In situ measurements of tropical cloud properties in the West African Monsoon: upper tropospheric ice clouds, Mesoscale Convective System outflow, and subvisual cirrus (original) (raw)
2011, Atmospheric Chemistry and Physics
In-situ measurements of ice crystal size distributions in tropical upper troposphere/lower stratosphere (UT/LS) clouds were performed during the SCOUT-AMMA campaign over West Africa in August 2006. The cloud properties were measured with a Forward Scattering Spectrometer Probe (FSSP-100) and a Cloud Imaging Probe 5 (CIP) operated aboard the Russian high altitude research aircraft M-55 "Geophysica" with the mission base in Ouagadougou, Burkina Faso. A total of 117 ice particle size distributions were obtained from the measurements in the vicinity of Mesoscale Convective Systems (MCS). Two or three modal lognormal size distributions were fitted to the average size distributions for different potential temperature bins. The measure-10 ments showed proportionate more large ice particles compared to former measurements above maritime regions. With the help of trace gas measurements of NO, NO y , CO 2 , CO, and O 3 , and satellite images clouds in young and aged MCS outflow were identified. These events were observed at altitudes of 11.0 km to 14.2 km corresponding to potential temperature levels of 346 K to 356 K. In a young outflow (developing 15 MCS) ice crystal number concentrations of up to 8.3 cm −3 and rimed ice particles with maximum dimensions exceeding 1.5 mm were found. A maximum ice water content of 0.05 g m −3 was observed and an effective radius of about 90 µm. In contrast the aged outflow events were more diluted and showed a maximum number concentration of 0.03 cm −3 , an ice water content of 2.3 × 10 −4 g m −3 , an effective radius of about 18 µm, 20 while the largest particles had a maximum dimension of 61 µm. Close to the tropopause subvisual cirrus were encountered four times at altitudes of 15 km to 16.4 km. The mean ice particle number concentration of these encounters was 0.01 cm −3 with maximum particle sizes of 130 µm, and the mean ice water content was about 1.4 × 10 −4 g m −3 . All known in-situ measurements of subvisual tropopause 25 cirrus are compared and an exponential fit on the size distributions is established in order to give a parameterisation for modelling. 11, 2011
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