3. DYNAMICAL AND THERMODYNAMICAL ASPECTS The proposed HLLV would emit approximately 1.08 x 1011 cal/s of thermal energy together with 2.02 x 107g/s of water to the atmosphere. Approximately 15s of exhaust would be contained in the ground cloud. The thermal energy provides sufficient buoyancy to lift the ground cloud and surrounding air to higher altitudes. During the course of rising, air cools through adiabatic expansion and, under certain conditions, reaches saturation to form water- saturated cloud. Cloud convection is, then, further enhanced through release of latent heat, and, in some situations, it could lead to precipitation. The phenomenon of a wet, saturated cloud formed by rocket exhaust has been observed on several occasions. Perhaps the most comprehensive and unique data are those obtained during a Tital III launch on December 13, 1978 at Kennedy Space Center. Temperature and dew point soundings prior to the time of launch indicate that air in the surface boundary layer is humid but potentially stable as shown in Fig. 1. Rocket effluents produced a saturated white cloud having the characteristics of a moderately-sized, vigorous cumulus cloud. Aircraft measurements taken 25 minutes after launch indicated that the ground cloud was still saturated with a liquid water content of about 0.1 g/m3. Thereafter, only portions of the ground cloud were found to be saturated; however, liquid water content was still detectable until 51 minutes after launch. Model calculations indicated that, under the same meteorological condition, the HLLV thermal effluent could generate a much more vigorous convective cloud than a Titan effluent did as shown in Fig. 2. The maximum cloud liquid water content in the HLLV cloud was predicted to be about 3 times that of the Titan cloud as compared in Fig. 3 (where an initial thermal energy of 9.4 x 1010 cal was assumed in the Titan cloud). Furthermore, a light precipitation with a maximum rainwater of 0.07 g/kg was predicted for the HLLV cloud, but the duration of a saturated cloud was shorter. Virtually all the liquid water and precipitation are from the atmosphere, not from the content of the HLLV and Titan rocket exhaust. The above relationships should not be used to scale predictions of HLLV effects. For example, under a potentially unstable condition with a deep surface boundary layer where the temperature lapse rate is adiabatic, quasisteady-state convective clouds with similar intensities could be generated by all types of rockets in which the exhaust thermal energies are different by two orders of magnitude. The predicted precipitations are slightly different in intensity for different types of rockets. In view of the nonlinearity and the relative insensitivity of the results to the rocket energy output in some situations, a climatology of the HLLV impacts should be conducted for a given launch site and for an updated HLLV reference information. Cloud modifications from SPS effluents are sensitively dependent upon the ambient meteorological conditions. Generally, the conditions that favor onshore flow without strong westerlies above the planetary boundary layer are conducive to greater inadvertent weather modification by SPS rocket launches in the Florida area. Characteristic synoptic weather regimes that would fall into this category were identified in a theoretical study of space-shuttle exhaust cloud.
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