Abstract:
Balloon-borne water vapour measurements in the
upper troposphere and lower stratosphere (UTLS) by means
of frost point hygrometers provide important information on
air chemistry and climate. However, the risk of contamination
from sublimating hydrometeors collected by the intake
tube may render these measurements unusable, particularly
after crossing low clouds containing supercooled droplets.
A large set of (sub)tropical measurements during the 2016–
2017 StratoClim balloon campaigns at the southern slopes
of the Himalayas allows us to perform an in-depth analysis
of this type of contamination. We investigate the efficiency
of wall contact and freezing of supercooled droplets in
the intake tube and the subsequent sublimation in the UTLS
using computational fluid dynamics (CFD). We find that the
airflow can enter the intake tube with impact angles up to
60 , owing to the pendulum motion of the payload. Supercooled
droplets with radii >70 μm, as they frequently occur
in mid-tropospheric clouds, typically undergo contact freezing
when entering the intake tube, whereas only about 50%
of droplets with 10 μm radius freeze, and droplets <5 μm radius
mostly avoid contact. According to CFD, sublimation
of water from an icy intake can account for the occasionally
observed unrealistically high water vapour mixing ratios
( H2O >100 ppmv) in the stratosphere. Furthermore, we use
CFD to differentiate between stratospheric water vapour contamination
by an icy intake tube and contamination caused by
outgassing from the balloon and payload, revealing that the
latter starts playing a role only during ascent at high altitudes
(p <20 hPa).
