Topographically Generated Cloud Plumes

Abstract

Abstract Two topographically generated cirrus plume events have been examined through satellite observations and real-data simulations. On 30 October 2002, an approximately 70-km-wide cirrus plume, revealed by a high-resolution Moderate Resolution Imaging Spectroradiometer (MODIS) image and a series of Geostationary Operational Environmental Satellite (GOES) images, originated from the Sierra Nevada ridge and extended northeastward for more than 400 km. On 5 December 2000, an approximately 400-km-wide cloud plume originated from the Southern Rocky Mountain massif and extended eastward for more than 500 km, the development of which was captured by a series of GOES images. The real-data simulations of the two cirrus plume events successfully capture the presence of these plumes and show reasonable agreement with the MODIS and GOES images in terms of the timing, location, orientation, length, and altitude of these cloud plumes. The synoptic and mesoscale aspects of the plume events, and the dynamics and microphysics relevant to the plume formation, have been discussed. Two common ingredients relevant to the cirrus plume formation have been identified, namely, a relatively deep moist layer aloft with high relative humidity and low temperature (≤−40°C near the cloud top), and strong updrafts over high terrain and slow descent downstream in the upper troposphere associated with terrain-induced inertia–gravity waves. The rapid increase of the relative humidity associated with strong updrafts creates a high number concentration of small ice crystals through homogeneous nucleation. The overpopulated ice crystals decrease the relative humidity, which, in return, inhibits small crystals from growing into large crystals. The small crystals with slow terminal velocities (<0.2 m s−1) can be advected hundreds of kilometers before falling out of the moist layer.