Primary ice nucleation in orographic cirrus clouds: A numerical simulationof the microphysics

Ice particle production by the processes of homogeneous freezing and hetero geneous nucleation in orographic cirrus clouds is studied using a simple ad iabatic ascending-ah-parcel model. Homogeneous freezing rates in the model are based on a formulation by Jeffery and Austin, which matches with observ ed rates, modified according to the solution effect. Two alternative ice nu clei (IN) activation spectra are applied, one dependent upon temperature an d the other upon ice supersaturation. Simulations are performed for air par cels with initial dew-point temperatures, T-d, in the range from -20 to -45 degrees C and constant vertical velocities, w, of 0.1 to 5.0 m s(-1). In m odelled clouds with T-d greater than or equal to -30 degrees C, heterogeneo us nucleation initiated by IN is found to dominate ice formation, since hom ogeneous freezing rates are so low. In modelled clouds with T-d less than o r equal to -35 degrees C, pure liquid-water homogeneous freezing rates are large, but inclusion of sufficient quantities of IN may completely suppress aqueous solution droplet growth, dilution and homogeneous freezing, with h eterogeneous nucleation then remaining dominant even at very low temperatur es. The IN number concentrations required for homogeneous freezing suppress ion, for a fixed cloud condensation nucleus (CCN) activation spectrum, are found to increase with increasing w-heterogeneous nucleation at low tempera tures is therefore much more viable at the low w values found in non-orogra phic compared with orographic cirrus clouds. The common conception of a def icit of IN aloft is challenged, and, because of the apparent sensitivity of cirrus cloud microphysical and radiative properties to these aerosol parti cles, IN measurements at cirrus levels, in addition to CCN measurements, ar e suggested as vital. The model results suggest that it may be possible to interpret measurements of peak liquid-water content, or possibly maximum dr oplet radius, in orographic cirrus clouds in terms of the dominant ice nucl eation mode.