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Title: Sensitivity of convectively driven tropical tropopause cirrus properties to ice habits in high-resolution simulations
Abstract. Cirrus clouds that form in the tropical tropopause layer(TTL) can play a key role in vertical transport through the uppertroposphere and lower stratosphere, which can significantly impact theradiative energy budget and stratospheric chemistry. However, the lack ofrealistic representation of natural ice cloud habits in microphysicalparameterizations can lead to uncertainties in cloud-related processes andcloud–climate feedbacks. The main goal of this study is to investigate therole of different cloud regimes and the associated ice habits in regulatingthe properties of the TTL. We compare aircraft measurements from theStratoClim field campaign to a set of numerical experiments at the scale of large-eddy simulations (LESs) for the same case study that employ differentmicrophysics schemes. Aircraft measurements over the southern slopes of theHimalayas captured high ice water content (HIWC) up to 2400 ppmv and iceparticle aggregates exceeding 700 µm in size with unusually longresidence 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) fromthe ERA5 reanalysis corroborates the presence of HIWC detrained from deep-convective plumes in the TTL but underestimates HIWC by an order ofmagnitude. In the TTL, only the scheme that predicts ice habits canreproduce the observed HIWC, ice number concentration, and bimodal iceparticle size distribution. The lower range of particle sizes is mostlyrepresented by planar and columnar habits, while the upper range isdominated by aggregates. Large aggregates with sizes between 600 and 800 µm have fall speeds of less than 20 cm s−1, which explains thelong residence time of the aggregates in the TTL. Planar ice particles ofliquid origin contribute substantially to HIWC. The columnar and aggregatehabits are in the in situ range with lower IWC and number concentrations. Forall habits, the ice number concentration increases with decreasingtemperature. For the planar ice habit, relative humidity is inverselycorrelated with fall speed. This correlation is less evident for the othertwo ice habits. In the lower range of supersaturation with respect to ice,the columnar habit has the highest fall speed. The difference in ice numberconcentration across habits can be up to 4 orders of magnitude, withaggregates occurring in much smaller numbers. We demonstrate and quantifythe linear relationship between the differential sedimentation of pristineice crystals and the size of the aggregates that form when pristine crystalscollide. The slope of this relationship depends on which pristine ice habitsediments faster. Each simulated ice habit is associated with distinctradiative and latent heating rates. This study suggests that a modelconfiguration nested down to LES scales with a microphysicalparameterization that predicts ice shape evolution is crucial to provide anaccurate representation of the microphysical properties of TTL cirrus andthus the associated (de)hydration process.  more » « less
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Journal Name:
Atmospheric Chemistry and Physics
Page Range / eLocation ID:
2393 to 2419
Medium: X
Sponsoring Org:
National Science Foundation
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