An annual cycle of Arctic cloud characteristics observed by radar and lidar at SHEBA

[1] The temporal distributions of cloudiness, vertical distribution of cloud boundary heights, and occurrence of liquid phase in clouds are determined from radar and lidar data sets collected from October 1997 to October 1998 during the Surface Heat Budget of the Arctic Ocean (SHEBA) project. The radar/lidar combination was necessary for comprehensive cloud detection over a variety of physical conditions and is significantly more detailed (5-9 s temporal resolution, 30-40 m vertical resolution) than measurements made by surface observers or satellites. The combined measurements revealed that clouds were almost continuously present, with an annual average occurrence of 85%, and displayed an overall annual trend of a cloudier summer and clearer winter. A monthly averaged cloud occurrence maximum of 97% was observed in September and a minimum of 63% was observed in February. Monthly averaged lowest cloud base heights were between 0.25 and 1.0 km above ground level (agl) and monthly averaged highest cloud top heights were between 2.5 and 5.5 km agl, and displayed no significant seasonal variation. The number of cloud layers was typically I or 2, with the summer months tending to be multilayered. The lidar utilized depolarization ratios to detect liquid water; the percentage of lidar-observed clouds containing liquid was 73% for the year. The least amount of liquid water phase was observed during December in 25% of the lidar-detected clouds and the maximum was observed during July in 95% of the lidar-detected clouds. Liquid was distributed in a combination of all-liquid and mixed phase clouds, and was detected at altitudes as high as 6.5 km agl and at temperatures as low as -34degreesC.