Subvisual cirrus cloud observations by using lidars in Kiribati and Indonesia

Subvisual cirrus clouds (SVCs), which are defined as cirrus with an optical thickness less than 0.03 (Sassen and Cho, 1992, JAM), generally exist at a height of around 17 km in the tropical tropopause layer (TTL). Boehm and Verlinde (2000, GRL) was a first paper to analyze lidar and radiosonde data and concluded that the SVCs were generated by negative temperature anomalies induced by Kelvin waves. Comstock et al. (2002, JGR) analyzed the same data over 7 months, including the month analyzed by Boehm and Verlinde (2000, GRL). They concluded that “High cirrus with a cloud base z > 15 km do not coincide with negative temperature anomalies as often as found by Boehm and Verlinde (2000).” Iwasaki et al. (2004, GRL) and Matsuura (2005, M.S.thesis) analyzed ship-borne lidar data and their studies were in agreement with Comstock et al. (2002, JGR). Though evaluation of their hypothesis is needed, extensive observations and analyses of SVCs with lidars and radiosondes have been performed by many researchers.

In order to research characteristics of SVCs, Soundings of Ozone and Water in the Equatorial Region/Pacific Mission (SOWER) group installed lidars in Tarawa (1 oN, 173 oE), Kiribati, and Biak (0 oN, 136 oE), Indonesia, and the other Japanese science groups installed two lidars in Kototabang (0 oN, 100 oE), Indonesia. Hence joint lidar researches are possible over thousands km. In addition, SOWER group implemented water vapor and ozone soundings as presented by companion papers in the same session of this conference.

Tarawa lidar has been installed on December 2005 and worked continuously except power failure. The Nd:YAG laser whose transmitted energy is 20 mJ per pulse at a wavelength of 532 nm and 10 Hz pulse repetition rate is used. Lidar return is received by a telescope of which diameter is 20 cm, then it is measured with two photomultiplier tubes (PMTs), where we observe two polarization at 532 nm. Biak lidar has been installed on January 2006, and its spec is the same as that of the Tarawa lidar.

Nakashima et al. (2006, MSJ fall meeting) showed the tendencies of cirrus cloud occurrence over Tarawa from January to July in 2006; cloud top height of cirrus clouds in TTL is basically the same as the cold point, the height of the cloud top and the occurrence frequency of the clouds are the highest on January.

We will show the summary of the annual lidar and meteorological data in Tarawa, including the data analyzed by Nakashima et al., and examine a correlation of subvisual cirrus occurrences in Tarawa and Biak.