R. Dupuy

Comparison of Airborne In Situ, Airborne Radar–Lidar, and Spaceborne Radar–Lidar Retrievals of Polar Ice Cloud Properties Sampled during the POLARCAT Campaign

AbstractThis study illustrates the high potential of RALI, the French airborne radar–lidar instrument, for studying cloud processes and evaluating satellite products when satellite overpasses are available. For an Arctic nimbostratus ice cloud collected on 1 April 2008 during the Polar Study using Aircraft, Remote Sensing, Surface Measurements and Models, of Climate, Chemistry, Aerosols, and Transport (POLARCAT) campaign, the capability of this synergistic instrument to retrieve cloud properties and to characterize the cloud phase at scales smaller than a kilometer, which is crucial for cloud process analysis, is demonstrated. A variational approach, which combines radar and lidar, is used to retrieve the ice-water content (IWC), extinction, and effective radius. The combination of radar and lidar is shown to provide better retrievals than do stand-alone methods and, in general, the radar overestimates and the lidar underestimates IWC. As the sampled ice cloud was simultaneously observed by CloudSat and C...

Ice Crystal Sizes in High Ice Water Content Clouds. Part II: Statistics of Mass Diameter Percentiles in Tropical Convection Observed during the HAIC/HIWC Project

High ice water content (IWC) regions in Mesoscale Convective Systems (MCS) are a potential threat to commercial aviation as they are suspected to cause in-service engine power-loss events and air data probe malfunctions. To investigate this, the High Altitude Ice Crystals (HAIC) / High Ice Water Content (HIWC) projects set up a first field campaign in Darwin (Australia) in 2014. The airborne instrumentation was selected to provide the most accurate measurements of both the bulk total water content (TWC), using a specially developed isokinetic evaporator, and the individual ice crystals properties using particle imaging probes. This study focuses on determining the size ranges of ice crystals responsible for the mass in high IWC regions, defined here as cloud regions with IWC greater than 1.5gm -3 . It is shown that for high IWC areas in most of the encountered MCS systems, median mass diameters (MMDs) of ice crystals range from 250 to 500μm and decrease with increasing TWC and decreasing temperature. At the same time, the mass contribution of the smallest crystals (below 100μm) remains generally low (below 15%). In contrast, data from two flight missions in a long-lasting quasi-stationary tropical storm reveal that high IWC values can also be associated with MMDs in the range 400-800 μm and peak values of up to 2mm. Ice crystal images suggest a major growth contribution by vapor deposition (columns, capped columns) even for such larger MMDs values.

Evaluation of Measured and Predicted Cloud Condensation Nuclei in Mace Head

Measurements of cloud condensation nuclei (CCN) were made in Mace Head during the winter and summer periods of Marine Aerosol Production (MAP) project in 2006. CCN measurements were performed using a DMT Continuous-flow CCN Instrument. The aerosol size distribution and aerosol microphysics measure- ments were measured with a TSI SMPS. Using the size and by implying inferred composition information, detailed CCN closure analyses were performed for the data sets from the winter and summer MAP campaigns. For the winter campaign, idealized compositions of pure sea salt and non-sea salt sulfates (nss-S) were assumed. The analysis indicates that in this case, there was good general agreement between the predicted and observed CCN concentrations and that the marine aerosol over this period comprised predominantly sea salt and nss-S. This approach was repeated for the data sets for the summer campaign. To achieve closure between predicted CCN and observed CCN for the summer period, an insoluble organic mass fraction of 0.6 had to be assumed, thereby confirming a significant organic fraction to marine CCN during periods of high biological activity.

The Formation of Radiatively Active Aerosol from Coastal Nucleation Events

Two similar aerosol nucleation events sampled at the Mace Head Atmospheric Research station during 2-3 September 2005 resulted in the produc- tion of >105 particles cm �3 . The contribution of coagulation of these particles to radiatively active sizes (>50 nm) were examined and were found to be radically different. The 3rd September event, with a coagulation factor two times the 2nd September, was found to have a strong impact on the radiative and CCN properties of the observed aerosol population.

Role of the Volatile Fraction of Marine Aerosol on its Hygroscopic Properties

The hygroscopic growth factor (HGF) of 85 and 20 nm particles marine aerosols was measured during January 2006 for a three-week period within the frame of the MAP (Marine Aerosol Production) winter campaign, using the TDMA technique. The results were compared to aerosol produced in a simulation tank by bubbling air through sea water sampled at the coastal site of Mace Head during the campaign, and through synthetic sea water (exempt of organic substances). This simulation was assimilated to primary production. The 85 nm HGF observed in the atmosphere during clean marine sectors were lower than the ones measured from the bubbling processes: the sea salt HGF mode is slightly lower and there is an additional 1.5 HGF mode on the atmospheric aerosol. This would indicate that either the sea water sampled near Mace Head was not as rich in hydrophobic matter as further up wind or that secondary processes have occurred during transport. The role of the volatile fraction of the aerosols was then studied by gently heating the particles at 90°C (without particle size change) and measuring the subsequent HGF change, with a combination of Volatility and Hygroscopicity TDMA (i.e., the VHTDMA). We observed that the volatilization of less than 10% by diameter of the particles lead to an inhibition of the 1.5 GF mode for 85 nm particles but not for 20 nm particles. This result would indicate that secondary condensing processes, implying volatile substances, would have influenced the 85 nm particles. These results only apply to low biological activity periods.