Supercooled liquid water and airframe icing

B. Geerts

4/'98


Liquid water occurs in supercooled form in both stratiform and convective clouds, because of a relative lack of ice nuclei in most environments. At temperatures at or below -39C, liquid water will freeze spontaneously, i.e. in the absence of an ice nucleus. Old clouds above the freezing level, especially those with tops colder than -39C, will contain very little supercooled liquid water (SLW), because the ice crystals that form at lower temperatures will grow at the expense of cloud droplets (the Findeisen-Bergeron process) and fall to lower levels, just above the freezing level, where no ice nuclei are active, and there they will either consume the SLW, or trigger the freezing of supercooled droplets. Apparently many ice crystals break up, resulting in many more crystals than ice nuclei (ice multiplication).

Nevertheless, in young updrafts, or slowly rising layered clouds with cloud tops that are not too cold (above about -20C), SLW is often observed, and this constitutes a problem to aviation. When droplets impact on the airframe, they freeze, and ice accumulation on the wings may reduce the lift and enhance the drag of the airfoil. Generally the supercooled liquid water content is fairly low (<0.3 g/m3). The figure on the right shows the distribution of SLW content (LWC) versus temperature in stratiform clouds sampled during the Winter Icing and Storms Project (WISP) in northeastern Colorado during the winters of 1990-1994 (1). Small liquid water concentrations are the most common. The temperature range of around -5 to -12C has the highest LWC values.

 

It is at these temperatures that airframe icing appears to be most likely. At lower temperatures (especially below 20C) SLW is rarely observed, because ice nucleus activity is strongly temperature dependent, and the number of active nuclei increases exponentially with decreasing temperature:

N = N0 exp (a DT)

where N is the number of active ice nuclei per unit volume of air at a certain temperature T, N0 is the number of active ice nuclei at T0 =0C, and DT= T0-T (this experimental relationship is known as Fletchers curve (2)). The coefficient a varies between 0.3 and 0.8.

The graph on the right (1) shows that most icing (as reported in PIREPS or pilot reports) tends to occur at temperatures between 0 and -20 C. Rime icing, which is white, porous and opaque, generally results from smaller droplets, therefore it can occur at lower temperatures. Larger droplets are more likely to contain an active ice nucleus at a given temperature. Clear icing on the airframe results from larger droplets, which dont freeze immediately, but spread a little first, thereby excluding air bubbles. In rime icing, air bubbles are trapped because the freezing is instantaneous.

 

References

  1. Politovich, M., 1996: Response of a research aircraft to icing and evaluation of severity indices. Journal of Aircraft, 33, 291-297.
  2. Fletcher, N.H. 1962. The Physics of Rainclouds (Cambridge) 386pp.