On the October 30, 2002, a high resolution (250m) MODIS image revealed a 40km-wide cloud plume trailing from the high Sierras massifs (Mountain Whitney) and extending north-eastward into the state of Nevada. The plume was more than 350km long and the cloud top height, estimated from the cloud shadow on the ground, was about 8.5km ASL. This spectacular mesoscale cloud plume was well captured by the COAMPS real data simulation with 55 vertical levels and three level nesting (grid sizes: 27km, 9km, and 3km). Satisfactory agreement is found between the model cloud ice field, and the MODIS snapshot and a series of GOES visible images. A second mesoscale cloud plume event occurred on December 05, 2000 and was characterized by a wide-spread cloud plume trailing over the lee of Sangre de Cristo Range and Sacramento Mountains as revealed by GOES visible images. The cloud plume spread over several states (Colorado, New Mexico, Oklahoma, Kansas, and Texas). A real data COAMPS simulation captures the formation and evolution of the cloud plume. COAMPS suggests that the cloud plume was approximately 9km ASL. In both events, the mesoscale cloud plumes were high above the surface, extended a long distance and were fairly persistent. These cloud plumes were clearly created/enhanced by the mesoscale terrain underneath. The physics and dynamics associated with such a spectacular mesoscale phenomenon have not yet been addressed.

In this study, two irreversible processes are identified as responsible for the cloud plume formation through diagnosis of the two COAMPS real data simulations, additional idealized simulations, and theoretical formulations. One is warming associated with latent heating and the second is ice-particle advection. As moist air (8-9km ASL) approaches a mesoscale mountain, it initially ascends in the upstream portion of the hydrostatic mountain wave and becomes saturated or supersaturated. At the level of 8-9km ASL, the temperature is favorable for the growth of ice crystals (~ -400C). Therefore, ice particles form with latent heat release over the ascending portion of the mountain wave. Some ice particles may survive the descent over the lee-slope, and therefore, the air is advected downstream with net heating and ice particles present that comprise the cloud plume. (isnt°¯ the cloud plume the same as the ice particles?).. Additionally, according to linear theory, the heated air will ascend downstream as the logrithmic function of the distance unless it reaches a new equilibrium state due to radiative cooling, rotation, or diffusion.