The formation of ice in a long-lived supercooled layer cloud

Article English OPEN
Westbrook, C. D. ; Illingworth, A. J. (2013)

This article focuses on the characteristics of persistent thin single-layer mixed-phase clouds. We seek to answer two important questions: (i) how does ice continually nucleate and precipitate from these clouds, without the available ice nuclei becoming depleted? (ii) how do the supercooled liquid droplets persist in spite of the net flux of water vapour to the growing ice crystals? These questions are answered quantitatively using in situ and radar observations of a long-lived mixed-phase cloud layer over the Chilbolton Observatory.\ud \ud Doppler radar measurements show that the top 500 m of cloud (the top 250 m of which is mixed-phase, with ice virga beneath) is turbulent and well-mixed, and the liquid water content is adiabatic. This well-mixed layer is bounded above and below by stable layers. This inhibits entrainment of fresh ice nuclei into the cloud layer, yet our in situ and radar observations show that a steady flux of ≈100 m−2s−1 ice crystals fell from the cloud over the course of ∼1 day. Comparing this flux to the concentration of conventional ice nuclei expected to be present within the well-mixed layer, we find that these nuclei would be depleted within less than 1 h. We therefore argue that nucleation in these persistent supercooled clouds is strongly time-dependent in nature, with droplets freezing slowly over many hours, significantly longer than the few seconds residence time of an ice nucleus counter.\ud \ud Once nucleated, the ice crystals are observed to grow primarily by vapour deposition, because of the low liquid water path (21 g m−2) yet vapour-rich environment. Evidence for this comes from high differential reflectivity in the radar observations, and in situ imaging of the crystals. The flux of vapour from liquid to ice is quantified from in situ measurements, and we show that this modest flux (3.3 g m−2h−1) can be readily offset by slow radiative cooling of the layer to space.
  • References (54)
    54 references, page 1 of 6

    Ansmann A et al. 2009. Evolution of the ice phase in tropical altocumulus: SAMUM lidar observations over Cape Verde J. Geophys. Res. 114 D17208

    Baldwin MP and Vonnegut B 1982. Automatic apparatus for nucleation investigations. Rev. Sci. Instrum. 53 1911-1914

    Barlow TW and Haymet ADJ et al. 1995. ALTA: an automated lag-time apparatus for studying nucleation of supercooled liquids. Rev. Sci. Inst. 66 2996-3007

    Bergeron T 1935. On the physics of clouds and precipitation Proces Verbaux de l2˘018Association de Mtorologie Lisbon, Portugal, International Union of Geodesy and Geophysics, 156-178.

    Bouniol D, Illingworth AJ and Hogan RJ, 2003. Deriving turbulent kinetic energy dissipation rate within clouds using ground based 94GHz radar. Proc. 31 AMS Conf. on Radar Meteorology, Seattle 192-196

    Brenguier JL, 1991. Paramaterization of the condensation process: a theoretical approach. J. Atmos. Sci. 48 264-282

    Broadley SL et al. 2012. Immersion mode heterogeneous ice nucleation by an illite rich powder representative of atmospheric mineral dust. Atmos. Chem. Phys. 12 287-307

    Crosier J et al.. 2011. Observations of ice multiplications in a weakly convective cell embedded in supercooled mid-level stratus. Atmos. Chem. Phys. 11 257-273.

    de Boer G et al.. 2011. Evidence of liquid-dependent ice nucleation in high-latitude stratiform clouds from surface remote sensors. Geophys. Res. Lett. 38 L01803.

    DeMott PJ et al.. 2010. Predicting global atmospheric ice nuclei distribution and their impacts on climate. Proc. Nat. Acad. Sci. USA 107 11217-22.

  • Metrics
    views in OpenAIRE
    views in local repository
    downloads in local repository

    The information is available from the following content providers:

    From Number Of Views Number Of Downloads
    Central Archive at the University of Reading - IRUS-UK 0 48
Share - Bookmark