MESOSPHERIC CLOUD FORMATIONS Jeffrey M. Forbes Space Data Analysis Laboratory, Boston College, Chestnut Hill, MA 02167 The natural existence of noctilucent clouds at high latitudes during summer suggests that deposition of large amounts of H2O by HLLV exhausts near the cold mesopause (85 Km) could result in the formation of mesospheric clouds. Studies of mesospheric cloud formation indicate that three conditions must be met in order to form and maintain clouds near the mesopause: (1) As discussed by Witt (1969), homogeneous nucleation is slow, and condensation nuclei are required for sufficiently fast growth of ice particles. (2) The ambient temperature must be less than the frost point temperature at which saturation occurs. (3) Sufficiently large vertical velocities might be required to ensure growth of ice to required dimensions (~.lp) for a given water vapor contents. Also, from the point of view of assessing SPS-related effects, the frequency and magnitude of H2O injections are factors to be considered. Witt (1969) points out that an effective source of condensation nuclei might be large water cluster ions, since the coulomb forces act to lower the free-energy barrier against the formation of small droplets, allowing particle growth to proceed relatively easily. In a companion paper (hereafter referred to as Paper I) in this volume entitled ”D- and E-Region Effects” it is shown that injections of H2O into the mesosphere resulting in volume mixing ratios greater than 100 ppmv could convert most of the 02+ and N0+ ions at 85 Km to ions of the type H+(H20)n (n=l to 7), and thus provide an abundance of condensation nuclei. In Paper I it is estimated that H2O mixing ratios greater than 100 ppmv can occur over areas of order 20,000 Km2 for realistic SPS scenarios. The terminal fall velocity at 85 Km of particles with density 1 g cm“3 and radius .Ip is about 1 msec_l. Updrafts of this magnitude might be required to ensure growth of ice to required dimensions (~.lp) for a given water vapor content. Updrafts of 1 msec"! are not thought to exist at midlatitudes, but only at high latitudes during summer. This is consistent with the observed seasonal and latitudinal occurrence of noctilucent clouds. However, it is shown by Reid (1975) that growth of needle-shaped or disc-shaped particles is much faster than growth of spherical particles, which could relax the requirement of vertical velocities of order 1 msec"l. Ice particles will grow (shrink) depending on whether the ambient temperature (Ta) is less (greater than the frost point temperature (Tf) at which saturation occurs. In Figure 1, the latitude variation of mesopause temperature during summer and winter are plotted with frost point temperatures at 85 Km indicated for 1, 10, 102, 103, and 104 ppmv. During winter, there does not seem to be any possibility of maintaining clouds, even for mixing ratios as high as 104 ppmv. During summer, Tf exceeds Ta above 67 deg N for mixing ratios of order 10 ppmv. At midlatitudes (30-45 deg), where SPS launch activities would likely occur, mixing ratios of order 10^ ppmv would be required for the maintenance of clouds at the mesopause. A crude estimate of possible effects has been obtained by calculating (see Paper I) the post-launch redistribution of H2O due to diffusion, transport by winds, and photodissociation, and determining over what area Tf exceeds Ta. Assuming HLLV second stage deposits of 7.0 x 10-51 molecules of H2O evenly distributed over 70-120 Km, and injection frequencies ranging from 2 week-! to 8 dayl, Tf is exceeded over areas of order 200 Km2 around the point of injection. This area estimate is probably correct to within an order of magnitude unless the scenarios examined
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