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[1] Measurements of the ice nucleating ability of aerosol particles in air masses over Florida having sources from North Africa support the potential importance of dust aerosols for indirectly affecting cloud properties and climate. The concentrations of ice nuclei within dust layers at particle sizes below 1 mm exceeded 1 cm À3 ; the highest ever reported with our device at temperatures warmer than homogeneous freezing conditions. These measurements add to previous direct and indirect evidence of the ice nucleation efficiency of desert dust aerosols, but also confirm their contribution to ice nuclei populations at great distances from source regions.

Abstract. On April 15 and 19, 1998, two intense dust storms were generated over the Gobi desert by springtime low-pressure systems descending from the northwest. The windblown dust was detected and its evolution followed by its yellow color on SeaWiFS satellite images, routine surface-based monitoring, and through serendipitous observations. The April 15 dust cloud was recirculating, and it was removed by a precipitating weather system over east Asia

[1] The cirrus clouds of the upper troposphere are globally widespread and are important regulators of the radiative balance, and hence climate, of the Earth-atmosphere system. Despite their wide distribution, however, cirrus are difficult to study from satellite radiance measurements or from scattered ground observing sites because they can occur as part of multilayered cloud systems and are characteristically optically thin. The need to better characterize the global distribution of cirrus clouds was therefore a major justification for the formation flying of the CloudSat and CALIPSO satellites, which support a cloud radar and polarization lidar, respectively. Measurements by these active remote sensors, when analyzed by appropriate algorithms, have the ability to identify and accurately measure the locations and heights of this category of clouds. The combined CloudSat/ Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data cirrus cloud algorithm used in this study is aimed at identifying those clouds that would likely be classified as cirrus by a surface weather observer: it is based on previous experience with multiple remote sensor approaches and knowledge gleaned from extensive surface lidar and radar observations of visually identified cirrus clouds with a minimum of a priori assumptions. We report on the global and seasonal frequencies of cirrus clouds, and on their heights and thicknesses obtained over the initial 1 year of data collected. We find a global average frequency of cirrus cloud occurrence of 16.7%. These new results are compared with other cirrus cloud climatologies and are interpreted in terms of local cirrus cloud formation mechanisms and the responsible global weather phenomena.

[1] A recent field experiment in southern Florida using aircraft and polarization lidar shows that mineral dust particles transported from Saharan Africa are effective ice nuclei, apparently capable of glaciating a mildly supercooled (À5.2°to À8.8°C) altocumulus cloud. These results are similar to those from Asian dust storm particles observed over the western US, suggesting that in the northern hemisphere major dust storms play a role in modulating climate through the indirect aerosol effect on cloud properties. If this is true of desert dusts in general, then even minor aeolian emissions could have an effect on regional weather and climate.