Effect of gravity wave temperature fluctuations on homogeneous ice nucleation in the tropical tropopause layer

Aurelien Podglajen* and Tra Dinh, Albert Hertzog, Bernard Legras, Riwal Plougonven
Laboratoire de Météorologie Dynamique, Paris, France

Observations of cirrus clouds in the tropical tropopause layer (TTL) have shown various ice number concentrations (INC). In particular, low INC have been frequently observed, which seemed hard to reconcile with homogeneous nucleation knowing the ubuquity of gravity waves with vertical velocity of the order of 0.1 m/s. Using artificial time series, Spichtinger and Krämer (2013) have illustrated that the variation of vertical velocity during a nucleation event could terminate it and limit the INC. However, their study was limited to constructed temperature time series.

Here, we carry out numerical simulations of homogeneous ice nucleation forced by temperature time series data collected during isopycnic balloon flights near the TTL. The balloons collected data at high frequency (30 s), so gravity wave signals are well resolved.

The simulations confirm that the time scale of temperature variations can be shorter than the nucleation time scale. They show the existence of two regimes for homogeneous ice nucleation : one limited by the depletion of water vapor by the nucleated ice crystals (vapor-limit events) and one limited by the reincrease of temperature after its initial decrease (temperature-limit events). Low INC may be obtained for temperature-limit events when the gravity waves produce a non-persistent cooling rate (even with large magnitude) such that the absolute change in temperature remains small during nucleation. This result is explained analytically by a dependence of the INC on the absolute drop in temperature (and not on the cooling rate).


Spichtinger, P. and Krämer, M.: Tropical tropopause ice clouds: a dynamic approach to the mystery of low crystal numbers, Atmos. Chem. Phys., 13, 9801–9818, doi:10.5194/acp-13-9801-2013, 2013.

Dinh, T., Podglajen, A., Hertzog, A., Legras, B., and Plougonven, R.: Effect of gravity wave temperature fluctuations on homogeneous ice nucleation in the tropical tropopause layer, Atmos. Chem. Phys. Discuss., 15, 8771-8799, doi:10.5194/acpd-15-8771-2015, 2015.


*email: aurelien.podglajen@lmd.polytechnique.fr
*Preference: Poster