On the origin of mid-latitude mesospheric clouds: The July 2009 cloud outbreak

Mid-latitude mesospheric clouds (MCs) are a rare phenomenon and their existence is not well understood, as the mesosphere at these latitudes is, in general, too warm for clouds to form. During the 2009 northern hemisphere summer season an unusually high number of these clouds were reported over both central and southern Europe, and the western contiguous United States. In this paper we investigate the mesospheric temperature field utilizing data from the Microwave Limb Sounder (MLS) instrument. We find that the temperature occasionally is near the frost point temperature and that the presence of planetary waves with periods of 2-, 5-, and 16-days combine to provide temperature anomalies of 1-1.5 K, lowering the temperature below the frost point for cloud formation and growth. Observed MCs are found to occur in close proximity to the 5-day wave anomaly. Model results show that the growth time to achieve visible particle sizes under the observed temperature and water vapor mixing ratio conditions are greater than similar to 20 h. Combined with climatological winds from a mid-latitude site, our study suggests that these clouds occur due to a combination of advection from higher and colder latitudes, and in situ wave growth.