Sensitivity of convectively driven tropical tropopause cirrus properties to ice habits in high-resolution simulations
Authors: Lamraoui Fayçal , Krämer Martina , Afchine Armin , Sokol Adam B. , Khaykin Sergey , Pandey Apoorva , Kuang Zhiming 
Published: February 2023

Abstract. Cirrus clouds that form in the tropical tropopause layer
(TTL) can play a key role in vertical transport through the upper
troposphere and lower stratosphere, which can significantly impact the
radiative energy budget and stratospheric chemistry. However, the lack of
realistic representation of natural ice cloud habits in microphysical
parameterizations can lead to uncertainties in cloud-related processes and
cloud–climate feedbacks. The main goal of this study is to investigate the
role of different cloud regimes and the associated ice habits in regulating
the properties of the TTL. We compare aircraft measurements from the
StratoClim field campaign to a set of numerical experiments at the scale of large-eddy simulations (LESs) for the same case study that employ different
microphysics schemes. Aircraft measurements over the southern slopes of the
Himalayas captured high ice water content (HIWC) up to 2400 ppmv and ice
particle aggregates exceeding 700 µm in size with unusually long
residence times. The observed ice particles were mainly of liquid origin,
with a small amount formed in situ. The corresponding profile of ice water content (IWC) from
the ERA5 reanalysis corroborates the presence of HIWC detrained from deep-convective plumes in the TTL but underestimates HIWC by an order of
magnitude. In the TTL, only the scheme that predicts ice habits can
reproduce the observed HIWC, ice number concentration, and bimodal ice
particle size distribution. The lower range of particle sizes is mostly
represented by planar and columnar habits, while the upper range is
dominated by aggregates. Large aggregates with sizes between 600 and 800 µm have fall speeds of less than 20 cm s−1, which explains the
long residence time of the aggregates in the TTL. Planar ice particles of
liquid origin contribute substantially to HIWC. The columnar and aggregate
habits are in the in situ range with lower IWC and number concentrations. For
all habits, the ice number concentration increases with decreasing
temperature. For the planar ice habit, relative humidity is inversely
correlated with fall speed. This correlation is less evident for the other
two ice habits. In the lower range of supersaturation with respect to ice,
the columnar habit has the highest fall speed. The difference in ice number
concentration across habits can be up to 4 orders of magnitude, with
aggregates occurring in much smaller numbers. We demonstrate and quantify
the linear relationship between the differential sedimentation of pristine
ice crystals and the size of the aggregates that form when pristine crystals
collide. The slope of this relationship depends on which pristine ice habit
sediments faster. Each simulated ice habit is associated with distinct
radiative and latent heating rates. This study suggests that a model
configuration nested down to LES scales with a microphysical
parameterization that predicts ice shape evolution is crucial to provide an
accurate representation of the microphysical properties of TTL cirrus and
thus the associated (de)hydration process.