Cirrus cloud appearance in a volcanic aerosol layer around the tropical cold point tropopause over Biak, Indonesia, in January 2011

[1] An aerosol layer was found 1–2 km around the tropical cold point tropopause by observations using ground-based lidar and balloon-borne optical particle counters (OPC) over Biak, Indonesia, in January 2011. The layer was observed throughout the survey period from 6 to 13 January. The backscattering coefficient of the layer was about 5 times larger than that of the background aerosols. The lidar-observed depolarization ratio of the layer was very low, and the wavelength dependence of the backscattering coefficients of the layer was similar to that of stratospheric aerosols. A layer of the particles at the size of the accumulation mode was also observed by an OPC at ambient temperature, but the particles were volatile at 200°C. These properties indicate that the aerosol layer was composed of liquid phase aqueous sulfuric acid solution particles and probably originated from a volcanic eruption. It was observed by lidar that a thin cirrus cloud layer appeared within this aerosol layer. Backward trajectory analysis and satellite-observed equivalent blackbody temperature indicate that the cirrus cloud layer probably formed in situ. The estimated upper limit of the number concentration of cloud particles was 105 m−3. The number concentration of the volatile aerosol particles (∼3 × 106 m−3) was 30 times larger than this upper limit. This upper limit, however, is comparable to the aerosol particle concentration observed by the OPC at 200°C. These results are consistent with the cirrus cloud formation with solid sulfate particles in tropical upper troposphere suggested by previous studies.