What controls the low ice number concentration in the upper troposphere?

Abstract. Cirrus clouds in the tropical tropopause play a key role in regulating the moisture entering the stratosphere through their dehydrating effect. Low ice number concentrations ( <  200 L⁻¹) and high supersaturations (150–160 %) have been observed in these clouds. Different mechanisms have been proposed to explain these low ice number concentrations, including the inhibition of homogeneous freezing by the deposition of water vapour onto pre-existing ice crystals, heterogeneous ice formation on glassy organic aerosol ice nuclei (IN), and limiting the formation of ice number from high-frequency gravity waves. In this study, we examined the effect from three different representations of updraft velocities, the effect from pre-existing ice crystals, the effect from different water vapour deposition coefficients (α  =  0.1 or 1), and the effect of 0.1 % of the total secondary organic aerosol (SOA) particles acting as IN. Model-simulated ice crystal numbers are compared against an aircraft observational dataset.

Including the effect from water vapour deposition on pre-existing ice particles can effectively reduce simulated in-cloud ice number concentrations for all model setups. A larger water vapour deposition coefficient (α  =  1) can also efficiently reduce ice number concentrations at temperatures below 205 K, but less so at higher temperatures. SOA acting as IN is most effective at reducing ice number concentrations when the effective updraft velocities are moderate ( ∼  0.05–0.2 m s⁻¹). However, the effects of including SOA as IN and using (α  =  1) are diminished when the effect from pre-existing ice is included.

When a grid-resolved large-scale updraft velocity ( <  0.1 m s⁻¹) is used, the ice nucleation parameterization with homogeneous freezing only or with both homogeneous freezing and heterogeneous nucleation is able to generate low ice number concentrations in good agreement with observations for temperatures below 205 K as long as the pre-existing ice effect is included. For the moderate updraft velocity ( ∼  0.05–0.2 m s⁻¹), simulated ice number concentrations in good agreement with observations at temperatures below 205 K can be achieved if effects from pre-existing ice, a larger water vapour deposition coefficient (α  =  1), and SOA IN are all included. Using the sub-grid-scale turbulent kinetic energy (TKE)-based updraft velocity ( ∼  0–2 m s⁻¹) always overestimates the ice number concentrations at temperatures below 205 K but compares well with observations at temperatures above 205 K when the pre-existing ice effect is included.