Colloquium, Fri., 5 Dec., 2:00 pm, EN6085A (NOTE: FRIDAY)
The Removal of Ultrafine Nuclei in Mountain Wave Clouds
The smallest detectable atmospheric aerosol particles - ultrafine nuclei (UFN) with diameter between 0.003 and 0.015 µm - were studied in association with mountain wave clouds formed over the Medicine Bow Mountains located in southeastern Wyoming. Aircraft measurements of the UFN were made upwind and downwind of the cloud, from which the removal of these particles was estimated, and an airborne Doppler cloud radar was used to measure the horizontal and vertical components of the wind vector. The study was conducted on four days: January 18, 2006, January 26, 2006, January 31, 2006 and February 2, 2006.
In model simulations, an air parcel trajectory obtained from the radar wind field and a cloud parcel model were used to simulate UFN removal through the cloud. The trajectory/parcel model was initialized with the measurements made in the cloud inflow region, and was validated with cloud property measurements made at the top of the trajectory and with observational values of the UFN removal.The model simulates the time-dependent size distributions of cloud condensation nuclei, cloud droplets, and ice particles. Based on the model, and its comparison to the observations, four conclusions are reached: 1) Approximately 50% of the UFN entering the cloud are removed during through-cloud transport, 2) the UFN are too small to initiate cloud droplet formation, 3) the removal of the UFN is dominated by their Brownian attachment to cloud droplets, and 4) the Brownian attachment of the UFN to ice particles contributes less than 10% to their removal.