Convective formation of pileus cloud near the tropopause

[摘要] Pileus clouds form where humid, vertically stratified air is mechanicallydisplaced ahead of rising convection. This paper describes convectiveformation of pileus cloud in the tropopause transition layer (TTL), andexplores a possible link to the formation of long-lasting cirrus at coldtemperatures. The study examines in detail in-situ measurements from off thecoast of Honduras during the July 2002 CRYSTAL-FACE experiment that showed anexample of TTL cirrus associated with, and penetrated by, deep convection.The TTL cirrus was enriched with total water compared to its surroundings,but was composed of extremely small ice crystals with effective radii between2 and 4 μm. Through gravity wave analysis, and intercomparison ofmeasured and simulated cloud microphysics, it is argued that the TTL cirrusoriginated neither from convectively-forced gravity wave motions norenvironmental mixing alone. Rather, it is hypothesized that a combination ofthese two processes was involved in which, first, a pulse of convectionforced pileus cloud to form from TTL air; second, the pileus layer waspunctured by the convective pulse and received larger ice crystals throughinterfacial mixing; third, the addition of this condensate inhibitedevaporation of the original pileus ice crystals where a convectively forcedgravity wave entered its warm phase; fourth, through successive pulses ofconvection, a sheet of TTL cirrus formed. While the general incidence andlongevity of pileus cloud remains unknown, in-situ measurements, andsatellite-based Microwave Limb Sounder retrievals, suggest that much of thetropical TTL is sufficiently humid to be susceptible to its formation. Wherethese clouds form and persist, there is potential for an irreversiblerepartition from water vapor to ice at cold temperatures.

[学科分类] 大气科学