ON THE FORMATION AND PERSISTENCE OF SUBVISIBLE CIRRUS CLOUDS NEAR THETROPICAL TROPOPAUSE

We have used a detailed cirrus cloud model to evaluate the physical pr ocesses responsible for the formation and persistence of subvisible ci rrus near the tropical tropopause and the apparent absence of these cl ouds at midlatitudes. We find that two distinct formation mechanisms a re viable. Energetic tropical cumulonimbus clouds transport large amou nts of ice water to the upper troposphere and generate extensive cirru s outflow anvils. Ice crystals with radii larger than 10 - 20 mu m sho uld precipitate out of these anvils within a few hours, leaving behind an optically thin layer of small ice crystals (tau(vis) similar or eq ual to 0.01 - 0.2, depending upon the initial ice crystal size distrib ution). Given the long lifetimes of the clouds, wind shear is probably responsible for the observed cloud thickness less than or equal to 1 km. Ice crystals can also be generated in situ by slow, synoptic scale uplift of a humid layer. Given the very low temperatures at the tropi cal tropopause (similar or equal to -85 degrees C), synoptic-scale upl ift can generate the moderate ice supersaturations (less than 10%) req uired for homogeneous freezing of sulfuric acid aerosols. In addition, simulations suggest that relatively large ice crystal number densitie s should be generated (more than 0.5 cm(-3)). The numerous crystals ca nnot grow larger than about 10 - 20 mu m given the available vapor, an d their low fall velocities will allow them to remain in the narrow su persaturated region for at least a day. The absorption of infrared rad iation in the thin cirrus results in heating rates on the order of a f ew K per day. If this energy drives local parcel temperature change, t he cirrus will dissipate within several hours. However, if the absorbe d radiative energy drives lifting of the cloud layer, the vertical win d speed will be about 0.2 cm-s(-1), and the cloud may persist for days with very little change in optical or microphysical properties. The f act that these clouds form most-frequently over the tropical western P acific is probably related (through the nucleation physics) to the ver y low tropopause temperatures in this region. Simulations using midlat itude tropopause temperatures near -65 degrees C suggest that at the h igher temperatures, fewer ice crystals nucleate, resulting in more rap id crystal growth and cloud dissipation by precipitation. Hence, the l ifetime of thin cirrus formed near the midlatitude tropopause should b e limited to a few hours after the synoptic-scale system that initiate d cloud formation has passed.