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Show where Q w liquid water content in gm - = C = qs = 3 vertical velocity saturation mixing ratio Using the Clapeyron equation, an expression for the variation of q with s height can be derived dq s dz where = qs (g + . 622 L dT] dz T - RT R = L T 3. 1. 3 (14) gas constant latent heat of condensation = temperature (0 A) Ice Phase Microphysics Assuming for the present that suitable sites are available for nucleation, the growth rates and trajectories of ice crystals can be computed in the model. Modeliing of the ice phase microphysics follows the development of Jiusto (196 7); details are presented in Appendix A of this report. Jiusto' s expressions for growth rates of crystals by diffusion, riming, and accretion are put into parameterized form and used to compute the evolution of total ice crystal water content. Particle trajectories are computed on the basis of the air flow model velocities and expressions for terminal fall velocity of the precipitation particles. 3. 1. 4 Model Examples The present version of the numerical mquntain precipitation model utilizes a vertical grid interval of 200 m and a horizontal grid interval of one kilometer. The required input data are vertical profiles of temperature, humidity, and wind velocity, and a cross section of the orographic barrier. The present model computes the purely orographic component of the precipitation only; a future version of the model will include a convective component and a large scale vertical velocity or storm component of precipitation. An illustrative examp le of the behavior of the model is provided by the case for 2 .January 1969. The input data are shown in Figure 19 for 1935 MST and 2250 MST. Note that the t emperature and moisture profiles are fairly similar. There is slightly less available moisture in th e 22 50 MST sounding. The wind velocity profiles, however, are markedly different below 200m. The 1935 MST sounding displays strong flow at lower levels, while the flow in the 2 2 50 MST sounding is considerably weaker in the lower layers. 43 |
