Abstract

Measurements and observations have been made on the development of ice in 90 cumulus (cumulus and cumulonimbus) and 72 stratiform (altocumulus, altostratus, nimbostratus, stratocumulus, and stratus) clouds. Ice particle concentrations significantly in excess of those to be expected from ice nucleus measurements (i.e., ice enhancement) were measured in 42 of the cumuliform and 36 of the stratiform clouds. For the complete data set, and for cloud top temperatures (TT) between −6° and −32°C, the maximum concentrations of ice particles (Imax in L−1) in the clouds were essentially independent of TT(r=0.32). However, Imax was strongly dependent on the broadness of the cloud droplet size distribution near cloud top. If the breadth of the droplet size distribution is measured by DT, such that the cumulative concentration of droplets with diameters ≥DT exceeds a prescribed value, then for −32≤TT≤−6°C:where n=8.4 and DO=18.5 μm for the cumuliform clouds and n=6.6 and DO=19.4 μm for the stratiform clouds.

When DT>D0 and TT≤−6°C, initial concentrations of ice were intercepted near the tops of clouds in the form of clusters ∼5–25 m wide. These clusters form strands of ice which, with increasing distance from cloud top, widen and merge and may eventually appear as precipitation trails below cloud base.

In light of these findings, it is postulated that ice enhancement is initiated during the mixing of cloudy and ambient air near the tops of clouds and that it is postulated with the partial evaporation and freezing of a small fraction (∼0.1%) of the droplets approximately >20 μm in diameter. Contact nucleation might be responsible for the freezing of these droplets. Under suitable conditions, this primary mechanism for ice enhancement may be augmented by other ice-enhancement mechanisms (e.g., ice splinter production during riming, and crystal fragmentation).

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