Recorded presentation1.6
Microphysical properties of subvisible cirrus
R. Paul Lawson, SPEC Inc, Boulder, CO; and B. Pilson, B. Baker, Q. Mo, E. Jensen, and P. Bui
Subvisible cirrus (SVC) have been shown to have a significant effect on the earth's radiation budget, and may play a role in freeze-drying of the tropical stratosphere. In situ microphysical observations of SVC between 8 degrees north latitude and the equator were collected using the NASA WB-57F research aircraft during the Costa Rica - Aura Validation Experiment (CR-AVE) in January 2006. The observations were taken near -85 C (17 to 18 km) and included ice particle size, shape, concentration, ice water content and relative humidity. Instrumentation included a cloud particle imager (CPI), a 2D-S (stereo) probe, a forward and backward scattering probe (CAS), the Harvard Integrated Cavity Output Spectroscopy (ICOS) water vapor probe and aerosol chemistry from the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument. These are the first in situ observations of subvisible cirrus that include a large dataset of digital images of the size and shape of tropical ice particles at -85 C, and the first image of ice particles in SVC since 1973. The data set is markedly different than the data set collected with a replicator in 1973, which suggested that the particles were mostly columnar and trigonal ice, which did not exceed about 50 microns in size. In contrast, the CR-AVE data shows that 84% of the particles are quasi-spherical in shape, and that the particles larger than about 65 microns are plates, with the largest particles having a maximum dimension up to 160 microns. The ice particles are almost always observed in an environment where the ICOS relative humidity w.r.t. ice (RHice) exceeds 120%, and sometimes exceeds 200%, which is where the largest particles are generally observed. Microphysical properties of the SVC clouds averaged over 1,700 km of flight data are presented and discussed. A numerical model that simulates ice particle growth in this environment suggests that the very high RHice (i.e., order 200%) is required to grow 100-micron ice crystals within 500 m of the tropical tropopause, where some of these large crystals are observed. This rekindles a long-standing discrepancy between airborne measurements (such as ICOS) and balloon-borne chilled-mirror measurements, which suggest that RHice is substantially less than the ICOS values. Chemistry measurements onboard the WB-57F show that the aerosols are mainly mixed organics and sulfates. The contributions of high RHice and unusual aerosol chemistry to the growth of these large particles, which are markedly different in size and shape from ice particles observed in SVC in 1973, may require cloud chamber experiments to understand.
Session 1, Recent Field Investigations of TTL
Monday, 20 August 2007, 9:00 AM-12:30 PM, Multnomah
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