Figure 6. Profiles of horizontally averaged mean ice crystal size for four different habits. The warm stable case is shown in the left two columns, and the cold stable case is shown in the right two columns (adopted from Liu et al., 2003b).

The corresponding radiative heating rates for the warm and cold stable cases are shown in Fig. 7. Although the ice water content profiles for columns and plates are similar (Fig. 5), the corresponding solar and IR heating rates are slightly larger for plates than for columns. It can be seen that the IR heating rate is more sensitive to ice crystal habit than the solar radiative heating rate is.

The differences among simulations with different ice habits may be explained on the basis of ice crystal capacitance. The growth rate of ice crystals is calculated based on the concept of electrostatic analogy as explained by Pruppacher and Klett (1997). In this analogy, the central quantity determining the ice growth rate is the capacitance, which is parametrized as a function of the maximum crystal diameter and habit in this model. A plot of the capacitance as a function of mass and habit is shown in Fig. 8. For ice particles with the same mass, bullet rosettes have the largest capacitance, hence the greatest diffusional growth rate, followed by plates and columns, whereas spheres have the smallest. We have seen that the ice water content is generally largest for rosettes, with columns ranked second, plates third, and spheres last (Fig. 4). Although the capacitance of columns is larger than that of plates, the resulting ice water contents are almost the same. This may be because the differential radiative heating induced by plates is greater than that for columns, resulting in slightly stronger upward vertical motion for the plates, and thus producing more ice water content.

The above discussions have made it clear that assuming spherical particles in cirrus modeling would have resulted in seriously underestimating the radiative impact. To make an accurate assessment of the radiative impact of cirrus clouds, it is important to have good

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