contrails might also evolve into, or trigger, widespread and persistent high-altitude clouds or enhance naturally occurring clouds are important questions. At high latitudes in summer, the meteorological conditions at the mesopause favor formation of tenuous night-luminous (noctilucent) clouds (NLCs) (4). However, at low latitudes, NLCs are rarely seen, except occasionally following rocket launches. Obviously, it would be most informative to test for the production of rocket clouds through appropriately designed atmospheric experiments. However, because of the large scale of the proposed rocket launches and the need to study cloud formation under a wide variety of conditions, it is also useful to have a detailed physical model to similate rocket contrails and clouds. Accordingly, we constructed a comprehensive noctilucent cloud model for this purpose. A description of the model is given in Sec. 2, including details of the physics and photochemistry employed. A comparison of simulated and observed noctilucent clouds is also made there. In Sec. 3, existing rocket cloud observations are reviewed. The effects of rocket water vapor emissions on local and global cloudiness are calculated specifically for the SPS HLLV in Sec. 4. Our findings are summarized in Sec. 5. 2. THE ICE CLOUD MODEL The mesospheric ice cloud model used in the present study was developed by Turco et al. (5). They give a complete description of the microphysical, photochemical and electrical processes treated. In summary, the model is built around four major physical elements: water vapor, ice crystals, meteoric dust, and air ionization. These elements, and some of their interactions, are illustrated schematically in Fig. 1. Each model element is analyzed over a vertical spatial grid which extends from 70 to 105 km (42 to 63 mi) in 0.5 km (0.3 mi) steps. The particles occupy 35 size bins ranging from 10 A to 2.6 jum radius; particle volume doubles between each bin. The model is time-dependent. Vertical transport occurs by eddy diffusion and gravitational sedimentation (for particles). Ions, however, are maintained in a local quasisteady state. The air temperature, density, and eddy diffusion coefficient profiles are nominally held fixed, but they may also be varied with time. Solar fluxes are determined at each height in 96 wavelength bins. These fluxes are employed to calculate photodissociation rates in the water/methane chemical system, and electron photoemission rates from particles. The v/ater vapor and methane concentrations are fixed at the lower boundary of the model (~70 km) (42 mi). They diffuse upward and are decomposed by sunlight (the methane contribution to the water budget above 70 km is negligible, however). Water vapor condenses on and evaporates from ice particles. Under the proper conditions, water vapor also initiates the nucleation of ions and meteoric dust. The ice crystal microphysics treated in the model includes nucleation, condensation, coagulation and sedimentation. Crystal shape effects on growth, coagulation and fall rates are accounted for. The equilibrium electrical charges on small particles embedded in an ionospheric plasma are also calculated and are used to correct particle coagulation rates. However, temperature effects due to latent heat release and sunlight absorption are not treated; the former effect is negligible for small, slowly-growing ice particles, while the latter effect has been shown to be relatively unimportant near the mesopause (6). Dust produced by ablating meteroids is analyzed in the manner of Hunten et al. (7). We assume that a 10 A radius smoke is generated in the trails of disintegrating
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