1. How do dehydration scenarios along forward trajectories differ for type A and B air masses in the ASM region?
Dehydration scenarios along forward trajectories differ for type A and B air masses in the ASM region. Type A air masses, which are freshly convected, experience strong dehydration, with mean/maximum values after 40 days of 5.0/11.3 ppmv (CLaMS-Ice) and 3.3/4.1 ppmv (FDM). In contrast, type B air masses, which have less than 1 ppm of ice at initialization time, experience weaker dehydration, with mean/maximum values after 40 days of 8.1/9.8 ppmv (CLaMS-Ice) and 6.5/7.9 ppmv (FDM). Only 14% (CLaMS-Ice) and 1% (FDM) of the initial observations did not experience any dehydration. The dehydration scenarios for type A and B are consistent with the respective frequency distributions of LDP temperatures from backward and forward trajectories. The strong dehydration of type A air masses is mainly due to the lowest temperatures (Lagrangian cold point) being experienced in the forward direction, while type B air masses can still experience significant dehydration in the future, well above the CPT, at the southern edge of the 75 anticyclone during the upward spiraling motion of the forward trajectories.
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2. How does CLaMS-Ice compare with CALIPSO observations?
CLaMS-Ice compares ice distribution during dehydration periods with CALIPSO observations, which detect ice mixing ratios larger than ~0.1 ppm. The results show a strong contribution over North India, while type B ice simulated by CLaMS-Ice has a larger horizontal spread over southeast Asia and the Maritime Continent. These signatures are related to isentropic mixing driven by Rossby waves. There are weak CALIPSO signatures of ice north of 35N, but warm temperatures raise doubts about their origin. The comparison with MLS observations shows a strong disagreement for type B data, even with enhanced ice nucleation in the model. Including dehydration in the backward direction improves the model's performance for all data types.
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