent with homogeneous mixing that occurs locally and intermittently. The dearth of observations of larger droplets in diluted cloud parcels suggested that mixing of entrained air did not promote droplet growth.

Characteristics of cloud-nucleating aerosols in the Indian Ocean region

During the Indian Ocean Experiment (INDOEX), cloud droplets were collected and evaporated using a counterflow virtual impactor (CVI). The nonvolatile residual particles were then analyzed by various instruments. Physical and chemical properties of the cloud droplets and their residual nuclei were compared with properties of the below-cloud aerosol to evaluate which aerosol particles act as cloud nuclei in different environments, and their effects on cloud microphysics. Four cases, ranging from clean Southern Hemispheric clouds to heavily polluted clouds near India, were analyzed. For the cleaner clouds, droplet concentrations were a much higher fraction of the available particle concentrations than for polluted clouds, but entrainment apparently acted to reduce droplet number concentrations in both regimes. For clean clouds the median critical supersaturation and size of the ambient particles and droplet residual particles were similar. In polluted clouds there were stronger differences between ambient and droplet residual distributions, and particles with lower critical supersaturations were favored as nuclei. Simple model calculations were used to show that polluted clouds are expected to achieve lower water supersaturations than clean clouds; thus only particles with relatively low critical supersaturations are likely to affect clouds in polluted regions. Soluble fractions for the ambient aerosol inferred from the size and cloud condensation nuclei measurements were in general agreement with another study in the region. Droplet residual particles did not necessarily have higher soluble fractions than the ambient aerosol, but did tend to have higher total amounts of soluble material per particle, particularly in the polluted cases.

A Numerical Study of Sea-Fog Formation over Cold Sea Surface Using a One-Dimensional Turbulence Model Coupled with the Weather Research and Forecasting Model

The formation mechanism of a cold sea-fog case observed over the Yellow Sea near the western coastal area of the Korean Peninsula is investigated using numerical simulation with a one-dimensional turbulence model coupled with a three-dimensional regional model. The simulation was carried out using both Eulerian and Lagrangian approaches; both approaches produced sea fog in a manner consistent with observation. For the selected cold sea-fog case, the model results suggested the following: as warm and moist air flows over a cold sea surface, the lower part of the air column is modified by the turbulent exchange of heat and moisture and the diurnal variation in radiation. The modified boundary-layer structure represents a typical stable thermally internal boundary layer. Within the stable thermally internal boundary layer, the air temperature is decreased by radiative cooling and turbulent heat exchange but the moisture loss due to the downward vapour flux in the lowest part of the air column is compensated by moisture advection and therefore the dewpoint temperature does not decrease as rapidly as does the air temperature. Eventually water vapour saturation is achieved and the cold sea fog forms in the thermal internal boundary layer.