Cirrus clouds, humidity, and dehydration in the tropical tropopause layer observed at Paramaribo, Suriname (5.8N, 55.2W)

In the framework of the European Project STAR the Mobile Aerosol
Raman Lidar (MARL) of the Alfred Wegener Institute (AWI) was
operated in Paramaribo, Suriname (5.8$^circ$N, 55.2$^circ$W) and
carried out extensive observations of tropical cirrus clouds during
the local dry season from 28 September 2004 to 16 November 2004. The
coverage with ice clouds was very high with 81\% in the upper
troposphere (above 12~km). The frequency of occurrence of subvisual
clouds was found to be clearly enhanced compared to similar observations
performed with the same instrument at a sation in the mid-latitudes. The
extinction-to-backscatter ratio of thin tropical
cirrus is with $26pm 7$~sr significantly higher than that of
mid-latitude cirrus ($16pm 9$~sr). Subvisual cirrus often occur in
the tropical tropopause layer (TTL) above an upper tropospheric
inversion. Our observations show that the ice-forming ability of the TTL is
rather high. The transport of air in this
layer was investigated by means of a newly developed trajectory
model. We found that the occurrence of clouds is well correlated
with the temperature and humidity history of the corresponding air
parcel. Air that experienced a temperature minimum before the
measurement took place was generally cloud free, while air that was
at its temperature minimum during the observation and thus was
saturated, contained ice. We also detected extremely tenuous cloud
layers slightly above the temperature minimum in sub-saturated air.
The solid particles of such clouds are likely to consist of nitric
acid trihydrate (NAT) rather than ice.